Document Number
Technical Report
No of Pages
The Three Gorges-Changzhou ± 500 kV DCTransmission Project
Prepared
1JNL100029-886 Rev. 01
Title
65
No of Attached PagesReg./Class no.
Hans Hillborg, 1999-08-31
Approved
Resp Dept
DC System Protection
PKC
WS-FJ-101D
Hans Tyskhagen, 1999-12-30Summary
This document is issued by means of a computerized system. The digitally stored original is electronically approved.The approved document has a name and date entered in the approved-field. A manual signature is not required.
This document describes the DC System Protection for The Three
Gorges-Changzhou ± 500 kV DC Transmission Project. The DC System Protectionincludes the following four groups of protections.• Converter protections.
• Pole protections (DC switchyard incl. pole and neutral busses, DC line and DC
filter).• Bipole protections (Bipole neutral and Electrode line).• Converter AC bus and converter transformer protections.
For each protective function the purpose of the protection, principle of protectionoperation, fault strategy and protection coordination, consequences of protectionoperation, redundancy or backup of protection and consequences oftelecommunication outage are described.
1
Rev ind
Rewritten
Revision text
Karl-Ola Jonsson2000-02-09
Prepared
Hans Tyskhagen2000-03-15
Approved
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Table of Contents
12344.14.24.34.44.4.14.4.24.4.34...74.84.94.104.114.1255.15.1.15.1.25.1.35.1.45.1.55.1.65.1.75.1.85.1.95.1.105.1.115.1.125.1.135.25.2.15.2.25.2.35.2.45.2.55.2.65.2.75.2.85.2.9
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Introduction........................................................................4Abbreviation.......................................................................4General Protection Philosophy.........................................5Fault Clearing Actions........................................................7Transfer to Redundant Protection System............................7Retarding of the Converter...................................................8Order Down to the Rectifier..................................................8Blocking of the Converter.....................................................8Type X - Blocking.................................................................9Type Y - Blocking.................................................................9Type Z - Blocking..................................................................9AC Circuit Breaker Trip.........................................................9Start Breaker Failure Protection..........................................10Set Lockout Relay for AC Circuit Breaker...........................10Runback.............................................................................10Pole Isolation......................................................................10Pole Balancing...................................................................10Reclose Transfer Breaker...................................................10Close Neutral Bus Ground Switch......................................11Protective Functions........................................................11Converter Protections.........................................................13Valve Short Circuit Protection.............................................14Commutation Failure Prediction..........................................14Commutation Failure Protection.........................................15Voltage Stress Protection...................................................16DC Overvoltage Protection.................................................17Valve Misfire Protection......................................................18Thyristor Monitoring............................................................18DC Overcurrent Protection.................................................19Back-up DC Overcurrent Protection...................................20High Angle Supervision......................................................21Valve DC Differential Protection.........................................22Auxiliary Power Supervision...............................................23Valve Cooling System Protection........................................23Pole Protections.................................................................24DC Harmonic Protection.....................................................25DC Pole Bus Differential Protection....................................25DC Neutral Bus Differential Protection................................26DC Pole Differential Protection...........................................27Electrode Line Open Circuit Protection...............................27DC Filter Overload Protection.............................................28DC Filter Capacitor Unbalance Protection..........................29DC Filter Differential Protection..........................................29DC Line Protections............................................................30
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5.2.105.2.115.2.125.2.135.2.145.35.3.15.3.25.3.35.3.45.3.55.3.65.3.75.3.85.3.95.45.4.15.4.25.4.35.4.45.4.55.4.65.4.75.4.85.4.95.4.105.4.115.4.125.4.135.4.145.4.156
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1JNL100029-886 Rev. 01
DC Undervoltage Protection...............................................34Open Line Test Operation..................................................35Open Line Test Supervision...............................................35Reverse Power Direction Protection...................................35Smoothing Reactor Protective Relays................................36Bipole Protections...............................................................41Bipole Neutral Bus Differential Protection...........................42Station Ground Overcurrent Protection...............................42Transfer Breaker Protections..............................................43Metallic Return Transverse Differential Protection..............47Metallic Return Longitudinal Differential Protection.............47Metallic Return Conductor Ground Fault Protection............48Electrode Line Overload Protection....................................48Electrode Line Impedance Supervision...............................49Electrode Line Unbalance Supervision...............................49Converter AC Bus and Converter Transformer Protections50Converter AC Bus Differential Protection............................51Converter AC Bus and Converter Transformer
Overcurrent Protection.......................................................51Converter AC Bus and Converter Transformer DifferentialProtection...........................................................................52Converter Transformer Differential Protection.....................52Converter Transformer Overcurrent Protection...................53Converter Transformer Thermal Overload Protection.........Converter Transformer Winding Differential Protection.......Converter AC Bus Overvoltage Protection..........................56Converter Transformer Neutral Shift Protection..................56Converter Transformer Zero Sequence Current Protection.57Converter Transformer Overexcitation Protection...............58Converter Transformer Saturation Protection.....................58Last Breaker Protection, Zhengping only............................59Converter Transformer Restricted Earth Fault Protection...59Converter Transformer Protective Relays...........................60References........................................................................65
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1 Introduction
This document describes the DC protections functions according to the
requirements in [1]. For all protections the following items are described in detail.• Purpose of the protection.• Principle of protection operation.
• Fault strategy and coordination between the DC control, DC protection and the
AC protection.• Consequences of protection operation, such as DC control and switching action
initiated at both converter stations.• Redundancy of protection or reference to what protection acts as back up.• Consequences of telecommunication outage.The following information is found in separate reports• Required accurancy of measuring signals is presented in [4]
• Detailed calculations of the protection settings together with limiting faults
cases and/or criteria that determines these settings is presented in [5] and [8].
2 Abbreviation
ACPCTDCCTGRTSHASIVDIVYMACH 2MCMRTBNBGSNBSOCTOLT
AC Control and Protection.Current Transformer.DC Current Transformer.Ground Return Transfer Switch.High Angle Supervision.
Three phased rectified AC currents ∆-winding.Three phased rectified AC currents Y-winding.Modular Advanced Control for HVDC and SVC.Main Computer
Metallic Return Transfer Breaker.Neutral Bus Ground Switch.Neutral Bus Switch.Optical Current Transformer.Open Line Test
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3 General Protection Philosophy
The purpose of the HVDC System Protection is to cause the prompt removal of anyelement of the transmission system from service, e.g. when it suffers a short circuitfault or when it starts to operate in any abnormal manner that might cause damageor otherwise interfere with the effective operation of the rest of the system. Theprotective system is aided in this task by the AC circuit breakers, which are capableof de-energizing the converter transformers, thereby eliminating the direct currentand voltage.
The latest protection system uses powerful computers and allows future adoptionof new protective functions and adjustments of existing protections in a flexiblemanner. The protection system is built in a modular way, which enables astructured design for complete system.
Redundant Control and Protection systems (Main 1 and Main 2) are provided.The HVDC control and protection systems are based on a fully redundant activestandby concept. In general, each system consists of primary and backupprotections.
SYSTEM OVERVIEWMAIN 1( CUBICLE A )MAIN 2( CUBICLE B )MC 1MC 1CONTROLDC SYSTEMPROTECTIONSET 1MC 2MC 2CONTROLDC SYSTEMPROTECTIONSET 1DC SYSTEMPROTECTIONSET 2DC SYSTEMPROTECTIONSET 2Figure 1 System overview.
Each of the two main systems consists of two main computers MC1 and MC2.MC1 contains the DC control and DC protections set 1 and MC2 contains DCProtections set 2.
In order to improve security for converter, pole and bipole protections which canoperate inadvertently for control system faults measurements, the concept of usingfast changeover from active to standby system is adopted. Before a trip order isissued from active protection system, a system change over is performed wheneverdelay due to such changeover is acceptable. If the redundant system issues a triporder as well, the order will be sent through. To separate between change over anda block/trip orders, both time and level separation is used.
The converter AC bus and the converter transformer protections are active in bothMain 1 and Main 2 protection systems.
In addition, each system has extensive self-supervision which further enhancessystem security. Any detected failures in the control and protection hardware will
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result in a request for change over, which will be executed if there is a stand-bysystem that is ready to take over.
The proposed protective scheme is designed to meet the following generalrequirements:
a)Fault conditions or other abnormal conditions that might expose equipment tohazards shall be detected. Also conditions that cause unacceptable disturbancesto operation should be detected and the faulty or overstressed equipment shouldbe taken out of service or relieved of stresses in a controlled way.b)The aim of the protection design is to detect every condition, according toabove, with at least two protections.c)Steps to minimize the possibility of fault in one converter causing protectiveaction in other converters shall be taken.d)The protections will be arranged into overlapping protective zones. For eachfault case, there should be a fast main protection with a limited protective zone.The main protection is normally supported with a slower or less sensitivebackup protection. The backup protection will, if possible, be based on adifferent measuring principle and, when applicable, with a more extendedprotective zone.e)The consequences of protection operating and protection coordination shall bearranged to avoid bipole outages.f)Auxiliary power supplies and sources to main 1 and main 2 protections shall beseparated.g)Tripping paths to the breaker shall be redundant. Trip coil on the breaker shallbe redundant and fed by two different auxiliary power supplies.h)The protections shall be arranged so that testing and maintenance can be carriedout without affecting the operation of the converter.Bipole protection philosophy:
The aim with the overall protection philosophy is to avoid undue bipole outageduring any circumstances.
Examples of possible faults and disturbances which are covered by the proposedprotection philosophy:
− Common mode disturbances and faults, such as AC system disturbances which
will give disturbances to both poles.− DC side disturbances related to common bipole equipment, such as the bipole
neutral bus, station earth, metallic return, electrode line, transfer breakers etc.− Domino effect, such as faults in one converter causing protective action in
other converter.According to above philosophy:
a) Protections related to equipment within a pole are separated between the poles,and have separate measuring devices.b) Bipolar protections are used for equipment common for the two poles. Eachpole has its own set of bipole protections using separated measuring paths.c) No protection, pole related or bipole related is allowed to trip the other pole.
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d) There are no single protection actions for bipole related faults or disturbanceswhich can lead to a bipole outage.It is recognized that during bipolar operation mode with temporary groundconnected, a fault in one of the two converter poles shall lead to a shutdown ofthe other pole too. This is done in order to avoid current in the station groundnet.
4 Fault Clearing Actions
The switching actions that are utilized to clear DC faults and DC breaker/switch
failures are:
a) Transfer to redundant control and protection system.b) Retard and blocking of the converters.c) Order down of the converter.d) Tripping of AC circuit breaker.e) Start breaker failure protection.f) Set lockout relay for AC circuit breaker.g) Runback.h) Pole isolation.i) Pole balancing.j) Reclose transfer breaker.k) Close neutral bus ground switch.These switching actions are explained below.
4.1 Transfer to Redundant Protection System
A protection action may be the result of an error in some part of the active controlsystem, such as a measurement failure, i.e. an overcurrent or overvoltage situationmay depend on a failure in the control system. To avoid unnecessary trips at
control failures, some protections will order a change over to the other Control andProtection system as the first action. This protective action is used when one
protection system is active and the other is in stand-by, except when instantaneousfault clearing is required.
If a control failure was the reason for the protection action, the trip condition willdisappear since the whole control is changed to a healthy system.
At the same time when the change over is executed, the other system will becomeactive. The system that initiated the change over will be set in a not Stand-bycondition which means that it is not possible to automatically order this system tobe in active mode again. A manual action must be taken to put the system back inStand-by mode. If a system is in service with no alarm but not set in Stand-bymode, an alarm will be given to the operator.
The protection system that is in Stand-by mode is continuously supervised. A
detected failure will set the system in not Stand-by mode, thus preventing a changeover to this system.
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4.2 Retarding of the Converter
Retarding means that as soon as it has been ordered, the next control pulses aredelayed with respect to the previous one. This increases the firing angle until thefull inverter operation that constitutes the limit for the firing angles has beenachieved.
The retarding will reverse the polarity of the rectifier converter voltage, therebyextinguishing the direct current.
4.3 Order Down to the Rectifier
An order down means retarding of the converter and after the retard that the firingangle be kept at minimum extinction angle until the ORDER DOWN is released.When the order down is released, the converter firing angles are decreased untilnormal operation is reached. This protective action is used to clear line faults withminimum disturbances on the transmission.
Order Down includes a preset number of restart attempts.
4.4 Blocking of the Converter
Blocking means removing the control pulses from the thyristors. When this is done,the valves will stop conducting as soon as the current reaches zero.
When blocking, it is sometimes desirable to provide a current path for the DC sidecurrent. If so, the bypass pairs (two opposite valves within the same six-pulsegroup connected to the same AC phase) are simultaneously fired. This provides aby pass across the converter and is normally employed when permanent earth faultsare detected.
CURRENT PATH AT BYPASS PAIR FIRING13621362Figure 6 Bypass pair firing.
The protective blocking actions can be categorized as X, Y, or Z type blocking. AnX-blocking always implies a blocking without simultaneous firing of by-pass pairsand as well a blocking with BPP when the AC breaker has opened in the inverter.A Z-blocking always implies a blocking with simultaneous firing of by-pass pairs.A Y-blocking is conditional and implies blocking without by-pass pairs in therectifier and with by-pass pairs in the inverter. All converter blocking orders areredundant and have redundant signal paths to the converter blocking sequences.
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4.4.1 Type X - Blocking
A category X-blocking is used during valve faults and it is mainly ordered byprotections which are initiated for a short circuit across a valve. X-blocking willalso be used for faults in the firing circuits where a correct selection of by passpairs cannot be achieved.
In rectifier operation, a valve short circuit gives rise to an initial large short circuitcurrent through a healthy valve in the three pulse valve group accommodating theshort circuited valve. To limit the overcurrent to one pulse only, the fault detectionmust be fast and the converter must be blocked without firing of by-pass pair.In inverter operation, a valve short circuit does not give rise to a large initial shortcircuit current. Therefore, it is not necessary to immediately block the converter. Inorder to limit the amplitudes of the short circuit currents, the converter is retarded,i.e. kept at minimum commutation margin angle until the converter is blocked.The complete X-blocking fault clearing sequence may be summarized as follows:
Rectifier:
− Immediate retarding of the converter.
− Subsequent blocking of the converter without firing of by-pass pairs.− Tripping of AC circuit breakers feeding the converter.
Inverter:
− Immediate retarding of the converter.− Tripping of AC circuit breaker.
− Blocking of the converter with by-pass pair when the AC breaker has started to
open.
4.4.2 Type Y - Blocking
The Y-blocking is generally used for DC side faults which do not expose theequipment to serious stresses, AC faults and manual blocking of the pole.− Immediate retarding of the converter.
− The converter is blocked directly with by-pass pairs in the inverter and
therefore resembles a category Z blocking. In the rectifier, the order to block isdelayed to bring about the extinction of the current before blocking. Theblocking is made without by-pass pairs in the rectifier if the DC current isbelow a voltage dependent limit. Otherwise, the converter is blocked with by-pass pairs.
4.4.3 Type Z - Blocking
The Z-blocking is generally used for earth faults or overcurrents related to the DCside.
Z-blocking always implies immediately retarding of the converter and blockingwith simultaneous firing of by-pass pairs.
4.5 AC Circuit Breaker Trip
The AC circuit breaker trip disconnects the AC side of the converter transformersfrom the AC power source. By doing this, the AC system (which is primarily a
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constant voltage source) is prevented from feeding a fault on the valve side of theconverter transformer. Also, the removal of AC voltages from the valves avoidsunnecessary voltage stresses especially when the valves have performed severecurrent stresses.
All protective trip orders to the AC circuit breakers will energize both trip coils inthe breakers through two different trip devices. The redundant trip orders will alsobe fed by two redundant auxiliary power supplies.
This action is generally activated by a serious fault in the converter pole.
4.6 Start Breaker Failure Protection
At the same time a trip order is sent to the AC breaker, generally an order is alsosent to start the breaker failure protection. If the breaker does not succeed in
opening, the breaker failure protection orders a trip of the next breaker further out.
4.7 Set Lockout Relay for AC Circuit Breaker
If a protection trip order has been sent to the AC breaker, then an order to thelockout relay will also be sent to prevent the breaker from being closed before theoperator has checked the cause of the trip. The lockout relay circuit is manuallyreset by operator.
4.8 Runback
The runback feature will, as quickly as possible, reduce the transmitted power to apreset maximum level in an attempt to keep the transmission in operation.This action is generally activated by overload conditions in the valve or in theelectrode line.
4.9 Pole Isolation
The isolate pole sequence implies disconnecting the DC bus from the DC line and
disconnecting the converter neutral from the electrode line.
This is done either manually during normal shut down or from protective actionsfor faults which trip the pole.
4.10 Pole Balancing
Current balance between the poles is achieved in normal operating mode. Shouldhowever the transmission be running in another operation mode initiated by theoperator, high electrode line current may occur. Current balancing can be achieved,initiated manually or from protection action. The pole balancing order is treated asa runback. The runback level in each pole, will be decided out from the lowestcurrent order, of the two poles.
This action is generally activated by a measured current in the electrode line duringbipolar operation.
4.11 Reclose Transfer Breaker
The transfer breaker reclose sequence is initiated from i.e. neutral bus switch
failure protection, to protect the pole equipment and station ground net in case theneutral bus switch fails to commutate the current over to the electrode line.
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4.12 Close Neutral Bus Ground Switch
The neutral bus ground switch close sequence is initiated from i.e. electrode lineopen circuit protection, to reduce possible high neutral voltages.
5 Protective Functions
This section describes the Main1 DC protections used for the Three
Gorges-Changzhou ± 500 kV DC Transmission Project. Main 2 protection setup isidentical to the Main 1 but with redundant measuring points.The following four main groups of protections are included.a)
Converter protections containing:
Valve short circuit protectionCommutation failure protectionDC overvoltage protectionBack-up DC overcurrent protectionValve DC differential protection
Set 1 Set 2 Valve short circuit protectionCommutation failure predictionCommutation failure protectionVoltage stress protectionValve misfire protectionThyristor monitoringDC overcurrent protectionHigh angle supervisionb)
Pole protections containing:
DC harmonic protection
DC neutral bus differential protectionElectrode line open circuit protectionDC filter overload protectionDC undervoltage protectionReverse power direction protection
Set 1 Set 2 DC pole bus differential protectionDC pole differential protectionElectrode line open circuit protectionDC filter capacitor unbalance prot. DC filter differential protectionDC line protection (travelling wave) DC line protection (derivative andlevel)
DC line longitudinal differential prot.Open line test supervision
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c)
Bipole protections containing:
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Set 1 Set 2 Bipole neutral bus differential prot.Transfer breaker protections (NBGS, NBS, GRTS, MRTB)
Metallic return longitudinal diff. prot.Electrode line overload protection
Station ground overcurrent protectionBack-up transfer breaker protections(NBGS, NBS, GRTS, MRTB)
Metallic return transverse differential prot.Metallic return conductor ground fault
protection
Electrode line unbalance supervision
d)− − − − − − − − − − − − −
Converter AC bus and converter transformer protections containing:Converter AC bus differential protection.
Converter AC bus and converter transformer overcurrent protection.Converter AC bus and converter transformer differential protection.Converter transformer differential protection.Converter transformer overcurrent protection.
Converter transformer thermal overload protection.Converter transformer winding differential protection.Converter AC bus overvoltage protection.Converter transformer neutral shift protection.
Converter transformer zero sequence current protection.Converter transformer overexcitation protection.Converter transformer saturation protection.
Converter transformer restricted earth fault protection.
In addition to the above four groups the following protections are also included,which are physically located in other distributed systems:− − − − − −
Auxiliary power supervisionValve cooling system protectionSmoothing reactor protective relaysElectrode line impedance supervisionConverter transformer protective relaysLast breaker protection, Zhengping only.
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5.1 Converter Protections
In general the converter protection measuring location and set up in each main
protection system is as presented in the figure below
MAIN 2MAIN 2 CONVERTER PROTECTION MEASURING LOCATION AND SETUP IS IDENTICAL TO MAIN 1.*) Zhengping converter station only.VALVEMISFIREPROTECTIONCOMMUTATIONFAILUREPREDICTION*)THYRISTORMONITORINGPF FIR INDMAIN 1 CONVERTER PROTECTION, SET 1DCOVERVOLTAGEPROTECTIONBACKUP DCOVERCURRENTPROTECTIONVALVE SHORTCIRCUITPROTECTIONCOMMUTATIONFAILUREPROTECTIONVALVE DCDIFFERENTIALPROTECTIONVALVE SHORT CIRCUITPROTECTIONVOLTAGE STRESSPROTECTIONFigure 2 Configuration of the converter protections
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YDYYTCMAIN 1 CONVERTER PROTECTION, SET 2DCOVERCURRENTPROTECTIONCOMMUTATION FAILUREPROTECTIONHIGH ANGLESUPERVISIONUDLUDNIDNEIDNCIDPIVDIVYIDNCIVDIVYIDPCWTIDNEIVDIVYIDNCIDPGAMMAGAMMAALPHAALPHAFREQIDNCIDNCIANCIDNEIANCIDNECWTUDI0UACUACUACTCPICNICNIVDIVDIVYIVYIDLIDLCPT4T4I0FPABB Power Systems
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5.1.1 Valve Short Circuit Protection
Purpose of the protectionTo protect the thyristor valve from stresses due to short circuits on the DC-side ofthe converter transformer.Principle of protection operationThe protective function uses the converter transformer currents IVY and IVD, theDC pole bus and neutral bus direct currents and the DC filter current. In normaloperation the currents are balanced.
Higher amplitudes in the converter transformer currents than in the direct current isa criterion on a valve short circuit or other phase to phase short circuits. At anexcess in AC side current, the converter is tripped instantaneously.Fault strategy and protection coordinationWhen a valve short circuit occurs, the high amplitude current is conducted by thefaulty valve and by a second healthy valve as it attempts to commutate. If a thirdvalve is fired in the same three pulse group, such a high current will also beconducted by this valve. This is avoided by a fast detection of the fault and byordering a blocking without by-pass pairs before the third valve is fired.Consequences of protection operation− X-block of the converter.− Trip of the AC circuit breaker.− Pole isolation order.
− Start of the breaker failure protection.− Set lockout of AC circuit breaker.Redundant and backup of protectionThe valve short circuit protection in the other set.Consequences of telecommunication outageNot applicable.
5.1.2 Commutation Failure Prediction
Purpose of the protectionTo reduce the number of commutation failures caused by AC net disturbances.Principle of protection operation7KHSURWHFWLYHIXQFWLRQXVHVWKH]HURVHTXHQFHFRPSRQHQWDQGWKHof the AC voltage.
FRPSRQHQWV
At an abnormal AC voltage the function will instantaneously order an increase ofthe commutation margin in order to avoid commutation failure.Fault strategy and protection coordinationNot applicable.
Consequences of protection operationIncrease of the inverter firing angle
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Redundant and backup of protectionCommutation failure protections.
Consequences of telecommunication outageNot applicable.
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5.1.3 Commutation Failure Protection
Purpose of the protectionTo detect commutation failures in the twelve-pulse converter that are caused fromAC net disturbances or other abnormal commutation conditions.Principle of protection operationThe protective function uses the converter transformer currents IVY and IVD, theDC pole bus and neutral bus direct currents and the DC filter current.
Commutation failures in a six-pulse bridge are characterized by a significantreduction of AC phase current amplitudes whereas the current on the DC side issubject to an increase. This condition is detected. A commutation failure is not afault as such but an indication of other faults such as control pulse transmissionfaults or AC network faults.
Persistent fault in one of the six-pulse bridges are most likely a result of valvemisfire (e.g. no control pulse or continuos control pulse to a valve) whereasintermittent faults in both bridges will occur at AC network disturbances. Thesefacts are used to distinguish between control pulse transmission fault and ACsystem faults.
At a commutation failure the protection will instantaneously order an increase ofthe commutation margin in the faulty converter to improve recovery.
At persistent commutation failures in one of the six-pulse bridges, the protectionwill trip the converter after a preset time delay. At persistent commutation failuresin both six-pulse bridges, the protection will trip the converter after another presettime delay.
Fault strategy and protection coordinationThe part that detects commutation failures in both six-pulse bridges, i.e. due to ACside disturbances must be coordinated with the longest clearing time for AC faults.Consequences of protection operationAt all commutation failures:
− Instantaneous advancing of firing angle in faulty converter to improve
recovery.− Lock order to the line protections in the rectifier station via telecom to avoid
line protection operation.− Start of the Transient Fault Recorder.
After 2 commutation failures, within a defined time window:− Transfer order to redundant Pole Control.
After 3 commutation failures, within a defined time window (Control pulsetransmission fault):
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− Order X-block of the converter (Bypass-pairs are not fired because the
commutation failures may be due to a valve short circuit. Bypassing anadjacent phase would result in short circuit current).− Trip of the AC circuit breaker.− Start breaker failure protection.
− Set lockout relay for the AC circuit breaker.
After 5 commutation failures totally in both bridges, within a defined time window(AC Network disturbance):− Order Y-block of the converter.− Trip of the AC circuit breaker.− Pole isolation order.
− Start breaker failure protection.
− Set lockout relay for the AC circuit breaker.Redundant and backup of protectionCommutation failure protection in the other set.Consequences of telecommunication outageNot applicable for this protection. However bad coordination with the DC lineprotection in the other station is possible.
5.1.4 Voltage Stress Protection
Purpose of the protectionTo protect all converter equipment exposed to AC voltage from excessivedielectric stresses by interlocking the converter transformer tap-changer.To avoid over-stressing the valve arresters and to avoid transformer overexcitation.
Principle of protection operationThe ideal no-load direct voltage Udi0 is calculated by the protection by using theAC converter bus voltage, the tap-change position and frequency. When Udi0exceeds a preset level, the protection will be initiated.
The frequency is added to the calculated Udi0 so as to compensate Udi0 in such waythat over excitation is avoided at lower frequencies than normal.Fault strategy and protection coordinationThe settings are chosen to avoid unnecessary operations in case of permanent ACnetwork voltage increase.
Consequences of protection operationFor a too high Udi0
− The tap-changer will be immediately blocked from increasing Udi0 further and
an order to the tap-changer to decrease the voltage will be given.− Transfer to redundant pole control.For an even higher Udi0:
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− Y-block of the converter.− Trip of the AC circuit breaker.− Set lockout of the breaker.Redundant and backup of protectionThe AC overvoltage protections.The converter transformer gas relay.
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The normal limitation of Udi0 is done in the reactive power and tap-changercontrols (RPC & TCC). The voltage stress protection serves as a backup forreactive power control failures. Thus the protection shall not operate when thereactive power control work properly.Consequences of telecommunication outageNot applicable.
5.1.5 DC Overvoltage Protection
Purpose of the protectionTo protect all equipment that is exposed to the DC voltage as a result of incorrecttap-changer operation or incorrect operation resulting in rectifier operation againsta blocked inverter.
Principle of protection operationThe protective function detects overvoltages on the DC line by measuring thedirect voltages in combination with the direct current and the firing angle alpha.Fault strategy and protection coordinationThe protection is coordinated with:
The overvoltage limiter in the pole control equipment.The DC line protection.
The voltage dependent current order limiter.Consequences of protection operation− If the total DC voltage Ud (Udl-Udn) exceeds the reference, the protection
orders a transfer to redundant control system.− If the total DC voltage exceeds the reference value and the Udl part exceeds
another reference, the protection orders a transfer to redundant control system,and after a further time delay a Z-block order (In the case of reverse voltage X-block order).− If the DC line voltage Udl exceeds a reference and the DC current IDNE is
below minimum current, the protection orders a Z-block order (in the case ofreverse voltage X-block order).Redundant and backup of protectionThe DC overvoltage protection in the other station.Consequences of telecommunication outageNot applicable.
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5.1.6 Valve Misfire Protection
Purpose of the protectionTo detect if a valve fails to conduct when a control pulse (CP) is applied. To detectunintentional valve firing. To prevent a valve failing to conduct from beingselected into bypass pair. To select into bypass pair a valve which is firingunintentionally.
Principle of protection operationThe control pulse generator in the converter firing control system delivers controlpulses (CP) to each valve with a duration corresponding to the required conductinginterval. To facilitate detection of faulty valves the CP is compared with
information about firing. Thereby valve firing outside the CP interval as well asfailure to fire within the CP interval will be detected.Fault strategy and protection coordinationThe protection is coordinated with:The commutation failure protectionThe DC harmonic protectionConsequences of protection operation− Transfer to redundant control system− X-block of the converter.− Trip of the AC circuit breaker.− Start breaker failure protection.
− Set lockout relay for the AC circuit breaker.Redundant and backup of protectionThe commutation failure protections.The DC harmonic protection.
Consequences of telecommunication outageNot applicable.
5.1.7 Thyristor Monitoring
Purpose of the protectionTo give an alarm if the number of faulty thyristors within any valve exceeds apreset level. To de-energize the converter transformer by tripping the AC sidecircuit breakers if the numbers of faulty thyristors in any valve are above a presetlevel when the converter is energized.Principle of protection operationIn the valve control units for each valve there is a function which detects if eachthyristor picks up voltage within a specified time. At the time the thyristor picks upvoltage an indication pulse (IP) is sent on an optical fiber to the valve control. Afailed thyristor does not pick up voltage and this condition is detected.Fault strategy and protection coordinationWe reserve all rights in this document and in theinformation contained herein. Reproduction, use ordisclosure to third parties without express authority isstrictly forbidden. ABB Power Systems (SE)
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Technical Report
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The time between energization of the converter transformer and deblocking of theconverters must be at least 5 seconds to give the thyristor monitoring time to scanall thyristor positions.
Consequences of protection operation− Alarm.
− Transfer to redundant control system and blocking of a deblock order− Trip.
Redundant and backup of protectionThe thyristor valve monitoring system is a self-supervising system. Internal faultsare indicated and therefore no backup monitoring system is required. A completeshort circuit of a valve will however be detected by the short circuit protection.Consequences of telecommunication outageNot applicable.
5.1.8 DC Overcurrent Protection
Purpose of the protectionTo detect and limit high temperatures that may over-stress the converterequipment, in particular the thyristor valves.Principle of protection operationThe protective function is limiting the current, when the calculated thyristor
temperature is too high. The thyristor junction temperature will be calculated basedon the measured DC current and the valve cooling water temperature.
Tj=Tmean+Ptmax⋅Rthja1+Rthja2(1−e−t/τ2)+Rthja3(1−e−t/τ3)Tmean
Tout+Tin=
2
ToutTin
temperature of water from thyristor valves
()where:
temperature of water incoming to thyristor valves
Thermal resistances
Time constants
Rthja1, Rthja2, and Rthja3
When the calculated temperature exceed a temperature reference, a runback orderwill be given to decrease the current by 5% until the temperature is under thereference.
A comparison between the current order and the measured DC current is includedin the protection.Fault strategy and protection coordinationThe protection is coordinated with the cooling properties of the valves and theback-up DC overcurrent protection.Consequences of protection operationRunback
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Redundant and backup of protectionThe back-up DC overcurrent protection.The overcurrent protections in the other station.The AC overcurrent protections.
The converter transformer thermal overload protection.Consequences of telecommunication outageNot applicable.
Technical Report
1JNL100029-886 Rev. 01
5.1.9 Back-up DC Overcurrent Protection
Purpose of the protectionTo detect overcurrents that may over-stress the converter equipment, in particularthe thyristor valves.
Principle of protection operationThe protection measuring the maximum current of the neutral direct current, theIVY and IVD currents.
One part of the protection is instantaneous, and is activated at high DCovercurrents.
The other part of the protection is a thermal overload protection for the thyristorvalves. The valve losses which can be expressed as PTHY=k2*Id+k1*(Id)2 are
calculated and fed into a network with a transfer function representing the coolingproperties of the thyristor. In this way the rise of thyristor temperature is obtained.The temperature is added to the cooling water temperature. When the resultingtemperature exceeds a preset reference level, a trip order is activated.Fault strategy and protection coordinationHigh overcurrents must be detected fast, hence coordination between the definitetime part and other protections is of low importance.
The protection must not operate for commutation failures in the inverter.Consequences of protection operationSlow overcurrent part:
− Transfer to redundant control system, and after a further time delay− Order Z-block of the converter− Trip of the AC circuit breaker− Pole isolation order
− Start breaker failure protection
− Set lockout relay for the AC circuit breakerFast overcurrent part:
− Order Z-block of the converter− Trip of the AC circuit breaker− Pole isolation order
− Start breaker failure protection
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− Set lockout relay for the AC circuit breakerThermal part:
Technical Report
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− Transfer to redundant control system, and after a further time delay− Order Z-block of the converter− Trip of the AC circuit breaker− Pole isolation order
− Start breaker failure protection
− Set lockout relay for the AC circuit breakerRedundant and backup of protectionThe overcurrent protections in the other station.The AC overcurrent protections.
Consequences of telecommunication outageNot applicable.
5.1.10 High Angle Supervision
Purpose of the protectionTo check and limit the stresses on the main circuit equipment due to operation atexcessive firing angles.Principle of protection operationThe stresses on the main circuit equipment from increased angle operation due tothe extreme requirements on increased firing and extinction angles are calculatedwith a function for High Angle Supervision.
The HAS calculates the limitations on the HVDC system. It includes a theoreticalmodel of the valve damping circuits, the arresters across the valve and the valvereactor.
The maximum allowed Udi0, Id and alpha/gamma are limited mainly by the
damping circuits, the arrester across the thyristor valve and the reactor in the valve.For the damping circuit the crucial point is the maximum allowed power dissipatedin the resistor and capacitor.
For the arresters, the commutation peak voltage is calculated in a model using afixed percentage of the commutation jump. The value is compared to the maximumallowed continues peak voltage across the arrester.
If the thyristor limitations have been exceeded at high angle operation togetherwith high Udi0, the HAS will give a decrease order of Udi0 with an hysteresis to thetap-changers and an alarm signal. The tap changers will be ordered to step until thestresses are below the limitations. The decrease order will bypass manual operationmode of the tap changers.
If the decrease of Udi0 is not enough, or if the tap changers are locked and thethyristor limitations still are exceeded an alarm signal will be given.
If the stresses on the thyristor valve will increase further, the HAS will after a timetrip the transmission.
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Please see above “Principle of protection operation”.Consequences of protection operationPlease see above “Principle of protection operation”.Redundant and backup of protectionThe voltage stress protection.DC overvoltage protection.DC line protections.
Consequences of telecommunication outageNot applicable.
Technical Report
1JNL100029-886 Rev. 01
5.1.11 Valve DC Differential Protection
Purpose of the protectionTo detect ground faults within the protective zone and take the faulty converterpole out of service.
Principle of protection operationThe pole direct current is measured in DCCT’s installed on the low voltage bus andthe DCOCT on the high voltage bus in the valve hall. A current difference betweenthe low voltage and the high voltage measuring points is a criterion to detect aground fault within the protective zone. The arrester across the six-pulse bridgewill only be exposed to overvoltage of impulse type, which give rise to arrestercurrent of too short duration to cause protection intervention. The valve DC
differential protection has one fast but relatively insensitive part and one slower butmore sensitive part.
Fault strategy and protection coordinationNot applicable
Consequences of protection operation− Transfer to the redundant control system− Order Y-block of the converter.− Trip of the AC circuit breaker.− Pole isolation order.
− Start breaker failure protection.
− Set lockout relay for the AC circuit breaker.Redundant and backup of protectionThe DC pole bus differential protection.The DC pole differential protection.Consequences of telecommunication outageNot applicable.
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5.1.12 Auxiliary Power Supervision
The protection and supervision systems for the auxiliary power system aredescribed in [6].
5.1.13 Valve Cooling System Protection
For information about the valve cooling system protection, see [7].
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DC System Protection
5.2 Pole Protections
MAIN 1 POLE PROTECTION, SET 1DC POLEDIFFERENTIALPROTECTIONOPEN LINETESTSUPERVISIONDC FILTERCAPACITORUNBALACEPROTECTIONDC FILTERDIFFERENTIALPROTECTIONDC LINEPROTECTION(Traveling Wave)DC LINELONGITUDINALDIFFERENTIALPROTECTIONDC LINEPROTECTION(Derivative andlevel)T1T4T4T1T4IDLIDLIDLIDLIDLICNUDLUDLIVDUDLIVYUDNIDNEIANCIDNEIDNCIUNBIDL *)IDNEIDNCALPHAUDLIDNEIDNEIANCICNT4T4T3IDNCIANCUDNUDLDCUNDERVOLTAGEPROTECTIONDCHARMONICPROTECTIONDC NEUTRAL BUSDIFFERENTIALPROTECTIONDC FILTEROVERLOADPROTECTIONELECTRODE LINEOPEN CIRCUITPROTECTIONREVESE POWERDIRECTIONPROTECTIONTechnical Report
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MAIN 1 POLE PROTECTION, SET 2IDNEWe reserve all rights in this document and in theinformation contained herein. Reproduction, use ordisclosure to third parties without express authority isstrictly forbidden. ABB Power Systems (SE)
ELECTRODE LINEOPEN CIRCUITPROTECTIONDC POLE BUSDIFFERENTIALPROTECTIONABB Power Systems
MAIN 2Figure 3 Configuration of the pole protection
SMOOTINGREACTORPROTECTIONS(***)**) Zhengping converter station only.In general the pole protection measuring location and set up in each mainprotection system is as presented in the figure below.
MAIN 2 POLE PROTECTION MEASURING LOCATION AND SETUP IS IDENTICAL TO MAIN 1.- 24 -**)*) From remote converter station.(***)PUMP AND FAN MOTORS PROTECTIONOIL LEVEL DETECTIONGAS DETECTIONOIL TEMPERATURE DETECTIONPRESSURE RELIEFOIL FLOW INDICATORWINDING TEMPERATUREABB Power Systems
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5.2.1 DC Harmonic Protection
Purpose of the protectionTo detect abnormal harmonics in the converter current which are generated byvalve disturbances, by AC network disturbances or by control equipmentmalfunctions.
Principle of protection operationThe direct current is filtered with respect to fundamental and second harmonicfrequencies and if the current amplitude exceeds a preset reference, the protectionis activated. For low amplitudes only an alarm is given after a preset time.For high amplitudes the converter is taken out of service.Fault strategy and protection coordinationThe protection are coordinated with the:Commutation failure protection.Valve misfire protection.
AC protections to determine longest clearing time of an AC fault.Consequences of protection operationHarmonics change over level:
− Transfer to redundant control system.Harmonics trip level:
− Order Y-block of the converter.− Trip of the AC circuit breaker.− Pole isolation order.
− Start of breaker failure protection.− Set lockout relay for AC circuit breaker.Redundant and backup of protectionThe DC filter overload protection.Consequences of telecommunication outageNot applicable.
5.2.2 DC Pole Bus Differential Protection
Purpose of the protectionTo detect ground faults within the protective zone. The protective zone of the DCpole bus differential protection is from the DCOCT installed on the pole bus (IDL)to the DC current measurement on the neutral bus (IDNC).Principle of protection operationA detected differential between the currents is the criterion for a ground fault. Theprotection operates due to a definite time function.Fault strategy and protection coordination- 25 -
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To be coordinated with the valve DC differential protection.Consequences of protection operationTechnical Report
1JNL100029-886 Rev. 01
Transfer to redundant control system, and after further time delay.− Order Z-block of the converter.− Trip of the AC circuit breaker.− Pole isolation order.
− Start breaker failure protection.
− Set lockout relay for the AC circuit breaker.Redundant and backup of protectionThe DC pole differential protection.Consequences of telecommunication outageNot applicable.
5.2.3 DC Neutral Bus Differential Protection
Purpose of the protectionTo detect earth faults within the protective zone. The protective zone of the DCneutral differential protection is from the DC current transducers installed on the lowvoltage bus in the valve hall (IDNC) to the DC current measurement on the neutralbus (IDNE).
Principle of protection operationA detected differential between the currents is the criterion for an earth fault.The protection operates due to a definite time function.Fault strategy and protection coordinationTo be coordinated with the DC pole differential protection and electrode line opencircuit protection.
Consequences of protection operationTransfer to redundant control system, and after further time delay,− Order Z-block of the converter− Trip of the AC circuit breaker− Pole isolation order
− Start breaker failure protection
− Set lockout relay for the AC circuit breakerRedundant and backup of protectionThe DC pole differential protection.Consequences of telecommunication outageNot applicable.
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5.2.4 DC Pole Differential Protection
Purpose of the protectionTo detect earth faults within the protective zone and take the faulty converter poleout of service. The protective zone of the DC pole differential protection is fromthe secondary side of the converter transformer to the DC current measurements onthe pole bus (IDL) and on the neutral bus (IDNE), including the DC filters.Principle of protection operationThe pole direct current is measured in DC current transducers installed on theneutral bus and on the pole bus in combination with AC currents from the surgearrester and the DC filter to detect earth faults in the protective zone.
The protection has one fast but relatively insensitive part and one slower but moresensitive part.
Fault strategy and protection coordinationTo be coordinated with the valve differential protection, the pole bus differentialprotection and the neutral bus differential protection.Consequences of protection operationTransfer to redundant control system, and after further time delay,− Order Z-block of the converter.− Trip of the AC circuit breaker.− Pole isolation order.
− Start breaker failure protection.
− Set lockout relay for the AC circuit breaker.Redundant and backup of protectionActs as backup for other differential protections within the zone.Consequences of telecommunication outageNot applicable.
5.2.5 Electrode Line Open Circuit Protection
Purpose of the protectionTo relieve the neutral bus equipment from overvoltages caused by the openelectrode line circuit.
Principle of protection operationThe voltage to earth is measured on the pole neutral bus and a significant persistentovervoltage is a criterion on an open circuit in the connection to the electrode. Attoo high voltage on the neutral bus, the protection will order closing of the stationearth switch.
If a combination of very high neutral voltage and low neutral current exists this isan indication of an open circuit to the electrode and current through the arresterclose to the converter.
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The protection should be coordinated with the neutral bus arrester properties and towithstand capability for DC voltage on the neutral bus equipment.Consequences of protection operationHigh UDN level 1:-Close order to the NBGS.
Transfer to redundant control system, and after a further time delay,− Order Z-block of the converter.− Trip of the AC circuit breaker.− Pole isolation order.High UDN level 2:
− Order Z-block of the converter.− Trip of the AC circuit breaker.− Pole isolation order.
High UDN level 3 and IDN level 1.− Order Z-block of the converter− Trip of the AC circuit breaker− Breaker failure protection− Lockout of the AC circuit breaker− Pole isolation order
Redundant and backup of protectionThe DC pole differential protection.Consequences of telecommunication outageNot applicable.
5.2.6 DC Filter Overload Protection
Purpose of the protectionTo detect overload of the DC filter components and relieve the filter from beingoverstressed.
Principle of protection operationThe current through the filter bank is measured and compared with references inthe protection. The trip is delayed enough to avoid inadvertent operation of theprotection during transient overload conditions.Fault strategy and protection coordinationThe protection settings will be coordinated with the thermal withstand properties ofthe filter components.
Consequences of protection operationTransfer to the redundant control system.Disconnection of the filter.
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Block of the converter at last filter or at high fault current.Redundant and backup of protectionThe DC filter differential protection.Consequences of telecommunication outageNot applicable.
Technical Report
1JNL100029-886 Rev. 01
5.2.7 DC Filter Capacitor Unbalance Protection
Purpose of the protectionTo detect faulty capacitor cans and to avoid avalanche failures of capacitor units inthe DC filter capacitor bank.Principle of protection operationThe unbalance current is measured at three frequencies, namely 150, 300 and 600Hz. Each unbalance current is compared with the main current of the samefrequency going through the whole filter. The protection continuously assesseswhich frequencies having useful level. A useful level is 0.8 p.u. of the main currentof that frequency. Alarm and disconnect levels are based on the ratio of unbalancecurrent to main current. Protective action is only allowed if the main currents of atleast two frequencies have reached 0.8 p.u. and two settings values of that specificfrequency.
Fault strategy and protection coordinationNot applicable
Consequences of protection operation− Alarm at level 1.
− Alarm and disconnection of the filter at level 2.
− Disconnection of the filter at level 3and block of the converter at last filter.Redundant and backup of protectionMain 2 DC filter capacitor unbalance protection.Consequences of telecommunication outageNot applicable.
5.2.8 DC Filter Differential Protection
Purpose of the protectionTo detect ground faults in the DC filter zone.Principle of protection operationThe harmonic current through the filter bank is measured at the pole and neutralside of the filter and compared with references in the protection.Fault strategy and protection coordinationThe DC pole differential protection.Consequences of protection operationDisconnection of the filter.
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Block of the converter at last filter or at high fault current.Redundant and backup of protectionDC pole differential protection.
Consequences of telecommunication outageNot applicable.
Technical Report
1JNL100029-886 Rev. 01
5.2.9 DC Line Protections
5.2.9.1 Travelling Wave ConceptPurpose of the protectionTo detect ground faults on the DC line and by means of control actions extinguishthe fault current and, if conditions permit, restore power transmission after the faulthas been cleared.
Principle of protection operationAccording to the theory of the wave equation the voltage and current can beconsidered as superimposed of a forward and backward travelling wave, whichpropagate with constant amplitude and at a speed slightly less than the speed oflight. These waves are usually denoted a(t) for the forward travelling wave and b(t)for the backward travelling wave. The relation between a(t) and b(t) and theinstantaneous current and voltage deviation can be written as:∆u(t) = ½[ a(t) - b(t) ], ∆i(t) = ½Z[ a(t) - b(t) ]or
a(t) = Z∆i(t) + ∆u(t), b(t) = Z∆i(t) - ∆u(t)
A sudden voltage break (earth fault) will cause the line to suddenly discharge andthus generate surges on the transmission system. The repeated reflection of thesewaves in the power system generates high frequency voltages and currenttransients. Thus by sampling the instantaneous current and voltage, and byknowing the surge impedance, the travelling waves can be calculated andfurthermore a DC line earth fault is detected.
For detection of the wave front, the difference between two samples is measured. Ifthe difference is greater than the threshold, the protection is initiated and threedifferent measurements will start to determinate if the wave has sufficientamplitude for a specified time. The first measurement calculates the wave
difference between just before the wave front and after 10 samples (0.2 ms). Thesecond and third calculate the wave difference between just before the wave frontand after 25 and 35 samples (0.5 and 0.7 ms). If all three measurements are greaterthan the threshold, a line fault is detected.
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35 SampleTechnical Report
1JNL100029-886 Rev. 01
25 Sample10 SampleAmplitude TimeTravelling WaveWave FrontOne SampleDue to the slow rise time of the ground wave in some cases, the wave front isdetected by the difference between a number of consecutive samples.Fault recovery strategyThe control action is to retard the rectifier firing control into full inverter operationmode. In this way both the rectifier and the inverter will act to discharge the DCline. After the deionization time a cautious restart attempt is made. When the faultis cleared, the DC voltage starts building up and the restart will continue and thusrestoring the transmitted power.Fault strategy and protection coordinationThe reference settings and the protection interlocking scheme should be
coordinated in such a way, that inadvertent protection operation is avoided in thefollowing cases:
a) At AC faults at the rectifier and at the inverter end.b) At start and stop of the pole.
c) At commutation failures in the inverter.Consequences of protection operationOrder down.
Redundant and backup of protectionThe derivative and level concept.
The DC line longitudinal differential protection.The DC undervoltage protection.
Consequences of telecommunication outageNot applicable.
5.2.9.2 Derivative and Level ConceptPurpose of the protectionTo detect ground faults on the DC line and by means of control actions extinguishthe fault current and, if conditions permit, restore power transmission after the faulthas been cleared.
Principle of protection operationWe reserve all rights in this document and in theinformation contained herein. Reproduction, use ordisclosure to third parties without express authority isstrictly forbidden. ABB Power Systems (SE)
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The protection is active at the rectifier terminal only. The protection monitors thedirect voltage and the direct current and has two different principles of protectionoperation, derivative and level. A ground fault on the DC line is characterized bythe fact that the direct voltage collapses to a low level at a certain comparativelyhigh rate (derivative).Fault recovery strategyThe control action is to retard the rectifier firing control into full inverter operationmode. In this way both the rectifier and the inverter will act to discharge the DCline. After the deionization time a cautious restart attempt is made. When the faultis cleared, the DC voltage is building up, the restart will continue and thus restoringthe transmitted power.
Fault strategy and protection coordinationThe reference settings and the protection interlocking scheme should be
coordinated in such a way, that inadvertent protection operation is avoided in thefollowing cases:
d) At AC faults at the rectifier and at the inverter end.e) At start and stop of the pole.
f) At commutation failures in the inverter.The derivative part:
The derivative part is very fast and provides a fault detection within 2-3 ms.The derivative detection is formed by a differentiator circuit and is compared withtwo reference values. The reference for the lower derivative enables the detectioncircuits, and if a derivative exceeding the higher reference the derivative criterionis met.
To distinguish between in station faults and DC line faults the measurement ofvoltage/time derivative (dU/dt) is combined with a measurement of the timederivative of the current in the DC line (dI/dt).
A high positive dI/dt (current increasing in normal current direction) indicates thatthe fault is located on the line side of the IDL measuring transducer, whereas a highnegative dI/dt implicates a fault in the DC yard.
To fulfil the derivative part completely also a low DC voltage level must bedetected.
The higher derivative reference and the level part are provided with time delays toprevent inadvertent operation due to voltage transients.Consequences of protection operationOrder down.Level criterion:
A low voltage level is detected if the low voltage condition persists for longer thanpredetermined times. The time delay is necessary to give the required selectivityagainst non-legitimate protective action for normal switching procedures as well asother disturbances than line faults which also result in a low direct voltage.Consequences of protection operationTransfer to redundant control system.Block of the converter.
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Trip of the AC circuit breaker.Set lockout of the breaker.Pole isolation.
Redundant and backup of protectionDC undervoltage protection.
Consequences of telecommunication outageThe derivative part:Not applicable.Level criterion:
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1JNL100029-886 Rev. 01
If the DC voltage is low on both poles it is most likely that the low levels are
caused by an AC fault in the inverter. In this case or in monopolar operation and ifthe telecommunication between the stations is out of operation, the pick up time forthe level criterion is increased so that the AC fault can be cleared without DC lineprotection operation. Normally the level criterion is interlocked through thetelecommunication system.
5.2.9.3 DC Line Longitudinal Differential ProtectionPurpose of the protectionTo detect ground faults on the DC line and by means of control actions extinguishthe fault current and, if conditions permit, restore power transmission after the faulthas been cleared.
Principle of protection operationThe pole line current (IDL) in both stations are measured and compared.
Compensation for possible time differences between the sampling of the measuredcurrents is taken.Fault recovery strategyThe control action is Order down which retards the rectifier firing control into fullinverter operation mode. In this way both the rectifier and the inverter will act todischarge the DC line. After the deionization time a cautious restart attempt ismade. When the fault is cleared, the DC voltage is building up, the restart willcontinue and thus restoring the transmitted power.Fault strategy and protection coordinationThe protection is to be coordinated with possible time deviations due to thecommunication between the stations, other line protections, start/stop sequencesand ramping.
Consequences of protection operationOrder down.
Transfer to redundant control system.Block of the converter.Trip of the AC circuit breaker.Set lockout of the breaker.Pole isolation.
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Redundant and backup of protectionDC undervoltage protection.
Consequences of telecommunication outageTechnical Report
1JNL100029-886 Rev. 01
The protection will be interlocked by the control system, so it doesn’t give aspurious protective order.5.2.9.4 Restart LogicPurpose of the functionThe purpose is to make a cautious restart attempt after fault clearing actions hasbeen initiated from the DC line protections.Principle of operationThe DC line fault protections are only active in the rectifier. When fault has beendetected “Order down” is sent to the current control to set the rectifier into fullinverter operation. By this action both the rectifier and the inverter will act todischarge the DC line and thus preventing the rectifier to support the fault current.After the deionization time a cautious restart attempt is made in order to minimizethe energy fed into the line. If the fault was cleared the restart logic will monitor aDC voltage build up and the restart will continue and thus restore the transmittedpower. The number of restart attempts are preselected. At the second restartattempt a transfer to the redundant control system is done.
The attempt at reduced voltage is done in order to keep the voltage stresses atlower level in case of insulation problems (e.g. polluted insulators etc).
5.2.10 DC Undervoltage Protection
Purpose of the protectionTo protect the converter pole against abnormal operation conditions due to linefaults.
Principle of protection operationThe protective function detects undervoltages on the DC line by measuring thedirect voltages and the direct current in combination with the firing angle alpha.Fault strategy and protection coordinationThe protection is coordinated with:The DC line protection.
The voltage dependent current order limiter.Consequences of protection operation− If the DC line voltage falls below a reference value the protection orders a
transfer to redundant control system, and after a further time delay a Z-blockorder.− If the firing angle is above a reference value and the direct current IDNE is
above its reference value the protection orders a transfer to redundant controlsystem.− If the firing angle and the direct current increases even further the protection
orders Z-block order.
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Redundant and backup of protectionActs as backup for the DC line protections.Consequences of telecommunication outageNot applicable.Technical Report
1JNL100029-886 Rev. 01
5.2.11 Open Line Test Operation
To achieve an optimised protection philosophy during open line test operation,some protective applications are automatically adjusted to a pre-selectedcharacteristic.
5.2.12 Open Line Test Supervision
Purpose of the protectionTo detect ground fault on the DC-yard of the local station and the DC line duringopen line test operation. To detect phase to phase short circuits or ground faultswithin the converter during open line test.Principle of protection operationIf the direct current exceeds a preset value or if the direct voltage does not rise asexpected, a ground fault is considered present. The protection operates also forexcess in current on the AC side. The AC current is compared with DC current toget selective fault indication.
Fault strategy and protection coordinationNot applicable.
Consequences of protection operation− X-block of the converter.− Trip of AC circuit breaker.− Set lockout of the breaker.Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.2.13 Reverse Power Direction Protection
Purpose of the protectionThe primary objective of the reverse power direction protection is to detect powerreversal due to control system failures in order to protect the feeding AC networkand the AC generators.Principle of protection operationThe protection measures the direct current and the direct voltage. If the powerchanges direction within a certain time window and the power exceeds a presetlevel the protection will take the pole out of operation. If the line voltage exceeds acertain minimum value and shifts polarity within a time window (typically 0.5 s),this will be taken as a criterion of power reversal.
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The power level will determine the protective action. At a low level a systemchangeover will take place. At two higher levels a slow and a fast trip will begenerated.
Fault strategy and protection coordinationNot applicable.
Consequences of protection operationThe power level will determine the protective action. At a low level a system
changeover will take place. At two higher levels a slow and a fast trip incl. breakerlockout will be generated.
Redundant and backup of protectionThe reverse power direction protection in the other station.Consequences of telecommunication outageNot applicable.
5.2.14 Smoothing Reactor Protective Relays
The protective relays for the smoothing reactor are:− Pump and fan motor protections.− Oil level detection.− Gas detection.− Oil temperature.− Pressure relief.− Oil flow indicator.− Winding temperature.− SF6 density switch.
The description given below is applicable to Longquan only.5.2.14.1 Pump and fan motors protectionsPurpose of the protectionTo detect pump and fan motor faults.Principle of protection operationThe pump and fan motor are provided with thermal motor protections.Fault strategy and protection coordinationNo coordination with the other protections is required. The setting of the protectionis coordinated with the thermal withstand of the motor.Consequences of protection operationIf the motor is overloaded the protection will trip the voltage supply to the motor.In case of a tripped motor, an alarm is given. An alarm is given for tripped motorand if the auxiliary power for the motors is out of service or if a motor fails to start.Redundant and backup of protectionOil temperature indicator.Winding temperature indicator.
Consequences of telecommunication outageNot applicable.
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5.2.14.2 Oil level detectionPurpose of the protectionTo detect oil level in the smoothing reactor conservator tank.Principle of protection operationTechnical Report
1JNL100029-886 Rev. 01
A float in the conservator tank is sensing the oil level. The movement of the floatarm is transmitted to a hydraulic transducer consisting of two counter-coupledbellows filled with hydraulic liquid. Each of these bellows is through capillarytubes connected to two similar bellows in the indicating instrument.
A change in the oil level in the conservator tank causes the float arm to move andthis will give a contraction of one of the bellows in the transductor and an
expansion of the other. The bellows in the indicating instrument are connected insuch a manner that they cooperate to give a proportional movement of the
indicating pointer. The dual bellows arrangement gives an automatic temperaturecompensation of the whole measuring system.Fault strategy and protection coordinationThe minimum and maximum oil level in the conservator tank.Consequences of protection operationAlarm is given.
Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.2.14.3 Gas detectionPurpose of the protectionTo detect faults in the reactor at an early stage, and prevent further damage. Fordetection of internal faults the reactor is equipped with a buchholz-relay with twolevels, one for alarm and one for trip.Principle of protection operationThe signal device consists of a float with a reed contact. When the oil level fallsdue to gas accumulation, the float will follow and the contact closes. The contactwill remain closed until the collected gas is released through the valve.Fault strategy and protection coordinationNot applicable.
Consequences of protection operation− Alarm
and for higher gas levels− Y-block of the converter.− Trip of the AC circuit breaker.− Set lockout of the breaker.− Pole isolation.
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Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.2.14.4 Oil temperature detectionPurpose of the protectionTechnical Report
1JNL100029-886 Rev. 01
The objective is to protect the smoothing reactor from overheating as aconsequence of overload or disturbances on the cooling equipment.Principle of protection operationTemperature meters measure the top and bottom oil temperature in the reactor.This meter/relay is doubled for redundancy. Each meter has two levels, one foralarm and one for Y-block and pole isolation.
The oil temperature measuring system is filled with a liquid that changes itsvolume with the oil temperature and actuates a pair of resilient bellows. The
movements of the bellows are transmitted by means of a link system to the pointerand signal contacts on the indicator. As the temperature variations around thethermometer bulb only should actuate the indicator pointer and signal device, thethermometer also includes a means for compensation the effect of the ambienttemperature on the thermometer housing and the bellows.Fault strategy and protection coordinationThe thermal withstand capability of the reactor.Consequences of protection operation− Alarm
and for higher temperature− Y-block of the converter.− Trip of the AC circuit breaker.− Set lockout of the breaker.Redundant and backup of protectionThe redundant temperature meter.Consequences of telecommunication outageNot applicable.
5.2.14.5 Pressure reliefPurpose of the protectionTo protect the reactor vessel from overpressure caused by internal faults.Principle of protection operationWhen the pressure in the smoothing reactor becomes too high, a valve operates andreleases the pressure. The operation of the valve is detected by a relay.Consequences of protection operationAlarm.
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Fault strategy and protection coordinationTechnical Report
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The setting of the pressure relay shall be chosen such that an overpressure islimited to a magnitude which is harmless to the tank.Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.2.14.6 Oil flow indicatorPurpose of the protectionTo detect a pressure drop across the cooler.Principle of protection operationThere is no oil flowing through the indicator. The reason is that the oil is reducedconsiderably when the temperature falls, because the viscosity of the oil is higher.The pressure drop in the cooler, however, is insignificantly reduced even for largevariations of the oil viscosity.
Fault strategy and protection coordinationNot applicable.
Consequences of protection operationAlarm.
Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.2.14.7 Winding temperaturePurpose of the protectionTo detect and protect the reactor from internal faults.Principle of protection operationThe winding temperature is calculated from temperature meters and the currentthrough the smoothing reactor. It will model the hot spot temperature of the reactorwindings.
Fault strategy and protection coordinationNot applicable.
Consequences of protection operation− Alarm
and for higher temperature− Block of the converter.− Trip of the AC circuit breaker.− Set lockout of the breaker.
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Redundant and backup of protectionOil temperature indicator.
Consequences of telecommunication outageNot applicable.
5.2.14.8 SF6 density switchPurpose of the protectionTechnical Report
1JNL100029-886 Rev. 01
To detect and protect the reactor bushings against low gas pressure.Principle of protection operationThe gas switch operates when the SF6 gas pressure is low. The switch has an alarmlevel and a trip level.
Fault strategy and protection coordinationNot applicable.
Consequences of protection operation− Alarmand for lower level− Block of the converter.− Trip of the AC circuit breaker.− Set lockout of the breaker.Redundant and backup of protectionOil temperature indicator.
Consequences of telecommunication outageNot applicable.
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5.3 Bipole Protections
In general the bipole protection measuring location and set up in each main
protection system is as presented in the figure below. Note that, the pole 2
protection setup is identical to pole 1 but with its own measuring interface for thetransfer breaker protection, electrode line overload protection, back-up transferbreaker protection and metallic return transverse differential protection.
MAIN 2MAIN 2 BIPOLE PROTECTION MEASURING LOCATION AND SETUP IS IDENTICAL TO MAIN 1.POLE 1 DC LINEMAIN 1, POLE 1 BIPOLE PROTECTION, SET 1ELECTRODE LINEIMPEDANCESUPERVISIONPOLE 2 DC LINE*) Measurements from remote stationTRANSFER BREAKERPROTECTIONBIPOLE NEUTRAL BUSDIFFERENTIAL PROTECTIONPOLE 1 NEUT. BUSFigure 4 Configuration of the bipole protections
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POLE 2 NEUT. BUSSTATION GROUNDOVERCURRENT PROTECTIONNBSNBSBACKUP TRANSFERBREAKERPROTECTIONMETALLIC RETURNTRANSVERSE DIFF.PROTECTION(Zhengping only)MAIN 1, POLE 1 BIPOLE PROTECTION, SET 2(Lonquan only)MRTBELECTRODE LINEOVERLOADPROTECTIONMETALLIC RETURNLONGITUDINALDIFFERENTIALPROTECTION(Lonquan only)GRTSNBGSMETALLIC RETURNCONDUCTOR GROUNDFAULT PROTECTION(Zhengping only)ELECTRODE LINEUNBALANCESUPERVISIONIEL 2IEL 1IEL 2IEL 1IDGNDIDMEIDP 1IEL 2IDMEIDNE 1IEL 1IDGNDIEL 2IDMEIEL 1IDNE 2IDNE 1X1IDME *)IDNE 2IDNE 1IDNE 1IDGNDIEL 2IDGNDIDMEIEL 1IANCEIEL 1IDNE 1IDMEIDMEIEL 1IEL 2IEL 2ABB Power Systems
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5.3.1 Bipole Neutral Bus Differential Protection
Purpose of the protectionTo detect earth faults between the electrode line and the pole neutral bus and clearthe fault current.
Principle of protection operationThe DC current is measured in the electrode line, the metallic return bus and thetwo pole neutral busses. A current difference between the measuring points is acriterion for an earth fault within the protective zone.Fault strategy and protection coordinationThe protection is disabled in metallic return operating mode.Consequences of protection operationBipolar operation:− Pole balancing.
− Transfer to redundant system (transmitted to the rectifier if inverter).Monopolar operation:
− Transfer to redundant system (transmitted to the rectifier if inverter).− Order down.
− Y-block of the converter.− Pole isolation.
− Trip of the AC circuit breaker.− Set lockout of the breaker.
When an earth fault has been detected, a down order is given and kept for a shorttime (enough for deionization). An attempt to restart is then made by releasing theorder down. If the earth fault persists an Y-block and trip order is given.Redundant and backup of protectionThe bipole neutral bus DC differential protection in the other pole.The station ground overcurrent protection.Consequences of telecommunication outageIf the fault is in the rectifier the operation of the protection is normal.
If the fault is in the inverter during monopolar operation, switch over to the
redundant control system is done in the inverter but can not be done in the rectifier.In case the fault persists the inverter is tripped.
5.3.2 Station Ground Overcurrent Protection
Purpose of the protectionTo detect a high current in the station ground and ground faults within theprotective zone, and to clear the fault current.Principle of protection operationWe reserve all rights in this document and in theinformation contained herein. Reproduction, use ordisclosure to third parties without express authority isstrictly forbidden. ABB Power Systems (SE)
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The direct current is measured in the electrode line, metallic ground return path andneutral bus.
A difference exceeding a preset level between the measured currents is a criterionfor a ground fault within the station ground.Fault strategy and protection coordinationNot applicable.
Consequences of protection operationBipolar operation:− Alarm.− Pole balancing.
− Transfer to redundant system, (transmitted to other station if inverter).− Delayed trip, if operating with station ground.Monopolar operation:− Y-block of the converter.− Pole isolation.
Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageSwitch over to the redundant system in the rectifier can not be made if theprotection action is in the inverter.
5.3.3 Transfer Breaker Protections
5.3.3.1 Neutral Bus Ground Switch ProtectionPurpose of the protectionTo protect the neutral bus ground switch if the commutation of current from thestation ground net to the electrode line fails.Principle of protection operationThe DC (IDGND) current in the ground switch is measured by a DCCT connectedin series with the switch. If the switch fails to commutate the current over to theelectrode line it will be reclosed by the protection.Fault strategy and protection coordinationThe protection shall be matched against the switch characteristic.Consequences of protection operationBipolar operation:
− Transfer to redundant control system.− Order reclose NBGS, set lockout.− Order pole balancing.Monopolar operation:
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− Transfer to redundant control system.− Order reclose NBGS, set lockout.− Order runback.
Technical Report
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− Order Y-block, pole isolation and trip of the AC circuit breaker.Redundant and backup of protectionThe backup neutral bus ground switch protection.Consequences of telecommunication outageNot applicable.
5.3.3.2 Back-up Neutral Bus Ground Switch ProtectionPurpose of the protectionTo protect the neutral bus ground switch if the commutation of current from thestation ground net to the electrode line fails.Principle of protection operationThe sum of the direct current in the metallic return and the direct current in the twoelectrodes lines are compared with the difference of the direct current in the neutralbus, thus the protection measures the direct current through the NBGS.Fault strategy and protection coordinationTo be coordinated with the neutral bus ground switch protection.Consequences of protection operation− Transfer to redundant control system.− Order reclose NBGS, set lockout.− Order runback.
− Order Y-block, pole isolation and trip of the AC circuit breaker.Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.3.3.3 Neutral Bus Switch ProtectionPurpose of the protectionThe purpose of the neutral bus switch is to commutate the DC current to theelectrode line after the pole has been taken out of service by protections. Theswitch interrupts the neutral connections of the pole, if the switch fails tocommutate the current it will be reclosed by the protection.Principle of protection operationThe pole direct current is measured in the neutral bus. If the current is not zerowhen the neutral bus switch is open a reclose order is given to the switch.Fault strategy and protection coordinationThe protection shall be matched against the switch characteristic.
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Consequences of protection operation− Transfer to redundant control system.− Order reclose NBS, set lockout.− Order close NBGS.
Protective function is enabled during pole isolation only.Redundant and backup of protectionThe backup neutral bus switch protection in the other set.Consequences of telecommunication outageNot applicable.
5.3.3.4 Ground Return Transfer Switch ProtectionPurpose of the protectionTechnical Report
1JNL100029-886 Rev. 01
To protect the ground transfer breaker if the commutation of current from metallicreturn to ground return fails.Principle of protection operationThe direct current through the GRTS is measured. If the direct current is not zerowhen the GRTS is open a reclose order is given.Fault strategy and protection coordinationThe protection shall be matched against the switch characteristic.Consequences of protection operation− Transfer to redundant control system.− Order reclose GRTS, set lockout.Redundant and backup of protectionThe backup ground return transfer breaker protection.Consequences of telecommunication outageNot applicable.
5.3.3.5 Back-up Ground Return Transfer Switch ProtectionPurpose of the protectionTo protect the ground return transfer switch if the commutation of current frommetallic return to ground return fails.Principle of protection operationThe sum of the direct current in the two electrode lines and the direct currentthrough the neutral bus ground switch are compared with the difference of thedirect current in the neutral bus, thus the protection measures the direct currentthrough the GRTS.
Fault strategy and protection coordinationTo be coordinated with the ground return transfer breaker protection.Consequences of protection operation− Transfer to redundant control system.
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− Order reclose GRTS, set lockout.Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.3.3.6 Metallic Return Transfer Breaker ProtectionPurpose of the protectionTechnical Report
1JNL100029-886 Rev. 01
To protect the metallic return transfer breaker if the commutation of current fromground return to metallic return fails.Principle of protection operationThe direct current in the two electrodes is measured. If the direct current is not zerowhen the breaker is open a reclose order is given.Fault strategy and protection coordinationThe protection shall be matched against the breaker characteristics.Consequences of protection operation− Transfer to redundant control system.− Order reclose MRTB, set lockout.Redundant and backup of protectionThe backup metallic return transfer breaker protection.Consequences of telecommunication outageNot applicable.
5.3.3.7 Back-up Metallic Return Transfer Breaker ProtectionPurpose of the protectionTo protect the metallic return transfer breaker if the commutation of current fromground return to metallic return fails.Principle of protection operationThe direct current in the metallic return and the direct current through the neutralbus ground switch are compared with the difference of the direct current in theneutral bus, thus the protection measures the direct current through the MRTB.Fault strategy and protection coordinationTo be coordinated with the metallic return transfer breaker protection.Consequences of protection operation− Transfer to redundant control system.− Order reclose MRTB, set lockout.Redundant and backup of protectionNot applicable.
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Not applicable.
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5.3.4 Metallic Return Transverse Differential Protection
Purpose of the protectionTo detect a ground fault on the metallic return conductor.Principle of protection operationThe metallic return transverse differential protection detects ground faults bymeasuring the line current, IDP1 or IDP2 according to which pole is running, andthe metallic return line current, IDME.Fault strategy and protection coordinationOnly active during metallic return and in the grounded station. To be coordinatedwith other protective functions inside the protective zone.Consequences of protection operation− Transfer to redundant control system.− Y-block of the converter.− Pole isolation.
− Trip of the AC circuit breaker.− Set lockout of the breaker.Redundant and backup of protectionThe metallic return conductor ground fault protection.Consequences of telecommunication outageNot applicable.
5.3.5 Metallic Return Longitudinal Differential Protection
Purpose of the protectionTo detect a ground fault on the metallic return conductor.Principle of protection operationThe metallic return longitudinal differential protection measures the metallic returnline current (IDME) in both stations.Fault strategy and protection coordinationOnly active during metallic return. To be coordinated with metallic return
transverse differential protection and the metallic return conductor ground faultprotection. Furthermore, possible communication delays will be measured andcompensated for.
Consequences of protection operation− Transfer to redundant control system.− Y-block of the converter.− Pole isolation.
− Trip of the AC circuit breaker.
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− Set lockout of the brealer.Redundant and backup of protectionMetallic return transverse differential protection.Metallic return conductor ground fault protection.Consequences of telecommunication outageThe protection will be interlocked.
Technical Report
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5.3.6 Metallic Return Conductor Ground Fault Protection
Purpose of the protectionTo detect a ground fault on the metallic return conductor.Principle of protection operationThe metallic return conductor ground fault protection detects ground faults bymeasuring the station ground current and the two electrode currents.Fault strategy and protection coordinationOnly active during metallic return and only in the grounded station. To becoordinated with the metallic return transverse protection and metallic returnlongitudinal differential protection.Consequences of protection operation− Transfer to redundant control system.− Y-block of the converter.− Pole isolation.
− Trip of the AC circuit breaker.− Set lockout of the breaker.Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.3.7 Electrode Line Overload Protection
Purpose of the protectionTo detect overload in the electrode line conductors.Principle of protection operationThe direct current in the electrode line conductors is measured. A definite timecharacteristic is used.
Fault strategy and protection coordinationThe electrode line withstanding overload level.Consequences of protection operation− Transfer to redundant control system.
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− Runback to a preset value.Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
Technical Report
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5.3.8 Electrode Line Impedance Supervision
Purpose of the protectionTo detect faults on the electrode line.Principle of protection operationAn injected high frequency current will be fed via a series resonance circuit to theelectrode line. The voltage produced over a resonance reactor and the currentinjected in the electrode line is measured after filtering and arranged to give theline impedance seen from the terminal. If the impedance is changing, that is takenas a criterion of a line fault.
Fault strategy and protection coordinationNot applicable.
Consequences of protection operationAlarm.
Redundant and backup of protectionThe electrode line unbalance supervision.Consequences of telecommunication outageNot applicable.
5.3.9 Electrode Line Unbalance Supervision
Purpose of the protectionTo detect uneven current sharing between the electrode line conductors.Principle of protection operationThe differential direct current between the electrode line conductors is measured. Acurrent difference between the two conductors constitutes a criterion on earth faultor open circuit on one of the conductors.Fault strategy and protection coordinationNot applicable.
Consequences of protection operationAlarm.
Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
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CONVERTER AC BUS AND CONVERTER TRANSFORMER MAIN 1 PROTECTION5.4 Converter AC Bus and Converter Transformer Protections
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Both Main 1 and Main 2 are active.
BUS 1CONVERTER ACBUSDIFFERENTIALPROTECTIONCONVERTERTRANSFORMERDIFFERENTIALPROTECTIONCONVERTERTRANSFORMEROVERCURRENTPROTECTIONCONVERTER ACBUS &TRANSFORMERDIFFERENTIALPROTECTIONCONVERTERTRANSFORMERTHERMALOVERLOADPROTECTIONCONVERTERTRANSFORMERWINDINGDIFFERENTIALPROTECTIONCONVERTERTRANSFORMERRESTRICTEDEARTH FAULTPROTECTIONFigure 5 Configuration of the converter AC bus and converter transformer protections
YYThere are six single-phase converter transformer units in a station for each pole.Three of them are connected in a ∆ on the valve side and the other three in a Y.Besides this there is no difference in the protection arrangement between thetransformers. The protection measuring location and set up in each protectionsystem is as presented in the figure below.
- 50 -YDUVYUVDCONVERTERTRANSFORMERZERO SEQUENCECURRENTPROTECTIONCONVERTERTRANSFORMERSATURATIONPROTECTIONCONVERTER ACBUS &TRANSFORMEROVERCURRENTPROTECTIONCONVERTER AC BUSOVEREXCITATIONPROTECTIONCONVERTER ACBUSOVERVOLTAGEPROTECTIONPROTECTIVERELAYS (*)(*)OIL TEMPERATURE RELAYPRESSURE RELIEF RELAYSOIL FLOW RELAYOIL LEVEL RELAYSGAS DETECTIONWINDING TEMP. INDICATORCOOLING SYSTEM FAILURELAST BREAKERPROTECTION(INCL LOGICS)BUS 2CONVERTERTRANSFORMERNEUTRAL SHIFTPROTECTIONTechnical Report
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5.4.1 Converter AC Bus Differential Protection
Purpose of the protectionTo detect faults on the converter AC bus.Principle of protection operationThe current flowing into the protective zone is compared phase to phase. Theprotection operates if the vectorial sum differs from zero, it is sensitive forfundamental current only and it is stabilized for through-currents.Fault strategy and protection coordinationThe settings should be chosen so as to properly protect the converter AC bus.Consequences of protection operation− Y-block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.− Start breaker failure protection.Redundant and backup of protectionConverter AC bus and converter transformer differential protection.Converter AC bus and converter transformer overcurrent protection.Consequences of telecommunication outageNot applicable.
5.4.2 Converter AC Bus and Converter Transformer
Overcurrent Protection
Purpose of the protectionTo detect faults on or in the converter AC bus and converter transformer.Principle of protection operationThe protection measures the converter AC bus current, and operates by a selectableinverse time characteristic.
Fault strategy and protection coordinationThe settings should be chosen to properly protect the equipment within the zone.Consequences of protection operation− Z-block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.− Start breaker failure protection.Redundant and backup of protectionProtective relays, such as oil, pressure, gas and temperature relays.Converter transformer zero sequence current protection.
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Consequences of telecommunication outageNot applicable.
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5.4.3 Converter AC Bus and Converter Transformer Differential
Protection
Purpose of the protectionTo detect faults on or in the converter AC bus and the converter transformers.Principle of protection operationThe currents flowing into the protective zone are compared phase by phase. Theprotection operates if the vectorial sum exceeds from the settings, it is sensitive forfundamental current only and it is stabilized for through-currents, inrush currentsand overexcitation.
Fault strategy and protection coordinationThe settings should be chosen so as properly protect the equipment within the zoneand to be coordinated with other protective functions in the same protective zone.Consequences of protection operation− Z-block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.− Start of breaker failure protection.Redundant and backup of protectionMain 2 converter AC bus and converter transformer differential protection.Converter AC bus and converter transformer overcurrent protection.Converter transformer restricted earth fault protection.Gas detection relays.
Consequences of telecommunication outageNot applicable.
5.4.4 Converter Transformer Differential Protection
Purpose of the protectionTo detect faults within the converter transformer from the CT located in theprimary bushings to the CT located in the secondary bushings.Principle of protection operationThe protection compares the converter transformer primary current and the
secondary side currents vectorially and with respect to turns ratio and tap changerposition. The protection detects a difference in the fundamental currents, andoperates if the equal ampere-turn criterion is not fulfilled. This is the criterion onan internal ground fault or a winding turn-to-turn short circuit.
The equal ampere-turns criterion is valid for steady state only. After a sudden riseof the AC voltage (e.g. when closing the circuit breaker) there will temporarilyflow a differential current. This inrush current always has a significant contents of
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2nd harmonic. The protection analyses the current and forms a 2nd harmonicrestraint operation.
The equal-turns criterion is valid only if the transformer is non-saturated. During asustained overvoltage the transformer might saturate, and there will flow adifferential current. The protection detects this overexcitation current due to itscontents of 5th harmonic, and restrains the operation.
There is also a fast non-restrained function which operates for a high differentialcurrent and without respect to harmonics.
The protection is also stabilized against through current, which might flow duringan external fault. The stabilization is especially important if the protection losesinformation about the tap changer.Fault strategy and protection coordinationThe settings should be chosen so as to properly protect the equipment within thezone.
Consequences of protection operation− Z-block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.− Start of breaker failure protection.Redundant and backup of protectionMain 2 converter transformer differential protection.
Converter AC bus and converter transformer differential protection.Converter AC bus and converter transformer overcurrent protection.Converter transformer restricted earth fault protection.Converter transformer zero sequence current protection.
Protective relays, such as oil, pressure, gas and temperature relays.Consequences of telecommunication outageNot applicable.
5.4.5 Converter Transformer Overcurrent Protection
Purpose of the protectionTo detect faults within the converter transformer.Principle of protection operationThe protection measures the transformer primary current, and operates by aselectable inverse time characteristic.Fault strategy and protection coordinationThe settings should be chosen to properly protect the equipment within the zone.Consequences of protection operation− Z-block of the converter.− Trip of the AC circuit breaker.
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− Set lockout of AC circuit breaker.− Start of breaker failure protection.Redundant and backup of protectionTechnical Report
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Protective relays, such as oil, pressure, gas and temperature relays.Main 2 converter transformer overcurrent protection.Converter transformer zero sequence current protection.
Converter AC bus and converter transformer overcurrent protection.Consequences of telecommunication outageNot applicable.
5.4.6 Converter Transformer Thermal Overload Protection
Purpose of the protectionTo detect overload of the converter transformer.Principle of protection operationThe protection measures the primary side current, and operates by a selectablethermal time constant.
Fault strategy and protection coordinationThe settings should be chosen to properly protect the transformer.Consequences of protection operation− Alarm.
Redundant and backup of protectionConverter transformer thermal overload protection in the redundant system.Consequences of telecommunication outageNot applicable.
5.4.7 Converter Transformer Winding Differential Protection
Purpose of the protectionTo protect the converter transformer windings against damage caused by internalearth fault.
Principle of protection operationPrimary winding:
The winding current is measured by two CT´s for each phase, one on each side ofthe winding. The protection operates due to a definite time function where thedifferential current is compared to a reference for each phase. See figure below.
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PRIMARY WINDINGTechnical Report
1JNL100029-886 Rev. 01
PROTECTIONValve side Y-winding:
The winding current is measured by two CT´s for each phase, one on each side ofthe winding. The protection operates due to a definite time function where theprocessed differential current is compared to a reference for each phase. See figurebelow.
VALVE SIDE Y-WINDINGPROTECTIONValve side ∆-winding:
The winding is measured by two CT´s for each phase, one on each side of thewinding. The protection operates due to a definite time function where the
processed differential current is compared to a reference for each phase. See figurebelow.
VALVE SIDE D-WINDINGPROTECTIONFault strategy and protection coordinationNot applicable.
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− Z-block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.− Start of breaker failure protection.Redundant and backup of protectionMain 2 converter transformer winding differential protection.
Technical Report
1JNL100029-886 Rev. 01
Converter AC bus and converter transformer differential protection.Converter transformer zero sequence current protection.Valve DC differential protection.
Converter transformer restricted earth fault protection.Consequences of telecommunication outageNot applicable.
5.4.8 Converter AC Bus Overvoltage Protection
Purpose of the protectionThe objective is to prevent severe sustained overvoltage conditions which couldcause damage to the converter transformer and converter bridge.Principle of protection operationThe voltage is measured per phase on the AC bus. The protection is sensitive forfundamental voltage, and voltages up to 7th harmonic. The phase-to-groundvoltage is compared to fixed references in order to detect abnormal overvoltageconditions.
Fault strategy and protection coordinationThe settings should be chosen so as to avoid unnecessary operation in case of ACnetwork switching overvoltages.Consequences of protection operation− Y-block of the converter.− Trip of the AC bus breaker.− Lockout of AC bus breaker.Redundant and backup of protectionThe main 2 system.
Consequences of telecommunication outageNot applicable.
5.4.9 Converter Transformer Neutral Shift Protection
Purpose of the protectionTo detect a ground fault on an AC conductor on the valve side of the convertertransformer.
Principle of protection operationWe reserve all rights in this document and in theinformation contained herein. Reproduction, use ordisclosure to third parties without express authority isstrictly forbidden. ABB Power Systems (SE)
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Technical Report
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For each six-pulse bridge the vector sum of the phase-to-ground voltage on thevalve side of the converter transformers is measured by means of the capacitivetaps in the valve hall transformer bushings. As long as the converter is blocked andno ground fault is present this sum is zero. In the case of a single phase to groundfault no heavy fault current will develop as long as the converter is blocked but asignificant zero sequence component will appear in the phase-to-ground voltages.This component is detected and compared to a preset reference.Fault strategy and protection coordinationThe protection must be disabled when the converter is deblocked, as the principleof protection operation are not applicable when the converter is deblocked.Consequences of protection operation− Interlock deblock order.− Alarm.
− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.Redundant and backup of protectionIf the converter is deblocked a phase to ground fault will be detected by the valveDC differential protection.
The main 2 converter transformer neutral shift protection.Consequences of telecommunication outageNot applicable.
5.4.10 Converter Transformer Zero Sequence Current Protection
Purpose of the protectionTo detect overcurrent in the neutral of the converter transformer.Principle of protection operationThe neutral currents are measured in the converter transformer neutrals and theinstantaneous values of the three phase measurements are added algebraically. Thesum is fed into the protection, hence the neutral overcurrent protection is sensitiveto the zero sequence current component. The protection analyses the current andforms an inrush (2nd harmonic) restraint operation.Fault strategy and protection coordinationThe settings should be chosen to avoid tripping for external AC faults.Consequences of protection operation− Z-block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.− Start of breaker failure protection.Redundant and backup of protectionThe main 2 converter transformer zero sequence current protection.Consequences of telecommunication outageWe reserve all rights in this document and in theinformation contained herein. Reproduction, use ordisclosure to third parties without express authority isstrictly forbidden. ABB Power Systems (SE)
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Not applicable.
Technical Report
1JNL100029-886 Rev. 01
5.4.11 Converter Transformer Overexcitation Protection
Purpose of the protectionTo detect overexcitation of the converter transformers.Principle of protection operationThe ratio between the AC bus voltage and frequency.Fault strategy and protection coordinationThe settings should be chosen so as to properly protect the converter transformer.The protection is provided with two setting levels, for alarm and for trippingrespectively.
Consequences of protection operation− Z-block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.− Start breaker failure protection.Redundant and backup of protectionVoltage stress protection.
Main 2 converter transformer overexcitation protection.Consequences of telecommunication outageNot applicable.
5.4.12 Converter Transformer Saturation Protection
Purpose of the protectionTo indicate and protect the convert transformers from DC saturation due to a DCcurrent entering the transformers through the neutral point.Principle of protection operationThe protection monitors the sum of the transformer primary neutral currents. Largeconverter transformer neutral DC currents may flow during unbalanced operationof the bipole or when the neutral ground switch is closed. Transformer DC neutralcurrent and the resulting DC saturation is characterised by the neutral currenthaving large periodically peaks. Value of the peaks varies linearly with the DCground current. The value of the peak current is passed on to an inverse timecharacteristic which has been specified by the transformer manufacturer.Fault strategy and protection coordinationThe setting should be chosen to properly protect the converter transformer. Theprotection is provided with two setting levels, for alarm and tripping respectively.Consequences of protection operation− Alarm.
− Y-block of the converter.
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− Trip of the AC circuit breaker.
Technical Report
1JNL100029-886 Rev. 01
− If the trip outputs are activated the breaker shall be effectively locked out, until
sufficient cooldown has occurred.Redundant and backup of protectionDC harmonic protection.
Main 2 converter transformer saturation protection.Consequences of telecommunication outageNot applicable.
5.4.13 Last Breaker Protection, Zhengping only
Purpose of the protectionTo monitoring the point of connection to the converter transformer and orders
immediate blocking of the converter pole if the last line in-feed is disconnected, thelast breaker feeding the converter transformer is tripped by any protection.Principle of protection operationThe protective function monitors the point of connection for relevant parts of theconnection scheme in combination with information as last line in-feed
disconnected or protective actions which will result in any in-feed disconnection.Fault strategy and protection coordinationThe protection shall operate for the following conditions;
− If one of the converter transformer breakers is tripped by the busbar protection
when one of the breakers in the bay is not closed.− If breaker failure protection operation on the middle breaker in the breaker and
one and a half bay feeding the converter transformer.− If breaker failure protection operation on the middle breaker at the bus side in
the breaker and one and a half bay feeding the converter transformer after thebusbar protection starts.− If tripping of last line in-feed to Zhengping converter station.Consequences of protection operation− Y- block of the converter.Redundant and backup of protectionThe protective function is located in the duplicated ACP cubicle.Consequences of telecommunication outageNot applicable.
5.4.14 Converter Transformer Restricted Earth Fault Protection
Purpose of the protectionTo protect the converter transformer primary winding against damage caused byinternal ground faults or winding faults.Principle of protection operationWe reserve all rights in this document and in theinformation contained herein. Reproduction, use ordisclosure to third parties without express authority isstrictly forbidden. ABB Power Systems (SE)
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Technical Report
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The protection is of low impedance type. The zero sequence current on the highvoltage primary side of the winding is compared to the zero sequence current in thetransformer neutral. If the differential current exceeds the preset level theprotection will operate.
Fault strategy and protection coordinationThe converter transformer differential protection.Consequences of protection operation− Z-block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.− Start breaker failure protection.Redundant and backup of protectionThe redundant system converter transformer restricted earth fault protection.Consequences of telecommunication outageNot applicable.
5.4.15 Converter Transformer Protective Relays
The protective relays for the converter transformers are:− Pump and fan motor protections.− Oil level detection.− Gas detection.− Oil temperature.− Pressure relief.− Oil flow indicator.− Winding temperature.− SF6 density switch.
− Pressure relief and oil flow (tap changer).
The description given below is applicable to Longquan only.5.4.15.1 Pump and fan motor protectionsPurpose of the protectionTo detect pump and fan motor faults.Principle of protection operationThe pump and fan motors are provided with thermal motor protections. If themotor is overloaded, the protection will trip the voltage supply to the motor. Thecooling system failure protection detects if the number of tripped motors mightcause overheating of the converter transformer.Fault strategy and protection coordinationNo coordination with the other protections is required. The setting of the protectionis coordinated with the thermal withstand capability of the motor.Consequences of protection operationWe reserve all rights in this document and in theinformation contained herein. Reproduction, use ordisclosure to third parties without express authority isstrictly forbidden. ABB Power Systems (SE)
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Technical Report
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If the motor is overloaded the protection will trip the voltage supply to the motor.An alarm is given for tripped motor and if the auxiliary power for the motors is outof service or if a motor fails to start.Redundant and backup of protectionOil temperature indicator.Winding temperature indicator.
Consequences of telecommunication outageNot applicable.
5.4.15.2 Oil level detectionPurpose of the protectionTo detect oil level in the converter transformer conservator tank.Principle of protection operationA float in the conservator tank is sensing the oil level. The movement of the floatarm is transmitted to a hydraulic transducer consisting of two counter-coupledbellows filled with hydraulic liquid. Each of these bellows is through capillarytubes connected to two similar bellows in the indicating instrument.
A change in the oil level in the conservator tank causes the float arm to move andthis will give a contraction of one of the bellows in the transductor and an
expansion of the other. The bellows in the indicating instrument are connected insuch a manner that they cooperate to give a proportional movement of the
indicating pointer. The dual bellows arrangement gives an automatic temperaturecompensation of the whole measuring system.Fault strategy and protection coordinationThe minimum and maximum oil level in the conservator tank.Consequences of protection operationAlarm is given.
Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.4.15.3 Gas detectionPurpose of the protectionTo detect winding short circuits, internal flashovers and excessive insulationdefects within the converter transformer.Principle of protection operationIf the insulation of the winding has been damaged in a spot, the intensity of theelectric field might be so high that gas is produced. Also high temperature in aspot, caused by overload, might decompose the oil and produce gas. The relaycollects this gas and operates if a certain volume has been collected. The relay alsooperates when it detects a shock wave in the oil due to an internal flashover.Fault strategy and protection coordinationWe reserve all rights in this document and in theinformation contained herein. Reproduction, use ordisclosure to third parties without express authority isstrictly forbidden. ABB Power Systems (SE)
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Not applicable.
Consequences of protection operation− Alarm
and for higher gas levels− Y-block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.− Start breaker failure protection.Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.4.15.4 Oil temperature detectionPurpose of the protectionTechnical Report
1JNL100029-886 Rev. 01
The objective is to protect the converter transformer from overheating as aconsequence of overload or disturbances on the cooling equipment.Principle of protection operationTemperature meters measure the top and bottom oil temperature in the transformer.This meter/relay has two levels, one for alarm and one for Y-block and poleisolation.
The oil temperature measuring system is filled with a liquid that changes itsvolume with the oil temperature and actuates a pair of resilient bellows. The
movements of the bellows are transmitted by means of a link system to the pointerand signal contacts on the indicator. As the temperature variations around thethermometer bulb only should actuate the indicator pointer and signal device, thethermometer also includes a means for compensation the effect of the ambienttemperature on the thermometer housing and the bellows.Fault strategy and protection coordinationThe thermal withstand capability of the transformer.Consequences of protection operation− Alarm
and for higher oil temperatures− Y-block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.Redundant and backup of protectionThe redundant temperature meter.Consequences of telecommunication outageNot applicable.
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5.4.15.5 Pressure reliefPurpose of the protectionTechnical Report
1JNL100029-886 Rev. 01
To protect the converter transformer vessel from overpressure caused by internalfaults.
Principle of protection operationWhen the pressure in the converter transformer becomes too high, a valve operatesand releases the pressure. The operation of the valve is detected by a relay.Consequences of protection operationAlarm is given.
Fault strategy and protection coordinationThe setting of the pressure relay shall be chosen so that an overpressure is limitedto a magnitude which is harmless to the tank.Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.4.15.6 Oil flow indicatorPurpose of the protectionTo detect a pressure drop across the cooler.Principle of protection operationThere is no oil flowing through the indicator. The reason is that the oil is reducedconsiderably when the temperature falls, because the viscosity of the oil is higher.The pressure drop in the cooler, however, is insignificantly reduced even for largevariations of the oil viscosity.
Fault strategy and protection coordinationNot applicable.
Consequences of protection operationAlarm.
Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
5.4.15.7 Winding temperaturePurpose of the protectionTo detect and protect the converter transformer from internal faults.Principle of protection operationThe winding temperature is calculated from temperature meters and the currentthrough the converter transformer. It will model the hot spot temperature of thetransformer windings.
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Fault strategy and protection coordinationNot applicable.
Consequences of protection operation− Alarm
and for higher temperature− Block of the converter.− Trip of the AC circuit breaker.− Set lockout of AC circuit breaker.Redundant and backup of protectionOil temperature indicator.
Consequences of telecommunication outageNot applicable.
5.4.15.8 SF6 density switchPurpose of the protectionTechnical Report
1JNL100029-886 Rev. 01
To detect and protect the converter transformer bushings against low gas pressure.Principle of protection operationThe gas switch operates when the SF6 gas density is low. The switch has an alarmlevel and a trip level.
Fault strategy and protection coordinationNot applicable.
Consequences of protection operation− Alarm
and for higher temperature− Block of the converter.− Trip of the AC circuit breaker.− Set lockout of the AC circuit breaker.Redundant and backup of protectionOil temperature indicator.
Consequences of telecommunication outageNot applicable.
5.4.15.9 Pressure relief and Oil flow switchPurpose of the protectionTo protect the tap changer tank from overpressure as a consequence of internalfaults.
Principle of protection operationFlow relay:
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Technical Report
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A rapid oil-flow in the pipe between the tap changer compartment and theconservator tank is detected.Pressure relay:
In case of an overpressure in the conservator tank, the pressure relay located on thetap changer compartment housing, will be activated.Fault strategy and protection coordinationThe setting of the pressure relay and the flow relay shall be chosen so that anoverpressure is limited to a magnitude which is harmless to the tank.Consequences of protection operationBlock of the converter.Trip of the AC circuit breaker.Start of the breaker failure protection.Lock out of the AC circuit breaker.Redundant and backup of protectionNot applicable.
Consequences of telecommunication outageNot applicable.
6 References
[1][2][3][4][5][6][7][8]
The Three Gorges-Changzhou ±500 kV DC Transmission Project,CPG/TG–HVDC9901SE, Appendix 2.
1JNL100022-972, Single line diagram, Main DC circuit Longquanconverter station.
1JNL100022-974, Single line diagram, Main DC circuit Zhenpingconverter station.
1JNL100031-908, Technical Report, Accuracy of measured signals inMACH 2.
1JNL100031-246, Technical Report, DC System Protection, Settings.1JNL100031-931, System Description, Station Service Power.1JNL100033-877, Functional Description, Valve Cooling System.1JNL100030-876, Technical Report, DC Filter Protection Settings.
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