1This document gives an overview of the HVDC Protection System.

The HVDC protections are grouped to:

DC protections

AC protections

Apparatus Protective Relays

Common features of the protection philosophy, the protective functions and the fault clearingactions are given. Also, tools for the system programming and supervision are briefly described.

2Table of Contents

1. INTRODUCTION 3

2. GENERAL PROTECTION PHILOSOPHY 3

3. DC PROTECTIONS 5

4. AC PROTECTIONS 10

5. APPARATUS PROTECTIVE RELAYS 15

6. FAULT CLEARING ACTIONS 16

7. SYSTEM SUPERVISION 18

31. INTRODUCTIONThis document describes the common featuresfor a complete HVDC Protection System.

The protection system uses powerful computersand allows both a future adoption of newprotective functions and adjustments of existingprotections in a very flexible manner. Theprotection system is built in a modular way, thusenabling a structured design of the system.

The detection principles and the protectivefunctions are briefly described, but for a moredetailed description a reference is made to theFunctional Descriptions of correspondingprotective function.

The backup protections are separately presentedonly in case the protective principles differ fromeach other.

2. GENERAL PROTECTIONPHILOSOPHYThe purpose of the HVDC protection systemis to promptly remove main circuit equipmentfrom service at short circuits or at abnormaloperation that might cause damage or interferewith the operation of the neighbouring system.

The HVDC control and protection system isdivided to two Redundant systems A and B,based on an Active Standby concept. Bothsystems include a complete set of control andprotection systems. Each protection systemconsists of both set1 and set2 protections.

As soon as a protection operates a fast changeover is executed from the active system to thestandby system, to ensure that the protectionaction is based on a correct externalmeasurement. The concept is equal for both theDC and the AC protections .

The protective scheme is designed to meet thefollowing general requirements:

a) Fault or other abnormal conditions, that mightexpose the equipment to hazards are

detected. Also, the cause of an unacceptabledisturbance should be detected and the faultyor overstressed equipment should either betaken out of service or relieved from stressesin a controlled way.

b) The aim of the protection design is to detectevery condition according to above with atleast two protective functions.

c) The protection setup should be arranged intooverlapping protective zones. For each faultcase, there should be a fast main protectionwith a limited protective zone. The mainprotective function should normally besupported with a slower or less sensitivebackup protective function. The backupprotective function should, if possible, bebased on a different measuring principle andwhen applicable, with a more extendedprotective zone. For those cases where themain/backup philosophy can not be appliedwithout difficulties, essential protectivefunctions should be doubled.

d) Steps shall be taken to minimize the possibilityof a fault in one converter causing protectiveactions in other converters.

e) The protection system shall be provided withrecording facilities. All signals used by theprotection system shall be recorded and timemarked. The recording facility shall alwayssave the latest event, even if there already areinformation stored.

f) Where applicable, the alarm and tripreferences shall be set so as to give level andtime separated operation.

g) The protection system shall be active/standbyas long as the power supply is OK and thereis no fault on the system.

h) Tripping paths to the breaker should beredundant and fed by two different auxiliaryvoltage supplies.

i) The protections shall be arranged so thattesting and maintenance can be carried outwithout affecting the operation of thetransmission.

4Bipole aspectsThe aim with the overall protection philosophy,especially for bipole related protective functionsshall be to avoid any undue bipole outage duringany circumstances.

Examples of possible faults and disturbanceswhich shall be covered by the protectionphilosophy:

Common mode disturbances and faults e.g. inthe AC system causing disturbances to bothpoles.

DC side disturbances related to commonbipole equipment i.e. the bipole neutral bus,station ground, metallic return, electrodeline/cable and transfer breakers.

Domino effect i.e. faults in one convertercausing a protective action in the otherconverter.

According to the above philosophy:

a) Protective functions related to equipmentwithin a pole shall be separated betweenthe poles and have separate measuringdevices.

b) There shall be a set of protective functionsfor equipment common for both poles in abipole, called bipole protections. Each poleshall have a separate set of bipoleprotections, using separate measuringdevices.

c) No protective function, pole related orbipole related, is allowed to trip the otherpole in a bipole.

d) There shall be no single protective actionsfor bipole related faults or disturbanceswhich can lead to a bipole outage.

e) A bipole related protective function shouldnot initiate protective actions on a singlecriterion.

53. DC PROTECTIONS

Converter protections

DC current measuring device

CONVPR.PRE

SET 1 CONVERTER PROTECTIONS, SYSTEM A

Y

Y

D

IVY *)

IVD *)

IDNE *)

IANC *)

IF1 *)

CP

UAC *)

IDNC *)

SHORT CIRCUITPROTECTION

HIGH ANGLE SUPERVISION

VOLTAGE STRESSPROTECTION

BACKUPUD SUPERVISION

BACKUP OVERCURRENTPROTECTION

BACKUP COMM-UTATION FAILUREPROTECTION

BACKUPSHORT CIRCUITPROTECTION

VALVE MISFIREPROTECTION

THYRISTORMONITORING

OVERCURRENTPROTECTION

IDL *)

SET 2 CONVERTER PROTECTIONS, SYSTEM A

CW

T

UAC

*) Redundant measuring points for system B

AC current transformer

FP

PR FIR IND

TC

ALP

GA

M CWT

ICN *)

**) **)**) Redundant measuring points between set1 and 2 for IVY and YVD.

**) **)

(Possible Control function)

COMMUTAION FAILUREPROTECTION

Figure 1

Valve short circuit protectionThe protection detects valve short circuits andother phase to phase short circuits on the DC-side of the converter transformer. The protectivefunction uses the AC conductor currents IVYand IVD and the DC pole line and neutral busdirect currents, IDL respectively IDNE. Higheramplitudes in the AC conductor currents than inthe DC current is a criterion of a valve or another phase to phase short circuit.

Commutation failure protectionThe Commutation failure protection detectsfailures in the twelve pulse converter due toabnormal commutation conditions by measuring

the AC currents IVY and IVD in combinationwith DC currents IDL and IDNE. Duringcommutation failures the DC current is higherthan the AC conductor current, which is detectedby the protection.

Voltage stress protectionThe Voltage stress protection protects theconverter equipment from dielectric stresses byinterlocking the converter transformer tapchangers. Dielectric stresses are caused by theAC side voltage.The interlocking prevents afurther increase of the ideal no-load directvoltage UDI0. The UDI0 is calculated by usingthe AC bus voltage measurement UAC, the tap-changer position and the frequency.

6Back up UD SupervisionThe back up UD supervision calculates the DCvoltage out of UDI0, a, g and the neutral buscurrent IDNE. The calculated DC voltage iscompared with the measured DC voltage and thedifference is used to detect and preventtransmission disturbances due to abnormalcontrol or measuring circuit conditions. Thefunction is coordinated with the UD calculation inthe control system.

Valve misfire protectionThe control pulse generator in the converterfiring control system delivers control pulses CP toeach valve with a duration corresponding to therequired conducting interval. To facilitatedetection of faulty valves the CP is comparedwith information about firing. Thereby, a valvefiring outside the CP interval as well as failure tofire within the CP interval is prohibited by thisprotection.

Thyristor monitoringThe thyristor valve monitoring system is a selfsupervising system. In every valve control unitthere is a function, detecting if the voltage ispicked up within a specified time by thecorresponding thyristor. At the time the thyristorpicks up the voltage an indication pulse IP is senton an optical fiber to the valve control. A failedthyristor does not pick up voltage and thiscondition is detected.

High Angle SupervisionThe stresses on the main circuit equipmentcaused by an increased angle operation, due tothe extreme requirements on increased firing andextinction angles, are calculated by the HighAngle Supervision HAS function.

The HAS calculates the limitations on the HVDCsystem. It includes a theoretical model of thevalve damping circuits, the arresters across thevalve and the valve reactor.

DC overcurrent protectionThe DC overcurrent protection protects thevalves and the converter equipment. The ACconductor currents IVY and IVD, the neutral buscurrent IDNE and the cooling water temperatureare measured. The protection contains two parts,one for the overcurrent and the other for thethyristor overtemperature detection.

7Pole protections

DC current measuring device

*) Redundant measuring points for system B

POLEP.PRE

SET 2 POLE PROTECTIONS, SYSTEM A

IDNE *)

IANC *)

IF1 *)

DC FILTER OVERLOAD PROTECTION

DC DIFFERENTIALPROTECTION

REDUNDANT DC FILTER OVERLOADPROTECTION

REDUNDANT DCDIFFERENTIALPROTECTION

DC LINE/CABLE GROUND FAULTPROTECTION

IDL *)

DC ABNORMALPROTECTION

AL

P

ICN *)IF2 *)

DC HARMONICPROTECTION

UDL

ELECTRODE LINEOPEN CIRCUITPROTECTION

SET 1 POLE PROTECTIONS, SYSTEM A

AC current transformer

UDN

Figure 2

DC abnormal voltage protectionThe protective zone includes all equipmentexposed to DC voltage and the thyristor valvesat bypass pair firing of the inverter. The DCabnormal voltage protective function detects bothover and undervoltages on the DC line bymeasuring the voltage UDL, the current IDL andthe converter firing angle.

DC harmonic protectionDC harmonic protection detects abnormalharmonics in the converter current, generated atcontrol equipment malfunction or during valveand AC network disturbances. The direct currentis filtered for the fundamental and secondharmonic frequencies.

DC differential protectionThe protective zone of the DC differentialprotection is from the converter transformersecondary bushings to the measurements of boththe line and electrode currents, at the DC side of

the converter. To detect ground faults within theprotective zone the direct currents IDL andIDNE are measured in combination with the ACconductor currents IVY and IVD neutral buscurrents IANC and ICN in addition with the DCfilter currents IF1 and IF2.

DC line ground fault protectionDetects ground faults on the DC line. Theprotection initiates control actions to extinguishthe fault current, and if conditions permit,restores the power transmission. The detection ismade by measuring the level of the DC linevoltage and the derivatives of both the direct linevoltage UDL and the direct line current IDL.

DC filter overload protectionThe protection detects an overload on a filtercomponent by measuring the two filter currentsIF1 and IF2. The protection calculates theheating in the resistors and the reactors causedby high currents and initiates control actions to

8prevent a further exposure, when one or severalcomponents has become overheated.

Electrode line open circuitprotectionElectrode line open circuit protection

protects the neutral bus equipment from dielectricstresses due to an electrode line open circuit.This is done by monitoring the neutral bus directvoltage UDN and the neutral bus direct currentIDNE.

DC switchyard protections

DC BIPOLE SWITCH YARD

BPP.PRE

ELECTR. LINE/CABLE

NBGS

POLE 1 DC LINE

MRTB

POLE 2 DC LINE

NBS 1

NBS 2

BIPOLE NEUTRALDIFFERENTIALPROTECTION

METALLIC RETURNGROUNF FAULTPROTECTION

TRANSFER BREAKERPROTECTION, NBS,NBGS, GRTB, MRTB

ELECTRODE LINELONGIT DIFF.PROTECTION

STATION GROUNDOVERCURRENTPROTECTION

METALLIC RETURNGROUNF FAULTPROT. BACKUP

TRANSFER BREAKERPROT. BACKUP, NBS, NBGS, GRTB, MRTB

ELECTRODE LINEIMPEDANCESUPERVISION

SET 1 BIPOLE PROTECTIONS, SYSTEM A, POLE 1

SET 2 BIPOLE PROTECTIONS, SYSTEM A, POLE 1

IDNE 2 *)

IDME *)IDEL *)

IDGND *)

X1 *)

DC current measuring device

*) To pole 2 bipole protection setup

POLE 1 NEUT.BUS

POLE 2 NEUT.BUS

GRTB

IDNE 1 *)

**) From electrode station if applicableIDEL **)

Figure 3

Bipole Neutral differentialprotectionThe electrode line conductor IDEL1&2, themetallic return path IDME, the station groundIDGND and the neutral bus IDNE1&2 currentsare measured. A differential current exceeding apreset level is a criterion for a ground fault withinthe protective zone. The protection is disabled inthe metallic return operating mode.

Metallic return conductor groundfault protectionMetallic return conductor ground fault protectionmonitors the ground current in the groundedstation. Depending on whether the station isgrounded by the two electrode conductors or thegrounding switch, the corresponding currentsIDEL1&2 or IDGND are measured. A detectedground current is a criterion for a ground fault onthe return conductor.

9Transfer breaker protectionProtects the transfer breaker in case thecommutation of the current fails during DC yardswitching.

Station ground overcurrentprotectionThe Station ground overcurrent protectionprotects the station ground, convertertransformer and the bus between the electrodeline and the pole neutral bus.

10

DC current measuring device ELECTRODE CABLE LONGIT. DIFFERENTIAL PROT.

ELECTRODE CABLEUNBALANCESUPERVISION

Ex: 12 )1 )

SET 1 SET 2

A1.2 **)

A2.1 **)

ELECTRODE LINE/CABLE PROTECTIONS, SYSTEM A

elcp1.pre

2) Remote Electrode Switchgear

1) Inside the HVDC Station Yard

**) Redundant measuring points for system B

A1.1 **)

A2.2 **)

Electrode cable longitudinaldifferential protectionThe protection device measures and comparesthe currents in both ends of the electrode cable.The detection is coordinated for possible timedifference between the sampling of the twomeasured currents. The protection shall only beactive if the communication with the remote endis in service.

Electrode line unbalancesupervisionThe direct current in each electrode lineconductor (IDEL1 and IDEL2) is measured.When the difference between the two measuredcurrents exceeds a preset level the criterion for aground fault or an open circuit on one of theconductors is fulfilled. The preset level has onefixed part and one part proportional to the neutralcurrent IDNE.

ELECTRODE LINEIMPEDANCE SUPERVISION

1 )

Ex: 2

X1

ELECTRODE LINE/CABLE PROTECTIONS, SYSTEM A

SET 2

elcp2.pre

1) Inside the HVDC Station Yard

11

Electrode line impedancesupervisionThe impedance to ground is measured byinjecting a high frequency signal on the electrodeline between the blocking filters, and from theresponse of the line the impedance is calculated.

12

4. AC PROTECTIONS

Converter transformer and AC bus protections

CB

CBUSCL.PRE

400 kV BUS

CONVERTER AC BUS AND TRANSFORMER PROTECTIONS, SYSTEM A

AC BUS & TRANSFORMER DIFFERENTIAL PROTECTION

XFMR AND AC BUS PROT. SET2

AC BUS DIFF& OVERCURRENTPROTECTION

AC BUS PROT. SET1

Y1Y2

D

UVY

UVD

RESTRICTED EARTH FAULTPROTECTION, Y1, Y2 AND D WINDING

PROTECTIVE RELAYSFOR THE TRANSFORMER

XFMR PROT. SET1

AC CONDUCTOR GND FAULT ANDXFRM NEUTRAL O/,PROTECTION.

PROTECTIVE RELAYSFOR THE TRANSFORMER

AC OVERVOLTAGEPROTECTION

IAC From AC filter & shunt measurement.

IAC From AC filter & shunt measurement.

( S1 )

( S2 )

*)

*)

*) *)

* )

S1 ) Oil temperature indicator, Oil level detector, Tap changer pressure, Oil flow relays and Gas detector relay.

S2 ) Transformer winding temperature indicator and cooling system failure protection.

* ) Redundant measuring point for system B.

*)

XMR PROT. SET2

Figure 5

AC conductor ground faultprotectionThe protection detects ground faults on the ACconductors by measuring the vector sum of thephase-to-ground voltages on the valve side of theconverter transformers. As long as the converteris blocked and no ground fault is present, this sumis zero. In the case of a single phase ground faultno fault current will develop as long as theconverter is blocked, but a significant zerosequence component will appear in the phase-toground voltages. This component is detected.

The detection principles are not applicable whenthe converter is deblocked and thus theprotection is disabled.

Converter transformer restrictedearth fault protectionProtects the converter transformer primary andsecondary windings against damage caused byinternal ground faults. At the secondary side thewinding current is measured by two CTs foreach phase, one on each side of the winding. Theprotection operates due to a definite time functionand the processed differential current iscompared to a preset value.

13

Converter transformer neutralground overcurrent protectionThe neutral currents are measured in thetransformer neutrals and the instantaneous valuesof the three phase measurements are added. Thesum is fed into the protection thus making theneutral ground overcurrent protection sensitive tothe zero sequence current component. Theprotection operates due to a definite time functionand/or a selective inverse time characteristic.

Converter AC bus & transformerdifferential protectionAll currents flowing into the AC bus and theconverter transformer are compared phase byphase. Measurements are made on both theprimary and secondary sides of the convertertransformer.

The transformer differential protective function ismonitoring the tap changer position, detectingboth the 2nd harmonics in the inrush currents andthe 5th harmonics due to an overexcitation of thetransformer, and restrains the operation. Theprotection is stabilized against a through current,which might flow during an external fault. Thestabilization is especially important if theprotection looses information about the tapchanger position.

The AC bus differential protective functionmonitors both the AC bus and the AC filtercurrents. There is also a fast non-restrainedfunction, operating for a high differential currentand without respect to harmonics.

Converter AC bus differential andovercurrent protectionDifferential part:

All currents flowing into the AC bus arecompared phase by phase. The protectionoperates if the vectorial sum differs from zero.The protection is sensitive for fundamentalcurrent only and is stabilized for through currents.

Overcurrent part:

The protection calculates the converter bus rmscurrent and operates by a preset inverse timecharacteristic and/or by a definite time function.

AC overvoltage protectionThe objective is to prevent severe sustainedovervoltage conditions, which could causedamage to the equipment connected to the ACbus. The voltage is measured per phase on theconverter AC bus. The protection is sensitive forfundamental voltage and/or harmonic voltages. Inorder to detect an abnormal overvoltage conditionthe phase to ground rms and/or peak voltage iscompared to a preset reference.

Commutation Capacitor (CC)Unbalance protectionThe capacitor cans are arranged in asymmetrical bridge, shaped like an H. In casea certain number of capacitor elements has beenshort circuited, the voltage stress on healthyelements may develop unacceptably highcurrents causing an operation of the fuses. Thecurrent is measured per phase in the midpoint ofthe filter bridge and if fuses have operated, therewill flow an unbalance current. The protectionorders a trip when the current exceeds a presetlevel. The protection is continuously compensatedfor actual load current by monitoring the CCphase current.

In general, the component tolerances alwayscause a small initial unbalance current.Therefore, to improve the sensitivity of theprotection, the initial unbalance current iscompensated by adding a fraction of the phasecurrent in the CC branch. This adjustment isdone only once, when energizing the CC bank forthe very first time.

Commutation Capacitor (CC)Varistor protectionThe protection detects CC varistor short circuits.In normal operation the varistor currents are zeroexcept when the varistor is expected to conduct,i.e. during temporary peaks in the capacitorvoltages. A thermal model of the varistor is used

14

to calculate for how long it can conduct a certainamount of current.

Commutation Capacitor (CC)Varistor protection, BackupInstead of the varistor currents the protectivefunction uses the AC conductor currents IVYand IVD and the corresponding voltages UVYand UVD, the DC line voltage and the voltage inthe neutral midpoint of the two six pulse bridges.

The detection is based on a capacitor voltagederivative measurement which is available sincethe voltage differs between the AC and DC sideduring certain time intervals of the commutation.The AC conductor currents are used to triggerthe measuring.

At normal operation the derivative is expected tobe at a certain level. If the varistor and therebyalso the capacitor is short circuited the derivativewill be zero. If the derivative is below a presetlevel during a certain amount of time theconverter is tripped.

AC filter and Shunt bank protections

SHUNT BANK PROTECTIONS, AVAILABLE CONFIGURATIONS SYSTEM A

AC CONVERTER BUS

*) Redundant measuring point for system B

***) To system B AC bus differential protections **) To system A AC bus differential protections

UAC*)CIRCUIT

BREAKER

SHC.PRE

* )**)

***)

SHUNT PROT. SET 1

CAPACITOR UNBALANCEPROTECTION

GROUND FAULTPROTECTION

BREAKER FAILURE PROTECTION

OVERCURRENT PROTECTION

SHUNT PROT. SET 2

BACKUP CAP. UNBALANCEPROTECTION

REDUNDANTGROUND FAULTPROTECTION

REDUNDANT OVERCURRENT PROTECTION

SHR.PRE

***)

*)GROUND FAULTPROTECTION

*)**)

PROTECTIVERELAYS FOR THE REACTOR

SHUNT REACTOR SHUNT CAPACITOR

REACT. PROT. SET 2

OVERCURRENTPROTECTION

DIFFERENTIALPROTECTION

PROTECTIVERELAYS FOR THE

REACTOR

BREAKER FAILUREPROTECTION

REACT. PROT. SET 1

Figure 6

AC filter / Shunt capacitorunbalance protection (Classic Hbridge)The capacitor cans are arranged in asymmetrical bridge, shaped like an H. In casea certain number of capacitor elements has beenshort circuited, the voltage stress on healthyelements may develop unacceptably high causingan operation of the fuses. The current ismeasured per phase in the midpoint of the filterbridge and if fuses have operated, there will flow

an unbalance current. The protection orders atrip when the current exceeds a preset level.

AC filter / Shunt capacitorunbalance protection, backupThe protection calculates the capacitance of thehigh voltage capacitor bank by measuring thefilter current and the AC bus voltage. The signalsare filtered and tuned for the fundamentalfrequency.

15

Shunt reactor / capacitorovercurrent protectionProtects the Shunt reactor/capacitor againstovercurrents caused by internal faults. Thereactor current measurement is made on the highand the capacitor current on the low potential.The protection is sensitive only for thefundamental frequency and it operates due to adefinite time function.

Shunt reactor differentialprotectionProtects the reactor against damages caused byground faultst. The reactor current is measuredby two CTs for each phase, one at high potentialand one at low potential.

AC FILTER PROTECTIONS, AVAILABLE CONFIGURATIONS SYSTEM A

AC CONVERTER BUS

*) Redundant measuring point for system B

***) To system B AC bus differential protections **) To system A AC bus differential protections

UAC*)CIRCUIT

BREAKER

AC FILTER (CLASSIC)

ACFCL.PRE

*)**)

***)

FILTER PROT. SET 2

BACKUP CAP. UNBALANCE PROTECTION REDUND. HARM.OVERLOADPROTECTION

REDUNDANTGROUND FAULTPROTECTION

OVER TEMPERATURE PROTECTION

AC FILTER (CONTUNE)

FILTER PROT. SET 2

OVERTEMPERATUREPROTECTION CAPACITOR UNBALANCE PROTECTION

FILTER PROT. SET 1

***)**)

*)

ACFCO.PRE

BREAKER FAILUREPROTECTION

CAPACITOR UNBALANCE ANDGROUND FAULTPROTECTION HARMONIC OVERLOAD PROTECTION

FILTER PROT. SET 1

UAC

*)**)

***)

ACFCOH.PRE

OVER. TEMPERATURE PROTECTION REDUNDANTGROUND FAULTPROTECTION

BACKUP CAP. UNBALANCE PROTECTION

CAPACITOR UNBALANCE PROTECTION

BREAKER FAILURE PROTECTION

GROUND FAULT PROTECTION

HARMONIC OVERLOAD PROTECTION

FILTER PROT. SET 1

FILTER PROT. SET 2

CAPACITOR UNBALANCE PROTECTION

BREAKER FAILURE PROTECTION

GROUND FAULT PROTECTION

HARMONIC OVERLOAD PROTECTION

FILTER PROT. SET 1

AC FILTER (CONTUNE, H-BRIDGE)

Figure 7

AC filter capacitor unbalanceprotection (Contune)Each capacitor can in the capacitor bank is builtup of a number of capacitor elements. Theelements are connected in series and parallel. Ifone or a certain number of capacitor elementshas been short circuited, the voltage stress onhealthy elements may develop unacceptably high.The same phase currents between two filters arecompared in the bottom of the filters. In order toreduce the influence of differences in harmonics

between filters the protection is sensitive only forthe fundamental frequency.

AC filter resistor and reactorthermal overload protectionProtects the filter reactors and resistors againstthermal damages. When applicable, the current ismeasured either through the component, or at hebottom of the filter. In the latter case the currentthrough the reactor and the resistor is calculatedby modeling the current sharing.

The protection calculates an equivalenttemperature by first calculating the dissipated

16

power for each component and then integratingthe power with respect to the thermal timeconstant of the component. Thus the thermalstress on each component is determined.

AC filter reactor overtemperatureprotection, backupProtects the reactor against overheating as aconsequence of regulator faults (Contune) or aninternal fault. Each reactor is equipped withtemperature measurement for both the windingand the core. There are two trip levels for eachmeasurement.

AC filter detuning supervisionDetects short circuits in the parallel circuitcomponents of the AC filter. The zero sequencecurrent is detected in the bottom of the filter. It issensitive for harmonic zero sequence current anddetects single phase changes of the filterimpedance.

AC filter / Shunt capacitor / Shuntreactor ground fault protectionThe AC filter/Shunt bank is protected againstdamages caused by internal short circuits. Theprotection will also detect open phases e.g. as aconsequence of non successful phase opening orclosing of the breakers.

The zero sequence current is detected in the lowpotential of the filter/shunt bank. In order toreduce the influence of sustained AC netdisturbances, the protection is sensitive only forthe fundamental frequency.

Breaker failure protectionThe protection action will isolate the faulty circuitbreaker from the AC system and trip the nextbreaker in line.

The three phase currents and the residual currentare fed to the protection via different inputamplifiers. The measured phase currents and theresidual current are compared with a presetlevel. The protection detects a fault based on atwo out of four decoding logic. Start of the

breaker failure protection is made by logicalredundant signals from other protections.

17

5. APPARATUS PROTECTIVERELAYSThe following guards are used to protect theConverter transformer, Shunt reactor and an oilcooled Smoothing reactor, when applicable.

Oil temperature indicatorThe oil temperature at the top of the convertertransformer is measured and an overheating, as aconsequence of a sustained operation duringoverload conditions or malfunction in the coolingequipment, is detected.

Reactor pressure and oil flowrelaysThe tap changer tank is protected againstoverpressure by two relays. A rapid oil flow inthe pipe between the tap changer compartmentand the conservator tank is detected by a Flowrelay. In case of an overpressure in theconservator tank the Pressure relay, locatedon the tap changer compartment housing, will beactivated

Low oil level detectorA low oil level in the converter transformer andtap changer conservator tank is detected by afloat.

Oil leakage detectorCapacitance changes of a dielectric sensor, in therubber bag of the conservator tank, is a criterionof an oil leakage inside the bag.

Gas detectorThe relay detects the gas produced by windingshort circuits and operates if a certain volumehas been collected. The relay also operates whenit detects a shock wave in the oil due to aninternal flashover.

Winding temperature indicatorTemperature and current indications are obtainedfrom the hot spot temperature meters of thetransformer windings and from a currenttransformer, located around the connection leador the bushing of the winding.

Cooling system failure protectionThe cooling fan motors are provided with athermal motor protection. If the motor isoverloaded the protection will trip the voltagesupply to the motor. The cooling system failureprotection detects an overheating of thetransformer caused by a tripped motor.

18

6. FAULT CLEARINGACTIONS

Transfer to redundant control& protection systemThe pole control and protection system is dividedto two Redundant systems; Active andStandby, each accommodated in a cubicle of itsown.

Before every execution of a trip order, anattempt to keep the system in service is made bytransferring the control from the active system tothe redundant standby control and protectionsystem, as shown in Figure 8 below. If a controlor protection failure was the reason for theprotection action, the trip condition will disappearsince the whole control is changed to a healthysystem. Thus the probability for unwarrantedprotective actions due to for examplemeasurement failures is essentially reduced.

In case the trip condition remains after thesystem change the trip order is executed by theactivated system. Thus a two out of fourselection logic is obtained. To separate betweenchange over and a block/trip orders both a timeand a level separation is used.

A minor fault in the Active system will result in asoft switch over, if the Standby system is OKand without minor faults. The previously Activesystem will be left in Standby mode.

A severe fault will reset also the Standby system.In the Active system it will also result in a systemswitch over if the other system is Standby.

Emergency faults will first order a reset of theStandby system or a system switch over. If theother system is unable to take over the control orhas the same kind of failure, the followingmeasures shall be taken:

Block of the converter or if the fault is in thefiring circuitry, initiate emergency BPP in twopretermined valves in the inverter.

Trip of the AC breaker.

After a protective switch over and a trip anAutomatic System Activation (ASA) system willset the inactivated system in Standby.

COPRINCPRE

CHANGE OVER REF

t

CHANGE OVER DELAY

>1

TRIP REF

t

TRIP DELAY

>1

ANALOG SIGNAL 1

ANALOG SIGNAL 2

SET 1 PROTECTIONS

SET 2 PROTECTIONS

PROTECTION SYSTEM A

S

RQ

CHANGE SYSTEM LOGIC

SYS A ACT

SYSTEM A ORDER CHANGE SYSTEM

SYSTEM B ORDER CHANGE SYSTEM

SYSTEM A TRIP ORDER

SYSTEM B TRIP ORDER

PROTECTION SYSTEM B

Figure 8

Control actions

Retarding of the converterThis is normally performed in the rectifier, whichnormally operates with firing angles around 15electrical degrees. Retarding means that thesuccessive control pulse is delayed in respect tothe previous one. This increases the firing angleuntil a full inverter operation is obtained and thusa limitation of the firing angles has beenachieved. The retarding will reverse the polarityof the rectifier converter voltage and therebyextinguish the direct current. To make theextinguishing of the current more efficient, theinverter will decrease the firing angle gamma.

Blocking of the convertersBlocking means removing the control pulses fromthe thyristors. When this is done, the valves willstop conducting when the current reaches zero.Blocking at high DC currents still running in themain circuit requires a safe bypassing currentpath across the converter. The bypass isprovided by simultaneously fired bypass pairs,meaning two opposite valves, within the same six

19

pulse group and connected to the same ACphase. This procedure is normally employedwhen a permanent ground fault is detected.

1 3

2

4

5

6

3

4 6

2

CURRENT PATH AT BYPASS PAIR FIRING

1

5

Figure 9

The protective blocking actions can becategorized as X, Y, Z or S type blocking. An X-blocking always implies a blocking withoutsimultaneous firing of bypass pairs. A Z-blockingalways implies a blocking with simultaneous firingof bypass pairs. A Y-blocking is conditional andimplies blocking without bypass pairs in therectifier and with bypass pairs in the inverter.Unlike X, Y and Z blocking the S blocking onlyimplies a power decrease (ramp) followed by aninitiation of the stop sequence. All converterblocking orders are redundant and haveredundant signal paths to the converter blockingsequences.

Converter actions Pole Isolate : The isolate pole sequence

implies disconnecting the DC bus from theDC line and disconnecting the converterneutral from the electrode line. This is madeeither manually during normal shut downs orby protective actions.

Runback: To keep the transmission inoperation a runback is ordered, thus reducingthe transmitted power to a preset level. Therunback level is defined from the lowestcurrent order.

Pole balancing: The pole currents arebalanced at normal operation. However, if thetransmission is running in an other operationmode, a high electrode current may occur.

The pole balancing is ordered either manuallyor by a protection.

Close the neutral bus ground switch: Inorder to reduce high neutral bus voltages, theneutral bus ground switch close sequence isinitiated by the electrode line open circuitprotection.

Reclose the DC yard transfer breaker: Incase the current commutation fails during amode shift, a reclose order is initiated by thecorresponding transfer breaker protection.

Disconnection from the ACsystemThe AC circuit breakers disconnect the AC sideof the converter transformers from the ACpower source. By this action the AC system,primarily being a constant voltage source, isprevented from feeding a fault to the valve sideof the converter transformer. Also, a removal ofthe AC voltages from the valves will preventunnecessary voltage stresses, especially after thevalves have been exposed to severe currentstresses.

All protective trip orders to the AC circuitbreakers will energize both trip coils in thebreakers via two separate trip devices. Theredundant trip orders will also be fed by tworedundant auxiliary power supplies.

+ V (A)

+ V (B)

- V (A)

- V (B)

AC BREAKER

TRIP COIL A

TRIP COIL B

TRIP CIRCUIT OVERVIEW

ACCB.PRE

PROTECTIONSET 1

PROTECTIONSET 2 (TRIP A)

(TRIP B)

OPT. DISTR. A

OPT.DISTR. B

SYSTEM A PROTECTIONS

(TRIP A)

(TRIP B)

PROTECTIONSET 1

PROTECTIONSET 2

(TRIP A)

(TRIP B)

(TRIP A)

(TRIP B)

SYSTEM B PROTECTIONS

OPT. DISTR. A

OPT.DISTR. B

CAN 1

CAN 2

CAN 1

CAN 2

ACT. A

ACT. B

TEST

TEST

Figure 10

Simultaneously with a trip order to the AC-breaker an order to start the breaker failureprotection is executed. If the breaker does not

20

succeed to open, the breaker failure protection isordering a trip of the next breaker in line.

A protective trip order to the AC breaker willalso set a lockout relay. The relay will preventthe breaker being closed until the operator haschecked for the cause of the trip. The lockoutrelay is reset by the operator.

7. SYSTEM SUPERVISIONThe HVDC control and protection system isprimarily designed to minimize the need for aperiodic maintenance.

For the protection system this means, that allparts of the protection system are continuouslysupervised and the need for the manual checks iscompletely eliminated.

All computing elements incorporate well provensupervisory techniques, such as:

Program execution control (stall alarm orwatch dog alarms), which will check thatcalculations are executed normally. Incombination with the predictable cyclicexecution and the HiDraw function blockprogram development tool, it will give anexcellent supervision of the individualmicroprocessors that makes up the HVDCControl and Protection system.

Memory checks

Flash PROM memories are checked by achecksum calculation. This makes itpossible to check that both the programcode and all protection settings remainunchanged.

Static RAMs are checked at regularintervals by writing and reading specialtest patterns to each memory cell.

Dynamic RAM memories are checked byeither continuous parity check or byECC (Error Correction Code)techniques.

These techniques ensure that all computers usedfor protections will behave correctly and that thesettings are intact.

To ensure that the measuring elements are notfaulty, all measurements are made with twoelectronic measuring circuits (aux. CTs or OCTs,input amplifiers, S&H circuits A/D convertersetc.). Although, only one of these measurementsis used for a particular protection, bothmeasurements should always show reasonablysimilar values, if no part of the system is faulty.When applicable, an additional comparison ismade with the results from other locationsmeasuring the same parameter. The control andprotection system is automatically monitoring allmeasurements and an alarm is generated if anerror is detectedand an alarm is generated if anerror is detected.

Indications from breakers and disconnectors aresupervised with the normal double pointtechnique (one closing and one opening contact)but they are also read by two independent switchcontrol units.

All indications and trip signals are transferred byredundant field busses, where both all units onthe bus and all segments are continuouslymonitored with alive messages.

To verify that a tripping signal is able to trip thebreaker, the corresponding switch control unitwill perform an automatic check of the outputcircuitry and the breaker tripping coil byenergizing one side of the trip coil at a time.

When a manual opening of the breaker isperformed the protection system alternatesbetween the two trip coils every other time,. Thismeans, that as long as a breaker is opened aseparate trip test from the protections is notneeded and in the rare case where a breaker isnever operated, a trip test is easily performedby twice opening and closing the breaker fromthe SCM system.

In conclusion, the supervision of the control andprotection system is thus regarded as completelyautomatic. This means not only a remarkableimprovement of the overall reliability of thesystem but also a total elimination of both the testswitches and handles.