dc protection

8/3/2019 DC Protection

http://slidepdf.com/reader/full/dc-protection 1/59

T&D UK Ltd. Power Electronic Activities Lecture Notes

AREVA HVDC DC Protection 1/59Confidential - This document must not be forwarded, copied or disclosed to others; its use and communication of its content is not permitted without priorauthorization from AREVA T&D Inc

Converter Protection





1. INTRODUCTION ......................................................................................... 3 2. PROTECTION PRINCIPLES........................................................................ 4 2.1 Selectivity .................................................................................................. 4 2.2 Reliability................................................................................................... 4 2.3 Stability...................................................................................................... 5 3. PROTECTION STRATEGY.......................................................................... 6 3.1 Protective Zones........................................................................................ 6 3.2 Protection Scheme .................................................................................... 7 4. PROTECTIONS..........................................................................................14 4.2 DC Pole Protections .................................................................................16 4.3 Valve & Ancillary Equipment Protections ................................................24 4.4 Converter Transformer Protections .........................................................35 4.5 Harmonic Filter Protections .....................................................................40 4.6 Circuit Breaker Fail Protection .................................................................44 4.7 Open Circuit Test Mode............................................................................44 4.8 Last Line Disconnect ................................................................................45 5 PROTECTIVE ACTIONS AND MARSHALLING...........................................47 5.1 Marshalling of DC Pole protections ........................................................48 5.2 Marshalling of Converter Transformer Protections .................................50 5.3 Marshalling of Harmonic Filter Protections .............................................50 5.4 Marshalling of Valve and Ancillary Equipment Protections.....................52 5.5 Marshalling of Unexpected Breaker Movement and Converter Isolation Detector

53 5.6 Marshalling of AC protections..................................................................53 6.1 Ground Faults and Short Circuits ............................................................54 6.2 Overvoltage ..............................................................................................56 6.3 Undervoltage ............................................................................................56 6.4 Ancillary Equipment Failure .....................................................................57 6.5 Commutation Failure ................................................................................58 6.6 Misfire .......................................................................................................58 6.7 Thermal Overload .....................................................................................59





1. Introduction

AREVA COGELEX is the supplier of the 3 x 600 MW back-to-back HVDC facility for the Al Fadhili 400kV-380 kV sub-station for the Gulf Cooperation Council Interconnection Authority.

The project consists of three HVDC Back-to-Back converter poles located at Al Fadhili in Saudi Arabia.On one side is the Saudi, 380kV 60Hz system and the other the 400kV 50Hz system connectingKuwait, Bahrain, Qatar, Oman and UAE.

The protection scheme is a set of functions which work in co-ordination to isolate promptly any part ofthe equipment under extreme stresses and to prevent undue disturbances to the system due to control

mal-operation. The scheme is deployed on a per pole basis with clearly defined protective zones andactions in accordance with the requirement of the Technical Specification stating that the control andprotection to be organised in an hierarchical fashion.

The protection schemes for both sides of all the three poles are identical except for the filter protectionand as such only the protection scheme for one side of a pole is described in this document. Any side orpole specific requirements is highlighted when necessary.

This document lays down the principles underpinning the protection design and describes the strategyemployed to implement the protection scheme. The objectives of the protection scheme are to limit thedamage to the faulted equipment, to isolate the faulted equipment from the rest of the system in order toallow the system to continue to operate, to minimise fire risk and to minimise hazards to personnel.

Contrary to ac networks, the majority of the practical fault cases on the DC side of the HVDC scheme do

not constitute immediate danger to the equipment involved, however, the high controllability and powertransfer capability of the dc link makes it necessary to have an additional objective which is to protect thesystem from the consequences of HVDC control mal-operation by prompt removal of the faultyequipment. Similar to the ac network, the two principal stresses on the equipment are overcurrents andovervoltages.

Surge arresters are installed across major circuit equipment such as the converter transformers and thethyristor valves to protect the equipment insulation against transient and temporary overvoltages. Thecontrol action to limit long term voltage stress are converter dynamic control, tap changer control andreactive power control. The high voltage operating condition of the equipment dictates that protection ofDC equipment against overcurrent using fuses is not a viable option, instead the transient overcurrentwithstand capability is accounted for in the main circuit design of the equipment ratings.

The HVDC control acts to limit DC side overcurrent and most of the ground faults do not give rise to

huge overcurrents as may be observed in ac networks. Thus, except for valve short-circuit or convertertransformer faults which may severely overstress the equipment, urgent circuit breaker tripping is notalways required. In ac networks, the protective action is limited to circuit breaker tripping and usuallyresults in removal of transmission capacity. The fast speed of response of the dc link makes it possibleto use valve firing sequences in addition to circuit breaker tripping and mechanical switching sequencesto implement fault clearing actions.





2. Protection Principles

The guiding principles for protective relaying in ac networks are generally applicable to dcprotection design.

2.1 Selectivity

Fault conditions or other abnormal conditions that might expose equipment to hazards as wellas conditions that will cause unacceptable disturbances to operation shall be detected and thefaulty or faulted equipment shall be taken out of service or relieved of stresses in a controlledmanner so that disturbance to the operation of the rest of the system is minimised. The aim ofthe protection design is to limit the amount of equipment removed when isolating a fault. Ideally

only the faulted equipment or the smallest possible zone containing the fault is disconnected.For example, in a multi-polar scheme the majority of faults are cleared by tripping one pole,leaving the other poles in operation. Selectivity is achieved by dividing the protection functioninto zones. The advantage of this approach is that the location of a fault can be determined andit makes it possible to disconnect the faulted equipment while leaving the rest of the system inoperation to limit the consequences of failures.

2.2 Reliability

There are various measures that can be employed to improve reliability to minimise the impactof faulty protective equipment and the primary measure is through redundancy. Redundancy isapplied to the entire tripping sequence and failure of any single element doesl not preventtripping. The protection has two independent tripping schemes and where applicable each fault

scenario is detected using different principles. The protections is arranged into overlappingprotective zones and for each fault case there is a fast primary protection with restrictedprotective zone, supported by a time-graded less sensitive backup protection based on adifferent measurement principle and with a more extended protective zone. For cases whereprimary/backup concept cannot be practically applied, the protection function is duplicated. Atypical example is the valve short circuit case where valve overcurrent capability dictates ashort fault clearing time which precludes the use of slow backup clearing times.

As mentioned before, the provision of primary and backup protection is to avoid failure of anysingle element preventing tripping. Possible causes are due to failures of one of the following :

• Power supply to the protection

• Power supply to the protective sequencing

•Measurement equipment

• Protective sequencing hardware and software

• Protection hardware and software

• Circuit breaker tripping circuits or breaker mechanism

For this reason, primary and backup protections is physically separated with independentauxiliary power supplies and, where applicable, independent measurement transducers. Thecircuit breakers are provided with breaker failure protection and the tripping and blocking pathsare duplicated and monitored by the control system.

The protection is as far as possible independent (not affected by control failure) of controlsystem i.e. separate power supplies and separate/buffered measurements to avoid common

mode failures. Control and protection functions in practice cannot be totally separated. In ac





networks, the protection action is restricted to circuit breaker tripping and usually results intaking out the transmission capacity. The controllability of HVDC systems makes it possible touse valve firing sequences to implement protective actions. Since the valve firing sequence isused for both control and protection purposes, it is not always possible to discriminate betweencontrol and protection by the action taken.

2.3 Stability

The protection is able to discriminate between external power system events or transient and genuineinternal faults so that undue disruption or disturbance to dc transmission can be avoided. A typicalexample is the requirement to avoid tripping on inverter commutation failure caused by ac networkfaults. The dc link in this case is designed to recover from commutation failures. On the other hand,repeated commutation failure caused by control mal-operation is not disregarded. Other protections may

also have to be coordinated under depressed ac voltage condition to avoid mal-operation. This is usuallyachieved by setting the trip operating delay longer than the expected duration of ac disturbance. Oftensuch delays are unacceptable and detection principles that can discriminate external and internal faultsare provided.

A protection acts upon a specific type of fault within a designated zone and is inert to other types ofdisturbances or fault external to the relevant zone. Stability and selectivity are achieved though time-grading and unit system. Protection settings and delays are chosen to avoid operation due to ac systemtransient disturbances and recoverable faults. Time guards and (Inverse time with Definite MinimumTime) IDMT characteristics are generally used for time grading. Unit system does not involve timegrading and therefore can be relatively fast in operation. This is achieved by means of a comparison ofquantities at the boundaries of AC-DC or DC-DC system.

The ac network is protected from the consequences of dc control mal-operation. The ac network can in

most cases tolerate the noncharacteristic harmonics that are created during dc system disturbances. Insome cases protection operating times may need adjustment to meet ac network requirements.





3. Protection Strategy

3.1 Protective Zones

The HVDC facility is divided into a number of separately protected and overlapping zones. Theprotective zones are illustrated in Figure 1 below.

Figure 1 Protection Zones

Figure 1 shows the arrangement for one of the poles relating to one side. The arrangement isidentical for all poles.

The protective zones are:

Zone 1: Cable Zone 2: Busbar Zone 3: Harmonic Filters Zone 4: Converter Transformer Zone 5: Pole Zone 6: Converter Zone 7: Lines (Outside Scope) Zone 8: Substation busbar (Outside Scope)

Zone 7 and 8 is outside the scope and is not described in this document.

Every credible fault within the protective zones is detected and acted upon either by a main protectionfunction or an independent back-up protection function generally using a different detection principle.Where different detection principles cannot be used, duplicated protections are used.





3.2 Protection Scheme

3.2.1 General

The circuit breaker tripping logic is implemented by two independent schemes. The circuit breakers areprovided with dual tripping facilities. The two protection circuits are energised from independent powersupplies.

Circuit breaker tripping initiated by a protection results in the circuit breaker(s) being locked out andinhibited from closing until the trip relays are reset manually by the maintenance/operation personnel.Where appropriate, the operation of ac protection signals the controls so that the converters will beblocked and/or signals sent to initiate the circuit breaker failure protection (see section 4.6).

Back-up and redundant protections is as far as possible be fed by independent transducers and allprotection trips is separately alarmed.

The protection scheme can be divided into two main categories :

AC Protections

These protections are mainly associated with the protection of the AC busbar, the AC cable and the ACline. Only protections for the AC busbar and AC cable are covered here, being in AC113 scope ofsupply. Protections included in this category are given in Table 1 below. Duplication is used to provideredundancy in order to ensure continuous operation or safe and orderly shut-down of equipment, asappropriate to the fault condition. The ac protection for the transformer (4) and filters (5) are dealt withunder DC System Protection.

Table 1 AC Protections

Protection Protective Zone Remarks

Busbar DifferentialProtection

Busbar Zone Zone 2

Cable DifferentialProtection

Cable Zone Zone 1

DC Protections

The HVDC related protection functions are referred to as DC protections. These protections areassociated with the protection of equipment used for HVDC transmission which includes the harmonicfilters, the converter, the converter transformer and ancillary systems. They can broadly be categorisedinto the categories described below:

DC Pole Protections

DC Pole protection independently oversees the system and ensures equipment safety. Asshown in Figure 2 below within each CCP lane there are two parallel identical DC protectionsystems Main1 and Main 2. It is as independent as possible from the control system andcontrol failure does not limit the functionality of these protections. Each of these protectionsystems provides the primary, backup and duplicated protection and are identical. Both arealways active and are powered by separate power supplies. This ensures continuous

operation or safe and orderly shut-down of equipment, as appropriate to the fault condition.





Side A Side B

Lane 1

VBESide A

Main 1Protection

Lane 1

Side A

ControlSystemLane 1

Side A

Main 2Protection

Lane 1

Side A

Lane 2

CCP

Main 1Protection

Lane 1

Side B

ControlSystemLane 1

Side B

Main 2Protection

Lane 1

Side B

CCP

Main 1Protection

Lane 2

Side A

ControlSystemLane 2

Side A

Main 2Protection

Lane 2

Side A

CCP

Main 1Protection

Lane 2

Side B

ControlSystemLane 2

Side B

Main 2Protection

Lane 2

Side B

CCP

InterfaceCubicleSide A

VBESide B

InterfaceCubicleSide B

Figure 2 Main 1 and Main 2 Protection Arrangement

For single lane operation, if either system detects a condition where equipment integrity may be at risk,protective action is performed. Ultimately a protective action results in a converter block and main circuitbreaker trip.

These DC protections are implemented on a microprocessor- based system, Series V and eachprotection function is capable of operating standalone and being configured through an Engineer’sInterface. The protections are designed so that it is possible to test protective functions during operationwithout affecting the operation of the transmission.

The protective zone for these protections is normally either the pole zone or the converter zone andincludes the protections described in Table 2:

Table 2 DC Pole ProtectionsProtection Protective Zone Remarks

Asymmetry Protection Pole Zone

DC Differential Protection Pole Zone

DC Overcurrent Protection Pole ZoneAdverse Firing AngleProtection

Converter zone

AC>DC Differential ( ShortCircuit) Protection

Converter zone

DC>AC Differential(Commutation Failure)Protection

Converter zone

AC Overcurrent Converter zone

AC Overvoltage Valve side Converter Zone/ConverterTransformer Zone

AC Overvoltage Line side Busbar Zone/ConverterTransformer Zone

Implemented within DCPole Protections but has

an extended protective





zoneAC Undervoltage Converter Zone

Tap Limits Converter Zone

Low Id Protection Converter ZoneThermal Model Converter Zone

Valve Hall AC ConductorGround Fault Protection

Converter Zone/ Converter TransformerZone

Described in convertertransformer protections

Valve & Ancillary System Protection

These protections are associated with equipment failure that may cause the power transfer level to be

compromised, harm the thyristors or compromise the integrity of the main components of the DCequipment, for example failure of the Converter Cooling Plant. They may not always be protections assuch, for example the change over unit (see section 4.3.6) but can when required block and trip the polein order to ensure that the integrity of main circuit components is not compromised. The protective zonefor these protections is normally within the Converter zone and includes the protections/equipment listedin Table 3:

Table 3 Valve & Ancillary Equipment

Protection ProtectiveZone

Remarks

Valve BaseElectronic

Refire

Thyristor Redundancy

Exceeded

Coolant Leak Detection

BOD Redundancy Exceeded

PSU Redundancy Exceeded

PCB Interlock Protection

Transceiver Card Failure

Backup Trip OT1 and OT2

VBE Fail

Invalid Changeover Command

Converter Zone VBEimplements a

set ofprotectivefeatures

Thyristor LevelProtection (Gate unit) Forward Recovery Protectiondv/dt ( rate of rise of forwardovervoltage)

Forward Overvoltage Protection

Reverse Overvoltage Protection

Electronic VBO triggering

Converter Zone The Gate unitimplements aset ofprotectivefeatures

Convertor CoolingControl

Valve Cooling Water Flow Rate

Valve Cooling ExpansionVessel Water Level

Valve Cooling WaterConductivity

Converter Zone ConverterCoolingControlimplements aset of

protective





Valve Cooling WaterTemperature

Valve Hall Dew Point

features

Valve Hall Fire Detection System Converter ZoneValve Hall HVAC System Converter Zone

Change Over Unit Converter Zone IncorporatesCCP failure

Power Supply Supervision Not zonedependent

Converter Transformer Protections

These protections are used to protect the converter transformer and include both electrical andmechanical protections. The list of converter transformer protections is given in Table 4.

Table 4 Converter Transformer Protections


Transformer DifferentialProtection

Converter TransformerZone

Transformer Overcurrentand earth fault protection


Restricted Earth FaultProtection


High Oil TemperatureProtection


Line Winding TemperatureProtection


Valve WindingTemperature Protection


Buchholz Surge Protection Converter TransformerZone

Tap Changer Oil SurgeProtection


Tap Changer SuddenPressure Protection


Valve Hall AC ConductorGround Fault Protection

Converter Zone/ Converter TransformerZone

Harmonic Filter Protections

The ac filters protection scheme is shown in Table 5.

Table 5 Harmonic Filter Protections


Main CapacitorOvervoltage Protection

Harmonic Filter zone

Filter Earth Fault Harmonic Filter zone





Filter OvercurrentProtection


Filter Overload Protection Harmonic Filter zoneFilter Resistor OverloadProtection


Filter Reactor OvercurrentProtection


Filter Capacitor UnbalanceProtection

Harmonic Filter Zone

Harmonic Filter DifferentialProtection


In addition to the protections listed above, two additional protections, Converter Isolation Detection

(Islanding) and Unexpected Breaker Movement (Opening) are supplied that do not properly reside withinany of the above categories but are needed as a final stage shutdown of the links. These are associatedwith what is defined as the “last line disconnect”. They are described in section 4.8.

Filter CBs are equipped with Circuit Breaker Fail protection. Main CBs are outside the scope of supplybut these also have Circuit Breaker Fail protection. Circuit Breaker Fail protection is describedseparately in section 4.6.

3.2.2 Measurements

3.2.2.1 Converter Transformer Protections

In order to derive the measurements required for the “electrical” Converter Transformer Protectionsdescribed in section 4.4 the arrangement inFigure 3 is deployed:

Figure 3 Converter Transformer Measurements





3.2.2.2 DC Pole Protections

In order to derive the measurements required for the DC Pole Protections described in section 4.2 thearrangement in Figure 4 is deployed:

DCDifferential

Asymmetry

AC>DCDifferential

DC>ACDifferential

ACOvercurrent

Line windingovervoltage

Valve windingovervoltage

ACUndervoltage

CID

Tap Limits

M

M

FilterBanks

MAdverse

Firing angle

DCOvercurrent

ZFCT

Voltage Divider

CT

CVT

1)

NOTES

1) Converter Isolation Detection is includedhere although it is treated separately.

Low ID

Figure 4 DC Pole Protection Measurements





3.2.2.3 Harmonic Filters

An example of the protection scheme applied to a double damped filter is given inFigure 5 below.

Figure 5 Protection Scheme for Harmonic Filter

As shown in Figure 5 above, CTs mounted are in the common neutral end of each phase and in thecentre limb of the main capacitor bank H connections to feed the capacitor unbalance protection.

Where appropriate, CTs are provided in the connections to the filter reactors and resistors to feed thereactor and resistor overload protection.

To feed the Harmonic Filter Differential Protection dedicated cores in the filter CB bushing CTs are usedin addition to CTs mounted at the common neutral end of each phase.





4. Protections

Descriptions of all protections listed in section 3.2.1 are provided in this section. The descriptions whereappropriate, include:

Protection - In case of protections implementing several protective

features, for example converter cooling control, thefeatures are described as separate functions and thisheading is used to indicate which protection isresponsible for the protective feature.

Protective Zone - The Protective Zone for the protection

Primary Objective

- The main objective of the protection.

Protective Action - The protective actions implemented by the protection to

clear a fault or alleviate stress on the equipment.

Backup Protection

- The Backup protection used to detect the fault or themeans to provide proper redundancy by for exampleduplication.

Functionality

- The detection principle and when required necessarybackground information.

Remarks - Additional information regarding the protection, for

example coordination with other protections.

Unexpected Breaker Movement and Converter Isolation Detection are described under section 4.8

Circuit Breaker Fail protection is described in section 4.6

All protections employed in the protection scheme are listed under this section. The protections areorganised by their respective group (AC Protection, Converter Transformer Protection etc)





4.1 AC Protections

AC protection is provided to protect against faults detected on the busbar leading up to themain equipment of the facility, i.e the converter transformer and harmonic filters.

4.1.1 Busbar Differential Protection

Protective Zone Busbar Zone

Primary Objective To detect earth or phase faults on the busbar and if detected takethe pole out of service.

Protective Action

Alarm Block DC Link Trip Main CBs

Backup Protection Duplicated

Functionality Busbar differential protection comprises CTs connected at the cableend, at the HV CBs of the harmonic filters and at the ConverterTransformer. The protection determines earth faults or faultsbetween phases by detecting differential current and will in responsetrip and block the pole.

Remarks

4.1.2 Cable Differential Protection

Protective Zone Cable Zone

Primary Objective To detect earth or phase faults on the cable and if detected take thepole out of service.

Protective Action Alarm Block DC Link Trip Main CBs


Functionality Cable differential protection comprises CTs connected at the mainfeeder CBs and at the connection between the cable and theconverter main busbar. The protection determines earth faults orfaults between phases by detecting differential current and will inresponse trip and block the pole.

Remarks





4.2 DC Pole Protections

These protections are implemented on a Series V microprocessor-based system that is asindependent from the control system as is practical.

4.2.1 Asymmetry Protection

Protective Zone Pole Zone

Primary Objective To detect persistent presence of fundamental and 2

ndharmonic

voltages between the dc terminals of the pole; caused by, forexample, valve misfire or commutation failure.


Backup Protection Duplicated DC>AC (Commutation failure) Protection

Functionality The measured HVDC voltage is band-pass filtered with respect tofundamental and second harmonic frequencies and when themagnitude of the RMS voltage exceeds preset thresholds theprotection is initiated. As soon as a fault is detected an alarm isinitiated. If the fault persists the pole is taken out of service. Theprotection has an inverse time characteristic.

Remarks 4.2.2 DC Differential protection


Primary Objective To detect ground faults on the dc side of the converter and the acconductor between the converter transformer and the valve hall and inresponse take the pole out of service.

Protective Action Alarm

Block DC Link

Trip Main CBs

Backup Protection

• Duplicated

• Valve Hall AC Conductor Ground fault ( If the converter isblocked)

Functionality The protection detects mismatch between the two HVDC currentinput signals, Id1 and Id2 measured either side of the dc circuit

earthing point, and operates after a delay. Possible causes of





mismatch are ground fault at the Neutral busbar, transformerstar/delta winding, or within the valve hall zone.

The protection has two levels of sensitivity, at the lower level the delay timebefore the protection is triggered is longer at typically 30ms. If themismatch exceeds the higher level the protection must act faster and thedelay before operating is brought down to typically 3ms.

The delay is introduced to avoid spurious triggering if the protection detectsmismatches during energisation caused by charging currents.

Remarks

4.2.3 DC Overcurrent


Primary Objective To detect overcurrent in the HVDC link and take the pole out ofservice if a fault is detected.


Backup Protection Duplicated AC Overcurrent Protection

Functionality The protection has an inverse time with definite minimum timecharacteristic (IDMT) with the response time (the length of time the fault isallowed to persist before protection operates) being inversely proportionalto the input signal for low overcurrents and a definite minimum time forlarge overcurrents.

Remarks

4.2.4 Adverse Firing Angle Protection

Protective Zone Converter Zone

Primary Objective To detect abnormal firing angle conditions that may result in thermalfailure of surge arresters connected between the thyristor valveterminals.


Backup Protection

Duplicated





Functionality The adverse firing angle protection computes when the voltage-dependent components are in danger of overheating by processingvarious system parameters within a mathematical model. Theoperating time depends on the firing angle and the dissipationcharacteristics of the surge-arresters.

Remarks

4.2.5 AC>DC Differential (Short Circuit)


Primary Objective To detect a valve short circuit, other phase-to-phase short circuitswhich give rise to high AC currents and low DC currents and inresponse take the pole out of service



Functionality The HVDC current is compared to the ac current and if the ac currentis greater than the equivalent HVDC current by a preset amount theprotection is initiated.

Remarks

4.2.6 DC>AC Differential (Commutation Failure)


Primary Objective To detect commutation failures within a twelve-pulse converterbridge and if the fault persists take the pole out of service

Protective Action Alarm Increase the firing angle ( performed before block and trip to aid

recovery) Block DC Link Trip Main CBs

Backup Protection Duplicated Asymmetry Protection

Functionality This is the opposite of the AC > DC Differential above. In this casethe sense of the difference indicates a commutation failure where dccurrent flow bypasses the ac connections.





On immediate detection of commutation failure:

The firing angle is advanced to aid recovery

A “commutation failure” signal is sent to thethermal model (see 4.2.13), so that the Id signal is used instead of the Ivw signals for the

junction temperature Tj calculations as the Ivw signal cannot be relied upon during acommutation failure

Normally the system shall be able to recover from a commutationfailure but if the commutation failure persists which indicates control

mal-operation the pole is blocked and tripped.

Remarks This protection needs to be coordinated with ac system protectionsas well as the asymmetry protection

4.2.7 AC Overcurrent


Primary Objective To detect overcurrents in any of the 12 valve winding connectionswhich can result from phase to phase valve connection faults orcontrol failure and initiate protective actions at detection.


Backup Protection Duplicated Transformer Overcurrent & Earth fault Protection

Functionality The protection has an inverse time with definite minimum timecharacteristic (IDMT) with the response time (the length of time the fault isallowed to persist before protection operates) being inversely proportionalto the input signal for low overcurrents and a definite minimum time for

large overcurrents.

Remarks

4.2.8 AC Overvoltage Line Side

Protective Zone Converter Transformer ZoneBusbar Zone

Primary Objective To detect overvoltages in the line winding side that could stress theequipment and in response take the pole out of service in case ofpersistent ac overvoltage






Backup Protection Duplicated Tap Limits ( Shall operate prior to this protection) AC overvoltage Valve side

Functionality Overvoltage is tolerated for a certain time, dependent on the voltagelevel, - if the overvoltage is removed within this time then the

protection does not operate. If the overvoltage persists, or if theovervoltage is removed but recurs within the cooling period allowed,then the protection operates.

The equipment is required to be protected for AC system voltageexcursions beyond the specified range resulting from a systemdisturbance. The overvoltage protection characteristic is defined herebased on the equipment capability.

The line side overvoltage characteristic is based on the worst case ofthe converter transformer overfluxing in the event of overvoltage.The converter transformer overfluxing characteristic associated withthe maximum tap at full frequency (60Hz or 50 Hz) is selected as itencompasses the requirement of all the equipment connected to theline terminal.

Remarks The operation of this protection is required to be coordinated with thetap changer control and the tap limit protection to allow for normaloperation. Provision shall also be made so that the settings do notgive rise to unnecessary alarms and trippings due to permanent acnetwork voltage changes or switching actions.

4.2.9 AC Overvoltage Valve Side

Protective Zone Converter ZoneConverter Transformer Zone

Primary Objective To detect overvoltages in the valve winding side that could stress theequipment and in response take the pole out of service in case ofpersistent ac overvoltage

Protective Action Alarm Block Trip Main CBs

Backup Protection Duplicated Tap Limits ( Shall operate prior to this protection) AC overvoltage Line Side





Functionality The voltage is measured on the line winding side of the convertertransformer and calculated for the valve winding using the measuredtap position.

Overvoltage is tolerated for a certain time, dependent on the voltagelevel, - if the overvoltage is removed within this time then theprotection does not operate. If the overvoltage persists, or if theovervoltage is removed but recurs within the cooling period allowed,then the protection operates.

The equipment is required to be protected for AC system voltageexcursions beyond the specified range resulting from a systemdisturbance. The overvoltage protection characteristic is defined here

based on the equipment capability.

For the transformer valve winding side overvoltage protection, thecharacteristic with respect to time is defined by the valve surgearrester capability, followed by the expected voltage on the eventualtripping of the AC filters and the consequent protective tapchangerlowering of the converter transformer.

Remarks The operation of this protection is required to be coordinated with thetap changer control and the tap limit protection to allow for normaloperation. Provision is also be made so that the settings do not giveraise to unnecessary alarms and trippings due to permanent acnetwork voltage changes and switching actions. (Line and shuntreactor switching).

4.2.10 AC Undervoltage


Primary Objective To monitor the line-to-line ac system voltage and inform Valve BaseElectronics that the ac volts are too low to maintain the charge onthe thyristor gate unit supply. If the ac volts should persist to be toolow for a fixed time when the pole is deblocked, the protection willblock and trip the pole.

Protective Action Inhibit Databack (performed prior to block and trip) to prevent

false reporting. Alarm Block DC Link Trip Main CBs


Functionality The voltage is measured on the line winding side of the convertertransformer and calculated for the valve winding using the measured tapposition.

If any valve winding line voltage falls below the set level and remains below

it for longer than a definite time, an indication to Databack that the ac





voltages are too low to maintain the charge on the thyristor gate unit powersupplies is sent.

Following restoration of the voltage level, Databack inhibit is held for apredefined period to allow time for the gate unit to be fully charged. If theconverter is deblocked, it produces block and trip after a fixed delay whenundervoltage is detected. The undervoltage detect time is selected to becomfortably shorter than the energy storage time of the gate unit powersupply, so that no corrupt signals are transmitted. The power supplystorage time itself exceeds the ac network protection time, so that there istime to carry out all necessary fault limitation, autoreclose and isolationfunctions and then to restart transmission before it expires.

If restoration is unsuccessful, e.g. if an autoreclose fails, the undervoltagedetection feature will time out, triggering Block and Trip. If charging voltageis restored before the end of the time-out, Databack inhibit will never beexerted, nor will the Block and Trip commands.

Remarks In practice for this scheme autoreclose action should not impact onthe converters due to the number of lines and in the case of the 60Hzsystem, the non-use of autoreclose.

Databack is part of the data gathering system for the Valve BasedElectronic System (VBE).

4.2.11 Tap Limits

Protective Zone Converter ZoneConverter Transformer Zone

Primary Objective To prevent long-term voltage stress that may cause harm to theequipment if they are allowed to persist. A typical example is over-excitation of the converter transformer.

Protective Action Alarm Inhibit Raise Force Lower

Backup Protection Duplicated AC Overvoltage Valve winding AC Overvoltage Line winding

Functionality Valve line side voltages are measured and compared with pre-setthresholds. For moderate voltage stress, the control is inhibited from raisingthe tapchanger position. For severe voltage stress the tapchanger is forcedto tap down to acceptable levels. The overfluxing of the convertertransformer is frequency dependent. The settings of this protection do notinterfere with normal tap changer control

Remarks





4.2.12 Low Idc Protection


Primary Objective The Low Idc (low dc current) protection prevents prolonged operationof either a rectifier or inverter operating into an open circuit, i.e.when one side fails to deblock.

Protective Action Alarm Block DC Link

Backup Protection Duplicated AC Overvoltage Valve winding AC Overvoltage Line winding

Functionality When the DC current goes below a set value for a set time theprotection operates. An integral timer is employed so that repeatedlow currents have a cumulative effect. The protection is alwaysactive but will not operate unless the pole is deblocked.

Remarks The settings are coordinated with the minimum deblocking current forthe scheme.

4.2.13 Thermal Model


Primary Objective To calculate the thyristor junction temperature and take protective actionshould the temperature exceed preset limits.



Functionality The thermal model has two temperature levels:

The OT1 level is deemed to be the maximum thyristor temperaturefor non-fault conditions. Smoothing is introduced to avoid tripping ontransient overtemperature.

The OT2 level results in a firing sequence which forces continuousconduction in order to prevent damage to the thyristors, which have adiminished forward voltage withstand capability above this temperature.

The valve has a chance to recover during the reverse voltage period





following the conduction period in which OT2 was exceeded, but if OT2 isstill exceeded at an instant shortly before the valve voltage goes positivethen a 'REFIRE' pulse is received from VBE, which then causes aProtective firing of the valve. A Block and Trip is then issued to theconverter and the other valves are blocked.

The Thyristor junction temperature as calculated by one thermal model inMain 1 (see Figure 2) is compared to the value calculated by the otherthermal model in Main 2. If a discrepancy in excess of a preset value isdetected for more than pre-set time then, a lane changeover is forced whenboth lanes are initially available and the original lane made unavailable, orthe converter is tripped if only one lane was initially available.

Remarks

4.3 Valve & Ancillary Equipment Protections

4.3.1 Valve Base Electronics (VBE)

The Valve Base Electronics includes several protective functions. Note that they are not separateprotections but simply protective functions carried out by Valve Base Electronics (VBE). A summary ofthe Protective functions are given below:

4.3.1.1 Refire

Protection Valve Base Electronic


Primary Objective To monitor the number of thyristors conducting and the number offailed to recover thyristors and issue a refire.

Protective Action Alarm Valve Refire


Functionality Independent Protective triggering of individual thyristor levels cansometimes lead to partial blocking of the valve, where some of the levelsare fired and others remain blocked. Partial valve conduction can arise forexample when some levels fail to satisfy the forward recovery requirementand the rest do or where all levels do recover but the re-applied forwarddv/dt is high enough to activate the protection on some, but not all levels. Inthis situation, if the valve voltage were to rise to a large positive value andthen reverse, the protectively triggered levels can potentially experience ahigher reverse voltage than they can handle. To prevent high reversevoltages, if more than a certain number of thyristors (taking into accountthe number of redundant levels) are protectively triggered, the entire valveis refired.





Refire relies on each individual thyristor level, signalling to ground level viadataback that it has been protectively fired (see section 4.3.2)

Remarks

4.3.1.2 Thyristor Redundancy Exceeded



Primary Objective

To monitor the number of failed thyristors and if too many thyristorsfail take the pole out of service.



Functionality If a preset number or more thyristors fail then the thyristor redundancyexceeded alarm is generated and the scheme is blocked and tripped.

Remarks

4.3.1.3 Coolant Leak Detection



Primary Objective To detect coolant leakages.



Functionality The leak detection system comprises a collection tray and detector locatedin the bottom stress shield of the quadrivalve (HVDC multi-valve unit).Coolant drips will collect in a bucket under the tray and when apredetermined amount of coolant has accumulated, will cause the bucketto decant. The number of operations per unit time is an indication of the

rate of coolant leakage.





The Leak Detector function comprises an optical transmitter which sendslight through a glass optical fibre to the detector and back to the opticalreceiver. The transmitter sends a continuous chain of light pulses, whichare interrupted when the bucket decants.

A minor coolant leak alarm will be generated by the bucket tipping and aBlock and Trip will be generated if the bucket stays down or if the rate ofleakage exceeds a predetermined amount.

Remarks

4.3.1.4 BOD Redundancy Exceeded



Primary Objective To take the pole out of service in case of prolonged BOD firing.



Functionality If healthy level BOD firing persists for a long time period the protectionoperates to take the pole out of service.

Remarks

4.3.1.5 PSU (Power Supply Unit) Redundancy Exceeded



Primary Objective To take the pole out of service in case the Power SupplyRedundancy limit is exceeded.







Functionality If insufficient power supplies are available for VBE the pole is blocked andtripped.

Remarks

4.3.1.6 PCB (Printed Circuit Board) Interlock



Primary Objective To ensure that the transceiver cards are in place


Backup Protection

Duplicated

Functionality VBE monitors that all transceiver cards are in place and if a card is pulledout this will cause a block and Trip.

Remarks

4.3.1.7 Transceiver Card Failure



Primary Objective To take the pole out of service in case of transceiver card failure







Functionality VBE monitors that the transceiver cards are healthy and should atransceiver card fail the pole is taken out of service.

Remarks

4.3.1.8 Backup Thermal Model Trip OT1 OT2



Primary Objective To provide a backup to thermal model to block and trip the pole incase of excessive junction temperature



Functionality VBE receives thermal junction temperature from the Thermal model (seesection 4.2.1.3) and provides a back-up in case Thermal Model does notblock and trip the pole.

Remarks

4.3.1.9 VBE (Valve Base Electronic) Fail

Protection Valve Base Electronic (watchdog)


Primary Objective To monitor the communication of firing word and thermal word and ifcomms fail, take the pole out of service. In addition at start up if thefirmware fails to load this will also cause a block and trip.







Functionality VBE continuously monitors the communication status of firing word andthermal word, even if the pole is blocked. Should the comms fail the pole istaken out of service. In addition at start-up of VBE, if the firmware fails to loadfrom the non-volatile memory the pole is taken out of service.

Remarks

4.3.1.10 Invalid Changeover Command



Primary Objective To ensure that the lane selection is correct



Functionality

VBE receives information of the control lane selection in form of twosignals, Select Lane 1 and Select Lane 2 and if both are truesimultaneously for a long time (1-2 s) the pole is blocked and tripped.

Remarks

4.3.2 Thyristor level protection

Triggering the thyristor into conduction is a useful mean of protecting individual thyristors. Thefollowing features are provided within the Valve Electronics and utilise this facility to protect thethyristor valves. These features also feed the Refire function of VBE described in section4.3.1.1 which is used to refire a whole valve.

Forward Recovery Protection

After a period of conduction the thyristor takes time to recover its forward blocking capability. Thethyristor is deemed to be conducting, if the fire latch is set when anode voltage is greater than minus10V. The turn-off time, tq, increases with junction temperature and the gate unit incorporates thischaracteristic.

The gate unit starts a recovery timer when the voltage on the thyristor anode becomes more negativethan minus 20V (the minus 10V threshold comparator has 10V hysteresis, resulting in a threshold ofminus 20V for a negative going signal and minus 10V for a positive going signal). The ‘fire latch’ is resetwhen the recovery timer reaches the determined value of tq. If however the voltage on the thyristoranode becomes more positive than minus 10V before the recovery timer stops, the timer resets andbegins counting from zero up to the time tq if anode voltage goes below minus 20V again. Also, if during





recovery the recovery time is updated, the recovery protection has the capability to revise the recoverytime “on the fly”.

If forward volts appear too early (before the recovery timer times out) across the thyristor when the firelatch is still set, the gating unit re-fires the thyristor, to avoid destructive breakdown of the device. Inaddition, a ‘thyristor conducting’ signal is transmitted via the databack fibre to the VBE. ‘Thyristorconducting’ is a logic signal obtained by ANDing the ‘fire latch’ and the output from the minus 10Vthreshold comparator. A change of state of this signal triggers a databack transmission. This enablesthe VBE to monitor the status of each thyristor level, so it can then prompt the whole valve to fire under amarginal recovery (section 4.3.1.1) case when some levels recover and some refire.

The next section covers dv/dt protection during forward recovery.

dv/dt Protection

The rate of change of voltage (dv/dt) that a thyristor can withstand and remain blocked is limited andvaries depending on the previous operating condition. After conduction, at the start of recovery, dv/dtwithstand is very low, and increases with time as the thyristor recovers, until it reaches a maximum whenthe thyristor is fully recovered. The rate of increase of dv/dt withstand, and the maximum withstand whenfully recovered, depends on junction temperature. At very high temperatures, the dv/dt withstand evenfor a fully recovered device will remain at a very low level. The gating unit models the thyristor dv/dtcapability in real time. A gate pulse is issued to fire the thyristor if the capability is exceeded whenforward volts appear, to prevent destructive breakdown.

During recovery when the dv/dt threshold is very low, the dv/dt protection can be susceptible to nuisancefiring due to high, but limited excursion dv/dt. Even at high dv/dt’s, a thyristor requires a minimumvoltage excursion before dv/dt induced turn-on becomes likely. The design of the dv/dt protection takes

this factor into consideration.

Large forward voltages can be established as a result of fixed delays in the system, such as gating unitpropagation delay and thyristor turn-on delay. To avoid this from happening particularly at high levels ofdv/dt, it is necessary to produce gate current while the thyristor voltage is still negative, i.e. to phaseadvance the gate pulse.

Forward Over-voltage (VBO) Protection

The gate unit delivers a gate pulse to the thyristor to prevent its forward voltage exceeding its saferating. Electronic VBO triggering is supplied. When a thyristor’s junction temperature is increased aboveabout 110°C, the forward blocking capability of the device is reduced. The gate unit models the thyristorforward blocking voltage as a function of junction temperature, which it uses to set the VBO threshold.

Reverse Over-voltage ProtectionIn some circumstances, the thyristor reverse voltage can reach the avalanche level. The thyristor willthen pass current in the reverse direction. The thyristor is chosen to survive the dissipation that results,but it will also need time to recover its forward blocking capability, as it does after a normal conductionperiod. When the anode voltage crosses the overvoltage protection threshold (-8kV for 8.5kVthyristor),the gate unit sets the fire latch, so that forward recovery protection can operate if necessary.

4.3.3 Converter Cooling Plant

The converter cooling plant includes several protective functions that can cause a block and tripof a pole. Each sensor can be switched out of service so that maintenance/replacement can take placewithout causing spurious protective operations. A summary of these functions are given below:





4.3.3.1 Valve Cooling Water Flow Rate

Protection Converter Cooling Plant


Primary Objective To detect low flow and give an alarm indicating that the flow is toolow and to take the pole out of service should the flow not beadequate to cool the converter.


Block DC Link Trip Main CBs

Backup Protection Duplicated sensors Thermal model

Functionality The valve cooling water has a “low speed” and a “high speed” flowdepending on the ambient temperature. Each of these states has acorresponding low flow alarm and a trip setting. Should the flow go belowthe Alarm setting an alarm is issued. Should the flow go below the tripsetting the pole is blocked and tripped.

Remarks

4.3.3.2 Valve Cooling Expansion Vessel Water Level



Primary Objective To ensure sufficient water within the system between top up eventsis available


Backup Protection Duplicated level transmitters





Functionality The cooling water circuit includes an expansion vessel to provide forcooling system volume change with temperature variation. This alsoprovides a reservoir for maintaining sufficient water within the systembetween top up events. The vessel has duplicated level transmitters thatwill issue an alarm if the water level should go below a predefined value ortrip the pole should the water level go below a preset value that is set lowerthan the alarm level.

Remarks This protection will also stop the pumps if the water level goes belowthe trip setting to prevent the pumps from running dry should the lowlevel be due to a major leak.

4.3.3.3 Valve Cooling Water Conductivity



Primary Objective to maintain an adequate insulation level in the thyristor valves.


Backup Protection Duplicated conductivity transmitters

Functionality The main cooling water circuit conductivity is continuously monitored tomaintain an adequate insulation level in the thyristor valves. The coolingcircuit has duplicated conductivity transmitters. If the conductivity shouldrise above the alarm level an alarm is issued. If the conductivity exceedsthe trip level the pole is tripped.

Remarks

4.3.3.4 Valve Cooling Water Temperature



Primary Objective To ensure the water temperature does not exceed preset limits sothat the maximum thyristor junction temperature is not exceeded. Italso ensures that the water temperature is not too low as this may

decrease the thyristors voltage level withstand level






Backup Protection Duplicated temperature transmitters Thermal Model

Functionality The cooling circuit has duplicated temperature transmitters to continuouslymonitor the main cooling water temperature before entering the thyristor

valves. The cooling controls maintain the water temperature within a range.Upper limits define the alarm level and trip level if the water is too hot.Lower limits define the alarm level and trip level if the water is too cold. Ifthe temperature of the water falls outside the boundaries defined by the triplevels, the pole is tripped.

Remarks

4.3.3.5 Valve Hall Dew Point



Primary Objective To monitor the temperature of the outlet air from the air conditioningplant in order to ensure that the risk of moisture “condensation”forming on the insulated cooling pipes in the valves, which couldresult in a flashover, is eliminated.


Backup Protection Duplicated dew point transmitters

Functionality The valve hall dew point temperature is continuously measured bymonitoring the outlet air from the air conditioning plant and the coolingwater temperature. The air circuit has duplicated dew point transmitters.The cooling control continuously compares the water temperature (andhence the temperature of the pipes in the valve hall) to ensure that it issufficiently greater than the dew point so that condensation will not form. Ifthe difference in temperature is not great enough an alarm is raised. If thedifference between the water temperature and dew point temperature isvery small then the pole is tripped before condensation can form.

Remarks





4.3.4 Valve Hall Fire Detection Protection

Each valve hall is equipped with a duplicated fire detection system to protect the converterstation against fire hazards.


Primary Objective To detect a fire or fire hazard within the valve hal l or HVAC room andtake the pole out of service when detected.

Protective Action

Alarm Block DC Link Trip Main CBs

Backup Protection Duplicated system with duplicated transmitters

Functionality When a fire hazard or fire is detected this will trip and block the pole.

Remarks

4.3.5 Valve Hall HVAC System

The Valve HVAC system is provided to ensure that the temperature in the valve hall does notgo above preset values.


Primary Objective To detect excessive valve hall temperature and in response take thepole out of service.


Backup Protection Duplicated system with duplicate sensors

Functionality Sensors mounted below the valve hall ceiling are used to measurethe temperature and in case of excessive temperature the pole isblocked and tripped.

Remarks





4.3.6 Change Over Unit

The controls are equipped with a Change Over Unit so that if one control lane fails due to forexample a sysfail (system failure detection), change over to the redundant lane occurs. Shouldthe redundant lane be unavailable due to for example maintenance or a fault within theredundant lane, the pole is blocked and tripped.

4.3.7 Power Supply Supervision

Protection cubicles for all protections, regardless of whether it is AC protections or Harmonic FilterProtections etc, are be provided with power supply supervision that will cause a block and trip if thepower supplies to a cubicle fails.

4.4 Converter Transformer Protections

The converter transformers are connected between the AC bus and the HVDC valves. Incontrast to a normal power transformer the converter transformer current is not purelysinusoidal. The line side has grounded wye-windings, while the valve side is not grounded andthe voltage level can be considerably high. These characteristics are taken care of in theprotection design. The following converter transformer protections are provided:

4.4.1 Electrical Protections

The converter transformer is provided with the following “electrical” protections:

4.4.2 Transformer Differential Protection

Protective Zone Converter Transformer Zone

Primary Objective To detect internal faults within the transformer and in response takethe pole out of service


Backup Protection Transformer Overcurrent and earth fault protection Restricted earth fault protection Duplicated

Functionality This differential protection is fed from the CTs on the transformer linewinding, valve winding bushings and neutral ground connection. Itdetects differential current and in response it will take the pole out ofservice.

Remarks





4.4.1.2 Transformer Overcurrent and Earth Fault Protection


Primary Objective To detect internal earth faults and overcurrents in the transformerand in response take the pole out of service


Backup Protection Restricted earth fault protection AC overcurrent protection (valve side only)

Functionality This protection is driven from a set of CTs mounted in the convertertransformer line windings to detect overcurrent and earth faults.

Overcurrent protection is not provided directly on the secondary(valve) windings as this is covered by the AC Overcurrent protection

Remarks

4.4.1.3 Restricted Earth Fault Protection


Primary Objective To detect ground faults on a given winding more sensitively thanoverall transformer protection is able to do and in response take thepole out of service


Backup Protection The protection is duplicated Transformer overcurrent and earth fault protection Transformer differential protection

Functionality This differential protection compares the residual line winding phasecurrent with the associated line winding star point to earth current todetect ground faults on the line winding, tap-changer compartment orAC conductors.

Remarks





4.4.1.4 Valve Hall AC Conductor Ground Fault Protection


Primary Objective To detect phase-to-phase short circuits or ground faults on the acconductors between the transformer and the converter.



DC Differential Protection (only if converter is deblocked)

Functionality This protection is only enabled if the converter is blocked, if the converter isdeblocked the protection used is the DC Differential Protection.

The protection operates by monitoring the valve winding voltage and if thevoltage goes below a preset level the protection operates

Remarks This protection operates as an interlock that prevents deblocking ofthe pole and will not cause a block and trip.

4.4.2 Mechanical Protections

In addition to the electrical protections mentioned above each converter transformer is alsoequipped with the following mechanical protections:

4.4.2.1 High Oil Temperature Protection


Primary Objective To protect the converter transformer from overheating as a result ofsustained overload or cooling system failures.



Functionality The oil temperature is measured at the top of the transformer and ifthe temperature is too high the protection operates

Remarks





4.4.2.2 Line Winding Temperature Protection




Backup Protection Duplicated Valve winding temperature protection

Functionality The hot spot temperature of the winding is derived using temperaturesensors and current measurement of the winding. When thetemperature exceeds pre-set values the protection operates.

Remarks

4.4.2.3 Valve Winding Temperature Protection




Backup Protection Duplicated Line winding temperature protection

Functionality The hot spot temperature of the winding is derived using temperaturesensors and current measurement of the winding. When thetemperature exceeds pre-set values the protection operates.

Remarks





4.4.2.4 Buchholz Surge Protection


Primary Objective To detect dielectric failure inside the transformer due to windingshort-circuit or internal flashover and in response take the pole out ofservice


Backup Protection Duplicated pressure relief device

Functionality A Buchholz relay is mounted in the pipework to an overhead oilreservoir, called a conservator, of the oil-filled converter transformerto detect dielectric failure inside the transformer due to windingshort-circuit or internal flashover.

When electric arcing due to insulation failure or overheating developsinside the transformer coils, gas is generated. The protection has twodifferent detection modes. On a slow accumulation of gas, due to forexample a slight overload, gas accumulates in the top of the relayand forces the oil level down. A float operated switch in the relay isused to initiate an alarm signal. This same alarm switch also operateon low oil level, such as a slow oil leak.

If there is an internal flashover, gas is accumulated rapidly and oilwill flow swiftly into the conservator. This flow of oil operates aswitch attached to a vane located in the path of the moving oil thatwill cause the protection to operate.

Buchholz relays have a test port to allow accumulated gas to bewithdrawn for testing. Flammable gas found in the relay indicatessome internal fault such as overheating or arcing, whereas air foundin the relay may only indicate low oil level or leak.

Remarks The alarm stage operates into the alarm scheme.

4.4.2.5 Pressure Relief Device


Primary Objective The pressure relief device is mounted on the transformer to relievedangerous pressure that may build up within the transformer tank.When a predetermined pressure is exceeded, a pressure reactionlifts the diaphragm and vents the transformer tank.





Protective Action Vent out excess pressure in the transformer tank Alarm Block DC Link Trip Main CBs

Backup Protection Bucholz surge protection This protection is duplicated

Functionality Internal flashover in a transformer tank causes a rapid oil pressurerise which will cause the protection to operate.

4.4.2.6 Tapchanger Sudden Pressure Protection


Primary Objective To protect the tapchanger from overpressure in the tap changerconservator due to internal flashover if t he pressure relief device failsto operate.


Backup Protection Diaphragm pressure relief device in tapchanger head

Functionality This protection is implemented by a pressure relay mounted in thetapchanger housing to detect conservator overpressure. Normally thepressure relief device should operate (4.4.2.5) but if this fails to operatewhen overpressure is detected the pole is blocked and tripped.

Remarks

4.5 Harmonic Filter Protections

4.5.1 Harmonic Filter Differential Protection

Protective Zone Harmonic Filter zone

Primary Objective To detect earth or phase faults within the Harmonic Filter zone and ifdetected take the filter out of service

Protective Action Alarm Trip Filter CB

Backup Protection Filter Earth Fault Protection





Functionality Harmonic Filter differential protection comprises CTs mounted at thefilter CB bushings and the neutral of the filters. The protectiondetermines earth faults or faults between phases by detectingdifferential current and will in response trip the filters.

Remarks

4.5.2 Main Capacitor Overvoltage Protection


Primary Objective

To protect the main capacitor banks of filters from overvoltage in theevent of for example load rejection or light load.


Backup Protection AC Overvoltage Line side

Functionality The protection utilises a set of cores in the filter neutral CTs and

measure line current (using appropriate treatment of the harmoniccurrents) in order to produce a signal representative of the voltagewaveform applied to the capacitor bank.

Remarks

4.5.3 Filter Earth Fault


Primary Objective To detect faults to ground in the filter components and bus work ofthe filter branch.


Backup Protection Harmonic Filter Differential Protection

Functionality This protection detects zero phase sequence current in the filterneutral and provides both IDMT and instantaneous trippingcharacteristic depending on the current level.

Remarks





4.5.4 Filter Overcurrent Protection


Primary Objective To protect the filter components from filter overload and short circuitseither due to harmonics or high fundamental frequency currents dueto a flashover across part of the capacitor elements units, whichdevelops into a partial short circuit of the high voltage capacitor.


Backup Protection Group B Filter Overcurrent Protection

Functionality This protection has an inverse time characteristic acting on the rmsvalue of the measured harmonic current. The Harmonic current in acfilters vary with dc power level, firing angles and filter configurations.

Remarks

4.5.5 Filter Overload Protection


Primary Objective To protect the filter components from thermal overload st ress.


Backup Protection Group B Filter Overload Protection

Functionality This protection has a definite time set acting on the thermal responsecurrent derived from a model the thermal response of the harmonicfilter to rms heating current.

Remarks Wide band harmonic detection required.

4.5.6 Filter Reactor Overcurrent Protection


Primary Objective To protect the filter reactor from thermal overload






Trip Filter CB

Backup Protection Not Applicable

Functionality This protection protects a filter reactor against overcurrent by athermal overload relay that is driven by a CT in series with thereactor. The relay shall respond to fundamental and harmonicthermal overload and be co-ordinated with the overloadcharacteristics of the reactor.


4.5.7 Filter Resistor Overload Protection


Primary Objective To protect the filter resistor from thermal overload


Backup Protection

Not Applicable

Functionality This protection protects a filter resistor against overcurrent by athermal overload relay that is driven by a CT in series with theresistor. The relay shall respond to fundamental and harmonicthermal overload and be co-ordinated with the overloadcharacteristics of the resistor.


4.5.8 Filter Capacitor Unbalance Protection


Primary Objective To detect one or more levels of failed capacitor units and protect thehealthy capacitor elements by indicating failure conditions that maylead to fault and by opening the filter circuit breaker if the conditionposes an immediate risk to the remaining healthy capacitorelements.


Backup Protection Filter Overcurrent Protection





Functionality The basic building blocks of a filter capacitor are capacitor units. Acapacitor unit is made up of series/parallel connected capacitorelements with or without internal fuses housed within a steelenclosure. By Correct rating of stress levels, the filter banks can bearranged such that it can tolerate loss of a specified percentage ofcapacitor units without causing overvoltage over the remaining units.In which case no further protective action is required.

However, if the specified limit is exceeded which usually involves amore severe fault such as leakages, the remaining healthy elementsmay be subjected to unacceptable high voltage stress.

The main capacitor bank of each filter is therefore protected by anunbalance scheme that gives an “H” Arrangement as shown in Figure5. Any unbalance between the four arms of the “H” connectionarrangement will result in a spill current flowing through the CT, thelevel of which gives an indication of how many capacitor elementsmay have failed, therefore a sensitive CT is required for thisapplication.

There are usually different stages of protective action with thecapacitor unbalance protection implemented to detect various levelsof capacitor elements failure and produce alarms for long-term faultsand trip for immediate faults. A short time delay is introduced toavoid spurious operations due to transient events.

Remarks

4.5.9 Filter Tripping

All filter protections described above for a specific filter bank operates into one hand resetlockout-relay for the filter bank where the trip was generated.

The Overcurrent protection (4.5.4) and Earth Fault protection (4.5.3) directly operates the triprelay. All other protections operate the trip relay after a time delay. The circuitry is designed toensure that if any of the fault detecting relays reset during the time delay the tripping stilloccurs.

Each filter circuit breaker has a circuit breaker fail protection. See section 4.6 for information

regarding circuit breaker fail protection.

4.6 Circuit Breaker Fail Protection

Each filter is provided with breaker fail protection. This protection is initiated when the filter trip relay isoperated. It measures the filter current and if the filter current has not gone below a preset value after apre-set time the protection operates. This will trip the main CBs and also trip the other healthy filters tominimise the duty on the feeder CBs and block the converters.

4.7 Open Circuit Test Mode

Open Circuit Test Mode is a mode of operation that the system can be selected to operate in.

When the system is set to this mode it is possible to apply a direct voltage to an open d.c.





circuit. This is used for commissioning only. This also means that some of the DC Poleprotections will need to be sensitised or desensitised as no current should flow through theconverter during this test. Below follows a list of the protections that are affected by this mode:

1. AC>DC Differential

-The DC signal input is set to zero to sensitise the protectionduring Open Circuit Test Mode.

2. AC Overcurrent protection

- During Open Circuit Test Mode no current should flow through theconverter. This protection is therefore sensitised during Open

Circuit Test Mode by shifting the IDMT charachteristic down to anew pick-up level.

3. DC Overcurrent protection

- During Open Circuit Test Mode no current should flow through theconverter. This protection is therefore sensitised during OpenCircuit Test Mode by shifting the IDMT characteristic down to anew pick-up level.

4. DC Differential protection

- This protection is sensitised when Open Circuit Test Mode isselected to protect against flashovers

5. Low Idc protection- The protection is disabled during Open Circuit Test.

4.8 Last Line Disconnect

The purpose of this section is to cover the principles for last line disconnect. There are mainlytwo reasons for having a principle on last line disconnect:

Islanding

Unexpected breaker opening

4.8.1 Islanding

When a HVDC scheme is isolated from its receiving ac system, the inverter may still be feedingpower into an islanded area of ac filters/capacitors which can resonate with the saturatedconverter transformer reactance, a phenomena known as ferroresonance. The resulting ringingvoltage provides a commutation voltage of nearly correct frequency that enables the inverter tocontinue to operate. This condition is defined as islanding. If this condition is allowed topersist, main circuit components may be damaged due to the ensued overvoltage and it istherefore necessary to detect this condition and take action to remove it.





The detection principle used is able to distinguish between the islanding condition and othersystem disturbances and is fast enough to ensure that the condition is removed before theequipment is harmed.

4.8.1.1 Converter Isolation Detection

The Converter Isolation Detection (CID) function is included in the protection scheme in order todetect and take action upon detection of “islanding”. On detection of islanding the CID shallforce the isolated inverter into rectification mode of operation in order to remove power from theisolated network and thereby alleviate the ensued overvoltage. In order to prevent overcurrentand minimise the disturbances on reconnection to the ac network through breaker(s) closing,the converter is bypassed until the ac voltages have recovered.

The primary method for detection of islanding is by measuring the line winding voltages andcompare the magnitude with pre-set limits. If the line winding voltage measurement exceedsthese pre-set limits for a pre-set time, the control action invoked is to order Enforced Rectification Mode for a pre-set time followed by converter bypassing. The bypass is onlyrevoked when the busbar voltages exceed 0.75 pu, which indicates that the remote breaker hasreclosed and normal operation can be resumed.

As a backup to the overvoltage detection for “islanding” a frequency out of range detector isprovided. If the ac system frequency is outside preset limits, control action is invoked asdescribed above.

4.8.2 Unexpected Breaker Opening

The HVDC control guards against inadvertent disconnection of the converter from the ac system

due to unintended breaker opening or an adjacent ac protection zone trip. Inadvertentdisconnection will cause commutation to stop, however the last two valves in a three-phasebridge may continue to conduct current and if a natural by-pass pair is not formed it is possiblefor the converter to feed DC current into the breaker. This may result in breaker damage if thebreaker is not able to extinguish the DC Current.

4.8.2.1 Unexpected Breaker Movement Protection

The Unexpected Breaker Movement protection is included in the protection scheme in order todetect and take action upon detection of inadvertent breaker movement.

The Unexpected Breaker Movement function guards against inadvertent disconnection of theconverter from the ac system due to unintended or adjacent protection initiated breaker

opening. The connectivity of the converter to the ac system is derived from the busbarenergised, isolators and circuit breakers statuses.

If there is current flowing in the circuit when the connectivity status indicates the converter isdisconnected from the ac system, this indicates the circuit breaker has opened unexpectedlyand appropriate protective actions have to be taken depending on the mode of operation. If theconverter is operating as an inverter the protective actions are to bypass the inverter and blockthe other side rectifier. The bypass action is revoked when the ac current is extinguished andthe inverter is blocked immediately. If it is operating as a rectifier both the rectifier and the otherside inverter are blocked.





5 Protective Actions and Marshalling

The protective actions normally employed to clear a fault or prevent a fault are:

Trip CB

The objective of this action is to isolate the HVDC equipment from the ACsystem and thereby clearing the fault and reducing the stress on the equipment.For an urgent converter fault such as a valve short circuit where the converter inimmediate danger, the operating time of the circuit breaker should be less thantwo and a half cycles and therefore the delay of the trip chain is selected to bewell within this range. For a non-urgent converter fault where the converter is notin immediate danger, it is desirable to trip the feeder circuit breaker when there

is no current flowing through the breaker. This can be achieved by checking thatthe valve winding currents are nearly zero after a protective block before issuinga trip command.

In the case of converter feeder CB tripping, the filters normally open at the sameinstant or earlier to assist t he opening of the feeder circuit breaker.

Block DC Link

Protective blocking is used to stop the flow of both AC and DC current in order tolimit the effect of the fault. This is achieved by simply removing the firing pulsesto the converter valves. Normally a protective block is followed by a trip assimply removing the firing pulses may not always stop conduction. If protectionoperates on one side resulting in a block the other side is also blocked.

Valve Refire

To prevent high reverse voltages, if more than a certain number of thyristors(taking into account the number of redundant levels) are protectively triggered,the entire valve is refired.

Inhibit Raise

For moderate overvoltages the tapchanger is inhibited from tapping up to ensurethat the overvoltage condition is not worsened due to tapchanger action.

Force Lower

For more severe overvoltages the tapchanger is forced to tap down to alleviatethe stress on the equipment due to the overvoltage.

Inhibit Databack to VBE (Data gathering system for the Valve Base ElectronicSystem)

If any valve winding line voltage falls below the set level and remains below it f orlonger than a definite time, an indication to Databack that the ac voltages are toolow to maintain the charge on the thyristor gate unit power supplies is sent.Following restoration of the voltage level, Databack inhibit is held for apredefined period to allow time for the gate unit to be fully charged.





Bypass

This action provides a short circuit across the converter and is achieved bysending start codes to valve pairs connected to one phase only in each converterbridge of a twelve pulse converter pole

Enforced Rectification Mode

Enforced Rectification mode means that all valves are turned on so that theconverter basically operates as a diode bridge.

Increase firing angle

On immediate detection of commutation failure the firing angle is advanced toaid recovery.

Force thyristor into conduction

In order to protect an individual thyristor from being destroyed it can be forcedinto conduction by the gate unit.

Rapid filter removal

To trip all the filters associated with a pole when a pole is protectively blocked.This is done to ensure that the filters are removed in advance or at the same

time as tripping of the bay and tie breakers to minimise the duty on the bay andtie breakers.

Alarm

Whenever a protection operates or protective action is taken a separatelyindicated alarm is given.

5.1 Marshalling of DC Pole protections

The protective actions invoked when a DC Pole protection is operated are described in Figure6. Protections that can carry out both Block & Trip actions as well as other protective actions

will in general try to use the protective actions at its disposal (for example increase firing angle)in an attempt to clear the fault before initiating a block and trip.





Figure 6 DC Pole Protection Marshalling





5.2 Marshalling of Converter Transformer Protections

The protective actions invoked when a Converter Transformer protections is operated aredescribed in Figure 7.

Figure 7 Converter Transformer protection marshalling

5.3 Marshalling of Harmonic Filter Protections

The protective actions invoked when a Harmonic Filter Protection is operated are described inFigure 8. Included here are also the Circuit Breaker Fail Protections. Note that the number offilters varies between the sides.





Figure 8 Harmonic Filter Protections





5.4 Marshalling of Valve and Ancillary Equipment Protections

The protective actions invoked when a Valve or Ancillary Equipment Protection is operated aredescribed in Figure 9.

Figure 9 Valve & ancillary system protection marshalling





5.5 Marshalling of Unexpected Breaker Movement and Converter Isolation Detector

In order to deal with islanding and unexpected breaker opening as described in section 4.8 4.8Last Line Disconnect, two additional protections that do not fit into any of the above

categories are deployed in the protection scheme. The marshalling for Unexpected BreakerMovement and Converter Isolation Detector can be found in Figure 10 below.

Figure 10 Converter Isolation & Unexpected Breaker Movement Protection Marshalling

5.6 Marshalling of AC protections

The only AC protection within AREVA Contract 113 scope is the cable and busbar differentialprotection. The 400kV 50Hz protection are provided under Areva Contract 116 while the 380kV60Hz protection are provided within SEC’s existing protection systems. The marshalling is givenin Figure 11.





Figure 11 Marshalling of AC protections

6 Fault Cases

This section is aimed at describing some of the most common fault cases or fault cases that may

severely compromise the integrity of the equipment and also show which protection(s) normallyoperates in response to the fault.

6.1 Ground Faults and Short Circuits

Figure 12 Ground Faults and Short Circuits

Figure 12 shows some possible cases of short circuits and ground faults. Table 6 gives the details ofthe faults and also the protection(s) that will operate in response to the fault.





Table 6 Ground faults and short circuits

FAULT Protection Remarks

Fault 1 – Converter Short Circuit DC Differential ProtectionDC Overcurrent

Fault 2 – Ground faults at a DC terminal ofa bridge

DC Differential ProtectionDC Overcurrent

Fault 3 – Faults across bridge terminals DC Differential ProtectionDC Overcurrent

Fault 4 – Fault across a valve AC>DC DifferentialProtectionAC Overcurrent

Fault 5 – Faults across AC phases on thevalve side of the converter transfomer AC>DC DifferentialProtectionDC Differential Protection

DC Overcurrent

Fault 6 – AC conductor ground fault

AC Conductor ground fault Only operates whenthe pole is blocked

Transformer DifferentialProtection

Transformer Overcurrent &Earth Fault Protection

Restricted earth fault

protection

Buchholz Surge Protection

Tapchanger Sudden OilPressure

Fault 7- Converter Transformer InternalFault

Tapchanger suddenpressure protection

Fault 8- Fault on the busbar or cable Busbar DifferentialProtectionCable Differential ProtectionHarmonic Filter DifferentialProtection

Filter Earth Fault Protection

Filter Resistor OvercurrentProtection

Fault 9- Fault in a Filter branch

Filter Reactor OvercurrentProtection





6.2 Overvoltage

6.2.1 Transient and Temporary Overvoltage

The equipment is protected against transient and temporary overvoltages by surge arrestersand converter control. The converter control acts to limit the voltage stress.

6.2.2 Long Term Overvoltage

The primary action used for overvoltage is to try to reduce the voltage stress by control actions. Thecontrol action to limit long term voltage stress are converter dynamic control, tap changer control andreactive power control detailed.

Should this not be sufficient to alleviate the overvoltage the protections in Table 7 below will detect andact upon overvoltage.

Table 7 Overvoltage protectionFAULT Protection Remarks

Tap Limits Will inhibit controls from tapping up to increase the voltagestress and if necessary force the tapchanger to tap down viahardwired signal. The settings of this protection shall notinterfere with normal tap changer control

Valve SideOvervoltage

AC OvervoltageValve SideAC OvervoltageLine side

This protection operates due to overvoltage on the busbarLine SideOvervoltage

Main CapacitorOvervoltageProtection

Trips the filter CB

6.3 Undervoltage

Table 8 Undervoltage

FAULT Protection RemarksSinglephasefault

Phase-to-phasefault

Threephasefault

ACundervoltageDC>ACDifferential

Under normal operating conditions the HVDC shall be able tocontinue operation during these fault conditions. Only severesustained fault conditions will operate the AC undervoltageprotection. These faults may also cause commutation failuredescribed in section 6.5





6.4 Ancillary Equipment Failure

Some protective functions included in subsystems or equipment require that the pole shall betaken out of service. Table 9 gives a description of possible equipment failures that may resultin block and trip of the pole.

Table 9 Equipment failureFAULT Protection Remarks

Low Water Level ConverterCooling Control

High WaterTemperature

ConverterCooling Control

High Water

Conductivity

Converter

Cooling ControlLow Water Flow ConverterCooling Control

Danger OfCondensation

ConverterCooling Control

Low water temp ConverterCooling Control

The Converter Cooling Control has separate protectivefunctions to detect these fault conditions.

Fire Hazard Valve Hall FireDetection System

Excessive convectionheat detected

Valve Hall HVACsystem

VBE Power Supplyfailure

VBE

Transceiver Cardfailure VBE

Thyristor failurecausing thyristorredundancy to beexceeded

VBE

Cooling leak detected VBE

Thermal Model Failsto block and trip

VBE

VBE has protective features to detect these faultconditions.

Change Over failure Change OverUnit

In response to a sysfail or power supply failure of alane the change over unit transfers to the redundantlane. If this lane is also unavailable due to for examplemaintenance the pole is blocked and tripped.

Line windingtemperatureprotectionValve windingtemperatureprotection

ConverterTransformer Coolingfailure

High oiltemperatureprotection

Protection cubiclepower supply failure

Power SupplySupervision





Low oil level inTransformerConservator

Low oil leveldetection withinBuchholz surgeprotection

Transformeroverpressure

Buchholz surgeprotectionPressure reliefdeviceTap changersudden pressureprotection

6.5 Commutation Failure

A commutation failure is the result of a failure to commutate current from an outgoing valve toan incoming valve before the driving voltage across the valves reverses its polarity, taking intoaccount the need for sufficient extinction time for charge recombination and carrier sweep-out inthe outgoing valve and regaining its blocking capability. This type of fault is mainly associatedwith ac system faults at the inverter end and if the fault is electrically close the inverter may notbe able to recover on its own.

Apart from AC system faults commutation failure can also be caused by valve misfire orinadequate extinction angle.

Normally the system recovers from a commutation failure due to ac system disturbances.

Should commutation failure persist due to control mal-operation protective action is required.

Table 10 Commutation failure


DC>ACDifferential

Will aid recovery by increasing firing angle. Only after repeatedcommutation failures will t he protection operate to trip and blockthe pole.

CommutationFailure

Asymmetry

6.6 Misfire

Misfiring is the term used when a valve fails to fire during a scheduled conducting period and isnormally the result of control and firing equipment failure.

Table 11 Misfire


Valve BaseElectronic

Misfire

Asymmetry




6.7 Thermal Overload

Normally the equipment is able to sustain short periods of overload. Normally the overloadcontroller assesses the thermal limits of the thyristor valves and the converter transformer andlimit the current in order to ensure that the thermal limits are not exceeded. Should the overloadhowever persist despite the action of the overload controller or the overload controller fails, itmay damage the equipment and protective action must therefore be taken.

Table 12 OverloadFAULT Protection Remarks

High valve windingtemperature

High line windingtemperature

Transformer thermal Overload

High oil temperatureprotection

Filter Overload Filter OverloadProtection

Not helped by the overloadcontroller

Filter resistor Overload Filter Resistor OverloadProtection


Filter thermal Overload Filter Reactor OverloadProtection


Thermal modelValve thermal overload

Converter CoolingControl

Thermal overload on Surge arrestersconnected to the terminals

Adverse Firing angleprotection

dc protection

Documents