HVDC CONTROL

HVDC Control & Protection

Operator Control Level

Control and Protection Level

Field Level

Operator Control LevelHMI System General Screen Display StructureHVDC Control & Protection

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Status Information Area

status of major components, status of operation and major selections and configurations

Operator Guidance Area

dynamic function key allocation

Warning Line

hints and warnings concerning operation sequences

Monitoring Area

detailed dynamic plant status

Switching Devices Control Flow Diagram

HVDC Control & Protection

Local Area Network (LAN)Communication and data exchange between all control and protection components connected to the LANInitiation and Monitoring SystemMMIC BoardRemote Control Interface Master Clock SystemAC and DC Station ControlPole ControlPrintersTopology is a Star ConfigurationRedundant Configuration (LAN1 and LAN2)LAN1 is the preferred system; LAN2 is active only in case of faults on LAN1communication automatically returns to the LAN1 after clearing the fault

Pole Control Field Bus Configuration

What are the basic control principles for HVDC Systems?HVDC Control & Protection

HVDC Control

HVDC Control

HVDC Control

Sending EndReceiving EndHVDC Control & Protection

Principles of HVDC ControlsControl of DC VoltageRectifier OperationInverter Operation

Principles of HVDC Controls Relationship of DC Voltage Ud and Firing Angle a

Principles of HVDC Controls Converter Control Functions, Fixed Firing Angles a

Principles of HVDC Controls Converter Characteristics, Fixed Firing Angles a Station A (Rectifier) Fixed ao (~15)

Station B, Inverter Operation

Principles of HVDC Controls Converter Characteristics, Fixed Firing Angles a DC Voltage reduction

Principles of HVDC Controls Converter Control Functions, Id control at RectiferaoFixed ao

Principles of HVDC Controls Converter Control Functions, Id control at Rectifer1.01.0Ud (p.u.)Id (p.u.)Rectifier Fixed amin (5)Inverter Fixed a (~140)Operating Point (e.g. alpha 15)Rectifier Id ControlOperating range with reduced AC Bus Voltage at rectifierNo Change in Operating Point but increased alpha (e.g. 30)No Change in Operating Point

Principles of HVDC Controls Converter Control Functions, DC Voltage Control

Principles of HVDC Controls Converter Control Functions, DC Voltage Control1.0Ud (p.u.)1.0Id (p.u.)Inverter Ud ControlOperating Point (e.g. aRect =15, aInv =140)Inv Fixed max (~160)Rectifier Fixed amin (5)Constant Operating Point for a wide range of inverter and rectifier side AC Voltage variationsRectifier Id Control

Principles of HVDC Controls Converter Control Functions, Id Margin ControlIDC Ref-

Principles of HVDC Controls Converter Control Functions, Id Margin Control1.0Ud (p.u.)1.0Id (p.u.)Operating Point (e.g. aRect =15, aInv =140)new Operating Point with inverter Id Margin ControlOperating Point without inverter Id Margin Control

Principles of HVDC Controls Converter Control Functions, Extinction Angle (g) ControlInverter Ud Control1.0Ud (p.u.)1.0Id (p.u.)Rectifier Fixed amin (5)Operating Point (e.g. aRect =15, aInv =140)Rectifier DC Current ControlInverter DC Current ControlInverter Gamma (g) min Control (17)

Principles of HVDC Controls Converter Control Functions, Extinction Angle (g) Control-IDC Ref

Principles of HVDC Controls Converter Control Functions, DC Voltage Limit ControlInverter Ud Control1.0Ud (p.u.)1.0Id (p.u.)Rectifier Fixed amin (5)Operating Point (e.g. aRect =15, aInv =140)Rectifier DC Current ControlInverter DC Current ControlInverter Gamma (g) min Control (17)Rectifier DC Voltage Limit Control

Principles of HVDC Controls Converter Control Functions, DC Voltage Limit ControlI DC RefI DC Control-+I DC ActIDC Ref

Principles of HVDC Controls Converter Control Functions, Converter Control Characteristic

Rectifier Id Cont.

0.1

0.2

0.3

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Inverter Id Cont.

Ud(pu)

ld (pu)

gd Contr.

Abbreviations:

Ud

DC Voltage

Id

DC Current

VDCOL

CEC

Voltage Dependent Current Limit

Current Error Characteristic

g

DC Line Drop

Inverter VDCOL

Rectifier VDCOL

Inverter Ud Cont.

CEC

Extinction Angle

Rectifier Ud Cont.

Minimum DC Current

Operation Point

Principles of HVDC Controls Converter Control Functions

Power Line Carrier and/or Microwave and/ or FO Link Remote Access LAN (100 Mbit/s) TDM (Time Division Multiplexing) Field Bus (Profi Bus DB) RCI (Remote Control Interface) HVDC Communication SystemsCommunication

Red. TDM Bus

Red. TDM Bus

Red. LAN100 MBit/s

Red. LAN100 MBit/s

Red. FO-Field Bus(Profi Bus DB)

Red. FO-Field Bus(Profi Bus DB)

Power Line Carrier Link

(Micro Wave or Fibre Optic Links)

Telefon/ Internet

Telefon/ Internet

HVDC PROTECTION

PROTECTION SYSTEM TASKSFastrecognising and removing the faultsSafealways switching-off, when a fault appearsFail-Safe to avoid unnecessary shut-downs of the plant - achieved e.g. by individual protection zones and self-monitoring functions Selectivityas less components as necessary will be isolated in case of a faultAssists to minimise the time needed to determine and eliminate the faultOverlapping Protection Zonesselectivity is mainly achieved by dividing the complete system into several overlapping protection zones

Protection System Tasks Redundancyin case of maintenance or fault of one protection system sufficient protection is ensured by either main / back-up or redundant protection systemsusing different measuring devicesusing different power supply systemsmain / back-up systems use different algorithmsredundant systems (1 and 2) are used where different algorithms are not possiblealways in service simultaneouslySeparated from other systemsto ensure independence of correct function of the controlsto ensure independence of correct function of telecommunication systems (especially differential protections)

HVDC PROTECTION SYSTEM

AC FILTER PROTECTION

CONVERTER TRANSFORMER PROTECTION

DC PROTECTION

DC FILTER PROTECTION

Protection Zones1 AC-Busbar Protection2 AC-Line Protection3 AC-Filter /Shunt Capacitor Protection4 Converter Transformer Protection5 Converter Protection6 DC-Busbar Protection7 DC-Filter Protection8 Electrode Line Protection9 DC-Line Protection10 Shunt Reactor Protection11 AC-Filter Connection Protection12 Converter Transformer Connection Protection

Trip signal flowPole Control system1Pole Control system2DCprotection system1DC protection system2Converter Transformer protectionBay Marshalling KioskTrip 1Trip 2VBE system1VBE system2

DC PROTECTION

CONVERTER PROTECTION

DC LINE PROTECTION

DC BUS BAR PROTECTION

ELECTRODE PROTECTION

CONVERTER TRANSFORMER PROTECTIONTRANSFORMER PROTECTION - SIEMENS MAKE NUMERICAL RELAYS (SIPROTEC)

OVER CURRENT AND EARTH FAULT PROTECTION (51 7SJ61)

DIFFERENTIAL PROTECTION (87T 7UT513)3 WINDING TRANSFORMER PROTECTION

OVER EXCITATION / V/F RELAY (24 7RW600)RATIO OF V & F

BRANCH DIFFERENTIAL PROTECTION (87M1 7UT513)PROTECTING THE BLIND ZONE BETWEEN TRANSFORMER & CONNECTED BAY

Converter ProtectionShort Circuit Protection (SCY,SCD)Over current Protection (Iac>)Bridge Differential Protection (BDD, BDY)Group Differential Protection (GD)DC Differential Protection (DDP)DC Over voltage Protection (59/37DC)DC Under voltage Protection (27DC)Fundamental Frequency Protection (81-50Hz, 81-100Hz)AC Valve Winding Supervision (ACVW)

Converter Protection Overview

Short Circuit Protection (SCY, SCD)

Short Circuit Protection (SCY, SCD)

Protection Task

Valve short circuit

Evaluation Principle

IacY MIN(IdH,IdN) > delta

IacD MIN(IdH,IdN) > delta

IacY: sum of the rectified AC currents Y-transformer

IacD: sum of the rectified AC currents D-transformer

Initiated Sequences

Short circuit at rectifier:

Block valve base electronic, ESOF,Open AC circuit breaker

Short circuit at inverter:

ESOF, Open AC circuit breaker

Settings

Stage-I :1000A 30mSec

Stage-II: 3400A - Instantaneous

Overcurrent Protection (Iac>)

Overcurrent Protection (Iac>)

Protection Task

Short circuits at rectifier/ inverter

Overload failures

Evaluation Principle

Max(IacY,IacD) > delta

Initiated Sequences

ESOF

Open AC Circuit Breaker

Typical settings

Stage-I: 2800A, 1900Sec

Stage-II:3300A, 5Sec

Stage-III:4000A, 200mSec

Stage-IV: 7000A, 2mSec

Bridge Differential Protection (BDD, BDY)

Bridge Differential Protection (BDD, BDY)

Protection Task

Slectivly detecting (six pulse group) of:

Commutation failures

Firing malfunction

Converter DC faults

Evaluation Principle

Max(IacY,IacD) IacY > delta

Max(IacY,IacD) IacD > delta

Initiated Sequences

1st stage: current reduction to 0.3 x Idref

2nd stage: ESOF + open AC circuit breaker

Typical settings

Stage 1:800A, 200mSec-Current reduction by 0.3 p.u

Stage-II: 200A / 80A, 200 / 500mSec- ESOF

Group Differential Protection (GD)

Group Differential Protection (GD)

Protection Task

All DC faults that are bypassing the inverter

Evaluation Principle

Max(IdH,IdN) Max(IacY,IacD) > delta

Max(IdH,IdN) Max(IacY,IacD) > delta

Initiated Sequences

ESOF

Open AC circuit breaker

Typical settings

Stage-I: 200 / 80A, 200msec / 700msec

Stage-II: 1000A, 10msec

DC Differential Protection (DDP)

DC Differential Protection (DDP)

Protection Task

Ground faults anywhere in the converter

Evaluation Principle

Abs(IdH IdN) > delta

Initiated Sequences

ESOF

Open AC circuit breaker

Block firing pulses at rectifier

Typical settings

140A / 80A, 5msec

DC Over voltage Protection (59/37DC)

DC Overvoltage Protection (59/37DC)

Protection Task

When rectifier tries to operate against an open DC-line or a blocked inverter

Pole Control malfunction

Evaluation Principle

UdL > thres.

Initiated Sequences

ESOF

Open AC circuit breaker

Typical settings

Three stages:

1st stage:

512KV, 40msec IdL

DC Undervoltage Protection (27DC)

DC Undervoltage Protection (27DC)

Protection Task

Inverter bypass without telecontrol

High voltage faults to neutral or ground

Evaluation Principle

UdL < thres

Initiated Sequences

ESOF

Open AC circuit breaker

Typical settings

Thres = 125KV

T = 1 s rectifier

T = 4 s Inverter

Fundamental Frequency Protection (81-50Hz, 81-100Hz)

Fundamental Frequency Protection (81-50Hz, 81-100Hz)

Protection Task

Commutation failures at inverter side

Single phase faults at the AC side

Evaluation Principle

IdL(50Hz) > thres

IdL (100Hz) > thres

Initiated Sequences

1st stage:

Current reduction to 0.3 x Idref

2nd stage:

Block pole

Typical settings

1st stage:

thres = 0.05 x IdL; T = 200 ms

2nd stage:

thres = 0.05 x IdL; T = 1100 ms

AC Valve Winding Supervision (ACVW)

AC Valve Winding Supervision (ACVW)

Protection Task

Valve winding ground fault when converter is blocked

Evaluation Principle

Max(UsumY; UsumD) > thres

IdH = 0

Initiated Sequences

Deblock inhibit

Typical settings

thres = 0.1 p.u.

DC Busbar ProtectionHV DC Busbar Differential Protection (87HV) Neutral Busbar Differential Protection (87LV)DC Differential Backup Protection (87DCB)

HV DC Busbar Differential Protection (87HV)

HV DC Busbar Differential Protection (87HV)

Protection Task

Faults to ground or neutral within protection zone

Evaluation Principle

Abs(IdL IdH) > thres

Initiated Sequences

ESOF

Open AC circuit breaker

Open HSNBS

Bypass inhibit on inverter side

Typical settings

Thres = 0.5 p.u.

T = 10 ms

Neutral Busbar Differential Protection (87LV)

Neutral Busbar Differential Protection (87LV)

Protection Task

Faults to ground or high voltage within protection zone

Evaluation Principle

Abs(IdN IdE) > thres

Initiated Sequences

ESOF

Open AC circuit breaker

Open HSNBS

Typical settings

Two stages

Thres = 0.05 ... 0.25 p.u.

T = 50 ... 1000 ms

DC Differential Backup Protection (87DCB)

DC Differential Backup Protection (87DCB)

Protection Task

Earth faults at DC busbars or pole equipment

Evaluation Principle

Abs(IdL IdE) > thres

Initiated Sequences

ESOF

Open AC circuit breaker

Open HSNBS

Typical settings

Two stages

Thres = 0.05 ... 0.25 p.u.

T = 50 ... 1000 ms

DC Line ProtectionTravelling Wavefront Protection (WFPDL)DC Line Undervoltage Sensing (27du/dt)DC Line Differential Protection (87DCM)AC/DC Conductor Contact Protection Remote Station Fault Detection (81-I/U)

DC Line Overview

Travelling Wavefront Protection (WFPDL)

Travelling Wavefront Protection (WFPDL)

Protection Task

DC line faults to ground

Evaluation Principle

Detection of the discharge wave

Initiated Sequences

DC line fault recovery sequence

Typical settings

Depends on the AC grid and the DC line characteristic

Travelling Wavefront Protection (WFPDL)Detection of the discharge waveEvaluating of the rates of change of voltage and currentProtection trips, when the following criteria are fulfilled: rate of change of voltage exceeds limittotal change of voltage exceeds limittotal change of current exceeds limit

Travelling Wavefront Protection (WFPDL)Detection of the discharge waveabsolut change of voltage in a fix timeabsolute change of current in a fix timeRate of change of voltage

DC Line Undervoltage Sensing (27du/dt)

DC Line Undervoltage Sensing (27du/dt)

Protection Task

DC line faults to ground or neutral

Evaluation Principle

Rate of change of DC voltage exceeds limit

DC voltage below limit

Initiated Sequences

DC line fault recovery sequence

Typical settings

Depends on the AC grid and DC line characteristic

DC Line Undervoltage Protection (27du/dt)Rate of change of voltageVoltage below limit

DC Line Differential Protection (87DCM)

DC Line Differential Protection (87DCM)

Protection Task

DC line faults to ground

Evaluation Principle

Abs(IdL-IdLos) > thres

Initiated Sequences

Line fault recovery sequence

Typical settings

Thres = 0.05 p.u.

T = 0.5 s

Fault Recovery SequencesAfter detecting line fault DC protection sends fault recovery request to Pole controlPole control starts recovery sequenceCounts the number of recovery sequencesPole control retards the firing angle and reduces the DC voltageTransient faults will be cleared in this processFirst restart after 200msecSecond restart after 250msecThird restart after 300msec at RVOIf unsuccessful Pole gets blockedDetection of fault any one side is sufficient

Unsuccessful DC line fault recovery sequence( Pole 1)

Unsuccessful DC line fault recovery sequence (Pole 2)

DC LINE FAULT LOCATOR

Principle arrangement

Line Fault Location Formula

The standard Line fault location formula is used.

D = L (v * t)

2

Where

D = Distance to fault

L = Line Length

v = wavefront propagation velocity

t = difference in arrival times of wavefronts

t = tremote - tlocal

Important EquipmentWave Front Detection Unit (FOWD) at grounding point of PLC capacitorsWave front detection transformer 1:55 turnsFOTX- FO TransmitterFORX FO Receiver

Display of fault locator

Electrode Line ProtectionElectrode Bus Differential Protection (87EB)Electrode Current Balance Protection (60EL)Open Conductor Electrode Line Protection (60OCEL)Electrode Overcurrent Protection (76EL)Overvoltage Protection (59EL)Station Ground Overcurrent Protection (76SG)

Pemo 200

Electrode Line Protection Overview

Overvoltage Protection (59EL)

Protection Task

Open electrode line

Evaluation Principle

UdN > Thres

Initiated Sequences

Close High Speed Ground Switch +

Bipolar Operation: Balanced Bipolar Mode

Monopolar Operation: Block Pole

Typical settings

Thres = 90 kV

Station Ground Overcurrent Protection (76SG)

Station Ground Overcurrent Protection (76SG)

Protection Task

Station ground overcurrent

Earth fault in Metallic Return operation

Evaluation Principle

Idee3 > Thres

Initiated Sequences

Bipolar and Ground Return operation:

Stage 1: Alarm

Stage 2: Block Pole

Metallic Return operation:

Stage 3: Block Pole

Typical settings

Stage 1:

Thres = 0.225 p.u.; T = 500 ms

Stage 2:

Thres = 0.45 p.u.; T = 3000 ms

Stage 3:

Thres = 0.225 p.u.; T = 200 ms

Pemo 2000

Pulse Echo Electrode Line Monitoring SystemNo additional blocking filters neededDetermination of fault location and fault type

Summary Main FeaturesNo additional blocking equipment necessary==>Insensitive to tolerances of blocking components==>Less primary components result in a higher availability and reliabilityInsensitive over a wide range of harmonic currents Determination of fault location possibleDetermination of fault type (open or ground fault) possible(high frequency (0.5 - 2 MHz) differential-mode voltage pulse injectionMulti-pulsing evaluation avoids maloperation during worst transient conditions

Evaluation AlgorithmSteady State EvaluationUsed for detection of long time developing faults (dirty insulator etc.)Steady State Evaluation monitors the reference echo Dynamic EvaluationDynamic Evaluation compares the actual incoming echo with the last reference echo

Impulse Response of a Healthy Electrode Line

Impulse Response during a Line to Earth Fault

Different Impulse Response

AC Filter ProtectionDifferential Protection (87ACF, CB)Inverse Overcurrent Time Protection (50/51ACF, 50/51CB)Capacitor Overload Protection (49/59ACF, 49/59CB)C1 Capacitor Unbalance Supervision (60/61ACFC1, 60/61CBC1)Overload Protection for Low Voltage Reactor (50/51ACFL)Overload Protection for Low Voltage Resistor (50/51ACFR)

AC Filter Protection Overview

AC Filter Protection Overview

Differential Protection (87ACF,CB)

Differential Protection (87ACF,CB)

PROTECTION TASK: FAULTS TO GROUND/NEUTRALEVALUATION PRINCIPLE: ABS(IT1(50HZ) IT3(50HZ)) > SVINITIATED SEQUENCES: TRIP FILTER CB

Inverse Overcurrent-Time Protection (50/51ACF, 50/51CB)

Inverse Overcurrent-Time Protection (50/51ACF, 50/51CB)

PROTECTION TASKTHERMAL OVERSTRESS OF THE CIRCUIT,SHORT CIRCUITSEVALUATION PRINCIPLETHERMAL CURRENT STRESS >SVINITIATED SEQUENCESTRIP FILTER CB

Capacitor Overload Protection (49/59ACF, 49/59CB)

Capacitor Overload Protection (49/59ACF, 49/59CB)

PROTECTION TASKVOLTAGE OVERSTRESS OF THE CAPACITOR EVALUATION PRINCIPLEINTEGRAL OF THE LINE CURRENT IS COMPARED TO THE INVERSE OVERVOLTAGE TIME WITHSTAND CURVE OF THE CAPACITOR C1

INITIATED SEQUENCESTRIP FILTER CB

C1 Capacitor Unbalance Supervision (60/61 ACFC1, 60/61 CBC1)

C1 Capacitor Unbalance Supervision (60/61 ACFC1, 60/61 CBC1)

PROTECTION TASKFAULTY CAPACITOR ELEMENTSSHORT CIRCUITS OF CAPACITOR ELEMENTSEVALUATION PRINCIPLEDELTA(IT2 / IT3) > SVINITIATED SEQUENCESSTAGE 1: ALARMSTAGE 2: ALARM, SWITCH OFF IN 2 HOURSSTAGE 3: TRIP THE CB INST.TYPICAL SETTINGSSTAGE 1: TWO CAPACITOR ELEMENTSSTAGE 2: THREE CAPACITOR ELEMENTSSTAGE 3: FOUR CAPACITOR ELEMENTS OR WHOLE UNIT

Overload Protection for Low Voltage Reactor 50/51 ACFL (double tuned filters only)

Overload Protection for Low Voltage Reactor 50/51 ACFL (double tuned filters only)

PROTECTION TASKOVERLOAD CONDITIONS IN REACTOR L2

EVALUATION PRINCIPLEIT42 IS COMPARED WITH THE INVERSE OVERCURRENT-TIME CHARACTERISTIC OF THE L2 INDUCTANCE

INITIATED SEQUENCESTRIP FILTER CB

Overload Protection for Low Voltage Resistor 50/51 ACFR (double tuned filters only)

Overload Protection for Low Voltage Resistor 50/51 ACFR (double tuned filters only)

PROTECTION TASKOVERLOAD CONDITIONS IN RESISTOR R2EVALUATION PRINCIPLEIT41 IS EVALUATED AGAINST THE INVERSE O/C-TIME CHARACTERISTIC OF R2 RESISTOR

INITIATED SEQUENCESTRIP FILTER CB

DC Filter ProtectionDifferential Protection (87DF)C1 Unbalance Supervision (60/61DF)Differential Overcurrent Protection (51C1DF)Inverse Overcurrent Time Protection (51DF)

DC Filter Protection Overview

Differential Protection (87DF)

Differential Protection (87DF)

Protection Task

Faults to ground or neutral within protection zone

Evaluation Principle

Abs(Isum IT2) > thres

Initiated Sequences

Block Pole

Open HSNBS

Typical settings

Thres = 0.3 x Max(Isum IT2)

T = 10 ms

C1 Unbalance Supervision (60/61DF)

C1 Unbalance Supervision (60/61DF)

Protection Task

Faults within capacitor unit

Evaluation Principle

Delta(Idelta / Isum) > thres

(Idelta / Isum) > thres

Initiated Sequences

Filter current below threshold:

Open DC filter disconnector HV side

Filter current above threshold:

Block pole

Typical settings

Stage 1:

Trip in case of faulty capacitor element group

Stage 2:

Instantaneous trip when whole unit is faulty

Differential Overcurrent Protection (51C1DF)

Differential Overcurrent Protection (51C1DF)

Protection Task

Short circuits, faults to neutral or ground

Evaluation Principle

Idelta > thres

Initiated Sequences

Block pole

Open HSNBS

Typical settings

Unbalance of one capacitor unit

AC Filter ProtectionDifferential Protection (87ACF, CB)Inverse Overcurrent Time Protection (50/51ACF, 50/51CB)Capacitor Overload Protection (49/59ACF, 49/59CB)C1 Capacitor Unbalance Supervision (60/61ACFC1, 60/61CBC1)Overload Protection for Low Voltage Reactor (50/51ACFL)Overload Protection for Low Voltage Resistor (50/51ACFR)

CPR97

AC Filter Protection Overview

AC Filter Protection Overview

C1 Capacitor Unbalance Supervision (60/61 ACFC1, 60/61 CBC1)

C1 Capacitor Unbalance Supervision (60/61 ACFC1, 60/61 CBC1)

Protection Task

Faulty capacitor elements

Short circuits of capacitor elements

Evaluation Principle

Delta(IT2 / IT1) > thres

Initiated Sequences

Stage 1:

Alarm Signal

Stage 2:

Alarm signal, switch off in 2 hours

Stage 3:

Switch off filter

Typical settings

Stage 1:

Two capacitor elements

Stage 2:

Three capacitor elements

Stage 3:

Four capacitor elements or whole unit

Realised in

Siamdyn D

SIMADYN DThe Digital Multi-Microprocessor System

Inverter side extinction angle () and Over lap angle () Valve Currents -StarAC Voltage Ph-N

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In the real HVDC system the DC voltage is varied by means of a converter bridgeIn rectifier operation the power flow is from the AC system to the DC systemThe power flow is changed from the DC system to the AC system by reversing the DC voltage. The DC current does not change its direction.The operating range of the ideal converter is in theory from 0 (+1.0 p.u. DC voltage) to 180 (-1.0 p.u. DC voltage). The operating range of a real converter is from approx. 5 to approx. 160.In 90 operation the DC voltage of the converter is 0.

*******************************************TYPICAL SETTINGSTHRES = 0.15 P.U.T = 0*****- EXCEEDING OF THE INVERSE OVERCURRENT-TIME CHARACTERISTIC- IT42(50HZ) EXCEEDS LIMIT (SHORT CURRENT)

**TYPICAL SETTINGSEXCEEDING OF THE INVERSE OVERCURRENT-TIME CHARACTERISTIC-IT42(50HZ) EXCEEDS LIMIT (SHORT CURRENT)**********