Overcurrent Protection

VA TECH Transmission & Distribution

1Cc/Hydro_Präs/Interne RS.ppt

PEEBLES REYROLLE

Overcurrent Protection

Fundamentals of Operation and Application

K. Hearfield



PEEBLES REYROLLE

1a) Overcurrent Condition

LoadLoad

Shortcircuit

Current increaseswith load

MOTOR MOTOR

Overcurrent protectionis provided for short

circuit clearance

Overload protection isrelated to therm al

capacity of the plant

Most com m only used overcurrent devices are fuses and relays

OVERCURRENT CONDITION OVERLOAD CONDITION



PEEBLES REYROLLE

2a) Overcurrent Protection for Phase Faults

3 PhaseO vercurrentRelay

Relays are used in conjunctionw ith circuit breaking device

Fuses

Detect and interrupt overcurrents3 Phase Fault

2 Phase Fault

2 PhaseO vercurrentRelay

Fault m agnitude lim ited byim pedances o f prim ary p lant

Can provide cost saving - w idely used ,but not suitab le for a ll app lications



PEEBLES REYROLLE

2b) Overcurrent Protection for Earth Faults

2 PH - E Fault

PH - E Fault

3 PH - E Fault

Residuallyconnectedrelay

CoreBalanceCT

Zero outputfor balanced

or phase faultconditions

EF current path thrugenerator earth

Fault m agnitude lim ited by:Prim ary p lant im pedanceM ethod of earthingSystem neutral resistance

Sensitive protection m ay be required- settings below load current

EF current path thrutransform er earth



PEEBLES REYROLLE

2c) Combined Overcurrent and Earth Fault Protection

2 Phase Overcurrent and Earth Fault 3 Phase Overcurrent and Earth Fault

Can provide cost saving - widely used,but not suitable for all applications



PEEBLES REYROLLE

3) Methods of System Earthing

SOLID

High EF Current

EARTHINGTRANSFORM ERUsed w here systemneutral not availab le

RESISTANCELow 400 - 1200AHigh 5 - 100A

REACTANCESm aller and lessexpensive than resistance.Can cause high over-vo lts .

PETERSON COILReactance chosen toequal system capacitance

ISOLATED NEUTRAL

Zero EF Current



PEEBLES REYROLLE

PROTECTION CO-ORDINATION AND GRADING



PEEBLES REYROLLE

4) Radial Distribution System

HVPow erSource

S /S tn A S /S tn D S /S tn ES /S tn CS /S tn B S /S tn F

Selective fault clearance is required forPH ASE-PH ASE and PH ASE-EAR TH faults

Instantaneous overcurrent protection at eachlocation w ould not provide d iscrim ination.

U N N AC C EPTAB LE TO SH U T D OW N W H OLESYSTEM FOR EVER Y FAU LT

R adial Pow er System

Pow er source feeds through a num ber ofsubstations - load taken from each S /Stn

LO A D LO A D LO A D LO A D

M VLoad



PEEBLES REYROLLE

4.1) Current Graded Protection

2900A2400A

8800A5400A

13100A6850A

BA

Relay at 'A' set tooperate for m ax. faultcurrent at rem ote end

8800A

630A600A1200A

1100A

(EquivalentHV Currents)

M VLoad

Faultcurrent

M ax.M in.

F1

F2

AIM - Protection co-ordinated to ensure m inim um unfaulted load isdisconnected

Unreliable Schem eCurrents F1 and F2 m ay be sim ilar - loss of discrim inationFor m inim um infeed A - B m ay be unprotected

NOTE: M ax. fault at S/Stn. E < m in. fault current between D-E

AD C B

630A8800A

2900A

1200A

F

E

DC

t

Amps

RADIALDIST RIBUT IO N

SYST EM



PEEBLES REYROLLE

4.2) Time Graded Protection

Infeed

BA

1.4s

MVLoad

A

D

C

B

F

E

DC

t

Am ps

1.0s 0.6s 0.2s

0.2s

1.4s

1.0s

0.6s

Operatingcharacteristic:Definite tim edelay

Relays nearer to power source are set tooperate in progressively longer tim es

DISADVANTAGELongest clearance tim e for faults nearestto source



PEEBLES REYROLLE

Inverse Definite Minimum Time (IDMT) Operating CharacteristicOperate current = 1.05 x setting

At 2x setting operate time = 10s

At 10x setting operate time = 3s3/10 operating curve

At 30x setting operate time = 2sDefinite minimum time

Numeric IDMT relay operating algorithm:

2s

3s

10s

2x 10x 30x

T ime M ultip lie r = 1.0

Op

era

tin

g T

ime

M u ltip le o f Current Setting

..

1

14.002.0 MT

II

t

S



PEEBLES REYROLLE

Current and Time Grading of IDMT Curves

2s

3s

10s

2A 10A 30A

0.2s

0.3s

1s

2A 10A 30A

Current Setting 1AT ime M ultip lie r 1 .0

Current Setting 1AT ime M ultip lie r 1 .5T ime M ultip lie r 0 .1

2.27s

3.62s

24.5s

2A 10A 30A

Current Setting 1.5AT ime M ultip lie r 1 .0

15s

4.5s

3s



PEEBLES REYROLLE

0 .1

1

10

100

1000

10000

Time(sec)

Long Time Inverse

Normal Inverse

Current (m ultiples of setting)

1 10 1002 3 4 5 6 8 20 30 50 70

Extremely Inverse

Very Inverse

IDMT Curve - General applications

..

1

14.002.0

MT

I

It

S

VIDMT CurveCloser grading where faultlevels differ significantly

..

1

5.13MT

I

It

S

EIDMT Curve – Grading with fuses

..

1

802

MT

I

It

S

LTIDMT Curve - Grading with liquid resistor

..

1

120MT

I

It

S

Characteristics can be supplemented with LS and HSStages e.g. to assist grading with complex characteristics

4.3a) IDMTL Curves - Time and Current Grading



PEEBLES REYROLLE

4.3b) Fuse Co-ordination

RequiredT ime

Delay"G rading

M arg in"

M in imumRelay

O peratingT ime

Pre-Arcing

Arcing

RelayO vershoot

0 m s Pre-Arcing

Arcing

SafetyM arg in

0 m s

Pre-Arcing

Betweenrelayandfuse

Betweentwo

fuses

Recom m ended current ratio 3:1- avoids overlap of characteristics

Recom m ended current ratio > 2:1

t' = 0.4tf + 0.15s

T.D. of 0.2s will ensuregrading at high fault levels

EIDM T relay &fuse characteristics

Fusecharacteristics



PEEBLES REYROLLE

4.3c) Co-ordination between Time Graded Relays

RequiredTim e

Delay"G rading

M argin"

M inim umRelay

O peratingTim e

M axim umRelayO peratingTim e

CT Error- IDM T only

CircuitBreakerTrippingTim e

O v ershoot(R2)

SafetyM argin

0 m s

RequiredTim e

Delay"G rading

M argin"

M inim umRelay

O peratingTim e

Pre-Arcing

Arcing

O v ershoot

0 m s

Betw eenrelay and

fuse

Betw eentw o

relays

Recommended current ratio 3 :1

R1R2



PEEBLES REYROLLE

4.3d) Time Grading - Nominating Grading Margin

CB T rip T im e 150m s 80m s

Safety Marg in 50m s 40m s

O ver-shoot 80m s 40m s

T im ing Error 7.5% 5%

Argus2T JM

O lderSystem

New erSystem

CT Error


Overshoot

SafetyMargin

0 m s

Max. Op. T im e

RequiredTim e

Delay"Grading

Margin"

M in. Op.Tim e

250ms

CT Error


Overshoot

SafetyMargin

0 m s

Max. Op. T im e

RequiredTim e

Delay"Grading

Margin"

M in. Op.Tim e

400ms

Older System

Newer System



PEEBLES REYROLLE

4.3e) System Study for IDMT Protection

Infeed

BA

F

EDC

300/5

50A165A 95A 80A

33/11kV10 MVA

33kV Bus 11kV Bus

390A400/5 225A 200/5 100/5

11kV/415V1 MVA

X = 7%

X = 4%1600A

130A

3ph Max. Fault3ph. M in. Fault

12596A6846A

5753A4156A

2882A2417A

1410A1289A

626A601A

Current Setting - Higher than m ax. load

Tim e graded @ m ax. 3ph. fault current

PROCEDURE

Grade: D with FuseC with DB with CA with B



PEEBLES REYROLLE

4.3f) IDMT Grading Calculation Results

Infeed

BA

F

EDC

300/5

50A

165A 95A 80A

33/11kV10 M VA

33kV Bus 11kV Bus

390A400/5 225A 200/5 100/5

11kV/415V1 M VA

X =7%

X =4%

1600A

130A

3ph M ax. Fault 12596 5753A 2882A 1410A 626A

@ D: P.S.M . = 626/(100 x 1.25) = 5For p .s.m . 5: N I operating tim e = 4.3s4.3 x T M S = 4.3 x 0 .05 = 0.215s

G rad ing m arg in steps 0.4s

Relay at 'A ' operates for a close up fault in 0.75s.IDM T fault c learance tim e < DT L tim e

DT L system relay at 'A ' operate tim e = 0.2 + 3 x 0.4 = 1.4s

t

Amps

A

D

CB

5753626

28821410

0.205

1.050.630.54

RelayCurrentSetting

T M S

A 125% 0.375

D 125% 0.05

C 75% 0.175

B 100% 0.275



PEEBLES REYROLLE

Power and Voltage Ratings

Equipment Impedances

CT Ratios

Fuse Ratings

Relay Characteristics

4.4a) Industrial System Protection Co-ordination Study

Single Line Diagram

11/0.4 kV1500 kVA

5%

280 M VA

2500/12500/1

1600/1

450A



PEEBLES REYROLLE

4.4b) Co-ordination Issues

Single LineDiagram

Norm al OperationEm ergencyOperation

No OperatingRestrictions

25kA

25kAF

25kA

25kA

44kA

22kA

Establish load flow and short circuit currents

Grading curves plotted beginning at low est voltage level and largest load.

Grade at the m axim um fault level that can be seen by both relays sim ultaneously.

Grade at fuse cut-off point if less than the above.

F F

1000A

500A

2000A

500A

1000A

500A



PEEBLES REYROLLE

Protection Grading - Software



PEEBLES REYROLLE

PROTECTION FOR LOW EARTH FAULT CURRENT LEVELS



PEEBLES REYROLLE

5.1) Sensitive Earth Fault Protection

CoreBalanceCT

Core sum m ates fluxes o f prim ary currents

O nly one core is used - CT m agnetis ing currentis reduced by approxim ately 3 to 1

Num ber o f secondary turns need not be relatedto rated current o f protected circuit - can beoptim ised to protection setting

SEF relay current setting as low as possib lelim ited by residual unbalance capacitance -estab lished by test

Low CT burden required - e .g . num eric re lay

DT L characteristic , back-up function

S.E.F . Relay

Low earth fau lt current

Sem i insulating ob ject



PEEBLES REYROLLE

5.2) Neutral Displacement ProtectionBalancedSystem

voltages

V RES = 0

R phase EF

V RES= V YR + V BR= 3V PH

EF on non-effectively earthed system

Faulted phase m ust not rem ain energised.

EF current very sm all- OC detection m ay be im practical

NDR detects residual voltage to earth

Operation not discrim inative- tim e delay required

NDR



PEEBLES REYROLLE

HIGH-SET INSTANTANEOUS PROTECTION



PEEBLES REYROLLE

6a) High Set Overcurrent Protection

Reach of h igh set700012500

A B

5700

Inverse time e lement

Inverse time e lement

Inversetime e lement

Reach of h igh set

Set above max. fau ltcurrent a t s/stn. B

Instantaneous h igh-setprimary setting 7000A Plain Feeder

Transform er Feeder



PEEBLES REYROLLE

6b) HSOC Example - Calculating System Impedances

Inverse tim e elem ent

Transformer Feeder33kVM ax fault 1000 M VAM in fault 650 M VA

20km (0.3 + j0 .43) O hm s/km

Instantaneous high-set

33/11kV24M VAZ = 22.5% 11kv

To determine system impedances:

OHL impedance = 20(0.3 + j0.43) = 6 + j8.6 Ohms

Source impedance M in = j330002/1000 x 106 = j1.09 OhmsM ax = j330002/650 x 106 = j1.68 Ohms

Transformer 100% impedance @ 33kV = j330002/24 x 106

= j45.38 Ohms22.5% impedance on 24M VA = 0.225 x 45.38

= j10.21 Ohms



PEEBLES REYROLLE

6c) HSOC Example - Establishing Relay Setting

IDM T elem ent

33kV SourceM ax im pedance = j1 .68 O hm sM in im pedance = j1 .09 O hm s 6 + j8 .6 O hm s

HSO C

33/11kVj10.21 O hm s 11kV

3ph m ax infeed917A @ 33kV

M in infeed:3ph fault 1601Aph-ph fault 1387A

W here com prom ise is necessary: Stability is preferred to high speed protection

M in. system im pedance to LV busbars = m in source + line + transform er= j1.09 +(6 + j8.6) + j10.21 = 20.78 Ohm s

M ax. LV 3-phase fault = 33000/(1.732 x 20.78) = 917A

M ax. system Z to transf. HV side = m ax source + line= j1.68 +(6 + j8.6) = 11.9 Ohm s

M in. HV 3-phase fault = 33000/(1.732 x 11.9) = 1601A

M in. HV PH-PH Fault = 0.866 x 1601 = 1387A

Best achievable grading m argin = (1387/917)^0.5 = 23%i.e. RELAY SETTING OF APPROX 1128A PRIM ARY



PEEBLES REYROLLE

APPLICATION OF OVERCURRENT PROTECTION

SPECIAL CONSIDERATIONS



PEEBLES REYROLLE

7.1a) Effect of System Capacitance in Resistance Earthed System

Instantaneous earth fault protection can be used to protectfeeder and un-earthed transform er w inding, how ever:

For an earth fault at F1:EF relay on healthy circuit m ay operate if not set above ICIC = IB C + IYC = 3 x nom inal charging current/phase

F1 D elta or U n -Earthed S tarW inding

IBC

IYC

IC

V R

IYC

IBC

IC

V YV B

N



PEEBLES REYROLLE

7.1b) EF Protection Applied to Non-Earthed Transformer Feeder

F1

IBC

IYC

IC

7km FeederCharging current - 1.8A/km

33kV

V R

IYC = 21.8A

IBC

IC = 37.8A

V YV B

N

On occurence of an EF (F1) the affected phase w ill be earthed. 33kVsystem is resistance earthed, healthy phase voltage rises to linevolts level.

Normal per phase capacitance increased by a factor of 1.732

For fault F1:EF relay on healthy circuit may operate if not set above I CIC = 7 x 1.8 x 3 = 37.8A primary

Set relay 2 to 3 x 37.8A i.e. 75 - 112.5A



PEEBLES REYROLLE

7.2) HSOC & EF Protection for Transformer Feeders

F2

F1F3

TRIP

E 1

P

E 2

P = Phase fault protection relayE 1 = Residual earth fault relayE 2 = Earth fault check relay

Earthed W inding

Provides inst. prot for transform er feeder phase and earth faults

For fault F3 E2 w ill not operateFor fault F2 E1 w ill not operate

P Set to> 1.5 x m ax. lv fault current> 2 x m in. in zone (hv) fault current

E 2 Set to< P (for faults at F2 E 2 m ay operate but P m ust not)> LV infeed to a fault at F3 (s ince E2 operated by +ve and -ve sequence currents)



PEEBLES REYROLLE

7.3) Faults on Star Delta Transformers

KI

3

KI

3

SO URCE

F

KI

3

KI

3

F

KI

3

SO URCE

I

II

I

I

I

K

I

3

2

Earth faults on star side of transformer is seen as a phase fault on Delta side

A phase fault on the star side of the transformer requires an additional current grading margin of :

Care must be taken if applying 2 phase overcurrent protection to HV side:2P protection suitable if minimum fault current/full load current > 4

%163

2 I

I



PEEBLES REYROLLE

7.4) - Two Stage Overcurrent Protection

IDM T

Stage1 T rip

DT LStage2 T rip

SeparateHV and LVO vercurrentProtection

2 Stage OvercurrentProtection

Im proved discrim inationReduced grading m argin

CT and Relay cost saving

Not applicable if there is apossibilty of LV infeed totransform er faults

IDM T

IDM T

HV Source HV Source



PEEBLES REYROLLE

7.5) - Interlocked Overcurrent Protection

Interlocked Overcurrent Relay

Normally Inhibited - UntilOperation of Busbar Prot.

Unit Feeder Protection

BusbarProtection

F1

F3

F2

Feeder

F1 - Cleared by busbar protection

F3 - Cleared by circuit protection

F2 - Operates busbar protectionW ill not be cleared by fdr. prot.



PEEBLES REYROLLE

7.6) Voltage Dependent Overcurrent Protection

Volts Contro lled O C

Inhib its overcurrent pro tection unlessvoltage is below set-po int:T ypically 0.7 - 0 .9 V n

NET W O RK

O vercurrent relay m ust be graded w ith netw ork protection

Close up generator faults - initia lly high current.M ay reduce rap id ly - lim ited by am plitude of exitationcurrent and d irect axis synchronous reactance

Volts Restrained O C Characteristic

Tap

Settin

g as %

of T

apS

etting

at Rated

Vo

ltage

25 1007550

25

50

75

100

Input Voltage (% of rated vo ltage)

G enerator



PEEBLES REYROLLE

7.7) Flashing Fault ConditionFAULTClashing conductors or re-sealing cab leR1R2

Loss of grading /discrim ination m ay occur:

If R2 Electrom echanical & R1 Inst. reset or if R2 DTL reset and R1 electrom echanical

Som e com prom ise m ay be necessary where m ore than two relay points are in series

R3D

isc

Tra

vel

E lectro -m echanical Relay

T im e

T RIP

% o

f A

lgo

rith

m

Argus (Inst. Reset)

T im e

Argus (DT L Reset)

T R IP



PEEBLES REYROLLE

DIRECTIONAL PROTECTION



PEEBLES REYROLLE

8a) Parallel Feeders - Application of OC Protection

LO AD51 51

5151

A

B

C

D

LO AD

51 51

5151

A

B

C

D

I1

I2

Faulted feeder:Fault current can flow in both d irectionsRelays C and D operate togetherBoth feeders w ill be tripped

I1 + I2

Conventional grad ing:G rade A w ith CG rade B w ith DA and B have the sam e settingC and D have the sam e setting

A & B

C & D



PEEBLES REYROLLE

8b) Parallel Feeders - Directional Protection

Solution to faulted feeder prob lem :D irectional contro l o f Relays C and DRelay D on unfaulted feeder does not operate

SET T ING PHILO SO PHY :

Load current alw ays flow s in non-operate d irection

Any current flow ing in operate d irection is ind icativeof a fault condition.

T herefore relays C and D m ay have sensitive setting ,fast operating tim e

C & D usually set to 50% full load , low T .M .S (0.1)

LO AD

67

6751 B

C

D

I1

I2

I1 + I2

51 A

51 E

G RADING PRO CEDURE

G rade A and B w ith E , assum ing onefeeder in service

G rade A w ith D (and B w ith C )assum ing both feeders in service



PEEBLES REYROLLE

9a) Ring System - Application of Time Graded OC Protection

LO AD

RM U

LO AD

RM U

67 67

LO AD

RM U

67 6751

LO AD

RM U

67 67

LO AD

RM U

67 6751 67

SourceSubstation

67

W ith ring closed both load and fault current may flow in either direction - directional relays are required

Directional relays look into feeders - aw ay from busbars

Non-directional relays can be used: At the source substationOn the RM U circuit w ith the longer time delay

A

F E D

CB

A'

B ' C ' D '

E 'F '

0.9

2.1 0.1 1.7 0.5 1.3 0.9

1.30.51.70.12.1



PEEBLES REYROLLE

9b) Grading of IDMT Relays in Ring Main SystemRM U

LO AD

RM U

67 67

LO AD

RM U

67 6751

RM U

67 67

RM U

67 6751 67

SourceSubstation

67

A

F E D

CB

A'

B ' C ' D '

E 'F '

Grading Procedure for IDM T Relays

Grading margins established at highest current level seen byboth relays

Highest fault level occurs w ith ring closedHighest branch current occurs w ith ring open

Open ring at AGrade A' - F' - E' - D' - C' - B'

Open ring at A'Grade A - B - C - D - E - F

Relays A, B, C, A', E', F' may be non-directional



PEEBLES REYROLLE

9c) Grading of Ring Main with Argus Numerical Relays

LO AD

RM U

RM U RM U

51

RM U RM U

51

SourceSubstation

Directional (Argus2) num erical relays can be set to tri-state, allowingtotally independent forward and reverse settings to be applied.

Outputs of Argus configured to trip relevant circuit breaker

Cost saving - less relays, CTs, installation etc.

67 67 67

6767



PEEBLES REYROLLE

9d) Problem of Grading Ring Main with Two Sources

Correct discrimination betw een directional overcurrent relays is not possible:

For F1 - B' must operate before A', A before D

For F2 - B' must operate after A', A after D

67 6767

A BB'

SourceSubstation

67

C'

67

A'

67

C

F1

F2

RMU

67 67

D D'



PEEBLES REYROLLE

9e) Grading Ring Main with Two Sources

67 6767

A BB'

Source 1Substation

67

C'

50

O ption 1T rip least im portantsource instantaneouslyT hen treat as ring m ainw ith sing le source

RMU

67 6767

A'

67

CD D'

8787

Source 2Substation

O ption 2F it p ilo t w ire protection tocircuit A - BConsider S/Stns A & B ascom m on source busbar



PEEBLES REYROLLE

10a) Establishing Current Direction for Phase Faults

Directional O C protection required if current can flow in both d irections through relay location e.g .Paralle l feeders, R ing m ain circuits

D irection of a .c. is by inference not an absolute quantity, it is m easured relative to som e reference quantity alternating atthe sam e frequency - the system vo ltage.

I = O PERAT ING Q UANT IT YReference V = PO LARISING Q UANT IT Y

Com paring phase current (e .g . IRED) w ith relevant phase vo ltage (e.g . V RED) w ould ind icate d irn o f current flow , how ever;

P.F . USUALLY LO W (Pow er system apart from loads is reactive)

USE O F FAULT VO LT AG E IS UNRELIABLE (System volts at po int o f fault w ill co llapse tow ards zero)

Each phase of d irectional overcurrent relays m ust be polarised w ith a vo ltage w hich w ill not be reduced excessively (c loseup 3-phase faults notw ithstanding)



PEEBLES REYROLLE

10b) Table Illustrating Relay Connections

E xam ple – R ed P h ase

V R

V YVB

V R -B

IR300

V R

VYV B

VR -B

IR -

Y 600

V R

V YV B

-VB -NIR

600

VR

V YVB

VY -B

IR

RedPhase

YellowPhase

BluePhase

APPLIEDCURRENT

APPLIEDVOLTAGE

APPLIEDCURRENT

APPLIEDVOLTAGE

APPLIEDCURRENT

APPLIEDVOLTAGE

APPLIEDCURRENT

APPLIEDVOLTAGE

300 Connection600 No .2 Connection

Risk o f m al-operationfor all fault types

600 No .1 ConnectionRequires deltaconnected CT s

900 Connection

V B-Y

V Y-R

V R-B IR-Y

V B-Y

V Y-R

V R-B

IB-R

IY-B

-V Y-N

-V R-N

-V B-N

V R-Y

V B-R

V Y-B

E xam ple – R ed P h aseE xam ple – R ed P h aseE xam ple – R ed P h ase

IR

IB

IY

IR

IB

IY

IR

IB

IY



PEEBLES REYROLLE

10c) 900 Relay Connection with Selectable MTA

For balanced system conditions:90 - 30 RelayM TA: Primary system volts leads primary system current by 600

Zero torque limits: Primary system volts leads primary system current by 1500

Primary system volts lags primary system current by 300

90 - 45 RelayM TA: Primary system volts leads primary system current by 450 W .K Sonnemann:Zero torque limits: Primary system volts leads primary system current by 1350 This relay gives the best characteristic

Primary system volts lags primary system current by 450 to fit the spread of possible phase angles

IT zero

IU PF

V

Im axT

IT zero

-450

1350

450

90 - 45 RelayBalancedprimarysystemconditions

RelayPhase

AppliedCurrent

AppliedVoltage

M TA(I w rt V)

Relay Circuit

R IR VY-B

30

B IB VR-Y

Y IY VB-R

R IR VY-B

45

B IB VR-Y

Y IY VB-R

I

I'

MTA

V300

I

I'

MTA

V450

M TA = d isplacem ent ofcurrent and voltageapplied to relay



PEEBLES REYROLLE

11a) Establishing Current Direction for Earth Faults

Require:O PERAT ING Q UANT IT Y, PO LARISING Q UANT IT Y

O perating SignalO btained from residual connection o f line CT s IOP = 3 Io

Polaris ing SignalP h - P h o r P h - E vo ltag es used in o vercurrent p ro tec tio n

becom e inappropriate.Residual vo ltage is used as the po laris ing quantity



PEEBLES REYROLLE

11b) DEF - Extracting Residual Voltage

To allow for the flow of zerosequence voltage com ponents:

VT prim ary m ust be earthedVT can be of 3 phase, 5 lim b construction or3 single phase units

VRES

VRES = VA-G + VB-G + VC - G = 3VO

O pen delta VTsecondary



PEEBLES REYROLLE

11c) DEF - Residual Voltage for Solidly Earthed Systems

Z S/Z L high

Z S/Z L low

SystemVolts

SystemVolts

ResidualVo lts

ResidualVo lts

S Z S R Z L F

V R

V R

V R

V R

M ay lim ituse o f vo ltage

polarised relays- m odern relaysvery sensitive

Sourceim pedance

Lineim pedance

V R =ResidualVo ltage

CHECK FO R SUFFIC IENT PO LARISING VO LT AG E!



PEEBLES REYROLLE

11d) DEF - Residual Volts for Non-Effectively Earthed Systems

ResistanceEarthed

SystemVolts

SystemVolts

ResidualVolts

ResidualVolts

S

ZS

R

Z

L

F

Insulated orPeterson

CoilEarthed

SR

SR

VR VR

VR(may

approach3VPH)

VR VRVR

SUFFICIENTPOLARISINGVOLTAGE SHOULDALW AYS BEAVAILABLE

Volts dropdue to earthresistance Neutral point

raised aboveearth potential

Neutralfullydisplaced



PEEBLES REYROLLE

11.2) DEF - Relay Connection with Selectable MTA

M TA (I wrt V)

O 0 -150 -900-650-450

Resistive Neutral Reactive Neutral

V RES

VRES

IRES

VRES

IRES

VRES/IRES = 00 VRES/IRES = 900

V RES

IRESIRES

IRES

IRES

IRES

V RESV RESV RES

ResistanceEarthedSystem s Distribution System

- Solid ly EarthedT ransm ission System- Solid ly Earthed

ReactanceEarthedSystem s

Earthing T ransform erw ith Resistor



PEEBLES REYROLLE

11.3a) Polarising with Neutral Current

Polarisingsignal

VOLTAGE POLAR IS IN G M AY N OT B E PR AC TIC AB LE

i) A solid ly earthed, h igh fault level (low source im pedance) systemm ay result in a sm all value of residual voltage at the relaying point.

ii) VTs m ay not be available

iii) VTs m ay not be suitable - no zero sequence path

If a reliable polarising signal is not available then the relay m ay bepolarised from a suitable current source:e.g . from a C T located in a suitable system neutral to earth .

For relay operation:Polarising and operating current should be in phase

N eutral current of a pow er or earth ing transform er m ay be used- but only if earth fault neutral current alw ays flow s tow ards the systemN eutral of S tar/D elta w ill alw ays flow in correct d irection for polarisingD ouble earthed Star/S tar and Auto-transform ers require study.

A

B



PEEBLES REYROLLE

11.3b) Current Polarising from 2 Winding Transformers

P OL OP

DEF Relay

INC O RRECT

P OL OP

DEF Relay

CO R REC T

P OL OP

DEF Relay

CO R REC T

P OL OP

DEF Relay

INC O RRECT



PEEBLES REYROLLE

11.3c) Current Polarising from 3 Winding Transformers

P O L O P

DEF RelayCORRECTif ZLO + ZSO is positive

SOURCE

H L

UnloadedDelta

LoadedDelta

Relaypolaris ingcircuit

Relaypolaris ingcircuit



PEEBLES REYROLLE

11.3d) Current Polarising of DEF Protection on Auto-Transformer Circuit

N e u t r a l c o n n e c t i o n i s s u i t a b l e f o r c u r r e n t p o l a r i s i n gi f e a r t h f a u l t c u r r e n t f l o w s ' u p ' t h e n e u t r a l f o r f a u l t so n H V s i d e

F o r c o r r e c t a p p l i c a t i o n , c h e c k :

1 L

H

TOLOSO

TO

V

V

ZZZ

Z

W h e r e :Z T O = T e r t i a r y w i n d i n g z e r o s e q u e n c e i m p e d a n c eZ L O = L V w i n d i n g z e r o s e q u e n c e i m p e d a n c eZ H O = H V w i n d i n g z e r o s e q u e n c e i m p e d a n c e

( N o t e t h e r e i s a l s o a p o s s i b i l i t y t h a tn e u t r a l c u r r e n t m a y b e z e r o )

SO URCE

PO L O P

DEF Relay

Z H Z L

Z T

Current polarising fromtransformer delta preferred

(if transf. has delta w inding)



PEEBLES REYROLLE

12) Application Consideration - 2 out of 3 Tripping

D irectional R elay C onnected in to H ealthy C ircuitFW D . = d irection of load

3450

2250

1650

1050

450

2850

B REV

C FW D

A REV

IB=IC=+1

IA =-2

HV Infeed

R - BFault

LV DOCRelays

For H V ph-ph fault one phase of relay m ay incorrectlyoperate in FW D dirn - depends on phase angle of fault.

Select 2-out-of-3 logic, then a m inim um of tw o phasesm ust be in itiated for a trip output to be issued

REV



PEEBLES REYROLLE

OVERCURRENT RELAYS



PEEBLES REYROLLE

13.1) - Electromechanical Induction Disc Relay - Reyrolle Type 2TJM

TO RQ UE

Upper electrom agnet

Low erelectrom agnet

Prim aryw inding

Plug bridge

Secondaryw inding

Disc

Electromagnetic system operates on a movable conductor– aluminium disc, on which a contact assembly is mounted

Torque is produced by interaction of two alternatingmagnetic fields mutually displaced in space and time

T = K. 1. 2.sin

Where:T = Torque

1 = Flux 1 i.e. flux produced by upper electromagnet

2 = Flux 2 i.e. flux produced by lower electromagnet

Single function device1 Current input2 o/p contactsFlag indicator



PEEBLES REYROLLE

13.2) Schematic Diagram of Numeric Relay - Reyrolle Type Argus

I> ISLowSet

HS1 HS2A

D

rm s

dc

DTL DTL DTL

Program m ableinterconnection

logic

Statusinput

O UTPUTS

DataCom m s

HM I

1 Phase of relay shown

Multifunction deviceUp to 4 Current inputsUp to 11 o/p contactsLED indicators

StartTrip

8 settings groupsUp to 9 status inputsLCDData storageData comms PC software

Overcurrent Protection

Documents

Transcript of Overcurrent Protection