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This document is the exclusive property of Alstom Grid and shall not betransmitted by any means, copied, reproduced or modified without the prior

written consent of Alstom Grid Technical Institute. All rights reserved.

GRID

Technical Institute

App l icat ion o f Direct ion al

Overcurrent

and Earthfaul t Protect ion


2/72

> Directional Overcurrent and Earthfault Protection2 2

Direct ional Protect ion


3/72


Need for Direct ion al Con trol

Generally required if current can flow in both directions

through a relay location

e.g. Parallel feeder circuits

Ring Main Circuits

2.1 0.50.9 0.11.31.7


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Ring Main Circuits

Grading has now been lost !

2.1 0.50.9 0.11.31.7


5/72






Ring Main Circuits

Relays operate for current flow in direction indicated

(Typical operating times shown)

0.9 0.10.5 0.90.50.1


6/72


Ring Main Circui t

With ring closed :

Both load and fault current may flow in eitherdirection along feeder circuits

Thus, directional relays are required

Note: Directional relays look into the feeder

Need to establish setting philosophy

51 67

51

Load

67 67

Load

67

67 67

Load


7/72> Directional Overcurrent and Earthfault Protection7 7

Ring Main Circui t

Procedure :

1. Open ring at A

Grade : A'

- E'

- D'

- C'

- B'

2. Open ring at A'

Grade : A - B - C - D - E

Typical operating times shown.

Note : Relays B, C, D, E may be non-directional.

1.30.1

0.1 0.90.5

0.9

0.5

B'

A'

B

E E'

A

1.7

D'

D

1.7

1.3

C' C



Ring System w ith Two Sources

Discrimination between all relays is not possible due to differentrequirements under different ring operating conditions.

For F1 :- B must operate before A

For F2 :- B must operate after A

Not

Compatible

B' B C' C

D D'

F1

B

F2

A

A'



Ring System w ith Two Sources

Option 1

Trip least important source instantaneously then treat as normal ringmain.

Option 2

Fit pilot wire protection to circuit A - B and consider as common sourcebusbar.

A

B

Option 1Option 1Option 1

Option 2 Option 2

50

PW PW



Parallel Feeders

Non-Directional Relays :-

Conventional Grading :-

Grade A with C

and Grade B with D

Relays A and B have

the same setting.

51

51

A

D

Load

51 B

51 C

A & B

C & D

Fault levelat F

Operat

ingTime



Parallel Feeders

Consider fault on one feeder :-

Relays C and D see the same fault current (I2). As C and

D have similar settings both feeders will be tripped.

51 A 51C

51 B 51D

LOAD

I1

+ I2

I1

I2



Parallel Feeders

Solution:- Directional Control at C and D

Relay D does not operate due to current flow in the reverse

direction.

51 A 67C

51 B 67D

LOAD

I1

+ I2

I1

I2



Parallel Feeders

Setting philosophy for directional relays

Load current always flows in non-operate direction.

Any current flow in operate direction is indicative of a fault

condition.

51A 67

E

51 B 67

C

D

Load

51



Parallel Feeders

Usually, relays are set :-

- 50% of full load current (note thermal rating)

- IDMT rather than DT

- Minimum T.M.S. (0.1)



Paral lel Feeder Applicat ion

P

B

B

D

D

Load

Load

If3

A C

If1If2/2 If2

BC

D

Ifmax

A

Grade A with B with C at If1

(single feeder in service)

GradeBwithDatIf3=If1

(upper feeder open at P)

Grade A with B at If2

(both feeders in service)

- check that sufficient margin existsfor bus fault at Q when relay A seestotal fault current If2, but relay Bsees only If2/2.

If1

If2

M

M = MarginM

M

Q



Advantage : Reduced number of grading

stages

6767

51

51

5151

P1

P1

S1

S1

S2

S2

P2

P2

Grid supply

Part ial Different ial Scheme



Establ ish ing Direct ion



Establ ishing Direct ion :- Polar ising Quant i ty

The DIRECTIONof Alternating Current may only bedetermined with respect to a COMMONREFERENCE.

In relaying terms, the REFERENCEis called the POLARISING

QUANTITY.

The most convenient reference quantity is POLARISINGVOLTAGEtaken from the Power System Voltages.


19/72



Direct ional Decision by Phase Comparison (2)

RESTRAINT when S2lags S1by between 90 and 270 :-

S2

S1

S2

S2

S2

S2S2

S2



Polarising Voltage for A Phase Overcurrent Relay

OPERATE SIGNAL = IA

POLARISING SIGNAL :- Which voltage to use ?

Selectable from

VA

VB

VC

VA-B

VB-C

VC-A



Directional Relay

Applied Voltage : VA

Applied Current : IA

Question :

- is this connection suitable for a typical power system ?

IA

VA

Operate

Restrain

VAF

IAF


23/72


Polar ising Voltage

Applied Voltage : VBC

Applied Current : IA

Polarising voltage remainshealthy

Fault current is near centreof characteristic

IA

VBC

ZERO SENSITIVITYLINE

VA

IAF

IVBC

VBC

MAXIMUM SENSITIVITY LINE

C


24/72


Relay Connect ion Angle

The angle between the current applied to the relay and thevoltage applied to the relay at system unity power factor

e.g. 90 (Quadrature) Connection : IA and VBC

The 90 connection is now used for all overcurrent relays.30 and 60 connections were also used in the past, but nolonger, as the 90 connection gives better performance.

IA

VA

90

VBC

VC VB

Relay Characteris tic A ng le (R C A )


25/72


Relay Characteris tic Ang le (R.C.A .)

for Electronic Relays

The angle by which the current applied to the relay must be

displaced from the voltage applied to the relay to produce maximumoperational sensitivity

e.g. 45

OPERATE

IAFOR MAXIMUM OPERATESENSITIVITYRESTRAIN

45

VA

RCA

VBC

90 C ti 45 R C A


26/72


90Connec tion - 45R.C.A .

RELAY CURRENT VOLTAGEA IA VBC

B IB VCA

C IC V

AB

IA

VA

90

VBVC

MAX SENSITIVITY

LINEOPERATE

IAFOR MAXSENSITIVITYRESTRAIN 45

45

135

VA

VBC VBC

90 C ti 30 R C A


27/72


90Connec tion - 30R.C.A .

RELAY CURRENT VOLTAGEA IA VBC

B IB VCA

C IC V

AB

IA

VA

90

VBVC

VBC30

30

OPERATE

MAXSENSITIVITYLINERESTRAIN

IAFOR MAXSENSITIVITY

150

VA

VBC

S l t i f R C A (1)


28/72


Select ion o f R.C.A . (1)

90 connection 30 RCA (lead)

Plain feeder, zero sequence source behind relay

Overcurrent Relays


29/72



Plain Feeder

900Connection

RCA = 300

Zero seq sourcebehind relay

rom

onnemans paper

S l t i f R C A (3)


30/72



90 connection 45 RCA (lead)

Plain or Transformer Feeder :- Only Zero Sequence Source is inFront of Relay

Transformer Feeder :- Delta/Star Transformer in Front of Relay


31/72



Plain Or

TransformerFeeder

900Connection

RCA = 450

Zero seq sourceIn front of relay

From

Sonnemans paper


32/72



Transformer

Feeder

900Connection

RCA = 450

/Y transformer

in front of relay

From

Sonnemans paper


33/72


Direct ional Earth faul t Pro tect ion

Directional Earth Fault


34/72


Directional Earth Fault

Requirements are similar to directional overcurrent

i.e. need operating signaland polarising signal

Operating Signal

obtained from residual connection of line CT's

i.e. Iop = 3Io

Polarising Signal

The use of either phase-neutral or phase-phase voltage as

the reference becomes inappropriate for the comparison withresidual current.

Most appropriate polarising signal is the residual voltage.

Residual Voltage (1)


35/72



May be obtained from broken delta V.T. secondary.

Notes :

1. VT primary must be earthed.

2. VT must be of the '5 limb' construction (or 3 x single phase units)

VRES= VA-G + VB-G+ VC-G = 3V0

A

BC

VRES

VC-GVB-GVA-G



36/72



Solidly Earthed System

Residual Voltage at R (relaying point) is dependant upon ZS/ ZL ratio.

3ExZ2ZZ2Z

ZV

L0L1S0S1

S0

RES

E S R FZLZS

A-G

VCVC VC

VB VBVB

V

RES

VAVA

V

RES

VBVCVCVC VBVB

VAVA



37/72



Resistance Earthed System

3Ex3ZZ2ZZ2Z

3ZZV

EL0L1S0S1

ES0

RES

VA-G

VA-GVA-G

VA-G

VB-GVC-G

G.FG.F

VB-G

V

RES

V

RES

RES

VC-GVC-GVC-G

VC-G VB-GVC-G

VB-G VB-G VB-G

E

N

G

S R FZLZ

S

ZE A-G

G.F

Relay Characteris tic A ng le (R C A )


38/72


Relay Characteris tic Ang le (R.C.A .)

Voltage Polarising Signal

Rotate VRESby 180O

to obtain voltage polarisation signal0O, -45Oor -60OR.C.A. applied for maximum sensitivity

e.g. -45V

A

V

C

V

B

VF

VRES

Rotate V

RES

by 180

MAX SENSITIVITYLINE

IRESFOR MAXSENSITIVITY-45

OPERATE

RESTRAIN

Residual Voltage Polarisat ion


39/72


Residual Voltage Polarisat ion

Relay Characteristic Angle

0 - Resistance/Petersen Coil earthed systems

-45 (Ilags V) - Distribution systems (solidly earthed)

-60 (Ilags V) - Transmission systems (solidly earthed)

+90 (Ileads V) - Insulated systems

Zero Sequence Network :-

V0 = 0 - I0 (ZS0+ 3R)

(Relay Point)

ZL0ZS0 I0

V03R


40/72

App lication (1/11)


41/72


App lication (1/11)

Typ ical UK Sub-Transm ission Protect ion System

Distance protection (21), without signalling, is commonly used at

sub-transmission levels

Intertripping is used to supplement the distance protection by

opening the LV breaker

F1

LV Load

67

Sub Transmission Network

LV Network

Intertripping

Channel

IT

21

CB1 CB2

CB4

Embedded

generation

CB3

Faults at F1 cleared by:

Distance protection at CB1and CB2

Intertripping to CB4

DOC (67) provides back-up in theevent of intertripping failure

App lication (2/11)


42/72


App lication (2/11)

Direct ional Protect ion

Protection is naturally insensitive to load current (IA-LOAD), by virtue of itsdirection

Since load current resides in the restraining region, a setting of 0.5In isoften selected

DOC protection without embedded generation :-

Operate

Restrain

VA

VBVC

RCA

VBC

(VPOL)

45

IA-LOAD

IAF

F1(A-B)

67

Sub Transmission NetworkCB1 CB2

CB4CB3

Normal

LoadDirection

(IA-LOAD)

IAF

IAF

App lication (3/11)


43/72


App lication (3/11)

Impact of Embedded Generat ion

Excess generation is exported back on to the sub-transmission network

Exported current (IA-EXP) resides in the operate region

Unless measures are taken, the DOC relay mal-operates during peakexport conditions

Increase threshold?

Operate

Restrain

VA

VBVC

RCA

VBC

(VPOL)

45

IA-LOAD

IA-EXP

67


CB4CB3

Normal

LoadDirection

(IA-LOAD) IA-EXPEmbedded

Generation

IA-EXP

App lication (4/11)


44/72


App lication (4/11)

Problem.

Increasing the current setting (IS

) to, say, 1.2In ensures stability of theDOC protection during peak export conditions. But.

Reducing the sensitivity creates a potential blind spot for the DOC

protection. This is a problem if :-

67


CB4CB3

Embedded

Generation

F

Potential

Blind Spot

IS> (IF1+IF2)

The intertripping scheme fails

to function and we are reliant

of the DOC relay to clear the

fault

The embedded generation isminimal or none existent

during the fault condition

IF1+

IF2

IF1

IF2

IF1

App lication (5/11)


45/72


App lication (5/11)

Solut ionDOC w ith Load Bl inding

Relay determines the difference between fault and load conditions bythe change in system impedance

DOC protection is:

Inhibited during load conditions, thus permitting export of excess generation

Allowed to operate for faults providing the correct direction

67

CB1 CB2

CB4CB3

Embedded

Generation

F

Potential

Blind Spot

IS> (IF1+IF2)

IF1+IF2

IF1

IF1

Load blinding originates from distance protection relays:

jX

R

Fault Impedance

(F)

Load Locus

(lagging VARs)

Load Locus

(leading VARs)

Load Blinder

Z behind

relay

Z in front

of relayVS

VS

App lication (6/11)


46/72


App lication (6/11)

Load Bl inder Character ist ic

4 main settings denote the shape and behaviour of the blindercharacteristic:-

ZMIN Minimum impedance threshold

Load angle setting

V< Voltage threshold to disable load blinder

I2

> Negative sequence threshold to disable load blinder

How to set?jX

R

Load Locus

(Import)

Load Locus

(Export)

1

2

3

4

ZMIN1ZMIN2

Application(7/11)


47/72


Application(7/11)

Load Bl inder Sett ings

ZMIN1 Minimum impedance threshold (Export)

Set below the minimum load impedance

Based upon rated current and rated voltage

Include safety margin if required

Example: for a 33kV system with a 600/5 CT (no margin):

ZMIN2 Not required as imported load is naturally in the restrainingregion of the DOC relay

inargmRatingPrimaryCT3

ph)-(phltagePrimary VoRated(primary)ZMIN

7.310063

1033(primary)Z

3

MIN

App lication (8/11)


48/72


App lication (8/11)

Load Bl inder Sett ing

Load angle setting

Set above worst case power factor angle

Include safety margin of typically 15

Equal in inductive and capacitive reactance regions ( 1 = 2)

Hence :-

Example: lowest power factor = 0.85:

15FactorPowerOSC -121

471585.0OSC -121

App lication (9/11)


49/72


App lication (9/11)

Load Bl inder Sett ing

V< Undervoltage threshold

Designed to disable the load blinder during fault conditions

Must disable load blinder for faults with minimum embedded generation(VFAULT 0.5VN)

Disabling load blinder for faults with maximum embedded generationless important due to increase in fault current

Recommended setting = 0.7VN

Hence :-

Example: For 33kV system:

i.e. Operates if any ph-nvoltage falls below 13.3kV

kV3.1331033.70V

3

jX

47

ZMIN=

31.7Restraining

region

47

Import region

naturally

blocked by

DOC blindercharacteristic.

Fault Impedance

(HV fault)

Restraining

region

R

App lication (10/11)


50/72



Load Bl inder Character ist ic Setting Criteria (I2>)

I2> Negative sequence current threshold

Designed to disable the load blinder during unbalanced fault conditions where thephase to neutral voltage collapse is insufficient (common with delta / startransformers)

Phase to Phase to Ground being the worst case

Calculation assumes zero arc resistance to ground resulting in lowest possible I2component

Sequence analysis gives the following setting guideline :-

Example: Assuming IS= 0.5 IFLC

i.e. Load blinder turns off ifI2component is above 0.166A sec

secA..

0.38I 1660600

52450

2

S0.38II2

67

30MVA

132kV

33kV

600/1 Full load current (IFLC)

= 524A at 33kV



51/72


pp ( )

Hyb rid Load B l inder / DOC Character ist ic

(A -Phase Element)

Import / Export LoadConditions

Fault Condition


52/72

Insulated Sys tems (2)


53/72


y ( )

Faulty Feeder

VRES

a b

cIca

Icb

Ic-3I

c

Healthy Feeders

VRES

Ic = Ica + IcbRCA

OperateRestrain

VPOL

-2IcRCA

OperateRestrain

VRES

VPOL

Peterson Co il Earthed Sys tems (1)


54/72


IL

y ( )

a b c

Source

IcbIca

Ic

IcbIca

Ic

IcbIca

2Ic

Location of CT's

3IcIc

IL

IL



55/72


y ( )



56/72


Negative Phase Sequence Voltage Polarisat ion


57/72


Transmission Systems

Directional earth fault used as back-up protection

Can form part of a directional scheme

VRESmight be unreliable due to mutual coupling

Unsuitable VT for VRESmeasurement (i.e. open delta, 3-limb)

Negative Sequence Network :-

V2 = 0I2 (ZS2)

ZL2ZS2 I2

V2

(Relay Point)

ZS1=ZS2

ZL1=ZL2

Current Polar ising


58/72


A solidly earthed, high fault level (low source impedance) system

may result in a small value of residual voltage at the relaying point. If

residual voltage is too low to provide a reliable polarising signal thena current polarising signal may be used as an alternative.

The current polarising signal may be derived from a CT located in a

suitable system neutral to earth connection.

e.g.

POL

OP

DEF Relay

Current Polarising (1)


59/72


POLDEF RELAY

INCORRECTOP

Direction of current depends on fault

position



60/72


POLDEF RELAY

CORRECTOP



61/72


POLDEF RELAY

CORRECT IFZLO+ ZSOISPOSITIVE

S

OP



62/72


POL DEF RELAY

OP

CORRECT

Virtual Curren t Polarising (1)


63/72


The faulted phase is not considered in the residual

voltage calculation.

The polarising quantity is in the same direction as

Vres.Applicable even where solid earthing immediately behind

the IED prevents residual voltage from being developed.

Faulted Phase Polarising

A VB + VC

B VA + VCC VA + VC

Au to Transfo rmers (1)


64/72


ZT

ZLZHSOURCE

ZSSOURCE

DEFRELAY

Neutral connection is suitable for currentpolarising if earthfault current flows up the

neutral for faults on H.V. & L.V. sides.

For LV Faults


65/72


T

H L

IN= 3 (ILO- IHO)

IH I

L

For HV Faults


66/72


T

H L

IN = 3 (IHO- ILO)

IH IL

Au to-Transform er Examp le


67/72


T

H L

IN = 3 (IHO- ILO)

ZS

ZS0ZL0ZH0IH0 IL0

I0

ZT0



68/72


kAn

kV

MVA

x

p.u.n

H

base

0

00

kAn

kV

MVA

x

Z

Z

p.u.n

Z

Z

L

base

0

L000

T0

0

L000

T0

L0



69/72


1

Z

Z

kV

kV

r

Z

Z

kV

1

kV

1

fes

Z

Z

kV

1

kV

1

3

.MVA

L000

T0

L

H

L000

T0

L

N

L000

T0

L

base

N



70/72


T

H L

IN= 3 (ILO- IHO)

ZS

ZS0ZL0ZH0IH0 IL0

I0

ZT0

IH0 = 0

IN = 3IL0 which is +ve.

Direct ional Contro l


71/72


Static Relay(MCGG + METI)

Characteristic Selectable

51 I

Overcurrent Unit(Static)

67

V

I

M.T.A. Selectable

Directional Unit(Static)

Numerical Relay Direct ional Characterist ic


72/72

Characteristic angle cc = -95 0 95

in 1 steps

Polarising thresholdsVp 2V to 320V

in 2V steps

VT supervisionselectively block operation

Zone of

forward start

forward operation

Reverse start

c - 90) c + 90)

c

+Is

-Is

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