Biased Differential Protection: -

Basic Principles
and Pilot Wire Differential Protection

Biased Differential Protection: -

Basic Principles
and Pilot Wire Differential Protection

GRID
Technical Institute

This document is the exclusive property of Alstom Grid and shall not be
transmitted by any means, copied, reproduced or modified without the prior
written
1 consent of Alstom Grid Technical
> Biased Institute.
Differential All rights reserved.
Protection
 Pilot Wire Differential Protection of Feeders

 Why Needed
 Circulating Current and Balanced Voltage Principles
 Pilot Wire Relays and Schemes
 Summation Transformers and Fault Settings
 Line Charging Currents
 Pilot Wire:
- Characteristics
- Isolation
- Supervision
 Overcurrent Check
 Intertripping/ Destabilising

2 > Biased Differential Protection

 Differential Feeder Protection

 Why Needed?
 Overcomes application difficulties of simple overcurrent
relays when applied to complex networks i.e. co-ordination
problems and excessive fault clearance times
 Basic Principle
 Involves measurement of current at each end of feeder
 And transmission of information between each end of feeder

Protection should operate for faults inside the protected zone

(i.e. the feeder) but must remain stable for faults outside the
protected zone
Thus can be instantaneous in operation

3 > Biased Differential Protection

 System Where Directional O/C Cannot Be Used

Discrimination between all relays is not possible due to different

requirements under different ring operating conditions.

Not
For F1 :- B’ must operate before A’
Compatible
For F2 :- B’ must operate after A’ B

F1
A B' B C' C

A'

F2 D D'

4 > Biased Differential Protection

 Use Of Pilot Wire Differential Protection

Option 1
Trip least important source instantaneously then treat as normal ring
main.
Option 2
Fit pilot wire protection to circuit A - B and consider as common source
busbar. B

A 50
Option 1 Option 1 Option 1

PW PW
Option 2 Option 2

5 > Biased Differential Protection

 Merz-Price Differential Or Unit Protection

Protected
Circuit Or Plant

 Boundaries of protection coverage accurately defined

 Protection responds only to faults in protected zone

6 > Biased Differential Protection

 Two Types of Scheme

End A End B End A End B

Relay
Relay

Circulating Current System Balanced Voltage System

 Basic Mertz-Price principle applies well where CT

secondary circuit can be kept short e.g. protection of
transformers, busbars, machines
 For feeder protection where boundaries of protection are
a distance apart a communication channel is required

7 > Biased Differential Protection

 Unit Protection Involving Distance Between
Circuit Breakers (1)

A B

Relaying
R
Point
Trip B

Trip A

Simple Local Differential Protection

8 > Biased Differential Protection

 Unit Protection Involving Distance Between
Circuit Breakers (2)

A B
Communication
Channel

Relaying Relaying
Point Point
R R

Trip A Trip B

Unit Protection Involving Distance

Between Circuits

9 > Biased Differential Protection

 Early Merz-Price Balanced Voltage
Systems For Feeders

R R

 2 Problems:
1) Mal-operation due to unequal open circuit secondary
voltages of the two transformers for thro fault currents
2) High output voltages of CT’s cause capacitance currents
to flow through relay Since capacitive currents are
proportional to pilot length, relay insensitive for all but
very short lines

10 > Biased Differential Protection

 Early Merz-Price Circulating Current
System For Feeders
End X End Y

a c
EY
Relay Relay
X Y
EX

b d
Relay Relay
a Voltage Level d
X Y
EX of Relay Pilot EY

Zero Voltage
b Across Centre c
of Outer Pilots Voltage Diagram
for External Fault
11 > Biased Differential Protection
 Non-Biased Relay & Biased Relay

Differential
Current External
Operate
Fault

Restrain

Through Current

Non-Biased Relay

Operate
Differential
Current
Restrain

Through Current

Biased Relay

12 > Biased Differential Protection

 Basic Pilot Wire Schemes with Bias (1)

B B
I

V OP OP V

Circulating Current

13 > Biased Differential Protection

 Basic Pilot Wire Schemes with Bias (2)

OP OP

V B B V

Balanced Voltage

14 > Biased Differential Protection

 Summation Current Transformer Output

a
b Ia
c l
c
n m
l b m+n

m Ic Ib m+n
output
a
n
Current Output
A–G=I+m+n
B–G=m+n
C–G=n
A–B=l
B–C=m
C–A=l+m
A – B – C = (m2 + m l) 1/2

15 > Biased Differential Protection

 Summation Transformer Sensitivity
for Different Faults (1)
IA
1
IB
1
IC Output for operation = K

IN

Let output for operation = K

(1) Consider A-E fault

for relay operation : IA (1 + 1 + 3) > K
IA > 1/5K or 20%K
16 > Biased Differential Protection
 Summation Transformer Sensitivity
for Different Faults (2)
(2) B-E fault
for relay operation : IB (1 + 3) > K
IB > 25%K

(3) C-E fault

for relay operation : IC x (3) > K
IC > 331/3%K

(4) AB fault
for relay operation : IAB x (1) > K
IAB > 100%K

(5) BC fault
for relay operation : IBC x (1) > K
IBC > 100%K

(6) AC fault
for relay operation : IAC (1 + 1) > K
IAC > 50%K
17 > Biased Differential Protection
 Summation Transformer Sensitivity
For Different Faults Cont.
 C-E Fault
For relay operation :
IA (1+1+3) + IB (1+3) + IC(3) > K
5IA + 4 2 IA + 3 IA > K
IA [ 5 + 4 (-1/2 – j √3/2) + 3 (-1/2 + j √3/2)] > K
√3 IA [√3 /2 – j ½ ] > K
√3 IA -30 > K
Effective turns 12 12 - 2 x 1 x cos 120
IA > K/ √3 OR 57.73% K
3
IA
4

5
1 3
IC IB
1 Effective turns

18 > Biased Differential Protection

 Type Of Relay Sensitivity Of
Fault Sensitivity E/M Pilot Wire Relay

A-E 20% K 22% In

B-E 25% K 28% In
C-E 331/3 % K 40% In
AB 100% K 90% In
BC 100% K 90% In
3 Phase 57.7% K 52% In

19 > Biased Differential Protection

 Zero Summation Transformer Output
For 2-1-1 Current Distribution

 Resultant bias for LV B-C fault is zero

 Problem can be alleviated by using unequal turns between
A-B and B-C taps

20 > Biased Differential Protection

 Line Charging Currents

Line charging currents flow in at one end of the feeder only

and is therefore potentially capable of unbalancing a
protective system.

Charging currents (or capacitance currents) of overhead lines

generally low.

Charging current levels of underground cables however can

be high enough to dictate minimum permissible operating
level of the protection.

21 > Biased Differential Protection

 Line Charging Currents

Line charging currents produce a certain amount of

unbalancing AT’s under normal steady state conditions
when balanced 3Ф charging current flows.
Unbalancing AT’s = 3Ic
AT’s of most sensitive fault setting = (2 + N) IA
Where IA = setting for A-E fault
For stability,
ICA
(2+N)IA > 3 Ic
ICB 1
ICC 1
IA > 3 I
c
(2 n) N
For N = 3, IA > 0.35Ic

22 > Biased Differential Protection

 Solid Earthed System

 Maximum unbalanced amp-turns is obtained when an

external double phase to ground fault on phases B & C
occurs.
 Charging current flows in phase A only since B & C
phase capacitances are shorted out.

I
C
1

1
N

23 > Biased Differential Protection

 Solid Earthed System

Unbalancing AT’s due to charging current

= (2 + N) IC
AT’s of most sensitive fault setting
= (2 + N) IA
Where IA = setting for A-E fault
For Stability,
(2 + N) IA > (2 + N) IC
I A > IC

Note: For EHV cables, one relay per phase is sometimes used
to avoid lack of sensitivity for phase faults

24 > Biased Differential Protection

 Resistance Earthed Systems

 Maximum unbalanced AT’s is obtained when an external

earth fault on the C phase occurs. This is because the
neutral is displaced so that the charging current in
phases A & B increases to √3 times the steady state
value
3 IC 60°
A
3 IC 1
1

C B

25 > Biased Differential Protection

 Resistance Earthed Systems

Phase A unbalancing AT’s = √3 IC 60 x (2 + N)

Phase B unbalancing AT’s = √3 IC x (1 + N)

For N = 3 : - AT’s = 13.5Ic , for operation AT > (1 + 1 +3 )IA

> 5IA

IA = A-Ph sensitivity, for stability IC / IA > 2.7 (3.2 with 20% margin)

For N = 6 :- AT’s = 22.5IC, for stability IC/IA > 2.81

26 > Biased Differential Protection

 Resistance Earthed System
With External C-N Fault
A

3 IC1 (2 + N)
sin 60°

C B
60°
3 IC1 (1 + N) 3 IC1 (2 + N) cos 60°

Summated amp turns = 13.5 IC for N = 3

= 22.5 IC for N = 6
Most sensitive E/F setting must be greater than = 2.7 for N = 3
the summated capacitive amp turns = 13.5 IC1 = 2.8 for N = 6
most sensitive fault turns 5

27 > Biased Differential Protection

 Selection Of Relay Sensitivity

Values of Ks and N are chosen such that:

IS (C - N) < 0.3 x Min. E/F Current
 For solidly earthed systems : -
IS (A - N) > 1.1 x Steady State Line Charging Current
 For resistance earthed systems : -
IS (A - N) > 3.2 x Steady State Line Charging Current
 For resistance earthed systems with one relay per phase : -
IS (A - N) > 1.9 x Steady State Line Charging Current

For systems where the steady state charging current is

negligible select Ks setting to give primary sensitivity.

28 > Biased Differential Protection

 Line Inrush Current

X Y
S L

CT CT

~ C

15a

+j

1
Wo = = Inrush Frequency
LC
Wo

-j Wo = ISOHZ – SKHZ
15b Therefore use HI – Q Filter to De-Sensitise
Protection to Inrush Current

29 > Biased Differential Protection

 Numerical Line Differential-
Charging Current Compensation

 Long lines charging current can be high can result in

differential protection operation
 Traditional techniques involves setting the differential
protection above line charging current
 Modern numerical line differential relays employ
techniques to subtract the charging current from the
differential calculations (Requires VT’s at both ends)
 This can allow for more sensitive relay settings and
increases the fault resistance coverage.
IL ZL IR

Relays requires cable

IchL IchR susceptance data to
VL VR correctly calculate
charging current

30 > Biased Differential Protection

 Pilot Wire

Resistance and shunt capacitance of pilots introduce

magnitude and phase differences in pilot terminal currents.

Pilot Resistance
Attenuates the signal and affects effective minimum operating
levels.

To maintain constant operating levels for wide range of pilot

resistance, padding resistor used.

R Rp/2 R

Rp/2

Padding resistance R set to ½ (1000 - Rp) ohms

31 > Biased Differential Protection
 Pilot Capacitance

 Circulating Current Systems:

 Pilot capacitance effectively in parallel with relay operating
coil
 Capacitance at centre of pilots has zero volts across them

 Balanced Voltage Systems:

 Relay operating coil connected in series with pilot
 Capacitance current therefore tends to cause instability

32 > Biased Differential Protection

 Effect of pilot capacitance and pilot isolation
transformers on setting

1.8
Factor by which the setting is

1.7 Without 15kV pilot isolating trans

With 15kV pilot isolating trans (Ks = 1.0 or 2.0)
With 15kV pilot isolating trans (Ks = 0.5)
1.6
1.5
increased

1.4
1.3
1.2
1.1
1
0 1 2 3 4 5
Pilot Capacitance (uF)

33 > Biased Differential Protection

 Methods Of Compensating For Pilot Capacitance

34 > Biased Differential Protection

 Pilot Isolation
Electromagnetic Induction

Field of any adjacent conductor may induce a voltage in the pilot

circuit.

Induced voltage can be severe when :

(1) Pilot wire laid in parallel to a power circuit.
(2) Pilot wire is long and in close proximity to power circuit.
(3) Fault Current is severe.

Induced voltage may amount to several thousand volts.

Danger to personnel
Danger to equipment

Difference in Station Earth Potentials

Can be a problem for applications above 33kV - even if feeder is

short.
35 > Biased Differential Protection
 Formula for Induced Voltage

e = 0.232 I L Log10 De/S

where I = primary line E/F current

L = length of pilots in miles
De = Equiv. Depth of earth return in metres = 655 . e/f
e = soil resistivity in .m
f = frequency
s = separation between power line and pilot circuit in
metres

Effect of screening is not considered in the formula.

If the pilot is enclosed in lead sheath earthed at each end, screening is
provided by the current flowing in the sheath.
Sheath should be of low resistance.

0.3 V / A / Mile Unscreened Pilots

0.1 V / A / Mile Screened Pilots

36 > Biased Differential Protection

 Arrangement Power System And Pilot Wire Relay
Circuit Under Earth Fault Conditions
Earth Wire

O/H Line

Fault

x
Pilot
Station Relay Isolating
Earth V Transformer
Vm
Resistance
RC
Pilot Pilot
OC
Circuit Circuit

Insulation
Barrier
Earth
Shield

True Earth
Potential

37 > Biased Differential Protection

 Isolation Transformers

Pilot circuits and all directly connected equipment should be

insulated to earth and other circuits to an adequate voltage
level.

Two levels are recognised as standard : 5kV & 15kV

Relay Case

5kV 15kV
Pilot
Terminal
Relay
Input Relay
Circuit
Pilot
Wire
2kV 5kV

38 > Biased Differential Protection

 Fig. Pilot Isolation Transformers

 Gives pilot isolation levels up to 15kV

 Primary taps to enable scheme to work with pilot
resistance up to 2500Ω

Km 0.8 1.0 1.2 1.5 2.5 Matching

Ratio
Loop 800 1000 1200 1500 2500 Ohms
Resistance
Capacitance 6.25 5.0 4.2 3.3 2.0 Microfarads

Km = (turns ratio)2
Rp
= 1000
Where Rp is the measured pilot loop resistance
Padding Resistor Setting
Rpp = 1 Rp
1000
2 Km

39 > Biased Differential Protection

 Supervision of Pilot Circuits

 Pilot circuits are subject to a number of hazards, such

as:
 Manual Interference
 Acts of Nature (storms, subsidence, etc)
 Mechanical Damage (excavators, impacts)

 Therefore supervision of the pilots is felt to be

necessary. Two types exist:
 Signal injection type
 Wheatstone Bridge type

40 > Biased Differential Protection

 Pilot Wire Supervision

Circulating Current Balanced Voltage

Schemes Schemes
Pilot Wire Open
Maloperate Stable
Circuited
Pilot Wire Short
Stable Maloperate
Circuited
Pilot Wire Crossed Maloperate Maloperate

 Maloperation occurs even under normal loading

conditions if 3 - phase setting < I Load
 Overcurrent check may be used to prevent
maloperation
 Overcurrent element set above maximum load current

41 > Biased Differential Protection

 Typical Pilot Wire Supervision Relay

PILOT

Cross Pilot
Detector Box

Unbalance
Detector
Circuit

A
Supervision
Supply

42 > Biased Differential Protection

 Supervision Features

 Detects open circuit, short circuit or crossed pilots

 Gives indication of loss of supervision supply

43 > Biased Differential Protection

 Connections For Pilot Supervision (5kV)

A1 A1

PILOTS

A2 A2
A3 A3

LVAC AC

44 > Biased Differential Protection

 Connections For Pilot Supervision (15kV)

P6 S2 S1 P1
X2 X1

PILOTS
X1 X2
S1 S2
P1 P6
S1 S2

P1 P2
E3 E4

LVAC AC

45 > Biased Differential Protection

 Overcurrent Check Relays

A
B
C
50
A
PILOT
50 WIRE
C RELAY
(87PW)
50
G

50A - 1 87PW - 1
+ Trip circuits
50C - 1 Isoc > Ifl / 0.9
Isef > 1.2 IZ
50G - 1 Isef > 0.8 x Ief

46 > Biased Differential Protection

 System Requiring Intertripping /
Destabilising Relay

Source

Feeder
Protection
Busbar
Protection

47 > Biased Differential Protection

 MVTW01 Destabilising Relay

P6 S2
17
PILOTS
P7 S1

MBCI
17 UN-1

18
18 UN-2
19
19
UN-3
20

I1 UN
V x (1) + 3
I2
V x (2) +
V x (3) + I3
- I4
MVTW01

48 > Biased Differential Protection

 Transformer Feeder - Intertripping

PILOT WIRES

PILOT WIRE
TRIP TRIP
PROTECTION PILOT WIRE TRANSFORMER
PROTECTION PROTECTION

DESTABILISE AND
INTERTRIP

49 > Biased Differential Protection

 MVTW03 Destabilising Relay

PILOT ISOLATION TRANSFORMER

P6 S2
17
PILOTS
P1 S1

MBCI
17

18
18
19
19
11 +VE
Trip send RL2
2
RL2-2

RL2-1
13 +VE 1
Vx 14 -VE
Power RL2 RL1-1 Supply
Supply 1
MVTW03 fail
2
Supply
healthy
Case earth
see note 2

50 > Biased Differential Protection

 Types of Transformer Feeder

51 > Biased Differential Protection

 Protection Of Transformer Feeders

HV LV CTs
CTs CTs CTs
x x

TRIP

FEEDER FEEDER TRANSFORMER

PILOTS
DIFFERENTIAL DIFFERENTIAL DIFFERENTIAL
PROTECTION PROTECTION PROTECTION

UNSTABILISE TRIP

TRIP

52 > Biased Differential Protection

 Fig. Transformer Feeders

FEEDER

PILOTS
PW PW

 For use where no breaker separates the transformer

from the feeder
 Transformer inrush current must be considered
 Inrush is a transient condition which may occur at the
instant of transformer energisation
 Mag. inrush current is not a fault condition therefore
protection must remain stable
 MCTH provides a blocking signal in the presence of
inrush current and allows protection to be used on
transformer feeders
53 > Biased Differential Protection
 Overall Protection Of Transformer Feeders Showing
Connections To MBCI Relays - Translay -S
Transformer (Dy 11)
Feeder

Star
Delta
MFAC MCTH
MCTH
N.E.R. Inrush A B C
Inrush A B C REF detector
detector

MBCI MBCI

1.25
1.25
1
1

6
Pilots

54 > Biased Differential Protection