09_Line Differential Protection

Pilot Wire Differential

Protection of Feeders

_{Why Needed}

_{Circulating Current and Balanced Voltage Principles}

_{Electromechanical Pilot Wire Relays and Schemes}

_{Solid State Pilot Wire Relays and Schemes}

_{Polar Diagrams}

_{Summation Transformers and Fault Settings}

_{Line Charging Currents}

_{Pilot Wire :}

_{Characteristics}

Isolation

Supervision

_{Overcurrent Check}

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.

System Where Directional O/C Cannot Be Used

I

₁

I

₂

I

₁

I

₁

I

₂

I

₁

+I

₂

I

₂

I

₂

I

₁

+I

₂

I

₁

+I

₂

I

₂

I

₁

+I

₂

I

₁

I

₁

4 must operate before 8

Use of Pilot Wire Differential Protection

, , ,  are pilot wire differential relays

 is non directional O/C relays

Merz-Price Differential or Unit Protection

R

Protected Circuit

or Plant

Boundaries of protection coverage accurately defined

Protection responds only to faults in protected zone

Relay

End A

End B

Circulating Current System

Balanced Voltage System

Basic mertz-price principle applies well where CT secondary circuit can be kept short, eg.

protection of transformers, busbars, machines.

Relay

Unit Protection Involving Distance

Between Circuit Breakers (1)

A

B

Relaying

Point

Trip A

Trip B

R

Unit Protection Involving Distance

Between Circuit Breakers (2)

Unit Protection Involving Distance

Between Circuits

A

B

R

R

Relaying

Point

Trip A

Trip B

Communication

Channel

Relaying

Point

Early Merz-Price Balanced Voltage Systems for

Feeders

R

R

2 Problems :

(1) Maloperation 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 thro’ relay.

Since capacitive currents are proportional to pilot length, relay insensitive for

all but very short lines.

Basic Pilot Wire Schemes with Bias (1)

B

B

I

V

OP

OP

V

I

Circulating Current

Translay Differential Protection

A

B

C

Summation

Winding

Secondary

Winding

Pilot

Bias Loop

End A

End B

MBCI Feeder Protection Circuit Diagram

T

_t

T2

T2

T

_r

T

_r

T1

T1

Ø

_C

Ø

R

_PP

R

_PP

PILOT

WIRES

T

_t

R

_S

R

_S

RVO

RVO

T

_o

T

_s

T

_s

v

v

A

B

C

A

B

C

T

_o

R

_o

R

_o

T1

- Summation Transformer

T

_s

-

Auxiliary Winding

T2

- Auxiliary Transformer

RVD -

Non Linear Resistor

T

_o

- Operating Winding

R

_o

-

Linear Resistor

Summation Current Transformer Output (1)

a

b

c

l

m

n

output

Summation Transformer

Sensitivity for Different Faults (1)

Let output for operation = K

(1)

Consider A-E fault

for relay operation :

I

_A

(1 + 1 + 3) > K

Output for

operation = K

1

1

3

I

_A

I

_B

I

_C

I

_N

(2)

B-E fault

for relay operation :

I

_B

(1 + 3) > K

I

_B

> 25%K

(3)

C-E fault

for relay operation :

I

_C

x (3) > K

I

_C

> 33

1/3

_%K

(4)

AB fault

for relay operation :

I

_AB

x (1) > K

I

_AB

> 100%K

(5)

BC fault

for relay operation :

I

_BC

x (1) > K

I

_BC

> 100%K

(6)

AC fault

for relay operation :

I

_AC

(1 + 1) > K

I

_AC

> 50%K

Summation Transformer

Sensitivity for Different Faults (2)

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

33

1/3

_{% K}

_{22% In}

AB

100% K

90% In

BC

100% K

90% In

CA

50% K

45% In

3 Phase

57.7% K

52% In

Fault Settings for Plain Feeders

Input transformer summation ratio is 1.25 : 1 : N

where N = 3 for normal use

and N = 6 to give low earth fault settings

Fault

Settings

N = 3

N = 6

A-N

0.19 x Ks x In

0.12 x Ks x In

B-N

0.25 x Ks x In

0.14 x Ks x In

C-N

0.33 x Ks x In

0.17 x Ks x In

A-B

0.80 x Ks x In

B-C

1.00 x Ks x In

C-A

0.44 x Ks x In

A-B-C

0.51 x Ks x In

Ks is a setting multiplier, variable from 0.5 to 2.0

In is the relay rated current 1 Amp or 5 Amps

Selection of Ks & N

Values of Ks and N are chosen such that I

_S

(C - N) < 0.3 x min. E/F current.

For solidly earthed systems

:-I

S

(A - N) > 3.2 x steady state line charging current.

For resistanced earthed systems with one relay per phase

:-I

_S

(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 required primary sensitivity.

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.

Padding resistance R set to ½ (1000 - R

_p

) ohms

R

_p

/2

R

_R

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.}

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

Formula for Induced Voltage

e = 0.232 I L Log

₁₀

D

_e

/S

where

_I

=

primary line E/F current

L

=

length of pilots in miles

D

_e

=

Equiv. Depth of earth return in metres = 655 . √

√√√e/f

e

=

soil resistivity in Ω

Ω

Ω.m

Ω

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

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

Relay

Input

5kV

15kV

2kV

_5kV

Relay

Circuit

Pilot

Terminal

Pilot

Wire

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

Pilot Wire Supervision

Circulating

Balanced

Current

Voltage

Schemes

Schemes

Pilot Wire

Maloperate

Stable

Open Circuited

Pilot Wire

Stable

Maloperate

Short Circuited

Pilot Wire

Maloperate

Maloperate

Crossed

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.

Pilot Wire Supervision Relay SJA

PILOT

Cross Pilot

Detector Box

B

Unbalance

Detector

Circuit

A

Supervision

Supply

MRTP Features

_{Detects open circuit, short circuit or crossed pilots.}

_{Gives indication of loss of supervision supply.}

Connections for Pilot Supervision (5 kV)

PILOTS

A1

A3

A2

LVAC

AC

A1

A3

A2

Overcurrent Check Relays (1)

50

G

50

C

50

A

A

B

C

PILOT

WIRE

RELAY

(87PW)

Overcurrent Check Relays (2)

+

I

_soc

> I

_fl

0.9

I

_sef

> 1.2 I

_Z

I

_sef

< 0.8 x I

_ef

87PW-1

50A-1

TRIP CIRCUITS

50C-1

50G-1

System Requiring Intertripping

Source

Feeder

Protection

Busbar

Protection

Destabilising Relay MVTW01

P6

17

P7

PILOTS

MBCI

V x (1) +

V x (2) +

V x (3) +

-UN-1

17

18

UN-2

18

19

19

UN-3

UN

3

20

I1

I2

I3

I4

S2

S1

MVTW01

MiCOM P521

Numerical Current Differential

Protection Relay

November 2002

End A

Communication Link

End B

A

I

I

F

I

_B

I

_A

+ I

_B

= 0 Healthy

I

_A

+ I

_B

≠ 0 (= I

_F

) Fault

Digital messages

0 I I I I I I 0 I 0 . . . 0 I 0 I I I I I I

0

A/

D

µ

µ

µ

µP

Digital communication interface

(electrical or fibre)

End A

End B

All Digital/Numerical Design

Current Differential

-Advantages

_{No voltage transformers needed}

_{Detect very high resistance faults}

_{Uniform trip time}

_{Clearly defined zone of operation}

MiCOM P521

Current Differential

-Signalling Options

_{Electrical communications}

EIA485 (direct or via PZ511 interface)

EIA232 / EIA485 Modems (requires single twisted pair)

_{Direct fibre optic}

850 nm multi-mode

1300 nm multi-mode

1300 nm single mode

Direct 4 Wire EIA485 Connection

Tx

1.2km max

R

x

MT RS485

MT RS485

2 Screened

Twisted Pairs

T

x

64kbps

Rx

Surge

Protection

4 Wire EIA485 Up To 10km

Tx

10km max

R

x

PZ511

Interface

2 Screened

Twisted Pairs

T

x

19.2kbps

Rx

NOTE:

EIA 485

PZ511

Interface

Pilot Wire Communications (1)

Tx

T

x

10km max

Rx

R

x

Leased

Line

Modem

Leased

Line

Modem

Twiste

d Pair

(Pilot

Cable)

19.2kbp

s

10/ 20kV isolation transformers available if required (2 required per

scheme)

NOTE:

EIA 485 or EIA

232

Pilot Wire Communications (2)

Tx

T

x

10km max

Rx

EIA 485

R

x

MDSL

Modem

_Twiste

_Modem

MDSL

d Pair

(Pilot

Cable)

64kbps

Same as

Fibre..!!

Condition Line Communications

Tx

T

x

No strict limits

Rx

R

x

Dial-up

Modem

_Modem

Dial-up

Conditioned

Telephone

Line

9.6 kbps

EIA 485 or EIA

232

OPGW

T

x

R

x

End A

R

x

T

x

End B

CH1

Communications Path for

Fibre Optic Application

Short Haul

Medium Haul

Key:

*

3dB allowance for joint loss/ageing

Optical Budgets for Direct Optical Connection

Between Relays

850nm Multi

Mode

1300nm Multi

Mode

1300nm Single

Mode

Min. Transmit

Output Level

-19.8dBm

-8.2dBm

-8.2dBm

Receiver

Sensitivity

-25.4dBm

-38.2dBm

-38.2dBm

Optical Budget

5.6dB

30.0dB

30.0dB

Less Safety

Margin (3dB)

2.6dB

27.0dB

27.0dB

Typical Cable

Loss

2.6dB/km

0.8dB/km

0.4dB/km

Max

Transmission.

Distance

1km

30km

60km

*

Interfacing to Multiplexers

Multiplexer

G.703, X21

or V.35

electrical

P591/2/3

interface

unit

850nm

multimode

optical fibre

Multiplexed Optical Link

34 Mbit/s

Multiplexer

Multiplexer

64k

bits/s

Earth wire optical fibre

Telephone

Telecontrol

Teleprotection

P521 current

differential protection

End A

End B

Multiplexed Microwave Link

Multiplexer

Multiplexer

64k

bits/s

Telephone

Telecontrol

Teleprotection

End A

End B

Propagation Delay Compensation

_{Synchronise sampling in both relays}

Direct comparison of samples

IRIG-B a possibility, but not always available

(= protection out of service)

_{Asynchronous sampling}

Continual time difference measurement

Current at B

Current received from A

Propagation delay

Relay A

Relay B

tA1

Data me

_ssage

Relays

B

A

Current

vectors

tA

1

tA2

tA3

tA4

tA5

tB1

tB2

tB3

tB4

tB5

tB

_*

tp1

Propagation Delay Time

Measurement - 1

Measured sampling time

tB3 = (tA - tp2)

_*

_*

Propagation delay time

tp1 = tp2 = 1/2 (tA - tA1) - td

_*

Data m

essage

tB1

tB2

tB3

tB4

tB5

tB

_*

tA1

tA2

tA3

tA4

tp1

tA5

Current

vectors

tA

1

tB3

_*

tA

_*

Curre

nt

vecto

rs

tB

3

tA

1

td

tp2

Propagation Delay Time

Measurement - 2

I (tA4)

I (tB3 )

*

θ

∆

θ

∆ θ = ω ∆

t

∆t = (tA4 - tB3 )

_*

If I (tB3 ) = Is + j Ic

_*

= I cosθ + j I sinθ

then I (tA4) = I (tB3 ) . (cos

∆ θ + j sin

∆ θ)

= I cos (θ + ∆ θ) + j I sin

*

Time Alignment of

Current Vectors

_{Dual slope bias characteristic}

_{Selectable operating time / characteristic}

Allows grading with tapped off fuse protected loads

Allows smaller CT’s to be used

_{Operating times when set to instantaneous:}

Current Differential

Baud rate (kbits/s) Max. Time (ms) Typical Time (ms)

9.6

19.2

56

64

100

80

45

45

90

70

30

30

Differential

current

I =

I + I

diff

A

B

Bias current

bias A B

I = 1/2 ( I + I )

I

_S1

Perce

ntage

bias k

1

I

No trip

IB

IA

Pe

rce

nta

ge

bi

as

k2

S2

Trip

Underground cables

Overhead lines

Line Volts

11kV

400kV

Line Volts

132kV

400kV

30

1.2

A/km

1

0.3

A/km

Line Charging Currents

•Capacitive current is only seen at one end of the line

•To prevent instability set Is1 setting to 2.5x steady state line charging curr

•Capacitive inrush current is rejected by the relay filtering methods

End A

End B

Comms Channel

CT Ratio Correction

600/1

500/1

To correct CT ratio mismatch a correction factor can

be applied to End A.

To maintain good sensitivity, correct to 1

pu:-Correction Factor = = 1.2

Max Load = 500A

0.83A

1.0A

1A

0.83A

Protection of Transformer Feeders

Power transformer

Virtual interposing CT

Vectorial

correction

Ratio

correction

Virtual interposing CT

Stability for Magnetising Inrush Current

Magnetising inrush current flows into the energised winding

at switch on

This current is not represented at the remote end of the line

A method of restraint is required to avoid trips on closure of

the breaker

:

Inrush current is rich in harmonics: 2nd, 5th etc..

Increase bias current by adding a multiple of 2nd harmonic

current = RESTRAINT

Inrush restraint facility can be enabled or disabled via a

dedicated setting

m

Φ

Φ

Φ

Φ

+

Switch on at voltage

zero - No residual flux

m

Φ

Φ

Φ

Φ

-m

Φ

Φ

Φ

Φ

2

Steady state

V

Φ

Φ

Φ

Φ

m

I

V

m

I

Φ

Φ

Φ

Φ

F

I

_{Differential protection can be IDMT or}

DT delayed to discriminate with tapped

feed protection:

Fused spurs

Tee-off transformer in-zone

End

A

End

B

Example MV Application:

Teed Feeder Protection

Direct Intertrip (DIT)

Transformer

Protection

DTT=1

Data Message

Relay A

_{Relay B}

+

-

+

-

_{Example shows interlocked overcurrent protection}

Feeder fault seen within busbar zone

Remote end trip after set delay for PIT & current > Is1

_{Current check can be disabled thus giving a second DIT channel}

Permissive Intertrip (PIT)

Busbar

Relay

PIT=1

Data

Message

Relay

A

Relay

B

F

IB

+

-

+