09_Line Differential Protection

Pilot Wire Differential Protection of Feeders 1

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1




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

2




Overcurrent Check




Intertripping / Destabilising

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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

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System Where Directional O/C Cannot Be Used I1

1

10 (v)

(v)

2 (i) (i) → (v) represents increasing time setting

9 (i)

3 (iv)

8 4 (ii)

(iv)

I2

5 7

6

(ii)

(iii)

(iii)

I1

I1 10

10

I1

I1+I2

I1+I2 4

8

I2

2

8

I1

6

4 must operate before 8 4

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I2

I1+I2

I2 4

I2

I1+I2

4 must operate after 8

4

Use of Pilot Wire Differential Protection 10 (iii) 1 2

9 (i)

8 (ii)

4

3

5 7

(ii)

6

(i)

(iii)


, , ,  are pilot wire differential relays  is non directional O/C relays , , , are directional O/C relays Operating times :- { and } > { and } > { and } > {
, ,  and } 5

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Merz-Price Differential or Unit Protection

Protected Circuit or Plant

R

Boundaries of protection coverage accurately defined Protection responds only to faults in protected zone 6

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End A

End B

Relay

Circulating Current System End A

End B

Relay Balanced Voltage System Basic mertz-price principle applies well where CT secondary circuit can be kept short, protection of transformers, busbars, machines.

7

eg.

For feeder protection where boundaries of protection are a distance apart, a communication channel is required. > Title of presentation - Date - References

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Unit Protection Involving Distance Between Circuit Breakers (1) A

B

Relaying Point

R Trip B

Trip A

Simple Local Differential Protection 8

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Unit Protection Involving Distance Between Circuit Breakers (2) A

B Communication Channel

Relaying Point

Relaying Point R

R

Trip A

Trip B

Unit Protection Involving Distance Between Circuits 9

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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. 10

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Basic Pilot Wire Schemes

B

with Bias (1)

B I

V OP

OP

I

V

Circulating Current 11

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Translay Differential Protection End A

End B

A B C Summation Winding Secondary Winding

Pilot

Bias Loop 12

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MBCI Feeder Protection Circuit Diagram A

A

B

B

C

C

T1

Tr

Tt ØC

13

T1 T2 To Tr Tt

RVO v

T2

T1

Tr PILOT WIRES

To

RS Ts

RPP

RPP

T2

Ro

- Summation Transformer - Auxiliary Transformer - Operating Winding - Restraining Winding - Reference Winding > Title of presentation - Date - References

ØC T t

To

Ro

Ts RVD Ro Rpp Øc

RS RVO v

-

Ts

Auxiliary Winding Non Linear Resistor Linear Resistor Pilots Padding Resistor Phase Comparator

13

Summation Current Transformer Output (1) a b c

l

m

output

n

14

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

IC 3 IN Let output for operation = K (1)

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Consider A-E fault for relay operation :

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IA (1 + 1 + 3) > K IA > 1/5K or 20%K 15

Summation Transformer Sensitivity for Different Faults (2) (2)

(3)

B-E fault for relay operation : C-E fault for relay operation :

IB (1 + 3) > K IB > 25%K IC x (3) > K IC > 331/3%K

(4)

(5)

(6) 16

AB fault for relay operation : BC fault for relay operation : AC fault for relay operation :

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IAB x (1) > K IAB > 100%K IBC x (1) > K IBC > 100%K IAC (1 + 1) > K IAC > 50%K

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17

Type of Fault

Relay Sensitivity

Sensitivity of E/M Pilot Wire Relay

A-E

20% K

22% In

B-E

25% K

28% In

C-E

331/3 % 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

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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 B-N C-N

0.19 x Ks x In 0.25 x Ks x In 0.33 x Ks x In

A-B B-C C-A A-B-C

0.80 1.00 0.44 0.51

x x x x

Ks Ks Ks Ks

x x x x

0.12 x Ks x In 0.14 x Ks x In 0.17 x Ks x In

In In In In

Ks is a setting multiplier, variable from 0.5 to 2.0 In is the relay rated current 1 Amp or 5 Amps 18

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Selection of Ks & N 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) > 3.2 x steady state line charging current. For resistanced 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 required primary sensitivity.

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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 20

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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 :

21




Relay operating coil connected in series with pilot.




Capacitance current therefore tends to cause instability.

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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

22

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short.

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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 Ω.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 23

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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 15kV

5kV Pilot Terminal

Relay Input

Relay Circuit Pilot Wire 2kV

24

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5kV

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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

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Pilot Wire Supervision

Pilot Wire Open Circuited Pilot Wire Short Circuited Pilot Wire Crossed

Circulating Current Schemes

Balanced Voltage Schemes

Maloperate

Stable

Stable

Maloperate

Maloperate

Maloperate

Maloperation occurs even under normal loading conditions if 3-phase setting < ILOAD. Overcurrent check may be used to prevent maloperation. Overcurrent element set above maximum load current.

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Pilot Wire Supervision Relay SJA PILOT

Cross Pilot Detector Box B Unbalance Detector Circuit

A Supervision Supply 27

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MRTP Features

28




Detects open circuit, short circuit or crossed pilots.




Gives indication of loss of supervision supply.

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Connections for Pilot Supervision (5 kV)

A1

A1 PILOTS

LVAC

29

A2

A2

A3

A3

AC

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Overcurrent Check Relays (1)

A B C 50 A

50 C

PILOT WIRE RELAY (87PW)

50 G

30

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Overcurrent Check Relays (2)

50A-1

87PW-1 TRIP CIRCUITS

+ 50C-1

50G-1

31

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Isoc > Ifl 0.9 Isef > 1.2 IZ Isef < 0.8 x Ief

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System Requiring Intertripping

Source Feeder Protection Busbar Protection

32

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Destabilising Relay MVTW01

P6

S2

P7

PILOTS S1

17

MBCI 18 19

17

UN-1

18 19

UN-2 UN-3

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

UN 3

MVTW01

33

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November 2002

MiCOM P521 Numerical Current Differential Protection Relay

34

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Current Differential Principle

End B

End A

IA

IF

IB

Communication Link IA + IB = 0 Healthy IA + IB ≠ 0 (= IF) Fault 35

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All Digital/Numerical Design

0IIIIII0I0.....0I0IIIIII Digital messages 0 End A

A/ D

End B

µP

Comms Channel

Digital communication interface (electrical or fibre) 36

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Current Differential Advantages


No voltage transformers needed
Detect very high resistance faults
Uniform trip time
Clearly defined zone of operation
Simple to set with no coordination problems

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MiCOM P521 Protection Comms

38

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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
Multiplexed communications

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Direct 4 Wire EIA485 Connection 1.2km max

64kbps

Tx MT RS485

Rx

MT RS485

2 Screened Twisted Pairs

R x T x

Surge Protection 40

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4 Wire EIA485 Up To 10km 10km max

19.2kbps

Tx PZ511 Interface

Rx

PZ511 Interface

2 Screened Twisted Pairs

R x T x

EIA 485 41

NOTE:10/ 20kV isolation transformers available if required (4 required per > Title of presentation - Date - References scheme)

41

Pilot Wire Communications (1) 10km max

19.2kbp R Leased s Leased x Line Line Modem Twiste Modem Rx T d Pair (Pilot x Cable) EIA 485 or EIA 232 Tx

42

NOTE:10/ 20kV isolation transformers available if required (2 required per > Title of presentation - Date - References scheme)

42

Pilot Wire Communications (2) 10km max

Tx Rx

Same as Fibre..!! 64kbps

MDSL Modem Twiste

d Pair (Pilot Cable)

R MDSL x Modem

T x

EIA 485 43

NOTE:10/ 20kV isolation transformers available if required (2 required per > Title of presentation - Date - References scheme)

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Condition Line Communications No strict limits

9.6 kbps

Tx Dial-up Modem

Rx

44

R Dial-up x Modem

Conditioned Telephone Line EIA 485 or EIA 232

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T x

44

Direct Optical Fibre Link

OPGW

45

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Communications Path for Fibre Optic Application

T x R x End A

46

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CH1

R x T x End B

46

Optical Budgets for Direct Optical Connection Between Relays 850nmMulti Mode

1300nmMulti Mode

-19.8dBm

-8.2dBm

-8.2dBm

-25.4dBm

-38.2dBm

-38.2dBm

Optical Budget

5.6dB

30.0dB

30.0dB

Less Safety Margin (3dB)*

2.6dB

27.0dB

27.0dB

2.6dB/km

0.8dB/km

0.4dB/km

1km

30km

60km

Min. Transmit Output Level Receiver Sensitivity

Typical Cable Loss Max Transmission. Distance

Short Haul

1300nm Single Mode

Medium Haul

Key: * 3dB allowance for joint loss/ageing 47

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Interfacing to Multiplexers

P591/2/3 interface unit

850nm multimode optical fibre

48

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Multiplexer

G.703, X21 or V.35 electrical

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Multiplexed Optical Link

Earth wire optical fibre

Multiplexer

Multiplexer 34 Mbit/s Telephone

64k bits/s

Telecontrol

End A

End B Teleprotection

P521 current differential protection 49

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Multiplexed Microwave Link

Multiplexer

Multiplexer

Telephone 64k bits/s

Telecontrol

End A

50

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Teleprotection

End B

50

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  Vector transformation in software

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Propagation Delay Problem

Relay A

Relay B Current at B

Current received from A Propagation delay 52

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Propagation Delay Time Measurement - 1

Relays A

tA1 tA2

Data mess Curre age vecto nt rs tA 1 tp1

tA3 tA4 tA5 53

B

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tB1 tB2 tB3

tB

*

tB4 tB5 53

Propagation Delay Time Measurement - 2 Propagation delay time Measured sampling time tp1 = tp2 = 1/2 (tA - tA1) - td tB3 = (tA - tp2)

*

tA1 tA2 tB3

* tA *

54

*

*

Current vectors

tp1

tA5

tB1 tB2 td

tA3 tA4

tA 1

tp2 td A t tB 1 3

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nt e r r u C ors vect ge a s s e Data m

tB3

tB

*

tB4 tB5

54

Time Alignment of Current Vectors I (tA4) θ ∆ θ

I (tB3 )

*

∆t = (tA4 - tB3 ) ∆θ=ω∆ t If then ∆ θ) 55

*

I (tB3 ) = Is + j Ic = I cosθ + j I sinθ I (tA4) = I (tB3 ) . (cos ∆ θ + j sin = I cos (θ + ∆ θ) + j I sin (θ + ∆ θ)

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*

*

55

Current Differential


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:

Baud rate (kbits/s) 9.6 19.2 56 64 56

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Max. Time (ms) 100 80 45 45

Typical Time (ms) 90 70 30 30 56

Current Differential Characteristic IA Differential current I diff

= Trip

I A + IB I S1

IB

g a t n e rc e P

2 k s a i eb

k1 s a i b age t n e c No trip r Pe I S2

Bias current I bias = 1/2 ( IA + I B ) 57

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Line Charging Currents

A/km

A/km

30

1

1.2

0.3 11kV

400kV Line Volts

Underground cables

132kV

400kV Line Volts

Overhead lines

•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

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CT Ratio Correction 500/1

600/1 0.83A

Max Load = 500A

End A

1.0A

Comms Channel

End B

To correct CT ratio mismatch a correction factor can be applied to End A. To maintain good sensitivity, correct to 1 pu:1A Correction Factor = = 1.2 0.83A 59

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Protection of Transformer Feeders

Power transformer

Ratio correction

Vectorial correction Virtual interposing CT

60

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Virtual interposing CT

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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 MiCOM-P540-61 61

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Inrush Current - Theory

+Φm

V

Φ

Im

Steady state - Φm Im

2Φm

Φ V

Switch on at voltage zero - No residual flux

MiCOM-P540-62 62

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Example MV Application: Teed Feeder Protection

End A


Differential protection can be IDMT or DT delayed to discriminate with tapped feed protection:

 

63

IF

End B

Fused spurs Tee-off transformer in-zone

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Direct Intertrip (DIT)

Relay A

Relay B Transformer Protection DTT=1

Data Message

64

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-

+ 64

Permissive Intertrip (PIT) IB F Relay A

Relay B

Busbar Relay

PIT=1

Data Message +




65

+

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 > Title of presentation - Date - References

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