1

©A

BB

AB

, 20

08

2008-02-05

Multi-terminal Line Differential Protection Installed on Series Compensated, 400kV Line with Five-Ends

Zoran Gajić

ABB AB

Vasteras, Sweden

Authors:Z. Gajić, ABB SwedenI. Brnčić, ABB Sweden F. Rios, Svenska Kraftnät

2

©A

BB

AB

, 20

08

2008-02-05

Application Description

The following data are valid for this application:

Positive sequence line impedance

Zero sequence line impedance

Total series capacitor reactance

this corresponds to the impedance of 400kV line with length of 265km

~211km

~92km

~74km~0.1km

400kV Station #1

400kV Station #2

400kV Station #3

-jXc

(0.018 0.275)j km

(0.26 0.982)j km

73j

3

©A

BB

AB

, 20

08

2008-02-05

Application Description

The following data are valid for this application:

Line three-phase reactive power generation 657.5 kVAr/km at 400kV (circa 350A per phase of charging current at 400kV over the whole length of the protected line). This corresponds to phase to ground fault with resistance of 650 Ohms primary.

Main CT involved in this scheme have ratio:

2000/2 in Station #1, 3000/1 in Station #2 2400/2 in Station #3.

~211km

~92km

~74km~0.1km

400kV Station #1

400kV Station #2

400kV Station #3

-jXc

4

©A

BB

AB

, 20

08

2008-02-05

For this installation master-master differential protection principle is used (i.e. every differential relay had all five currents available and is able to perform the differential protection algorithm).

Distance protection is included in each differential relay in order to provide reserve protection for the line.

The line differential protection scheme uses a telecommunication SDH/PDH network with unspecified route switching. Therefore differential relays utilize the GPS for the time synchronization. In the substations there are 16 x G.703 64 kbit/s channels. The following SDH/PDH configuration is used:

SDH multiplexing (STM-1 or STM-4)à 8 x 2 Mbit/s (E1) à PDH multiplexing à 16 x 64 kbit/s (E0).

Communication Setup

5

©A

BB

AB

, 20

08

2008-02-05

Overall System Setup

SDH/PDH network

~211km

~92km

~74km~0.1km

400kV Station #1

400kV Station #2

400kV Station #3

-jXc

6

©A

BB

AB

, 20

08

2008-02-05

Differential Operating Characteristic

Differential current (Operate current) is the vectorial sum of measured currents at all line ends and it is calculated separately for each phase

Bias current (Restrain current) is considered as the greatest phase current in any line end and it is common for all three phases

Dual slope characteristic is used

Unrestraint differential level is available

IdMinHigh used during initial line energizing

Section 1

UnrestrainedLimit

Section 2 Section 3

Restrain

Operate

unconditionally 5

4

3

2

1

0

0 1 2 3 4 5

IdMin

EndSection1

EndSection2

Restrain current[ in pu of IBase]

Operate current[ in pu of IBase]

SlopeSection2

SlopeSection3

Operate

conditionallyIdMinHigh

AC

B

7

©A

BB

AB

, 20

08

2008-02-05

External/Internal Fault Discriminator

Based on theory of symmetrical components

It utilize the negative sequence current component from all ends of the protected line

Directional comparison principle is applied

Negative sequence current component from the local end is compared with the sum of the negative sequence current components from all remote ends

When these two phasors are in phase fault is internal

When these two phasors are in contra-phase fault is external

NegSeqROA (Relay

Operate Angle)

0 deg180 deg

90 deg

270 deg

120 deg

IMinNegSeq

If one or the other of currents is too low, then no measurement is done, and 120 degrees is mapped

External fault region

Internal fault region

Internal/external fault boundary

8

©A

BB

AB

, 20

08

2008-02-05

Problem of Charging Current for Long Lines

R

UB

I

C2

C2

ICBICAUA

CT CTL

Capacitance of the protected line causes false differential current to be measured by IED

Π – equivalent circuit can be used to estimate the charging current if voltage measurement on both end of the line is available

Charging currents are symmetrical (i.e. they exist mostly as positive sequence current component)

9

©A

BB

AB

, 20

08

2008-02-05

Problem of Charging Current for Long Lines

By using voltage derivative charging current can be estimated It is only APROXIMATE method because the Π – equivalent circuit

is too simplified and do not represent properly the line especially during transient conditions

Problem for more complex line configurations: Series compensation Multi-terminal lines HV cables

+

-

S Fourierfilter

Communicationunit

Differentialprotectionalgorithm

Trip

dUdt

CIcc

U

I

10

©A

BB

AB

, 20

08

2008-02-05

Why Charging Current is Required?

To make line differential protection sensitive for high resistance faults

Is there any more general method which will work for independently from the primary system set-up?

11

©A

BB

AB

, 20

08

2008-02-05

Assumption for New Charging Current Algorithm

During high resistance faults voltage drop will be quite small therefore the charging current just before and after the fault inception will be approximately the same

For heavy external or internal faults influence of charging current will be practically negligible

12

©A

BB

AB

, 20

08

2008-02-05

New Charging Current Algorithm

Differential function learns the false symmetrical differential current over a period of time (i.e. it consider all three phases simultaneously)

Once this value is learned it is simply subtracted from the presently measured RMS currents in every phase

Any change in measured currents freezes charging current estimation Algorithm is adaptive and does not need to know anything about primary

system setup, however it needs some short time before it becomes effective

Voltage measurement is not required Only end user setting is to enable or disable it (i.e. On/Off)

13

©A

BB

AB

, 20

08

2008-02-05

Charging Current Compensation Principle

0 20 40 60 80 100 120 140 160 180-100

-50

0

50

100

time since start in ms

Inst

anta

neou

s di

ff.

curr

.

Instantaneous differential currents, and fundamental frequency diff. currents

instDifCurr L1

instDifCurr L2

instDifCurr L3

0 20 40 60 80 100 120 140 160 1800

20

40

60

time since start in ms

Fun

d. f

req.

diff

. cu

rr.

difCurr L1

difCurr L2

difCurr L3

L1 L2 L3function ON

differentialfunctionswitchedON

Currents low,symmetricaland not changing

DFFtransients

Charging currentscompensated in approx. 100 ms

Compensationstarts step bystep

Note that only RMS differential current are compensated, not the instantaneous differential current values

Both of them are available from IED as service value and to the built-in disturbance recorder

14

©A

BB

AB

, 20

08

2008-02-05

Recorded Internal Faults

Two captured recordings in this installation are presented in the paper.

Both recordings were captured by line differential protection IED installed in Station #3.

The first recording is an internal L2-L3-Gnd fault which was caused by lightning. The fault location was estimated to be 99km from Station #1 and estimated fault resistance was around 24 Ohms primary.

The second recording is an internal L2-Gnd fault. The fault location was estimated to be 7km from Station #1 and the fault resistance was estimated to be around 8 Ohms primary. The cause of this fault is unknown.

Differential protection has operated properly for both internal faults.

15

©A

BB

AB

, 20

08

2008-02-05

Internal Fault No 1; Recorded Diff Currents

L1-DIFF L2-DIFF L3-DIFF t/s-0,075 -0,050 -0,025 0,000 0,025 0,050 0,075 0,100 0,125one-10000-500005000

Instantaneous Diff Currents

BIAS IDIFF L1 RMS IDIFF L2 RMS IDIFF L3 RMS NEG SEQ DIFF t/s-0,075 -0,050 -0,025 0,000 0,025 0,050 0,075 0,100 0,125I/kA0246

Diff RMS Quantities

Present before the fault

Zero before the fault

16

©A

BB

AB

, 20

08

2008-02-05

Internal Fault No 2; Recorded Diff Currents

BIAS IDIFF L1 RMS IDIFF L2 RMS IDIFF L3 RMS NEG SEQ DIFF t/s-0,075 -0,050 -0,025 0,000 0,025 0,050 0,075I/kA0,02,55,0

L1-DIFF L2-DIFF L3-DIFF t/s-0,075 -0,050 -0,025 0,000 0,025 0,050 0,075one-10000-500005000

Diff RMS Quantities

Instantaneous Diff Currents

Present before the fault

Zero before the fault

17

©A

BB

AB

, 20

08

2008-02-05

L1-DIFF L2-DIFF L3-DIFF t/s-0,08 -0,06 -0,04 -0,02 0,00 0,02 0,04 0,06 0,08one-500005000

BIAS IDIFF L1 RMS IDIFF L2 RMS IDIFF L3 RMS t/s-0,08 -0,06 -0,04 -0,02 0,00 0,02 0,04 0,06 0,08I/kA024

Internal Fault No 3 Summer 09; Recorded Diff Currents

Diff RMS Quantities

Instantaneous Diff Currents

Present before the fault

Zero before the fault

18

©A

BB

AB

, 20

08

2008-02-05

The proposed charging current compensation method, independent from voltage measurements, seems to work very well for such long, series-compensated overhead line configurations.

It has been shown that the multi-terminal line differential protection is a good solution for protection of long, series-compensated, high-voltage lines with more than two ends.

Combination of multi-terminal line differential protection and distance protection provides good protection solution for such lines.

Differential protection is in service for almost two years with correct behavior

Conclusion

19

©A

BB

AB

, 20

08

2008-02-05

СПАСИБО

ЗА ВНИМАНИЕ!

THANK YOU!