1. Zoran Gajić ABB AB Vasteras, Sweden
Multi-terminal Line Differential Protection Installed on Series Compensated, 400kV Line with Five-Ends
Zoran Gajić
ABB AB
Vasteras, Sweden
Authors:
Z. Gajić, ABB Sweden
I. Brnčić, ABB Sweden
F. Rios, Svenska Kraftnät

2. 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

3. Application Description
The following data are valid for this application:
Line three-phase reactive power generation 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.

4. Communication Setup
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 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).

5. Overall System Setup

6. 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

7. 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

8. Problem of Charging Current for Long Lines
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. 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

10. 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. 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. 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. Charging Current Compensation Principle
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. 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. Internal Fault No 1; Recorded Diff Currents
Instantaneous Diff Currents
Present before the fault
Diff RMS Quantities
Zero before the fault

16. Internal Fault No 2; Recorded Diff Currents
Instantaneous Diff Currents
Present before the fault
Diff RMS Quantities
Zero before the fault

17. Internal Fault No 3 Summer 09; Recorded Diff Currents
Instantaneous Diff Currents
Present before the fault
Diff RMS Quantities
Zero before the fault

18. Conclusion
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

19. СПАСИБО
ЗА ВНИМАНИЕ!
THANK YOU!