Zoran Gaji ć ABB AB Vasteras, Sweden

1. 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 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.

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.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).

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

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