MULTI-TERMINAL LINE DIFFERENTIAL PROTECTION WITH INNOVATIVE CHARGING CURRENT COMPENSATION ALGORITHM Z. Gajić*, I. Brnčić*, F. Rios¤ *
ABB AB; SA Products, Sweden;
[email protected] ¤ Svenska Kraftnät, Sweden;
[email protected]
Keywords: Line differential, Charging current compensation.
Abstract Multi-terminal lines are more often used in modern power systems than before. They are becoming particularly popular in sub-transmission networks (i.e. in networks with rated voltages between 60kV - 160kV) but also in HV networks [3, 4]. Practical experience with such protection from a commercial installation in Sweden will be presented.
1 Introduction In Swedish 400kV national grid Station 2 has been built and existing series compensated 400kV overhead line previously connected between two stations was converted into line with five-ends, as shown in Figure 1. 400kV Station #1
The following data are valid for this line application: positive sequence impedance (0.018+j·0.275)Ω/km zero sequence line impedance (0.26+j·0.982)Ω/km three-phase reactive power generation is 657.5kVAr/km at 400kV (circa 350A per phase of charging current at 400kV over the whole length of the protected line) series capacitor reactance max value is -j·73Ω and it is variable due to thyristor control and switching facilities CT involved in this scheme have ratio 2000/2 in Station #1, 3000/1 in Station #2 and 2400/2 in Station #3. For this installation master-master differential protection principle is used (i.e. every differential relay have all five currents available and it is able to perform the differential protection algorithm). Distance protection is used in each differential relay providing 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 used SDH/PDH configuration is: SDH multiplexing (STM-1 or STM-4)à 8 x 2 Mbit/s (E1) à PDH multiplexing à 16 x 64 kbit/s (E0).
2 Description of the line differential function
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Figure 1: Five-end, series compensated, 400kV OHL
The installed protection is a multi-terminal line differential relay [1] consisting of a traditional unrestrained/restrained differential function in combination with an internal/external fault discriminator. The restrained differential function has a dual biased slope characteristic according to Figure 2. The function is phase segregated except for the case when a power transformer is included in the protected zone. The differential current (Operate current) is the vectorial sum of all measured currents taken separately for each phase and the bias current (Restrain current) is considered as the greatest phase current in any line end and it is common for all three phases. If a fundamental frequency differential current is above the restrain characteristic a start signal is issued for that phase. The instantaneous differential current of the phase is analyzed regarding the 2nd and 5th harmonic contents.
Figure 2: Line differential protection characteristic If the function has started and the content of these harmonics are below defined levels the function will trip. Otherwise the function will be restrained from operation as long as the harmonics are above the preset levels. The blocking affects the phase where a high level of harmonics has been detected. However with the cross-blocking feature enabled, the 2nd and the 5th harmonic presence in one phase will also block the differential function operation for the other phases. There is also an unrestrained differential function without any stabilization from the 2nd and the 5th harmonic. An additional fault position discriminator distinguishes between internal and external faults and is based on an analysis of the negative sequence current component at all ends of the protected circuit. It works in such a way that the phase angle of the negative sequence current component from the local end is compared with the phase angle of the sum of the negative sequence current components from the remote ends. The characteristic for this fault discriminator is shown in Figure 3, where the directional characteristic is defined by the two setting parameters IminNegSeq and NegSeqROA [1].
Figure 3: Operating characteristic of the internal/external fault discriminator
The reference direction of currents is considered to be towards the line. Thus, with this reference direction used at all ends, the phase angle difference between two phasors shall ideally be zero degree when an internal fault can be suspected. In the opposite case, when one current is entering and the other is leaving the protected object, the phase difference will ideally be 180 degree out of phase and an external fault can be expected. In case either the local or the sum of the remote negative sequence currents, or both, is below the set minimum current level, IminNegSeq, the fault discriminator will not make any fault classification and the value 120 degree is set. This value is an indication that negative sequence directional comparison has not been possible to do and the classification is neither internal fault nor external fault. When a fault is classified as internal, a trip is issued under the condition that the dual slope restrained function has started. In most cases the harmonic blocking is overridden. A classification as external fault results in an increase of the restrained characteristic trip values from IdMin to IdMinHigh.
3 Charging current compensation The normal charging currents of overhead lines and cables are capacitive and of the positive-sequence nature. They are related and proportional to the distributed line capacitances between phases, and between each phase conductor and earth. For long high-voltage power lines and particularly HV cables such charging currents may have relatively big magnitude. Such charging current are seen as false differential currents by the line differential protection. In case of a line with two ends, it is possible to compensate for these currents by measuring voltage at both line ends and assuming a Π-equivalent circuit for the protected line [2]. However for a multi-terminal line such type of compensation would be quite complicated and it would require exact description of the line topology. Here a new approach is proposed which is not dependent on the voltage measurement and it can be used for arbitrary multi-end line configuration. Simply the line differential protection learns the amount of symmetrical false differential current value over the time and subtracts it from the presently measured fundamental frequency, RMS differential currents. By doing so the sensitivity of the line differential protection is increased for the high resistance internal fault, while the operation of the line differential protection is not much disturbed for all types of external faults and heavy internal faults. Note that only the symmetrical pre-fault charging currents are subtracted from the fundamental frequency differential currents in a phasewise manner. The simplified algorithm description can be summarized as follows. As long as no disturbance has been detected, false fundamental frequency differential current magnitudes are stored over a period of last five cycles in relay internal memory. Then the average value over the oldest three stored cycles is calculated and used as charging current magnitude. Values of this charging current magnitude is not updated under faults, or under disturbed operating conditions. The updating process is resumed five cycles after normal through-
load conditions have been restored and at the beginning it will be done in couple of steps. Main principles of this algorithm are shown in Figure 4. The estimated value of the charging current is available in primary amperes as a service values from the multi-terminal differential protection function [1]. By continuously applying such charging current compensation method the RMS value of the fundamental frequency differential currents in all three phases will practically be zero or very close to zero during normal through-load condition. Thus the calculated fundamental frequency differential currents during the fault condition will not be affected by the line charging current. Note that this is an approximate method. However it is a very efficient way to increase the sensitivity of the line differential protection for high resistive internal faults which is the prime task of the charging current compensation algorithm. As shown in this paper such approach even works for long high-voltage overhead lines with variable series compensation. function ON
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Figure 4: Charging current compensation algorithm operating principles
4 Operational experience in the field The multiterminal line differential protection has been in service on this line for more than two years. Both external and internal faults have happened and protection has behaved correctly. Three internal faults captured in this installation will be presented here. All 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. The third recording is an internal L2-Gnd fault. The fault location was estimated to be 190km from Station #3. The cause of this fault is also unknown. Differential protection has operated properly for all internal faults. Note that on all presented traces differential protection trip instant is shown as vertical line at t=0,000 s (i.e. triggering signal for the disturbance). In
figures 5, 6 & 7 the following traces captured by the line differential protection IED installed in Station #3 are shown: a) Phase to ground voltage waveforms in Station #3 in kV b) Three-phase current waveforms in Station #3 in kA c) Bias current RMS value, three-phase differential current RMS values and negative sequence differential current RMS value in kA as calculated by the IED d) Three-phase instantaneous differential current waveforms in primary amperes as calculated by the IED From Figure 5 it is obvious that the first fault was L2-L3-Gnd but that its location is remote from Station #3. Note that instantaneous differential currents are present before the fault with equivalent magnitude of approximately 380A in all three phases as shown in Figure 5d, while the RMS differential currents are practically zero, as shown in Figure 5c, due to charging current compensation method previously described. Note also that the RMS differential current for non-faulty phase remains zero during the fault. The voltage RMS value before the fault was 410kV at Station #3. Figure 6 shows that the second fault was L2-Gnd but that its location is remote from Station #3. It is interesting to notice that instantaneous differential currents are present before the fault with equivalent magnitude of approximately 380A in all three phases as shown in Figure 6d, while the RMS differential currents are practically zero, as shown in Figure 6c, due to charging current compensation method previously described. Note that the RMS differential currents for nonfaulty phases remain zero during the fault. The voltage RMS value before the fault was 409kV at Station #3. From Figure 7 it is obvious that the third fault was L2-Gnd but that its location is also quite remote from Station #3. Instantaneous differential currents are present before the fault with equivalent magnitude of approximately 385A in all three phases as shown in Figure 7d, while the RMS differential currents are practically zero, as shown in Figure 7c, due to charging current compensation method previously described. Note that the RMS differential currents for non-faulty phases remain zero during the fault. The voltage RMS value before the fault was 414kV at Station #3 (see Figure 7a). Finally the behaviour of the charging current algorithm during normal load condition is shown in Figure 8. This recording was captured by IED installed in Station #2. Note that instantaneous differential currents are present with equivalent magnitude of approximately 385A primary in all three phases while the RMS differential currents calculated by IED [1] are less than 15A primary, as shown in Figure 8c and 8d. Current in Station #3 had RMS value of 685A and voltage in station #2 was 426kV at that point in time (see Figure 8a and 8b).
5 Conclusion In this paper 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. The innovating charging current compensation method, independent from voltage measurements, seems to work very well for such long overhead line configurations. Combination of differential protection and distance protection provides good protection solution for such applications.
References [1] ABB Document 1MRK 505 186-UEN, "Application manual, Line differential protection IED RED 670", Product version: 1.1, ABB Power Technologies AB, Västerås, Sweden, Issued: March 2007 [2] ABB Document 1MRK 506 081-UEN, "Technical reference manual, Line differential and distance protection Terminal REL 561", Product version: 2.3, ABB Automation Products AB, Västerås, Sweden, Issued: 2001
[3] Z. Gajić, I. Brnčić, T. Einarsson, B. Ludqvist: “Practical Experience from Multiterminal Line Differential Protection Installations”, International Conference on Relay Protection and Substation Automation of Modern EHV Power Systems, Moscow – Cheboksary, Russia, September 2007. [4] S. Holst, I. Brnčić, D. Shearer, R. Mangelred, K. Koreman: “Problems and Solutions for AC Transmission Line Protection under Extreme Conditions caused by Very Long HVDC Cables”, Study Committee B5 Colloquium, Madrid, Spain, October 2007
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Figure 5: First internal fault (L2-L3-Gnd) located 99km from Station #1
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