METHOD AND DEVICE FOR PREVENTING UNDERSTABILIZATION OF LONGITUDINAL DIFFERENTIAL PROTECTION IN CASE OF EXTERNAL FAULT AND CURRENT TRANSFORMER SATURATION

Abstract: A method and a device for preventing understabilization of longitudinal differential protections in case of external faults and current transformer saturation. The invention comprises an extension of the state of the art with regard to stabilization of longitudinal differential protections in such a way that the values (A1, A2, ... An), ( phi 1, phi 2, ... phi n), obtained via current measurement (C1, C2, ... Cn) and Fourier filters (4, 5, 6), for determining the function characteristic of the protection under certain conditions as regards current amplitude and current transformer saturation are given corrected values (A1k, A2k, ... Ank), ( phi 1k, phi 2k, ... phi nk).

Description: Method and device for preventing understabilization of longitudinal differential protection in case of external fault and current transformer saturation TECHNICAL FIELD A differential protection is a type of protection used for preventing damage which may arise in connection with inter¬ nal faults on busbars, generators, motors, transformers, power lines, etc. The requirements on the protections are somewhat different depending on the object to be protected. An unwanted protective function must be prevented and this can be done by stabilizing the protections. This means that attempts are made to make the protections unsusceptible to certain internal and external electrical transients and to external electrical faults. It is necessary, for example, to prevent that inrush current surges, overmagnetization currents, continuous currents, saturation of current transformers, etc., lead to an unwanted protective function. However, there is always a certain risk that the stabilization measures are insufficient in the case of external faults. The present invention relates to a special stabilizing problem which may arise in a longitudinal differential protection when current transformers included in the protection become saturated owing to an external fault, that is, when the protection is to be stabilized so as not to enter into operation. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows, in principle, how, according to the state of the art, a differential protection changes its degree of stabilization when current transformer saturation occurs. Figure 2 shows how, according to the invention, the basis for determining the current differential and stabilizing currents is changed when current transformer saturation occurs. BACKGROUND ART, THE PROBLEM The stabilization measures for longitudinal differential protections for different fault conditions have been described in a large number of patent specifications. A relatively comprehensive description of the prior art is given in patent application SE 9200406-8. From this it is clear, inter alia, that known problems in connection with differential protections are that they exhibit deficiencies with respect to stability for external faults and for networks with a large short-circuit power. In addition, false difference currents may arise in case of external faults because some of the current transformers becomes saturated. However, current transformer saturation may be detected in accordance with several known methods, see, for example, SE 9100917-5. Information that saturation has occurred may be used for stabilization of a differential protection, which, however, entails a certain risk of both overstabilization in case of an internal fault and under- stabilization in case of an external fault. Differential protections are often designed such that two different circuits can be distinguished, namely, a differen¬ tial circuit and a stabilizing circuit. The protection functions such that the difference current ID, which upon an internal fault arises between incoming and outgoing currents, can be separated with the aid of the differential circuit. The current which is used to stabilize the protection, Is, in some way emanates from the stabilizing circuit and it may be different for different protections, depending on what is primarily to be stabilized against and on the technique used as well as the design of the protection in other respects. For example, the stabilizing current may be a reflection of the incoming current, obtained by means of a current trans- former, for busbars, for example, the sum of reflected incoming currents, any of its components, or the like. The function characteristic of a differential protection constitutes a graphic representation of the differential current as a function of stabilizing current, stabilizing degree S and a function margin K, which is often expressed as I D = S-Is + K To prevent operation of the longitudinal differential protection when saturation occurs and when saturation has been detected, the stabilizing degree is changed to a higher value such that a higher differential current is needed to obtain operation. The above-mentioned method has its deficiencies, in particular if only one of the current transformers has become saturated. This is due to the fact that in such a case a large measured differential current may be obtained which may exceed the level of stabilization according to the function characteristic which is obtained in the case of a saturated transformer. This leads to a situation with understabilization of the longitudinal differential protection. A problem which arises in this connection is to effect a suitable stabilizing signal. Admittedly, it is possible to determine in various ways whether a current transformer as become saturated. However, to achieve an optimal stabili¬ zation, that is, neither overstabilization nor understabili¬ zation of the longitudinal differential protection in case of an external fault which entails current transformer saturation, it would be desirable to obtain a signal which is in some way connected to the degree of saturation. One way of obtaining a relevant signal is disclosed in patent application SE 9200011-6, in which, with the aid of discrete-time measurements of the secondary current of a current transformer, both the primary current and the secondary current are reconstructed upon saturation via a signal model and a current transformer model. This solution, however, requires access to a computer with a very high calculation capacity. By an extension, according to the invention, of the prior art described, it is, however, possible by relatively simple means to achieve a considerable reduction of the risk of understabilization in the case of current transformer saturation. To be able to describe the invention, a somewhat more detailed description of the state of the art is needed than the summary description above where it is stated that, when saturation is indicated, an increase of the stabilizing degree takes place. Therefore, the state of the art will now be described with reference to Figure 1. A differential protection is associated with a number, n, of terminals with current measurement in each phase. To simpli¬ fy the description, Figure 1 only shows one current trans¬ former Cl, C2 ... Cn from each terminal. The measured values of the current transformers are converted into sampled digital values in the A/D converters 1, 2 and 3. Via Fourier filters FFl, FF2 and FFn with reference numerals 4, 5 and 6, the fundamental component of each of the measured currents is obtained as a current vector characterized by an amplitude and a phase angle, that is, as Al and φl, A2 and φ2 as well as An and φn. The amplitudes obtained in this way will hereinafter be referred to as "Fourier amplitudes". Since there may be quite a significant geographical variation between the locations of the current transformers, a synchronization of the measured phase angles is normally needed, which takes places with the aid of a global clock 7. The synchronization is performed with the amounts Δφl, Δφ2 and Δφn via the synchronization units 8, 9 and 10. The phase angles φll, φ21 and φnl thus adjusted are supplied together with the Fourier amplitudes and the current stabilizing degree to a decision unit 11. This comprises the previously mentioned circuits for obtaining differential and stabilizing currents and determines the current function characteristic depending on whether or not any of the current transformers exhibits saturation. Information about this is obtained via any of the saturation detectors MCI, MC2 and MCn with reference numerals 12, 13 and 14. If any of these signals saturation, the stabilizing degree is switched, via an OR element 15, from the lower stabilizing degree Si to the higher stabilizing degree S2 with the aid of a contact system 16. When the limits of the function characteristic are exceeded, the necessary protective measures are activated via the contact 36. As mentioned above, the amplitude values which are obtained via the Fourier filtering represent the amplitude of the fundamental component, that is, they are freed from any superimposed harmonics, d.c. components and other noise. In these contexts it is common for the Fourier filtering to be based on some 20 samples per cycle. There are also other methods for amplitude determination of the measured currents. One such method comprises finding out the peak value with the aid of two consecutive sample values for each cycle. Such a method is described, inter alia, in "High-Speed Distance Relaying Using a Digital Computer, Part 1-System Description", IEEE Trans, on Power Apparatus and Systems, Vol-91, No. 3, May/June 1972, pp. 1235-1243 by G.B. Gilchrest, G.D. Rockefeller and E.A. Udren. The peak values which are obtained in this way under normal conditions, that is, before any saturation occurs, are relevant measured values which correspond to the Fourier amplitudes. However, when during saturation or other states, harmonics, d.c. components etc. occur, peak values determined by this method will be influenced. SUMMARY OF THE INVENTION, ADVANTAGES As already mentioned, the invention relates to a method and a device in the form of an extension of the state of the art with regard to stabilization of a longitudinal differential protection during current transformer saturation. When saturation has been indicated in any of the current trans¬ formers included in the protection, the extension comprises, inter alia, under certain conditions as regards measured amplitude values, performing a correction of the amplitude and phase angle of the current vectors obtained via the Fourier filters. The values thus corrected are supplied to the circuits for determining the differential and stabili¬ zing currents prevailing at the time. This will also cause the function characteristic to become corrected. Indepen¬ dently of whether the above-mentioned conditions are ful¬ filled or not, a higher stabilizing degree will be switched in at current transformer saturation, as is the case in the state of the art. According to the invention, in addition to determining the amplitude via the Fourier filters, a parallel amplitude determination takes place with the aid of the above- described method where the peak value is found out with the aid of two consecutive sample values for each cycle. Via holding circuits, the last determined peak values, herein¬ after called "holding amplitudes", are stored consecutively. These values are compared with the respective Fourier amplitudes. If a holding value is somewhat greater than the corresponding Fourier amplitude and if at the same time saturation has been detected, the above-mentioned correction of the amplitude and phase position of the current vector(s) concerned is carried out. The extension described above, that is, the correction of the amplitude and phase angle of the current vector, obtained via the Fourier filters, which is carried out according to the invention, means a simple way of conside- rably reducing the above-mentioned risk of understabili- zation at current transformer saturation in case of an external fault. This can be explained as follows: Owing to the technique used to obtain the Fourier amplitude, depen¬ ding on where during the cycle a saturation occurs, the Fourier amplitude may have a relatively low value even though saturation has been detected. To avoid understabili- zation in such a case, the current vector is corrected by giving the Fourier amplitude a higher value and reducing the phase angle to a lower value. In this way, also the function characteristic will be increased while at the same time the stabilizing degree is increased because of the detection of saturation. The decision as to whether this correction is to be carried out is based on, in addition to saturation being detected, the magnitude of the holding amplitude in relation to the Fourier amplitude. If the holding amplitude is greater than the Fourier amplitude multiplied by a first constant, this is equivalent to the Fourier amplitude having a relatively low value, and then correction of the amplitude and phase angle of the current vector is to take place. If the opposite situation prevails, no correction of amplitude and phase angle is needed although saturation has been detected. In this case it is sufficient to increase the stabilizing degree. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a device according to the invention for preventing understabilization of longitudinal differential protections in case of external faults and current trans- former saturation is shown in Figure 2. Since the invention constitutes an extension of the state of the art with regard to stabilization of longitudinal differential protections when current transformers become saturated, the device and Figure 2 comprise certain parts from Figure 1 which have been described in connection with the description of that figure. The parts shown jointly in Figures 1 and 2 consist of the current transformers Cl, C2 and Cn, the Fourier filters 4, 5 and 6, the global clock 7 for synchronization, the synchronization units 8, 9 and 10, the decision unit 11, comprising, inter alia, the circuits for obtaining differential and stabilizing current, saturation detectors 12, 13 and 14, an OR element 15 and the contact system 16 for switching of the stabilizing degree S. The extension means that a device according to the inven- tion, in addition to the above-mentioned parts, comprises peak value formers Tl, T2 and Tn with reference numerals 17, 18 and 19 for determining the peak values of the currents with the aid of two consecutive samples. The output signals from these are designated All, A21 and Anl. Via holding circuits Hi, H2 and Hn with the reference numerals 20, 21 and 22, the last determined peak values are stored, which according to the above are called holding amplitudes and are designated AllH, A21H and AnlH. The holding amplitudes are each supplied, together with the corresponding Fourier amplitudes, to a comparison means 23, 24 and 25 to decide on the inequalities AllH > a-Al, A21H > a-A2 and AnlH > a-An, where "a" is a first constant greater than 1. Now, if one or more of the inequalities is/are true, this means that a change has taken place with respect to the relationships between the holding amplitudes and the Fourier amplitudes. If, in addition, saturation has been detected, the con¬ clusion can be drawn, as stated under the summary of the invention, that the Fourier amplitude has a relatively low value when saturation has been detected. This then means that both the criteria for correction of the Fourier amplitudes Al, A2 and An and the phase angles φll, φ21 and φnl are fulfilled and that the correction can be initiated via the AND elements 26, 27 and 28. The correction means that the phase angles are reduced by the phase changes Δφll, Δφ21 and Δφnl in the phase angle correction units 30, 31 and 32. The output signals from here constitute the corrected phase angles φlk, φ2k and φnk which are supplied to the decision unit 11. Further, the correction means that the Fourier amplitudes are multiplied by the constants KAl, KA2 and KAn in the amplitude correction units 34, 35 and 36. The output signals from here constitute the corrected Fourier amplitudes Alk, A2k and Ank which are also supplied to the decision unit 11. In the same way as shown in Figure 1, the necessary protective measures are activated via the contact 36 when the limits of the function characteristic are exceeded. The phase changes Δφll, Δφ21 and Δφnl can be set at one and the same value Δφ between 0 and -30 degrees to simplify the design. In similar manner, the constants KAl, KA2 and KAn can also be set at one and the same second constant KA between 1 and 2. The first constant "_a", which is part of the basis for decision whether correction is to take place or not, suitably assumes a value between 1 and 2.

Claims: CLAIMS 1. A method for preventing understabilization of longitu¬ dinal differential protections in case of external faults and current transformer saturation, which method comprises an extension of a method wherein the measured, sampled currents of the longitudinal differential protection, obtained via current transformers (Cl, C2, ... Cn) and A/D converters (1, 2, 3), are filtered by means of Fourier filters (4, 5, 6) for producing the Fourier amplitudes (Al, A2, ... An) and phase angles (φl, φ2, ... φn) of the respective current vectors, and after synchronization the phase angles together with the Fourier amplitudes are used for determining the function characteristic of the longitudinal differential protection, and the sampled currents are also used for detecting whether any current transformer has become saturated and if this is the case, the stabilizing degree (S) included in the function characteristic is increased, the method according to the extension being characterized in that the peak values (All, A21, ... Anl) of the sampled currents are determined with the aid of two consecutive sample values for each cycle and that the holding amplitudes (AllH, A21H, ... AnlH) of the currents are determined as the last determined peak values consecutively stored in holding circuits, and that if one or more of the holding amplitudes is/are greater than the respective Fourier amplitude multiplied by a first constant (a) and if at the same time saturation has been detected, the Fourier amplitude is multiplied by a second constant (KA) into a higher value (Alk, A2k, ... Ank) and the phase angle is reduced by a phase change (Δφ) into a lower value (φlk, φ2k, ... φnk) and that the values of Fourier amplitude and phase angle thus corrected are used for determining a corrected function characteristic. 2. A method for preventing understabilization of longitudi¬ nal differential protections in case of external faults and current transformer saturation according to claim 1 and which is characterized in that the first constant (a) is assumed to have a value between 1 and 2, that the second constant (KA) is assumed to have a value between 1 and 2 and that the phase change (Δφ) is assumed to have a value between 0 and -30 degrees. 3. A device for carrying out the method for preventing understabilization of longitudinal differential protections in case of external faults and current transformer satura- tion according to claim 1, which device is an extension of a design comprising current transformers (Cl, C2, ... Cn) , A/D converters (1, 2, 3) and Fourier filters (4, 5, 6) for determining the Fourier amplitudes (Al, A2, ... An) and phase angles (φl, φ2, ... φn) of measured and sampled currents, a decision unit (11) with circuits for producing the differential and stabilizing current of the longitudinal differential protection as well as the function characteris¬ tic of the protection, saturation detectors (12, 13, 14) which upon a detected saturation via a switching system (16) change the stabilizing degree (S) of the function characte¬ ristic, the extension being characterized in that it comprises peak value formers (17, 18, 19) for determining the peak values (All, A21, ... Anl) of the measured currents with the aid of two consecutive samples, holding circuits (20, 21, 22) for consecutively storing the holding ampli¬ tudes (AllH, A21H, ... AnlH) in the form of the last determined peak values, comparison means (23, 24, 25) for determining the inequalities of the holding amplitudes in relation to the Fourier amplitudes multiplied by a first constant (a) and, if the inequality is true while at the same time current transformer saturation is detected, the Fourier amplitudes are adapted to be multiplied in amplitude correction units (34, 35, 36) by a second constant (KA) and the phase angles are adapted to be reduced in angle correc- tion units (30, 31, 32) by a given phase change (Δφ) and that the corrected Fourier amplitudes (Alk, A2k, ... Ank) and the phase angles (φlk, φ2k, ... φnk) are arranged as input signals to the decision unit. 4. A device for preventing understabilization of longitudi¬ nal differential protections in case of external faults and current transformer saturation according to claim 3, characterized in that the first constant (a) may be i assumed to have a value between 1 and 2, that the second constant (KA) may be assumed to have a value between 1 and 2, and that the phase change (Δφ) may be assumed to have a value between 0 and -30 degrees.

Inventors: SAHA MURARI MOHAN, WENNERLUND PER, SAHA, MURARI, MOHAN, WENNERLUND, PER
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1993
Main classification: H02H1
IPC codes: H02H3