METHOD AND DEVICE FOR PROTECTING A POWER TRAMSMISSION LINE

Abstract: Method and device for range limitation and direct trip­ ping for protection in the case of a fault on a power trans­ mission line located between two stations (P, Q) with a traveling wave model located in one of the stations (P), which travelling wave model, with the said of measured currents and voltages in the station, calculates the voltage distribution at a number of control points along the line. The range of a protection device is indicated as the distance between the measuring station and that control point for which a voltage difference (δu) becomes equal to zero. The voltage difference consists of the difference between the absolute value of a voltage difference (Δu), formed as the sum of a voltage value (U″) calculated with the travelling wave model for the control point at a certain time, and the corresponding voltage value (U′) one half-period earlier and the absolute value of the latter voltage value. When the voltage difference becomes greater than zero, a direct trip­ ping is initiated.

Description: The invention relates to a method and device for range limitation and direct tripping for protection in case of a fault on a power transmission line according to the precharacterising part of Claim 1. Protection devices in power supply and distribution networks normally cope with a plurality of different types of fault. An impedance or distance protection device, for example, often has several properties, in addition to the impedance measurement capacity, which exercise a protective effect. These may be in the form of directional or range properties, and they may also be used as starting means for overcurrent protection, in protection systems with absolute selectivity, as protection against oscillations in networks, etc. The range properties and range limitation of an impedance protection device, according to the prior art, are related to the so-called model impedance which is directly or indirectly included in such protection devices. If their is a load, the impedance of which falls within an impedance range determined with the aid of the model impedance in an R-X plane, it is assumed that a fault has occurred in the network. The range properties are then coupled to the resistance and inductance values of the model impedance, which values can be adjusted individually. This then means that the impedance range which implies a fault on the network may have different shapes. A protection device based on this principle and composed of analog components is described, inter alia, in US-A-3,629,658. This protection device includes, among other things, analog filters, which have their limitation as to selectivity and rapidity. The provision of the desired protective function characteristic, among other things range limits, may be a difficult problem owing the frequency dependence of the components included, etc. EP-A-150814 also describes a protection device based on the same principle but designed with digital components. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 a power transmission line in a faultless state between two stations P and Q which are supplied from two power sources A and B, Figure 2 the measured voltage distribution, or the voltage distribution calculated with a travelling wave model, along the line according to Figure 1, Figure 3 the same power transmission line as in Figure 1 with a fault F having occurred between the stations P and Q, Figure 4 an example of a voltage distribution along the faulty according to Figure 3, Figure 5 a voltage distribution, calculated with a travelling wave model at P, along the line according to Figure 3 when the line is subjected to a fault at F, Figure 6 the same voltage distribution as in Figure 5 with indication of a calculated voltage at a control point D, Figure 7 an embodiment of a device for carrying out the method according to the invention. In the EP-A-164711, a method and a device are described for locating a fault point on a line based on voltage waves emanating from a measuring point towards a fault point and the corresponding waves reflected from the fault point. The method comprises carrying out, at certain regular time intervals, a measurement of the instantaneous values of the current and the voltage at the end point of the line, for example at a station. With the aid of these values and a travelling wave model, the voltage at a number of control points along the line can be calculated. It is the travelling wave model described in the latter application that forms the basis of this invention, i.e. achieving range limitation in an impedance protection device and direct tripping in the case of a fault. Figure 1 shows a power line between two stations or measuring points P and Q. In the case shown the line is powered from two power sources A and B. The line PQ may be conceived to be a transmission line between two networks A and B, a line in a meshed network, or a line which connects a power station A with a power drain. Figure 2 shows the real voltage distribution along the line in a faultless state. E A and E B , respectively, are the emf's of the respective power sources and U' P and U' O , respectively, are the voltage measured at P and the voltage measured at Q, respectively. With the aid of the current and voltage values measured in station P and Q, control voltages or the voltage distribution along the line from P and Q and from Q and P, respectively, can be calculated by means of the travelling wave model described in EP-A 164711. In a faultless state, this provides the same voltage distribution as in Figure 2 within the limits determined by errors in measurement in measured value transformers and by uncertain parameter knowledge. When the line according to Figure 1 is subjected to an internal fault, i.e. a fault between the points P and Q (see Figure 3), a real voltage distribution according to Figure 4 can be obtained, depending on the fault resistance, etc.. The real, measured voltage at P and at Q is now designated U P and U Q , respectively. The value of the voltage at the fault point, i.e. U F , depends on the resistance of the fault point, which also influences U P and U Q . A voltage distribution along the line, calculated with a travelling wave model in station P, is shown in Figure 5. A comparison with Figure 4 shows that the voltage distribution of the model is correct between P and F. It will now be shown that the voltage distribution calculated with the travelling wave model can be utilized, besides for the correct part of the voltage distribution, also for the voltage distribution from F towards and past Q in accordance with this invention for range limitation and direct tripping in the case of a fault on the line. With the aid of a travelling wave model at P, a measure of the voltage in station Q can be obtained, for example as an instantaneous value at a certain time t and the corresponding voltage one half-period earlier. Now, if, for the time being, the last measured voltage is designated U'' Q and the preceding voltage one half-period earlier is designated U' Q , the voltage difference in a stationary and faultless state will be ΔU Q = U'' Q + U' Q (1) equal to zero. When a new measured value becomes available, a new U' Q is formed from a saved measured value one half-period earlier than the new measured value, and the new measured value forms a new U'' Q . This shifting and updating take place continuously. When a fault occurs, the value of ΔU Q will be different from zero. With the aid of the absolute value of the voltage difference ΔU Q and the absolute value of the voltage prior to a fault, i.e. |U' Q |, the voltage difference is formed as follows δU Q = |ΔU Q |-|U' Q | (2) It will now be clear from Figure 5 that when δU Q becomes greater than zero, there is a fault between P and Q, i.e. and internal fault. On the other hand, the value of δU Q says nothing as to where on the line between P and Q the fault has occurred, since the value is also dependent on the fault resistance at the fault point F. So far a purely general description of voltages at control and fault points has been given. It will be shown later on that the conclusions drawn are valid both to phase voltages and to principal voltages. To obtain manageable voltage values, it is suitable to utilize some characteristic measure of the voltages, for example in the form of mean values. This will also be described below. As will be clear from the above, the information that δU Q > 0 can therefore be utilized, together with the information about the direction to the fault and after determination of the faulty phase or the faulty phases, for direct tripping of the circuit-breaker(s) in question. In the case described, the protection device has a range which corresponds to the entire line section PQ. As will be clear from the above, the voltage along the line can be calculated with the travelling wave model at an optional number of control points along the line. A calculation of voltages at a control point D between P and Q according to Figure 6 shows that δU D = |ΔU D |-|U' D |< 0 in the case of a fault at F. In this way, range limitation can be obtained by choosing the control point for calculating δU. If a protection device with a range corresponding to PD is desired, a fault at F according to Figures 5 and 6 is a fault which lies outside the area of protection, and consequently a fault at F shall not be remedied via station P. The fundamental method described for direct tripping and range limitation, in order to operate in practice, must be described somewhat more detailed according to the followings. The previously mentioned voltage difference ΔU between the calculated voltage subsequent to a fault and prior to a fault is formed, as mentioned, as the instantaneous value of the voltage at a certain time t and the value of the same voltage one half-period earlier, i.e. ΔU = u(t) + u(t-T/2) (3) where T is the period. In a sampled system there is obtained, in similar manner, for the k th measurement per period Δu k = u k + u k-N/2 (4) where N = T/Δt and Δt = the time between the samples. The further implementation of the method as well as of a device for carrying out the method according to the invention in based on sampled measurement. An adaptive filter, in the sense that N is determined with the aid of the zero passages of the preceding periods, can be used. In that case the influence of slow frequency changes upon oscillations is prevented. Also, it is presupposed in the following description that the desired range for the protection device at P is the whole line section PQ. The voltage difference in station Q, calculated in station P, according to equation (4) can then be indexed as Δu qk = u qk + u q(k-N/2) (5) As will be clear from the above-mentioned EP-A-85107101.9, both phase voltages and principal voltages can be calculated with the travelling wave model. To indicate that the difference relates to one of the phase voltages, the index "ρ" may be used, i.e. equation (5) is changed into Δu ρqk = u ρqk + u ρq(k-N/2) (6) In a corresponding way, the principal voltage difference may be indexed "ρσ" to indicate any of the principal voltages RS, ST or TR between phases R, S and T, whereby equation (5) is changed into Δu ρσqk = u ρσqk + u ρσq(k-N/2) (7) To indicate the voltage difference according to equation (2) and indicate that it relates to any of the phase voltages R, S, or T, index "ρ" is used in a corresponding manner, whereby δu ρqk relates to the k th voltage difference of any of the phases calculated for the station or control point Q. Equation (2) is thereby transferred to δu ρqk =|Δu ρqk |-|u ρq(k-N/2) |(8) The corresponding equation for the principal voltages is δu ρσqk =|Δu σqk -Δu ρqk | - | u σq(k-N/2) - u ρq(k-N/2) |(9) To obtain manageable voltage differences, the mean value of the δu-values obtained is suitably generated. This can be done, for example, by forming, respectively, δu ρqk = δu ρqk - δu ρq(k-N/2) + δu ρq(k-1) (10) and δu ρσqk = δu ρσqk - δu ρσq(k-N/2) + δu ρσq(k-1) (11) If any of the mean values δu according to equation (10) or (11) becomes greater than zero, this means that a protection device at P has a fault within its area of protection PQ of the line, i.e. an internal fault. By combining this information with information about the direction to the fault and about the faulty phase or faulty phases, if any, direct tripping of the circuit-breaker(s) in question can be made. This means that a protection device for directional comparison can be provided with a direct-tripping operating mode, which is independent of the communication channel included in the protective system. A device for carrying out a method according to the invention can be designed in accordance with Figure 7. Phase currents and phase voltages in the station in which the protection device is located are supplied to a travelling wave model 1. In the example shown, the protection device is situated in station P, and consequently phase currents i RP , i SP and i TP and phase voltages u RP , u SP and u TP are supplied to the travelling wave model. If the protection device is specially made for a certain given line section or range, a simplified version of the travelling wave model, which only calculates the control voltage at the end point of the section, may be used. If the protection device is to be adjustable for different ranges, a travelling wave model with a possibility to calculate voltages at several control points must be used. This is exemplified by the outputs u p and u k of the travelling wave model. The voltage differences Δu as regards phase voltages, i.e. Δu ρqk according to equation (6), are obtained by adding the voltage values of two consecutive half-periods. The time shift is obtained by means of the time delay elements 2,3 and 4 and the summation is obtained in the summation members 5,6 and 7. The voltage differences Δu with respect to the principal voltage, i.e. Δ ρσqk according to equation (7), is obtained by generating, in the summation members 8,9 and 10, the difference between the respective phase voltages. The voltage differences Δu can also be conceived to be generated by comparing the voltage value for one period with the corresponding value during a preceding period. The time delay elements 2,3 and 4 must then provide a displacement corresponding to the time T for a whole period. To generate the difference voltage Δu, the time-shifted value must then be supplied to the summation member with a negative sign. A device in accordance with this method is not shown in Figure 7. To generate the voltage differences δu with respect to phase voltages, i.e. δu ρqk according to equation (8), the numerical value of the Δu ρqk -values is first formed in the numerical value generating units 11,12 and 13 and the numerical value of the corresponding voltages one half-period earlier is formed in the numerical value generating units 14,15 and 16. The difference between the numerical values obtained according to equation (8) is then formed in the summation members 17,18 and 19. To form the voltage differences δu with respect to principal voltages, i.e. δu ρσqk according to equation (9), the numerical value of Δu ρσqk is first formed in the numerical value generating units 20,21 and 22. The value of u ρσq(k-N/2) is formed in the summation members 23,24 and 25 and the respective numerical values are formed in the numerical value generating units 26,27 and 28. The differences according to equation (9) between the numerical values obtained are then formed in the summation members 29,30 and 31. The mean value generation of δu with respect to phase voltages, i.e. δu ρqk according to equation (10), is obtained by adding, in the summation members 32,33 and 34, the value of δu ρqk to a mean value of δu ρq one sample earlier, i.e. δu ρq(k-1) , and subtracting a value of δu ρq one half-period earlier, i.e. δu ρq(k-N/2) . The half-period time shift takes place in the time delay elements 35,36 and 37, and the time shift Δt takes place in the time delay elements 38,39 and 40. The mean value generating of δu with respect to principal voltages, i.e. δu ρσqk according to equation (11), is obtained by adding, in summation members 41,42 and 43, the value of a δu ρσqk to a value of a mean value of δu ρσq one sample earlier, i.e. δu ρσq(k-1) , and subtracting a value of δu ρ σq one half-period earlier, i.e. δu ρσq(k-N/2 . The half-period time shift takes place in time delay elements 44,45 and 46, and the time delay Δt takes place in time delay elements 47,48 and 49. If any of the mean values obtained is greater than zero, according to the above description of the invention a fault is present within the range of supervision of the protection device. The comparison takes place in the comparator elements 50,51,52,53,54 and 55, the Boolean output signals of which, DR,DRS,DS,DST,DT,DTR, are supplied to an "OR"-element 56 which emits a signal DF when the protection device is to enter into operation. The components included in the device, such as travelling wave model, summation members, comparison members, time delay elements, etc., can be formed as more or less integrated solutions in analog or digital technique.

Claims: Method for range limitation and for direct tripping of a protection device for a power line (PQ) included in a multi-phase distribution or transmission system, comprising measurement of current and voltage in each phase in a measuring station (P) near the power line, and supplying the measured values to a travelling wave model, located in the measuring station, by means of which wave model the voltage distribution at a number of control points along the line can be obtained, characterized in that a voltage difference Δu is formed as the sum of a voltage value U'', calculated for a control point at a certain time, and a corresponding voltage value U' one half-period/period earlier, that a voltage difference δu is formed as the difference between the absolute value of the voltage difference Δu and the absolute value of the voltage U' for the same control point, and that the range for a protection device in the measuring station (P) is determined as the distance from the measuring point to the control point for which δu is greater than zero when a low resistance fault occurs at the corresponding point of the real transmission line. Method according to Claim 1, characterized in that when the voltage difference δu for the control point becomes greater than zero, a signal is obtained for direct tripping of a protective circuit-breaker for the line section between the measuring station and the control point. Device for carrying out the method according to any of claims 1 or 2, which device is located in a measuring station (P) near the power line and comprises a travelling wave model (1), time delay elements, summation members, numerical value generators, comparison elements and an "OR"-element, characterized in that for a control point Q, with the distance between P and Q constituting the range of the device, a first summation member (5) is adapted to form a voltage difference Δu Rq equal to the difference between an R-phase voltage, calculated with the travelling wave model, and the corresponding voltage one half-period/period earlier, a second summation member (6) is adapted to form a voltage difference Δu Sq equal to the difference between an S-phase voltage, calculated with the travelling wave model, and the corresponding voltage one half-period/period earlier, a third summation member (7) is adapted to form a voltage difference Δu Tq equal to the difference between a T-phase voltage, calculated with the travelling wave model, and the corresponding voltage one half-period/period earlier, a fourth summation member (17) is adapted to form a voltage difference δu Rq equal to the difference between the numerical value |Δu Rq | and the numerical value |u Rq | one half-period/period earlier, a fifth summation member (18) is adapted to form a voltage difference δu Sq equal to the difference between the numerical value |Δu Sq | and the numerical value |u Sq | one half-period/period earlier, a sixth summation member (19) is adapted to form a voltage difference δu Tq equal to the difference between the numerical value |Δu Tq | and the numerical value |u Tq | one half-period/period earlier, a seventh summation member (32) is adapted to form a mean value δu Rq of the voltage difference δu Rq , an eighth summation member (33) is adapted to form a mean value δu Sq of the voltage difference δu Sq , a ninth summation member (34) is adapted to form a mean value δu Tq of the voltage difference δu Tq , a tenth summation member (8) is adapted to form a voltage difference Δu RSq equal to the difference between Δu Sq and Δu Rq , an eleventh summation member (9) is adapted to form a voltage difference Δu STq equal to the difference between Δu Tq and Δu Sq , a twelfth summation member (10) is adapted to form a voltage difference Δu TRq equal to the difference between Δu Rq and Δu Tq , a thirteenth summation member (23) is adapted to form a voltage difference u RSq equal to the difference between values of u Sq and u Rq one half-period/period before the respective Δu-values, a fourteenth summation member (24) is adapted to form a voltage difference u STq equal to the difference between values of u Tq and u Sq one half-period/period before the respective Δu-values, a fifteenth summation member (25) is adapted to form a voltage difference u TRq equal to the difference between values of u Rq and u Tq one half-period/period before the respective Δu-values, a sixteenth summation member (29) is adapted to form a voltage difference δu RSq equal to the difference between the numerical value |Δu RSq | and the numerical value |u RSq |, a seventeenth summation member (30) is adapted to form a voltage difference δu STq equal to the difference between the numerical value |Δu STq | and the numerical value |u STq |, an eighteenth summation member (31) is adapted to form a voltage difference δu TRq as the difference between the numerical value |Δu TRq | and the numerical value |u TRq |, a nineteenth summation member (41) is adapted to form a mean value δu RSq of the voltage difference δu RSq , a twentieth summation member (42) is adapted to form a mean value δu STq of the voltage difference δu STq , a twenty-first summation member (43) is adapted to form a mean value δu TRq of the voltage difference δu TRq , that the produced mean values are each adapted to be compared in a respective comparison element (50,51,52,53,54,55) with the value zero, and that the device is adapted to deliver a signal, via an "OR"-element (56), indicating the occurrence of a fault within the range of the protection device when any of the mean value is greater than zero.

Inventors: NIMMERSJOE, GUNNAR, SAHA, MURARI MOHAN, Nimmersjö, Gunnar
Applicants: ASEA AB
Application year: 1987
Main classification: H02H7
IPC codes: H02H7, H02H3