A METHOD AND A DEVICE FOR DETERMINING THE DISTANCE FROM A MEASURING STATION TO A FAULT ON A TRANSMISSION LINE

Abstract: A method and a device for determining the distance from a measuring station to a fault on a transmission line based on a fault model of a transmission network while taking into consideration the zero-sequence impedance and, where assuming a fault current, while taking into consideration the feeding of fault current to the fault point from both ends of the transmission line.

Description: A method and a device for determining the distance from a measuring station to a fault on a transmission line TECHNICAL FIELD For protection of cables and overhead lines, so-called dis¬ tance protections are used. These may be based on different technical principles, often starting from a certain region of function in an impedance plane or in the form of a wave detector-based design. In this connection it is normally desired to find out the distance from a measuring station to a fault and also to be able to determine the magnitude of the fault resistance. The present invention comprises a method and a device for determining the distance to a fault from a measuring station and the magnitude of the fault resistance. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a transmission line between the stations P and Q, on which a fault on any of the phases to ground has occurred at F. Otherwise, the figure comprises the voltages, currents, impedances, etc. which are mentioned in the following description. Figure 2 shows how a distance protection with a fault model according to the state of the art is connected to the trans¬ mission network according to Figure 1. Figure 3 shows how a distance protection with a fault model according to the invention is connected to a transmission network. Figure 4 shows a device for fault determination according to the invention. BACKGROUND ART, THE PROBLEMS The principles of fault location and determination of fault resistance in the event of a fault on a protected line dis- tance are known from a number of publications, of which some will be described in the following. The basic material con¬ sist of measured values obtained with the aid of instrument transformers for voltage and current in a measuring station near the protected line. These measured values are applied to a model of the network in question, the model being built into the distance protection. Current technique comprises analog-to-digital (A/D) conversion and filtering of the measured values which then, via different distance protec¬ tion equations for the model, determine the distance to the fault and the magnitude of the fault resistance. US 4,906,937 describes a method which is based on a network configuration according to Figure 1. The faultless line impedance Zpς> of the protected line distance between two measuring stations P and Q is sensed. After a fault has been determined, the voltages Up and UQ and the currents Ip and IQ can be measured in the respective stations. The voltages and the currents can belong to anyone of the phases R, S and T. Normally, however, in order to avoid communication between the stations, the starting-point are measured values at one of the stations. If in accordance with Figure 1, the assump¬ tion is made that a current IF flows through a fault resis¬ tance Rp, whereby a voltage Up is applied across the fault resistance, the following voltage balance equation for the network can be set up: Up = Upp + Up = α-ZpQlp + Rp-lp (1) Here, α = 0 - 1 is a measure of the fault location assumed for the time being. Equation (1) cannot be directly solved because of too many unknown parameters and it is therefore necessary to make certain assumptions and use special methods for solution, for example in the form of linear regression, to be able to solve it. In an article in IEEE Tutorial Course: "Computer relaying", 79 EH 0148-7-PWR, page 22, equation (18), the assumption is made that the protected line can be described as an R-L series circuit. The voltage balance equation (18), with designations from this article, has the following form: V = R e ff i + L βf f di/dt (2) where R eff relates to the sum of the line resistance up to the fault point plus the fault resistance and L eff relates to the line inductance from the measuring station and up to the fault point. The above two modes of representation of a transmission network and the equations which can be derived therefrom are used as distance protection equations and form the direct basis of the fault model according to Figure 2 which is arranged in existing distance protections. Figure 2 shows, in principle, how the distance protection 1 with the fault model in question, in addition to being supplied with infor¬ mation about the data of the network and an assumption about the fault current via instrument transformers 2 and 3 for voltage and current, is also continuously supplied with all the phase currents and phase voltages, which values are fil¬ tered and signal-processed. Because of the transformation of the measurement signals, the line impedance Zpς is replaced by a transformed line impedance ZL in the fault model. Tripping of the circuit breaker 4 for disconnection of the network takes place when specified criteria built into the distance protection are fulfilled. In an article entitled "Microprocessor-based algorithm for high-resistance earth fault distance protection", by Q. S. Yang et al in IEE Proc, Vol. 139, Pt.C, No. 6, Nov. 1983, a voltage balance equation for a network is described, star- ting from a simplified network configuration where capaci- tive powers have been omitted. The article makes the assump¬ tion that the fault current is equal to the current change which takes place between the current during one cycle before the occurrence of a fault and the current after the occurrence of the fault. The voltage balance equations are evaluated with respect to the distance to the fault, star¬ ting from measured values from the power network at two adjacent points in time and based on known network data. The method described requires relative extensive resources to be able to save measured currents before the occurrence of a fault and difficulties also arise when these current values are to be matched together with currents measured during the fault. These difficulties are also amplified if the frequency of the network deviates from the nominal one. Since the calculations are based solely on measured values taken at two adjacent points in time, the accuracy is limited where the derivative of the current is small, that is, at the peak of the current curve. The article therefore recommends that the calculations be performed with measured values taken close to the zero crossing of the current. The assumption of a fault current according to the state of the art, as is also clear from the above, is based on feeding the fault current IF to the fault point only via the station where the measurement takes place, that is, only from one end of the line. This results in a fault source since feeding of fault current to the fault point from the other end of the line is not taken into consideration. In an article entitled "Numerical distance protection for sub-transmission lines" by A. Engqvist, L. Eriksson, Cigre, 34-04, 1988, a technique is described which shows how measured sampled phase currents and phase voltages are processed in various filters to be subsequently used in the fault models used. The filters comprise, inter alia, low- pass filters and recursively operating Fourier filters. The filter technique described also means that unwanted harmonic signals are eliminated as well as an orthogonal processing of the measurement signals for a division thereof into real and imaginary parts. SUMMARY OF THE INVENTION The invention relates to a distance protection and comprises a method and a device for determining the distance from a measuring station to a fault on a transmission line. The invention is clear from Figure 3 and is based on a fault model of a transmission network in which, in contrast to prior art fault models, the zero-sequence impedance has been taken into consideration. The fault model according to the invention will therefore be described below as an extended fault model. In the same way as the transformed line impe¬ dance Z L , the zero-sequence impedance consists of a trans¬ formed zero-sequence impedance and is here designated Z N . With these designations, the voltage balance equation (1) according to the above for the extended fault model will be written as U = α(Z I + Z N I N ) + I F R F (3) where I is the transformed phase current and I N the trans- formed zero-sequence current. If the impedances are replaced by their respective resistances and inductances, that is, where the transformed line impedance Z L is replaced by the transformed line resistance R L and the transformed line inductance L L and where the transformed zero-sequence impe- dance Z N is replaced by the transformed zero-sequence resis¬ tance R N and the transformed zero-sequence inductance L N , the voltage balance equation (3) for each phase will have the following form U = α(R L I + L dI/dt + R N IN + L N dI N /dt) + I F R F (4) If the inductance of the line is expressed by means of the reactance XL of the line at the network frequency in question, (Do, that is. L = X L /ω 0 and, in a corresponding way, if the zero-sequence inductance is expressed by means of the zero-sequence reactance XN at the network frequency in question, α>o, that is, L = XN/G>O while at the same time setting dl at dl = ii - lo that is, the difference between two consecutive sample values of the current I while at the same time setting di N at dι N = INI - INO that is, the difference between two consecutive sample values of the zero-sequence current and defining "dt" as the time between two consecutive sample values, which also means that (ωodt) corresponds to the angle between two consecutive samples, then the voltage balance equation (4) can be written as u = α(R i+X (iι-io) (ωodt) +R N IN+XN(INI-IN O )/ (ωodt) ) +ι F R F (5) The voltage balance equation (3) comprises three different currents, namely, I, IN and Ip. Of these, I is directly measurable, the zero-sequence current I can be obtained, inter alia, as the sum of all the phase currents, whereas for the fault current Ip an assumption has to be made. In contrast to the state of the art, according to the present invention, when assuming a fault current, feeding of fault current to the fault point from both ends of the line is also taken into consideration. One way of achieving this is described in Swedish patent application 92019728 entitled "Fault current filter". This shows that a measure of the fault current can be obtained by summing the measured current samples for each phase at two adjacent points in time, the value of each current sample being multiplied by a respective coefficient which is chosen such that the fault current gets into phase with the positive-sequence and nega¬ tive-sequence current changes. If calculations are to be carried out on an extended fault model according to Figure 3 starting from corresponding voltage balance equations to obtain a measure of "α" and Rp, it is also necessary to have access to calculated values of the transformed line impedance ZL and the zero-sequence impedance Z with their respective resistive and inductive components. To obtain a measure of "α" and Rp, any of voltage balance equation (4) or (5) can be made the starting-point, as shown above. However, this presupposes access to the derivatives of both I and IN- The derivatives can either be obtained by pure differentiation or with the aid of the current diffe¬ rence between two consecutive sample values, that is as shown in equation (5) . Thus, besides having access to the parameters ZL and ZN of the extended fault model, access is also had to six of the remaining eight varying signal parameters in the equation, namely, U, I, I and hence also dl/dt and dlw/dt and the fault current Ip. To be able to solve the parameters "oc" and Rp, according to the invention the possibility of orthogonal processing, as mentioned above, is made use of. By this processing, that is, obtaining the real part and the imagi¬ nary part of the signal parameters, for each measurement two voltage balance equations (4r) and (4i) can be set up as follows, where the index "r" indicates the respective real part and the index "i" indicates the respective imaginary part: U r = α(RLl r + LdI r /dt + RNINΓ + N dlN r dt) + IFΓ^F ( 4r ) Ui = α(RLli + L L dIi/dt + RNI I + L N dI N i/dt) + IFIRF (4i) Without going into any intermediate calculations, it can be shown that "α", that is, the relative distance from the measuring station to the fault F, can be written as α = (Ur'Ipi-Ui-lF r )/(U H r-lFi-UHi-lFr) (6) where UHΓ = kι-I r +k2-l r+k3-dI r +k4-dlNr (7) U H i = kι-Ii+k2-I N i+k3-dIi+k4-dlNi (8) the coefficients ki, k 2 , k 3 and k 4 being capable of being determined with the aid of the parameters RL, LL, RN and L of the extended fault model according to the following: kι=R L , k =R N , k 3 =x L /ωodt=L L /dt, k =X N /ω 0 dt=L N /dt From the voltage balance equations (4r) and (4i) a value of the fault resistance Rp can be obtained in a corresponding way. In practice, however, this value is of no interest since the criteria for solution of a transmission network, in addition to controlling whether the value of "oc" lies between 0 and 1, is normally based on a control whether IZI = IUA/IAI < Z in that is, via a normal underimpedance relay with a setting Zin lower than the normal load impedance. The connection of the distance protection to the trans¬ mission network takes place in the same way as for the transmission network according to Figure 1 and is clear from Figure 4, where the equivalent diagram of the extended fault model used is shown. The extended fault model is described by a distance protection equation which is based on any of the previously described equations, where the zero-sequence impedance has been taken into consideration. In addition, when assuming a fault current, feeding of the fault current to the fault point from both ends of the line has been taken into consideration. The signal processing and the method of calculation for obtaining the basic data for the criteria for tripping of the line are based on the same filtering and orthogonal processing of the measurement signals as are mentioned in the above-mentioned Cigre report. DESCRIPTION OF THE PREFERRED EMBODIMENTS A device for carrying out the method for obtaining a rela- tive measure "α" of the distance from a measuring station to a fault on a transmission line with the aid of an extended fault model according to Figure 3 is clear from Figure 4. The device is a realization of the voltage balance equa¬ tions, obtained under the "Summary of the invention" above, which are based on the voltage balance equations (4r) , (4i) , (7) and (8) . The device according to Figure 4 illustrates an embodiment for one of the phases, the current I and the voltage U of which may belong to any one of the phases R, Ξ and T. Further, it is assumed that all the phase currents are available to be able to obtain a measure of the zero- sequence current IN which arises upon a fault on the trans¬ mission network, and that a fault current IF obtained with the aid of the above-mentioned fault current filter is available. All the input signals consist of sampled numeri- cal values. As before, designations indexed "0" and "1" relate to two consecutive sample values. In addition, known values of the impedance of the transmission network and the zero-sequence impedance with their respective resistance and reactance parts are made the starting-point, to be able to choose suitable settings of the fault model. Thus, in a distance protection according to the invention, a device according to Figure 4 is arranged for each phase. The device is continuously supplied with measured values or values of the phase voltage U, the phase current I, the zero-sequence current I and the fault current Ip obtained in some other way. As will be clear from the above equa- tions, information about the derivatives of both the phase currents and the zero-sequence current is needed. According to the state of the art, a number of different methods are available to obtain a measure of these derivatives. Figure 4 indicates a method which relates to the voltage balance equation (5) , that is, that a measure of the derivatives is obtained with the aid of two consecutive sample values. As far as the phase current is concerned, this takes place via the time-lag element 5 and the dl-generator 6 and as far as the zero-sequence current is concerned, this takes place with the the time-lag element 7 and the di N -generator 8. As stated under the background art, it is known, inter alia, from the Cigre report 34-04, 1988, by A. Engqvist and L. Eriksson how sampled measured values can be filtered and processed such that the measurement signals can be divided into their respective real and imaginary parts. All six of the above-mentioned available sampled signals are now supplied to a "filter and orthogonal generator unit" 9 for filtering and processing to obtain the real and imaginary parts of these signals, whereby access will be had to values Of U r , Ui, I r , Ii, dl r , dli, I Γ - iNi- dlNr- dI N i, Ipr and Ipi. This means that together with known data for the trans¬ mission network, all the necessary parameters for deter¬ mining the relative distance to the fault, " ", are available. As will be clear from the above summary of the invention, the coefficients ki, k 2 , k 3 and k 4 can be determined with the aid of the parameters R < L , RN and N of the transmission network. By placing ki in the coefficient units 10 and 11, k 2 in the coefficient units 12 and 13, k 3 in the coefficient units 14 and 15 and k 4 in the coefficient units 16 and 17, in accordance with equation (7) the quantity UH Γ can be determined via a first sum ator 18 which is composed of kι-I r , k 2 -l N r k 3 *dl r and k 4 -dl r and, in a corresponding way, according to equation (8) the quantity UHI can be determined via a second summator 19 which is composed of kχ-Ii, k 2 -l N i k3*dli and k 4 -dlNi- Then, by multiplying UHΓ in a first multiplier 20 by the imaginary part of the fault current, that is Ip r , for forming a first product Uκr"lFi and multiplying UHI in a second multiplier 21 by the real part of the fault current, that is Ip r , f° r forming a second product Uπi-l F r and thereafter, in a difference generator 22, forming the difference between the fist and second products, a value of the denominator α n in equation (6) is formed. By multiplying in a third multiplier 23 the real part U r of the phase voltage by the imaginary part of the fault current, that is Ipi, for forming a third product U r -Ipi and multi¬ plying in a fourth multiplier 24 the imaginary part Ui of the phase voltage by the real part of the phase current, that is Ip r , for forming a fourth product Ui-Ip r , and then in a difference generator 25 forming the difference between the third and fourth products, a value of the numerator a t in equation (6) is formed. With values of a t and α n thus obtained, a measure of the relative distance from the measuring station P to the fault F can be obtained via a first quotient generator 26. Figure 4 also shows the conditions, of a more practical nature, for the fault resulting in tripping of the circuit breaker 3. A first criterion for the fault being interpreted as a fault on the line between P and Q is that the value of "α" obtained lies within the limits 0 < α< 1. In practice, however, the higher value is set at a value "k" somewhat lower than 1. The drawing-up of these limits can be per¬ formed with the aid of a first selector element 27. As mentioned under the summary of the invention, it is nor¬ mally assumed that at least two criteria should be fulfilled for the line or phase in question to be disconnected. The second criterion is often checked with the aid of a conven- tional underimpedance relay where, via a second quotient generator 28 with the aid of the phase voltage U and the phase current I, the impedance represented by the transmis¬ sion network is found out. If the impedance thus obtained is lower than the permissible underimpedance Zi n , which can be checked via a second selector 29, also the second criterion for tripping is fulfilled. Tripping of the circuit breaker 4 can then be executed via an AND-element 30.

Claims: CLAIMS 1. A method in connection with a fault on a transmission line for determining the relative distance "α" to the fault from a measuring station P located close to one of the ends of the transmission line, wherein the method is based on a fault model built into the distance protection representing the transmission network in question and an assumption of a fault resistance Rp and a fault current Ip fed to the fault point via the measuring station, and wherein the distance protection is supplied with information about the impedance Z , the resistance R and the inductance L L of the trans¬ mission line transformed to the fault model and is con¬ tinuously supplied with sampled measured values of the phase currents I and the phase voltages U of the transmission network, and wherein the distance protection is associated with a unit (9) for A/D conversion, filtering and orthogonal processing of the values obtained, the method being characterized in that an extended fault model is formed which comprises the zero-sequence impedance Z N , the zero- sequence resistance R N and the zero-sequence inductance L N of the transmission network transformed to the fault model, and the corresponding transformed zero-sequence current I N and that it is assumed that the fault current is fed to the fault point also from the other end of the transmission line, and that the relative distance to the fault is deter¬ mined on the basis of the voltage balance equation which can be set up for the extended fault model. 2. A method in connection with a fault on a transmission line for determining the relative distance "α" to the fault from a measuring station P located close to one end of the transmission line according to claim 1, the method being characterized in that for the extended fault model the voltage balance equation can be expressed as u = α(z ι + Z N IN ) + IF F (3) or as U = α (R L l + L L dl /dt + R N IN + L N dl N /dt ) + IF F ( 4 ) or as u = α (R L ι+X L ( iι-io ) / (ωodt ) +R N ι N +x N ( iNi-iNo) / (ωodt ) ) +I F RF ( 5 ) where LL is replaced by X L /ωo where XL is the line reactance at nominal frequency ωo and where LN is replaced by XN/ o where XN is the zero-sequence reactance at nominal frequency ωo and where dl is replaced by the difference between two consecutive sample values lo and lι of the phase current I and where dlw is replaced by the difference between two consecutive sample values INO and INI of the zero-sequence current IN and where dt is equal to the difference in time between two consecutive samples and where (α>odt) is the angle between two consecutive samples. 3. A method in connection with a fault on a transmission line for determining the relative distance "α" to the fault from a measuring station P located close to one end of the transmission line according to claim 1, the method being characterized in that the fault current in the voltage balance equation is set as the sum of measured current samples for each phase at two adjacent points in time and where the value of each current sample is multiplied by a respective coefficient which is chosen such that the fault current gets into phase with the positive-sequence and negative-sequence current changes. 4. A method in connection with a fault on a transmission line for determining the relative distance "α" to the fault from a measuring station P located close to one end of a transmission line according to claim 1, the method being characterized in that with the aid of the orthogonal generator in the distance protection, the real part and the imaginary part of the measurement signals are formed for each sample occasion, whereby for each measurement two voltage balance equations can be set up, which starting from U = α(R l + L L dl/dt + R N IN + L N dl N /dt) + I F RF (4) will have the form U r = α(R l r + LLdI r /dt + RNINΓ + L N dI N r/dt) + IFΓ F (4r) Ui = α(R Ii + L L dIi/dt + R N lNi + L N dl N i/dt) + IFI F (4i) where "r" indicates the respective real part and "i" the respective imaginary part, and where the solution of these equations with respect to the relative distance "α" from the measuring station to the fault gives α = (U r -lFi-Ui-I Fr )/(U H r-lFi-UHi-lFr) (6) where UHΓ = kι-I r +k2-lNr+k3-dI r +k 4 -dlNr ' ( 7 ) UHi = kι-Ii+k2-l i+k3-dIi+k 4 -dI N i (8) where the coefficients ki, k2, k3 and k 4 are determined with the aid of the parameters of the transmission network according to ki = R L , k 2 = R k 3 = L L /dt, k 4 = L N /dt 5. A device for carrying out the method according to claim 1 for determining, in connection with a fault on a transmiss¬ ion line, the relative distance "α" to the fault from a measuring station P located close to one end of the trans- mission line, which device for each phase is included in a distance protection and which is associated with input sig¬ nals in the form of sampled measured values of phase current I, phase voltage U, zero-sequence current IN and fault current Ip and which is provided with information about the impedance ZL, the resistance RL and the inductance LL of the transmission line, transformed to the fault model, as well as the zero-sequence impedance ZN, the zero-sequence resis- tance RN and the zero-sequence inductance LN of the trans¬ mission network, transformed to the fault model, characterized in that it is adapted to execute the equation α = ( U r -lFi-Ui-I Fr ) / (UHr'lFi-UHi-lFr ) ( 6 ) where UHr = kι-I r +k2-lNr+k3"dI r +k4-dlNr ( 7 ) U H i = kι-Ii+k 2 -I N i+k3-dIi+k 4 -dlNi (8) where ki = R L , k 2 = R N , k 3 = L L /dt, k 4 = L N 7dt for which it comprises time-lag elements (5, 7), a dl- generator (6) and a dl N -generator (8), a filter and ortho¬ gonal generator unit (9) for forming the real part U r and the imaginary part Ui of the phase voltage, the real part l r and the imaginary part Ii of the phase current, the real part dl r and the imaginary part dli of dl, the real part IN Γ and the imaginary part i N i of the zero-sequence current, the real part dl r and the imaginary part dl N i of dl , the real part I Fr and the imaginary part Ipi of the fault current, coefficient units (10, ...17) for the coefficients ki, k 2 , k 3 and k 4 , a first summator (18) for forming Uπr according to equation (7), a second summator (19) for forming Uπi accor¬ ding to equation (8) , a first multiplier (20) for forming a first product U H r'l F i a second multiplier (21) for forming a second product UHI-IF Γ a first difference generator (22) for forming the difference between the first and second pro¬ ducts, a third multiplier (23) for forming a third product U r -l F i, a fourth multiplier (24) for forming a fourth product Ui-Ipr' a second difference generator (25) for forming the difference between the third and fourth products, and a first quotient generator (26) for forming the quotient between the output signals of the second and first diffe¬ rence generator, which value constitutes the relative dis¬ tance α to the fault.

Inventors: ERIKSSON LEIF, SAHA MURARI MOHAN, ERIKSSON, LEIF, SAHA, MURARI, MOHAN
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1993
Main classification: G01R31
IPC codes: G01R31