METHOD FOR DETECTING HIGH RESISTANCE GROUND FAULTS ON A POWER TRANSMISSION LINE

Abstract: A method and a device for detecting high resistance ground faults on a power transmission line located between two stations (P, Q), the line being included in a multi-phase distribution or transmission system. The voltage distribution along the transmission line, seen both from station P to sta­ tion Q and from station Q to station P, is obtained with the aid of a device comprising a travelling wave model of the line located in measure stations (P,Q) at either end of the line. A voltage ΔU Qp is formed as the difference between the voltage in station P, calculated in the travelling wave model in station Q and the corresponding voltage one period/half-period ear­ lier. A voltage ΔU Pp is formed as the difference between the voltage in station P, calculated in the travelling wave model in station P and the corresponding voltage one period/half-­ period earlier. The corresponding voltage differences ΔU Pq and ΔU Qq are calculated for station Q. The voltage difference ΔP is then formed as the difference between ΔU Qp and ΔU Pp , and the corresponding voltage difference ΔQ is formed as the difference between ΔU Pq and ΔU Qq . Fault detection is ob­ tained when ΔP > ε p or when ΔQ>ε q , where ε p is the accep­ ted voltage difference at P and ε q is the accepted voltage dif­ ference at Q.

Description: The invention relates to a method for detecting high resistance ground faults on a power transmission line according to claim 1. The invention also refers to a device for carrying out the method. The invention is designed for networks for the distribution or transmission of electric power, in which supply of power to the transmission line to be protected takes place at least from one of the end points of the transmission line, and in which that part of the network which is to be protected is provided with measuring devices in a station or the like at either end of the object to be protected. Distribution and transmission networks have to be protected against faults in such a way that a faulty component or transmission line is automatically disconnected with the aid of an associated circuit-breaker. This is necessary to minimize the risk of personal injury and fire, and it also applies to high impedance faults with relatively small fault currents. Ground faults, in particular, may have high fault resistances. Ground fault protection devices of prior art are determined primarily by the size of the network, its grounding and the regulations which apply to electric heavy current installations with regard to permissible voltages in the case of ground faults on a grounded component. In view of the different appearances of the networks, therefore, several different types of measurement criteria are currently used. An ungrounded network may be used when the total length of line of the network is not too great. In that way, the ground fault current is limited by the capacitive reactance between the network and ground and by a possible transition resistance at the site of the fault. In this connection, directional current relays are used which are sensitive to ground fault currents having a capacitive phase shift relative to the neutral point voltage. Direct-grounded networks are seldom used at the distribution level since the ground fault current may assume very high values. On the other hand, direct grounding is commonly used for the transmission networks. Since direct grounding implies that the neutral point voltage of the network is zero, the ground fault current is utilized only for selective ground fault detection or for disconnection of a faulty transmission line. Grounding via neutral point resistors larger than 50 Ω is used in small and medium-sized networks. The neutral point resistor is chosen such that a sufficiently great active or - as it is also called - resistive current, that is, a current being in phase with the neutral point voltage, is obtained in the case of a ground fault. The network is protected by directional current relays which are sensitive to ground fault current and which are supplied with the current which passes through the neutral point resistor. Grounding by means of a neutral point reactor and a neutral point resistor occurs in large networks, in which the capacitive ground fault current would otherwise become too high. The capacitive ground fault current is compensated for by the neutral point reactor so as to obtain a tuned network. For selective disconnection of a faulty component, current direction relays are used which are sensitive to resistive ground fault current, that is, the current through the neutral point resistor. In some applications a special automatic system is available which takes care of the disconnection and connection of the neutral point resistor so as to provide a chance of self-extinguishing of the fault before a relay starts operating and disconnects the line. Grounding by means of neutral point reactor occurs in large distribution networks. Otherwise, the same conditions apply as in the case of grounding by means of a reactor and a resistor; however, it is presupposed that the internal resistance of the neutral point reactor shall be sufficiently high as to enable the evaluation of a resistive current component. The components and systems for ground fault measurement, identification and disconnection which are used at present cannot, as is partly clear from the above description, be made identical because of different grounding principles. Nor do they offer the desirable sensitivity and speed. In direct-grounded networks, however, it is theoretically possible to achieve a relatively high sensitivity of the ground fault protection devices with a possibility of selective detection of faults with high transition resistances. This type of grounding primarily applies to the transmission networks. Ungrounded networks also provide relatively good possibilities of selective detection of ground faults with a high transition resistance. However, ungrounded networks are less common, since only small networks may come into question. In addition, it is normally desired to avoid ungrounded networks in view of the risk of intermittent ground faults. In networks which are resistance-grounded corresponding to a ground fault current of 2-15 A and networks with reactor grounding together with resistance grounding, directional current relays which are sensitive to resistive ground fault currents give varying sensitivity to ground faults with a high transition resistance. The sensitivity is largely dependent on the size of the network. Generally, however, it can be said that large networks give limited possibilities of good selective detection of high-ohmic ground faults. The three most important reasons for the limited sensitivity of current direction relays which are sensitive to resistive ground fault current are as follows: In the case of low ground fault currents, where the capacitive or the inductive component of the current is predominant, the angular fault of the current transformer may cause incorrect measurement. In the case of faults with a low degree of propagation, power and current direction relays have difficulties in measuring I x cos α in the case of great values of α, that is, in the range α = 80° - 90°. For practical reasons it is not possible to set the sensitivity of the relays at an arbitrarily high level. The reasons may be leakage currents in insulators, spontaneous ground contacts through vegetation or salt storms near sea shores. Too high a sensitivity of the ground fault protection devices would therefore result in unjustified trippings. It is an express wish on the part of the distributors of electric power to detect ground faults with a higher transition resistance at the site of the fault than what is possible at present (∼3 kΩ). In many transmission networks ground faults with high fault resistances may also occur. These faults are difficult to detect with, for example, impedance relays. It is a well known fact that ground faults which are not located and disconnected in time may lead to personal injury or fire. Of special interest in this connection are reversed ground faults, that is, faults in which a phase is interrupted and a ground fault arises in the phase after the point of interruption involving feeding via the load object. In the present situation these faults may remain undiscovered for a long period of time. In addition to the above general description of the various designs of ground fault protection devices and the associated problems, the description of the prior art should also include a reference to EP-B-0 082 103 (Jan Bergman). This document describes a method and a device for detecting ground faults in networks for distribution of electric power from a power station from which a number of transmission lines emanate and in which the ground fault current of each line is measured. According to this method, that line is selected which exhibits the greatest active ground fault current or the greatest ground fault current or the greatest change in ground fault current. Thereafter, the measured ground fault current, or the change therein, in the selected transmission line is compared with at least one predetermined reference value. A fault indication takes place when the measured value exceeds the reference value. 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 faultless condition between two stations P and Q which are fed from two power sources A and B, Figure 2 the real voltage distribution along the transmission line in Figure 1 between the two power sources, Figure 3 the same voltage distribution, in which the voltage values have been calculated by means of a travelling wave model located at P or at Q, based on voltage and current values measured at P and Q, Figure 4 the same transmission line as in Figure 1 with a fault at F between P and Q, Figure 5 the measured voltage along the transmission line after the occurrence of a high resistance fault, Figure 6 the voltage difference ΔU along the transmission line between the measured or calculated voltage prior to a fault and the voltage calculated with a travelling wave model after a fault, Figure 7 an embodiment of a device for detection of a high resistance fault according to the invention with a digital travelling wave model. Figure 1 shows a power line between two stations or measuring points P and Q. In the example shown the line is fed from two power sources A and B. More generally the line between P and Q can 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 complex power consumer B. Figure 2 shows the real voltage distribution along the line in a faultless state. E A and E B , respectively, are the E.M.F.s of the respective power sources, and U' P and U' Q , respectively, are the measured voltage at P and at Q, respectively. In EP-A-0164711 it is described how a so-called travelling wave model of a transmission line can be used for calculating the voltage distribution along that line. These calculations are based on the propagation of voltage waves along the line. 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 in a station. With these values and with the aid of the travelling wave model, the voltage can be calculated at a number of control points along the line, thus obtaining the voltage distribution along the line. With the aid of the current and voltage values measured in the stations P and Q, the control voltages or the voltage distribution from P towards Q along the line and from Q towards P in a faultless state can be calculated with the travelling wave model (see Figure 3). This means that the same voltage distribution is obtained as in Figure 2 within the limits which are determined by errors in measurement in measuring transformers and by an uncertain knowledge of parameters. In an ideal transmission system, the zero sequence voltage and the negative sequence voltage are zero under faultless conditions. In practice they can be assumed to be small. This applies particularly to that part of the zero sequence voltage and the negative sequence voltage which is caused by asymmetry in the monitored transmission line. The voltage change which occurs as a result of a fault may either be conceived to be the difference voltage ΔU which, in accordance with the theorem of Helmholtz-Thevenin, arises at the different points of the networks as a result of the fault, or only the zero sequence component and the negative sequence component, respectively, of this change. The conclusions drawn are valid for all interpretations of ΔU. In the faultless case, thus, ΔU = O at all measuring points. When the transmission line according to Figure 1 is subjected to an internal fault, that is, a fault located between the measuring points P and Q as indicated at F in Figure 4, a real voltage distribution is obtained according to Figure 5. The greatest voltage change ΔU F occurs at the site of the fault F. The actual measured voltage at the points P and Q are now represented as U'' P and U'' Q . In the following description the sign ' will indicate the values of quantities prior to the occurrence of a fault and the sign '' will indicate the values of quantities after the occurrence of a fault. Further, the following indexing system is used to define the quantities in question: An index containing P indicates that the quantity in question has been calculated with values measured at the measuring point P. An index containing Q indicates that the quantity in question has been calculated with values measured at the measuring point Q. An index containing p indicates that the value of the quantity in question is calculated for the point P. An index containing q indicates that the value of the quantity in question is calculated for the point Q. This means, for example, that U Pq indicates a voltage at Q calculated with values measured at P. With the aid of this system, the following difference voltages will now be defined: ΔU Pp = U' P - U'' P (1) ΔU Qq = U' Q - U'' Q (2) ΔU Pq = U' Pq - U'' Pq (3) ΔU Qp = U' Qp - U'' Qp (4) The zero sequence component in the changes is obtained by forming the sum of the phase voltages U' R , U' S and U' T . Prior to occurrence of a fault U'' R + U'' S + U'' T = O , and a simple calculation of (ΔU Pp ) O , (ΔU Qq ) O , (ΔU Pq ) O and (ΔU Qp ) O in all the cases is obtained as (ΔU) O = U' O = U' R + U' S + U' T . The negative sequence component can be obtained by means of some known type of negative sequence filter. In Figure 6 the voltage differences according to equations (1), (2), (3) and (4) are plotted graphically. It can be determined that a device at P for calculating the voltage distribution from P towards Q has a line model which is correct between P and F but incorrect at point F, and therefore the calculated voltage distribution is incorrect for the distance FQ. In similar manner, a device at Q provides a correct picture of the voltage distribution between Q and F but an incorrect picture of the voltage distribution between F and P. The two devices together give a correct distribution of the voltage namely the device at P for the distance PF and the device at Q for the distance QF. F is the only point for which the two devices give the same model voltage. The voltage differences ΔP and ΔQ, as they are represented in Figure 6, that is, ΔP =ΔU Qp - ΔU Pp (5) ΔQ = ΔU Pq - ΔU Qq (6) constitute respectively an error in measurement at Q as regards the voltage change at P and an error in measurement at P as regards the voltage change at Q, because of the fact that the models in the case of a fault at F are not correct for the whole transmission line. ΔP and ΔQ can therefore be referred to as model error voltages which are always zero if the real transmission line and the model are in accordance. The model error voltage can now be utilized for ground fault indication, since in a faultless state both ΔP and ΔQ shall be zero. In equations (5) and (6), the mean value, the peak value or the R.M.S. of the difference voltages may be used. From a practical point of view, fault detection occurs when ΔP > ε p (7) or when ΔQ > ε q (8) where ε p is the accepted voltage difference at P and and ε q is the corresponding value of Q. By trimming these values under normal operation, it is possible, especially when changes in zero sequence voltages are to be sensed for ground fault detection, to set the protection device for high-ohmic faults which give greater deviations of ε p and ε q , respectively. Since the mathematical model for calculating the control voltage along the transmission line gives an accurate description of the voltage on the line within the limits determined by errors in measurement etc., this means that also high resistance faults can be discovered. This means that the prior art problems with detection of high resistance faults, as described above, can be overcome. As stated, the complete travelling wave model provides control voltages along the whole transmission line. As will be clear from the description of the invention, what is necessary for the fault detection is really only access to the control voltage at the two end points calculated with measured values for the respective opposite end point of the protected line. This means that it is also possible to use a simplified version of the travelling wave model and that the device for carrying out the method according to the invention will therefore be of a considerably simpler design than if a complete voltage distribution with the model is to be achieved. The method and the device according to the invention can be applied to phase voltages, principal voltages as well as to zero sequence and negative sequence voltages. An embodiment of a device for carrying out the method according to the invention is shown in Figure 7. Voltages u RP , u SP , u TP and currents i RP , i SP , i TP , measured in analog manner in station P are converted to digital signals in the measured value convertor 1. The corresponding conversion of measured values in station Q is performed in the convertor 2. The digital measured values are each supplied to a respective digital travelling wave model 3 and 4. As will be clear from the above description of the method, the fault detection can take place with the aid of a phase voltage, a principal voltage, a zero sequence voltage or a negative sequence voltage. Thus, the voltage values U delivered from the travelling wave model can be anyone of the afore-mentioned voltages; however, the choice of the kind voltage must be the same for all voltage values, for example all values U must be principal voltages. The value of U'' P , which may be a mean value of the voltage in question during one period, is compared in the summator 5 with U' P , that is, the mean value of the same voltage measured for the preceding period. In stationary and faultless state, the difference ΔU Pp is zero. When a new measured value for the next period becomes available, an existing U'' P is transferred into a new U' P and the new measured value forms a new U'' P . This shifting and updating takes place continuously up to the point where a fault occurs, whereby ΔU Pp receives a new value different from zero. With the aid of voltage and current values measured at P, the voltage U'' Pq , that is, the voltage in station Q, as well as the value of U' Pq , that is, the value one period earlier, can be calculated with the travelling wave model. These two values are compared with each other in the summator 6, which delivers the difference ΔU Pq . Instead of forming the voltage differences from values in one period and one period earlier, the time interval between the values may also be only half a period, depending on how the filtration is performed. To execute the equations (5) and (6), that is, to form the differences ΔP and ΔQ, the values of ΔU Qq and ΔU Qp must be transferred to station P. This transfer takes place by means of a transmitter 9 in station Q and a receiver 10 in station P. Of course, the execution may instead take place in station Q, whereby the values of ΔU Pp and ΔU Pq must be transferred in a respective manner from station P to station Q. In accordance with equation (5) the difference voltage value ΔP is then formed in a summator 11 and the corresponding value ΔQ is formed in a summator 12. If ΔP exceeds the accepted difference voltage ε p in station P, a fault detection signal F p is obtained from the comparison element 13, and in similar manner a fault detection signal F q is obtained from the comparison element 14 when ΔQ exceeds ε q . Since a transfer of ΔU Qq and ΔU Qp cannot take place without channel time delay, ΔU Pq and ΔU Pp are delayed by a corresponding time interval before ΔP and ΔQ are formed. The components included in the device, such as analog-digital convertors, travelling wave model, summators and comparison elements, can of course be integrated to differing degrees according to modern techniques. In an alternative analog design, the units 1 and 2 consist of analog measured value convertors/interface units and the units 2 and 4 of analog travelling wave models. The further signal processing is then also suitably performed in analog units. As stated in the above description of the method, alternative embodiments of the device comprise using a complete travelling wave model by means of which the voltage distribution can be calculated along the whole transmission line between P and Q, or using a simplified model which only calculates a control voltage in the respective opposite station.

Claims: Method for detecting high resistance ground faults on a power transmission line located between two stations P, Q, which line is included in a multi-phase distribution or transmission system, with measuring of current and voltage in each phase at the two end points P, Q of the transmission line, with supplying the measured values to a travelling wave model (2, 3) in the respective station and with obtaining the voltage distribution along the transmission line, seen both from station P to station Q and from station Q to station P, with the aid of the travelling wave models, whereby ΔU Pp is formed as the difference between the voltage U' P , measured in station P or calculated by means of the travelling wave model, and the corresponding voltage U'' P one period or one half-period later, ΔU Qq is formed as the difference between the voltage U' Q , measured in station Q or calculated by means of the travelling wave model, and the corresponding voltage U'' Q one period or one half-period later, ΔU Pq is formed as the difference between the voltage U' Pq in station Q, calculated in the travelling wave model in station P with values of currents and voltages measured in station P, and the corresponding voltage U'' Pq one period or one half-period later, ΔU Qp is formed as the difference between the voltage U' Qp in station P, calculated in the travelling wave model in station Q with values of currents and voltages measured in station Q, and the corresponding voltage U'' Qp one period or one half-period later, ΔP is formed as the difference between ΔU Qp and ΔU Pp , ΔQ is formed as the difference between ΔU Pq and ΔU Qq and fault detection is obtained when ΔP > ε p or when ΔQ > ε q where ε p is the accepted voltage difference at P and ε q is the accepted voltage difference at Q. Method according to claim 1 , characterized in that ΔU Pp , ΔU Qq , ΔU Pq and ΔU Qp are formed as differences of phase voltages or principal voltages for the same period/half-period. Method according to claim 1 or 2, characterized in that the travelling wave models are designed such as to allow the calculation of the voltage distribution along the entire transmission line, seen from the respective station. Method according to claim 1 or 2, characterized in that the travelling wave models are designed such as to allow only the calculation of the voltage in each separate station and its respective opposite station. Method according to any of the preceding claims, characterized in that voltages delivered from the travelling wave models consist of phase voltages. Method according to any of claims 1 to 4, characterized in that voltages delivered from the travelling wave models-consist of principal voltages. Method according to any of the preceding claims, characterized in that voltages delivered from the travelling wave models consist of zero sequence or negative sequence voltages. Device for carrying out the method according to any of the preceding claims comprising two members located in both stations, these members consisting of analog-digital convertors (1, 2), digital travelling wave models (3, 4), summators (5, 6, 7, 8, 11, 12), a transmitter (9) and a receiver (10) and elements (13, 14) for comparison with predetermined values (ε p , ε q ), wherein a first summator (5) is arranged to form a difference voltage ΔU Pp equal to the difference between the voltage (U' P ), measured in station P or calculated by means of the travelling wave model, and the corresponding voltage (U'' P ) one period or one half-period later, a second summator (7) is arranged to form a difference voltage ΔU Qq equal to the difference between the voltage (U' Q ), measured in station Q or calculated by means of the travelling wave model, and the corresponding voltage (U'' Q ) one period or one half-period later, a third summator (6) is arranged to form a difference voltage ΔU Pq equal to the difference between the voltage (U' Pq ) in station Q, calculated in the travelling wave model in station P with values of currents and voltages measured in station P, and the corresponding voltage (U'' Pq ) one period or one half-period later, a fourth summator (8) is arranged to form a difference voltage ΔU Qp equal to the difference between the voltage (U' Qp ) in station P, calculated in the travelling wave model in station Q with values of currents and voltages measured in station Q, and the corresponding voltage (U'' Qp ) one period or one half-period later, the difference voltages (ΔU Qp , ΔU Qq and ΔU Pp , ΔU Pq , respectively) are adapted to be transferred, via a transmitter and a receiver, from station Q to station P and/or from station Q to station P, a fifth summator (11) is arranged to form the difference ΔP = ΔU Pp - ΔU Qq , a sixth summator (12) is arranged to form the difference ΔQ = ΔU Pq - ΔU Qq , the device is arranged to deliver, via a first comparison member (13), a first signal (F p ) indicating a high resistance ground fault when ΔP is greater than the voltage difference ε p accepted at P, and the device is arranged to deliver, via a second comparison member (14), a second signal (F q ) indicating a high resistance ground fault when ΔQ is greater than a voltage difference ε q accepted at Q. Device according to claim 9, characterized in that the members included are arranged for analog signal processing.

Inventors: NIMMERSJOE, GUNNAR, Nimmersjö, Gunnar
Applicants: ASEA AB
Application year: 1986
Main classification: G01R31
IPC codes: G01R31, H02H3