METHOD AND APPARATUS FOR FAULT IDENTIFICATION IN ELECTRIC POWER TRANSMISSION SYSTEMS

Abstract: A method of identifying faults in a polyphase alternating current electric power transmission system wherein determination of the type of fault occurring is effected by monitoring the amplitudes and relative phases of fault current quantities derived from the phase currents of the system. The current quantities are preferably phase fault currents and quantities derived by addition and subtraction of phase fault currents. Apparatus for carrying out the method is also disclosed.

Description: This invention relates to methods and apparatus for protecting polyphase alternating current electric power transmission systems against faults. Such methods and apparatus are required to identify the type of fault that has occurred so that appropriate action to protect the transmission system can be taken. To this end many different schemes of varying degrees of sophistication have been proposed. Whilst many such schemes operate satisfactorily for most types of faults, a comprehensive fault identification method which will satisfactorily identify all likely types of fault is not presently available. Thus existing methods frequently exhibit shortcomings such as: incorrect discrimination between fault and load currents; incorrect identification of fault type, particularly in respect of double phase to earth faults; load encroachments; and excessive time to identify faults. Consequently, known schemes capable of reasonably comprehensive fault identification are necessarily highly sophisticated and complex, and frequently employ different fault identification methods for different types of fault. It is an object of the present invention to provide a method and apparatus for identifying faults in a polyphase alternating current electric power transmission system wherein this problem is overcome. According to one aspect of the present invention there is provided a method of identifying faults in a polyphase alternating current electric power transmission system wherein identification of the type of fault occurring is effected by monitoring the amplitudes and relative phases of fault current quantities derived from the phase currents of the system. In a preferred arrangement in accordance with the invention said current quantities are phase fault currents of the system and quantities derived by addition or subtraction of phase fault currents. In one such preferred arrangement for use in a three-phase system, said current quantities are at least some of the quantities Ir, Iy, Ib, Ir + Liy) + Ib, Ir + Iy, Iy + Ib, Ib + Ir, Ir - Iy, Iy - Ib and Ib - Ir, where Ir, Iy and Th are the respective phase fault currents. The particular methods which may be utilised to identify particular types of faults and the particular phase or phases involved are further explained below. In one particular method in accordance with the invention the direction of an identified fault is determined by comparing the phases of a post fault current relating to the faulted phase or phases and the corresponding pre-fault voltage. In addition to being capable of identifying all likely types of fault, a method according to the present invention has the advantages that; except where fault direction is determined, no voltage transformers are required; it suffers less from travelling wave distortion than methods using voltage quantities; and it is particularly adapted for execution by a digital computer, or other computer means capable of logical analysis of inputs. Hence, according to a second aspect of the invention there is provided an apparatus for identifying faults in a polyphase alternating current electric power transmission system comprising: a plurality of current transducer means for producing respective output signals representing the phase currents of the system; and computer means programmed to identify how many phases a fault condition occurring on the system involves and whether ground is involved in the fault condition, utilising said output signals as its sole input variables. Preferably the computer is further programmed to identify which particular phase or phases are involved in a fault condition. The computer may be further programmed to determine the direction of a fault. By way of further explanation of the invention, and by way of example, one method and apparatus for detecting faults in a three phase transmission system will now be described with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a three phase electric power transmission system; Figure 2 is a diagrammatic representation of the system of Figure 1 in a faulted condition; Figures 3(a) to 3(f) are Thevenin fault current diagrams for various types of fault condition in the system; Figure 4 is a block schematic diagram of the apparatus; Figures 5 and 6 are flow charts illustrating a programme carried out by a computer forming part of the apparatus; and Figures 7 is a diagram of a circuit used in the apparatus in identifying the direction of faults. The method is based on the realisation by the inventors that when a fault occurs on a transmission system the resulting fault currents can be analysed to determine what type of fault has occurred. This can be demonstrated, as follows: Referring to Figure 1, a three-phase power transmission system may be considered as comprising a length of line of impedance e per phase connecting two sources A and B producing red, yellow and blue phase voltages ERA' F A EBA and MX h Bs EBB, the sources A and B having impedances ZSA and ZSB per phase. The impedances ZSOA, E 0 and ZSOB represent the mutual impedances of the system whilst the impedances URSA' e and ZSB represent the self impedances of the system. By application of Thevenin's theorem, the fault currents which flow when a fault occurs, e.g. between the blue phase and ground as indicated in Figure 1, can be determined by considering the voltage sources A and B to be short-circuited, and a voltage source equal to the voltage at the fault point before the fault occurred connected across the fault point in series with the fault impendance ZF. This is shown in Figure 2 where Vpf is the red phase pre-fault voltage at the fault point, a is the phase operator - 0.5 + j 0.866, and Ir, Iy and Ib are the red, yellow and blue phase fault currents respectively, current Ib having oppositely directed components IbA and IbB on opposite sides of the fault point, and the ground current having similar components IeA and Ires. It will be appreciated that the fault currents Ir, ly and Ib are the changes in the phase currents which occur when a fault occurs, i.e. the differences between the pre-fault (load) phase currents and the post-fault phase currents, i.e. the total currents in the system under fault conditions. Figure 3(a) is a Thevenin fault current diagram for a ground fault on the blue phase with the impedances omitted for simplicity. For such a fault the following conditions hold: Ir + Iy + Ib exists (i.e. is finite) Iy - Ib and Ib - Ir exist and are in-phase or antiphase with Ir + Iy + Ib Ir - Iy is zero The presence of a blue phase ground fault can thus be identified uniquely by determining that Ir + It + Ib exists, that the two difference currents Iy - Ib and Ib - Ir involving the blue phase exist and are in phase or antiphase with Ir + Iy + Ib, and that the other difference current Ir - Iy is zero. Similar sets of conditions uniquely identify the presence of red phase to ground and yellow phase to ground faults. Referring to Figures 3b if one source is not grounded so that Ir + Iy + Ib does not exist, the following condition identifies the presence of a single phase to ground fault. Ir + ly, Iy + Ib and Ib + Ir all exist and are respectively in phase or antiphase with Ib, Ir and Iy. The particular faulted phase red, yellow or blue can then be identified, as before, by examination of Ir Iy - Ib and Ib - Ir. It will be noted that, in the case of single phase faults the current, quantities Ir, Iy, Ib, Ir + Iy, Iy + Ib, Ib + Ir, and Ir + Iy + Ib are all in phase or anti-phase with one another although, depending on whether the system sources are grounded and whether the fault point is to the left or right of the current monitoring point, some of these quantities may not exist. Thus, to establish phase relations, the various current quantities are interchangeable. Figure 3(c) is a Thevenin fault current diagram for yellow phase to blue phase fault when the following conditions hold: Ir + Ib + Iy does not exist Iy + Ib does not exist Ib + Ir exists and is anti-phase with Ir + ly. The presence of a yellow phase to blue phase fault can thus be uniquely identified by determining that Ir + Ib + Iy and the sum current Iy + Ib do not exist, and that the other sum currents Ib + Ir and Ir + Iy exist and are in anti-phase. The other phase to phase faults can be similarly identified. It will be noted that in general for a phase to phase fault, on17 positive and negative sequence quantities exist, and the positive sequence current is anti-phase with the negative sequence current. The sound phase conductor appears to the fault generator like an extra earth conductor and since no zero sequence current flows, no sound phase current exists. Thus, the identification of a phase to phase fault could also be made using the following set of conditions, the case of a yellow and blue phase fault again being considered by way of example. Ir + Iy + Ib does not exist Ir does not exist Iy exists and is anti-phase with Ib. The fact that Iy and Ib exist indicates the fault is between these phases. The following set of conditions could also be used Ir + Iy + Ib does not exist Iy - Ib does not exist Ib - Ir exists and is anti-phase with Ir - ly. The fact that the finite quantities both involve Ir indicates that the red phase is sound and the fault is therefore between yellow and blue phases. Figure 3(d) is a Thevenin fault current diagram for a yellow and blue phases to ground fault when the following conditions exist. Ir + Iy + Ib exists Ir + ly, Iy + Ib and Ib + Ir all exist but only Iy + Ib is in phase or anti-phase with Ir + IY + Ib. The presence of a yellow and blue phases to ground fault can thus be uniquely identified by determining that Ir + ly + Ib, Ir + Iy, Ly + Ib and Ib + Ir all exist and that only the sum current Iy + Ib is in phase or anti-phase with Ir + Iy + Ib. Referring to Figure 3(e) if one source is not grounded so that Ir + ly + Ib does not exist, the following condition can be used to detect the existence of a yellow and blue phases to ground fault. Ir + Iy, Iy + Ib and Ib + Ir all exist but only Iy + Ib is in phase or anti-phase with Ir. The presence of such a fault can thus be uniquely identified by determining that the sum currents all exist and that only the sum current Iy + Ib is in phase or anti-phase with the other phase current Ir. The other two phase faults to ground can be similarly identified. In general terms, for a two phases to ground fault both sound phase and earth return currents are zero sequence quantities so that, by girchoff's current law, the sum of the currents in the faulted phases must also be a zero sequence quantity. Figure 3(f) is a Thevenin fault current diagram for a three phases to ground fault when the following conditions exist Ir + Iy + Ib does not exist Ir + Iy, Iy + Ib, Ib + Ir exist and are at 1200 separations. This condition uniquely identifies a three phase fault to ground fault. It will be appreciated that this applies equally to a three phase fault between phases not involving ground. In general terms for a three phase fault all sets of corresponding current quantities are at 1200 separations. Thus a three phase fault can alternatively be identified by determining that Ir + Iy + Ib does not exist and Ir, Iy and Ib, or Ir - Iy, Iy - Ib and Ib - Ir all exist and are at 1200 separations. It will be appreciated that in a practical implementation of the above fault identification method the fault phase currents Ir, Iy, Ib and the current quantities derived -therefrom will be considered to exist only when they exceed certain set minimum values. Similarly, quantities will be considered to be in phase or anti-phase or at 120 separation so long as their phase difference is within a set amount of 00, 1800 or 120 respectively. The set values for determining existance of quantities are suitably determined by considering a single phase to ground fault of maximum fault resistance at the far end of the protected line section with minimum source MVA. The set value for Ir + Iy + Ib is suitably taken as this fault current times the attenuation introduced by the current measuring apparatus used. The set values for Ir, Iy, Ib, Ir - Iy, Iy - Ib and Ib - Ir are suitably made the same, and the set values for Ir + ly, ly + Ib and Ib + Ir are suitably made twice that for Ir + Iy + Ib. One apparatus for detecting three phase transmission system faults using the above described method will now be described. Referring to Figure 4, the apparatus includes three current transformers 1, 3 and 5 by means of which currents representative of the red, yellow and blue phase currents of the system are respectively supplied to three transactors 7, 9, and 11. The transactors convert their input currents to corresponding voltages and remove any d.c. offsets which may be present. The transactor output voltages are supplied to analogue to digital converters 13, 15 and 17 via respective low pass filters 19, 21 and 23 which remove any high frequency components which may be present. The outputs of the converters 13, 15, and 17, which comprise digital signals representing samples of the instantaneous values of the phase currents, are supplied to a micro computer 25. In the micro computer 25 fault identification is carried out using fault current quantities. To obtain these quantities the computer memory stores phase current instantaneous value samples supplied by the converters 13, 15 and 17 in respect of at least a quarter of a cycle of the system waveform, typically one half cycle. As each new value sample is received it is subtracted from the stored value sample relating to the same point on wave as the new sample, and the stored sample is then discarded and replaced in the memory by the new sample. The results of the subtractions are passed to the central processing unit of the computer 25 where they are analysed as further described below. It will be apparent that for a period following a fault equal to that over which samples are stored, the results of the subtractions indicate phase fault currents. The computer is programmed to calculate from the phase fault currents Ir, Iy, Ib the other fault current quantities required for fault identification i.e. Ir + Iy + Ib, Ir + Iy, Iy + Ib, Ib + Ir, Ir - Iy, Iy - Ib and Ib - Ir. The amplitude and phase of each fault current quantity relative to a common time reference is then calculated by the computer using the algorithms EMI9.1 and EMI10.1 where is the phase of the current quantity relative to the time reference A I is the amplitude of the current quantity il and i2 are samples of the instantaneous values of the current quantity at times t1 and tl + ht after the time reference and Z is the angular frequency of the system. The computer monitors the current quantity amplitudes using the algorithm B (for which no time reference is required) and indicates when these meet the operating limits determined by the set values described above. Once a limit has been exceeded the relative phase angles required for fault detection are also calculated using the algorithm A and using the time at which the limit was first exceeded as the time reference. Having established whether the various quantities exist and their relative phase angles the computer then utilises the above described method to identify the type of fault that has occurred, and the phase or phases involved in the fault. Figure 5 is a flow chart which gives a general idea of the process involved in fault identification whilst Figure 6 is a flow chart which shows the actual logic steps which the computer is programmed to carry out. Taken together with the above description of the conditions for identifying the different types of fault, Figures 5 and 6 are self-explanatory. Having identified the type of fault that has occurred the computer determines the actual phase or phases involved in the fault, using the method described above. Thus, for single phase to ground faults the faulted phase is indicated by the absence of the faulted phase current in the difference current quantity which is zero. For two phase faults the sound phase is the phase whose current appears min the two sum currents which exist. For two phase to ground faults the faulted phases are the phases for which the sum current is in phase or anti-phase with Ir + Iy + Ib, or in the case of non-earthed sources, the other phase current. For three phase faults no phase selection is required. Having identified the phase or phases involved in the fault, the computer passes appropriate command signals to circuit breaker control equipment (not shown). In addition to identifying the type of fault and the phase or phases involved, the computer may be arranged to indicate the fault direction. The method used is to compare the phases of the appropriate postfault current and the corresponding pre-fault voltage. For single phase to ground faults a fault on the line side is taken to have occurred if the postfault, faulted phase current lags the pre-fault, faulted phase voltage by less than 1350 or leads it by less than 45 . A fault on the source side is thus indicated if this current leads the voltage by more than 450 or lags it by more than 1350. For phase to phase and phase to phase to ground faults the same test is applied to the post-fault line current and pre-fault line voltage between the faulted phases. For three phase faults the same test is applied to any post-fault phase or line current and the corresponding pre-fault voltage. In order to enable the computer 25 to carry out the above phase comparisons the phases of the various quantities concerned are determined relative to a particular reference point on a pre-fault voltage quantity waveform as opposed to a point defined by the moment when a set value is first exceeded. The phases of the current quantities relative to the latter point are, of course, available by use of the algorithm described above for fault type identification. A circuit for defining the required reference point at a positive going zero crossing in the red phase pre-fault voltage is shown in Figure 7. The circuit operates to determine the phase angle between the moment when a set value is first exceeded and the previous positive-going zero crossing in the red-phase pre-fault voltage. Referring to Figure 7, the circuit includes a waveform squaring circuit 27 to which a voltage corresponding to the red phase voltage is applied via a voltage transformer (not shown). The output of the squarer 27 is applied to two integrators 29 and 31, one integrator 31 being supplied with the squarer output via an inverter 33. The output of the integrator 31 is applied to one input of a summing circuit 35 whose other input is supplied by an attenuator 37 whose input is derived from the output of the inverter 33. The outputs of the summing circuit 35 and the integrator 29 are summed in a further summing circuit 39 whose output is fed to the computer 25. In operation the integrators 29 and 31 are reset at each zero crossing of the input signal and produce voltage ramps of the same polarity during positive and negative half cycles of the input voltage respectively. The attenuator constant K is set so that the attenuator output during negative half cycles is equal to the output of the integrator 29 at the end of a half cycle when it is integrating. The output of the summing circuit 39 is thus a ramp voltage rising continuously over each full cycle of the input voltage, being reset at each positive-going zero crossing. When the computer detects a fault the output of the summing circuit 39 is immediately read by the computer, this output being representative of the pEase angle between the moment when a set value is first exceeded and the preceding positive-going zero crossing in the red phase pre-fault voltage. Using this output the phase of any particular pre-fault voltage required for fault direction determination can easily be determined with respect to the reference point by simple vector relationships. Similarly, the phase of any particular postfault current required for fault direction determination can easily be determined with respect to the reference point since the phase of all such currents relative to the moment that a set value is first exceeded is already available. However, allowance must be made for any phase shift at system frequency produced by the transactors:7, 9, 11 and the filters 19, 21, 23. Having thus established the phases of the required pre-fault voltage and post-fault current with respect to a common reference point, their phase difference and the fault direction can be determined as described above. The circuit of Figure 7 may be of digital or analogue form, but in the latter case the output of the summing circuit 39 must, of course, be digitised for use by the computer 25. It will be understood that in the method and apparatus described above by way of example fault detection and direction is carried out on the basis of fault current quantities. However, it will be appreciated that in methods and apparatus for use in a system wherein fault currents greatly exceed load (pre-fault) currents, e.g. a lightly loaded system, post-fault currents differ very little from fault currents and may be used instead of fault currents. Moreover, by appropriately adjusting the set values, post-fault currents can be used even when the load currents may be an appreciable proportion of the postfault currents. It should thus be understood that the term fault current quantity as used in the present specification embraces any current quantity, e.g. post-fault current, which can in the context of a particular system, be considered as equivalent to fault current. It will be appreciated that the present invention is theoretically applicable to fault identification in polyphase power transmission systems having other than three phases, although in practice the number of phases is rarely, if ever, other than three.

Claims: CLAIMS 1. A method of identifying faults in a polyphase alternating current electric power transmission system characterised in that identification of the type of fault occurring is effected by monitoring the amplitudes and relative phases of fault current quantities derived from the phase currents of the system. 2. A method according to Claim 1 wherein said current quantities are phase fault currents of the system and quantities derived by addition or subtraction of phase fault currents. 3. A method according to Claim 2 for use in a three-phase system wherein said current quantities are at least some of the quantities Ir, Iy, Ib, Ir + ly + Ib, Ir + Iy, Iy + Th, Ib + Ir, Ir - ly, Iy - Ib and Th - Ir, where Ir, Iy and Ib are the respective phase fault currents. 4. A method according to Claim 3 wherein the presence of a ground fault on a single phase of the system is identified by determining that Ir + Iy + Th exists and that two of the difference currents Ir Iy - Ib and Ib - Ir exist and are in-phase or antiphase with Ir + Iy + -Ib and the other difference current does not exist. 5. A method according to Claim 3 wherein the presence of a ground fault on a single phase of the system is identified by determining that the sum currents Ir + ly, Iy + Ib and Ib + Ir all exist and are in phase or antiphase with Ib, Ir and Iy respectively. 6. A method according to any one of Claims 3 to 5 wherein the presence of a phase-to-phase fault is identified by determining that Ir + Iy + Ib does not exist and that of the three sum currents Ir + Iy, Iy + Ib and Ib + Ir, the three difference currents Ir - Iy, Iy - Ib. and Ib --Ir or the three phase currents Ir, Iy and Ib, only one does not exist and the other two exist and are antiphase. 7. A method according to any one of Claims 3 to 6 wherein the presence of a two phases to ground fault is identified by determining that Ir + Iy + Ib and the sum currents Ir + Iy, Iy + Ib and Ib + Ir all exist but only one sum current is in phase or antiphase with Ir + Iy + Ib. A L method according to any one of Claims 3 to 6 wherein the presence of a two phases to ground fault is identified by determining that the sum currents. Ir + Iy, Iy + Ib and Ib + Ir all exist but only one sum current is in phase or antiphase with the phase current Ir, ly or Ib not involving that sum current. 9. A method according to any one of Claims 3 to 8 wherein the presence of a three phase fault is identified by determining that Ir + Iy + Ib exists and that the three sum currents Ir + ly, Iy + Ib and Ib + Ir, the difference currents Ir - liy, ly - Db and Ib - Ir or the three phase currents Ir, Ty and Ib all exist and are at 120? phase separations. 10. Method according to any preceding claim wherein the direction of an identified fault is determined by comparing the phases of a post fault current relating to the faulted phase or. phases and the corresponding prefault voltage. 11. A method according to Claim 15 wherein for a single phase to ground fault a fault on the line side of the monitoring position is taken to have occurred if the post-fault, faulted phase current lags the pre-fault, faulted phase voltage by less than 1350 or leads it by less than 45 . 12. A method according to Claim 10 wherein for phase to phase faults and two phases to ground faults a fault on the line side of the monitoring position is taken to have occurred if the post-fault line current between the faulted phases lags the pre-fault line voltage between the faulted phases by less than 1350 or leads it by less than 45 . 13. An apparatus for identifying faults in a polyphase alternating current electric power transmission system characterised by a-plurality of current transducer means (1 to 23 Figure 4) for producing respective output signals representing the phase currents of the system; and computer means (25) programmed to identify how many phases a fault condition occurring on the system involves and whether ground is involved in the fault condition, utilising said output signals as its sole input variables. 14. An apparatus according to Claim 13 wherein the computer means carries out fault identification from fault current quantities obtained by storing samples of the transducer output signals in respect of at least a quarter of a cycle of each output signal waveform, and subtracting each new sample of an output signal from the stored value of the corresponding output signal relating to the same point on wave as the new sample. 15. An apparatus according to Claim 14 wherein the amplitude and the phase of each fault current quantity required for fault identification relative to a common time reference is calculated by the computer using the algorithms and EMI17.1 where ¯ is the phase of the current quantity relative to the time reference is the amplitude of the current quantity and i2 are samples of the instantaneous values of the current quantity at times tl and tl + At after the time reference and w is the angular frequency of the system. 16. An apparatus according to Claim 15 wherein the computer continuously monitors the fault current quantity amplitudes and, from the moment when any of these exceeds a predetermined set limit, calculates the fault current amplitudes and relative phase angles using the time at which a set limit was first exceeded as the common time reference. 17. An apparatus according to Claim 16 wherein the computer is further programmed to indicate fault direction by comparing the phases of a post fault current relating to the faulted phase or phases and the corresponding prefault voltage, the phase of the relevant post fault current relative to the relevant pre-fault voltage'being obtained by determining the phase angle between the moment when a set limit is first exceeded and a predetermined point on the relevant pre-fault voltage and adding this to the phase relation between said relevant post fault current and said time reference.

Inventors: OWEN, MICHAEL, SANDERSON, JOHN VICTOR HAROLD
Applicants: THE GENERAL ELECTRIC COMPANY, P.L.C.
Application year: 1980
Main classification: G01R31
IPC codes: G01R29, H02H7, G01R31, H02H3