METHOD OF AND APPARATUS FOR TRANSMITTING INFORMATION ABOUT FLOW OF FAULT CURRENT IN A FAULTY SECTION OF AN ELECTRIC POWER TRANSMISSION SYSTEM

Abstract: A method and an apparatus for indicating, at a receiver (37), the location of sections of a cable or line, in an elec­ tric power system, which have carried a fault current, by providing, for each section, a transmitter (20) including energy storage means charged by the fault current flow via a current transformer (16) and operative, on cessation of the fault current, to initiate, after a predetermined delay different for each section, the injection by transmitter (20) of a high frequency pulse signal into the cable or line (10, 11) also through the current transformer (16) so that, at the receiver (37), a record of the times of receipt of the signals identifies all the sections which have passed the fault current.

Description: METHOD OF AND APPARATUS FOR TRANSMITTING INFORMATION ABOUT FLOW OF FAULT CURRENT IN A FAULTY SECTION OF AN ELECTRIC POWER TRANSMISSION SYSTEM This invention relates to the location of a faulty section in an electrical power transmission system. In a typical electrical power transmission system in which power is transmitted at a very high voltage and is transformed down to say 11 kV at sub-stations, the feeders from the sub-stations would typically comprise several sections of cable with a section switch between each section to enable a faulty section to be isolated. If a fault occurs on this feeder causing a fault current to flow, it is necessary to determine which section is faulty and the common practice is to provide, at each switch, a latched relay which will operate on the passage of fault current. It is then necessary for the investigating engineer to inspect these relays, which give a visual indication, to determine which of the section switches along the feeder have passed the fault current. The fault is in the section immediately beyond the furthest switch to have recorded the passage of the fault current. Such switches are commonly at unattended locations and the engineer has to visit each section switch in order to locate the faulty section of cable. The present invention, from one of its aspects, is concerned with an improved method and apparatus for transmitting to a central location information identifying each section which has passed a fault current, making use of the power transmission cable or line for transmitting the information. It will be appreciated that, following a fault, the feeder may have been isolated by tripping of a circuit breaker and there is then no mains power supply available at the locations from which the information is to be transmitted. According to one aspect of the present invention, a method of transmitting information from each section of a cable or line in an alternating current power transmission system having a plurality of sections, comprises the steps of sensing the passage of a fault current at a point in each section, utilising the sensed fault current at said point to charge capacitance means, energising a transmitter with the stored energy to transmit a signal or signals at a predetermined time after the fault current into the cable or line, the transmitters at the different points being arranged to transmit the signal or signals at different predetermined time intervals after the cessation of the fault current, and receiving the various signals transmitted along said cable or line and recording the received signals to indicate in which sections the fault current has been sensed. As explained above, the cable or line would typically have switches between the sections, to enable any faulty section to be isolated, and conveniently the passage of the fault current is sensed adjacent each section switch. Using this technique, the information about the passage of fault current in the various sections is transmitted and recorded at a central location without any necessity for providing batteries or other power supplies at the location of the transmitters and without any separate communication link apart from the power transmission line. The fault current may be sensed and the capacitance means charged using a current transformer to extract power from the fault current. The signal transmission is conveniently effected by means of a pulse or pulses of a high frequency carrier. The same carrier frequency may be used for all the signals. Each transmitter may be arranged to transmit for example a single pulse of a carrier at a frequency of several kHz. The signal injection into the cable may be effected using a current transformer. If the fault current is sensed using a current transformer, the carrier frequency may be injected into the transmission line ot the same current transformer or through a separate current transformer. In a feeder in a power distribution system, the fault current may have tripped a circuit breaker at the input substation and hence the feeder will be opencircuit at this point. The receiver conveniently is located at this substation and it is therefore preferred to arrange the receiver to detect a voltage signal rather than a current signal. Thus, at the receiver, voltage detecting means may be provided for detecting the signal, for example using a capacitive coupling. The invention furthermore includes within its scope apparatus for detecting the location of a faulty section of a power transmission line or cable which has a plurality of sections, which apparatus comprises, for each section, means responsive to a fault current arranged to charge capacitance means forming an energy storage, a transmitter arranged for energisation with the stored energy and to transmit into said transmission line or cable a signal at a predetermined time after the cessation of the fault current, said predetermined times differing for the different transmitters whereby the signals may be received in succession at a receiver responsive to voltage signals on the line. The transmission line would commonly be a cable having an outer sheath or shield which is earthed. The fault current may be detected using a current transformer coupled around the cable, with the outer sheath or shield interrupted adjacent the current transformer, a bypass circuit being provided for electrical continuity across the interruption, the bypass circuit being linked with the current transformer to give a flux in opposition to that from the current in said sheath or shield. In this case, for each section, the transmitter may be arranged to feed said signal into said current transformer to be coupled thereby into the cable or into a separate current transformer. Such apparatus may have a receiver coupled to said cable or line to he responsive to said signals, the receiver having means for indicating the relative times of reception of the signals whereby the signals received from the various transmitters a-re identifiable. The invention furthermore includes within its scope apparatus for transmitting a signal representative of a fault current flow-ing in a conductor or conductors of a power transmission cable having a metal sheath or shield, which apparatus comprises a current transformer around the cable, the shielding of the cable being interrupted adjacent the transformer with a bypass circuit to provide electrical continuity extending around the interruption, said bypass circuit being coupled into the current transformer to provide coupling of the bypass current in opposition to the direct coupling from the current in the sheath or shield, which sheath or shield passes through the current transformer, and a transmitter responsive to the flow of fault current and arranged for injecting a high frequency signal into said current transformer for coupling to a conductor or conductors in the cable. The following is a description of one embodiment of the invention, reference being made-to the accompanying drawings in which: Figure 1 is a diagram illustrating part of a power supply system with a transmitter and a receiver for fault location indication; and Figure 2 illustrates in further detail a transmitter employed in the arrangement of Figure 1. Referring to Figure 1 there is shown diagrammatically part of a cable comprising a power transmission line, this cable being shown diagrammatically as a conductor 10 with an outer earthed sheath and armouring 11. Typically the cable would have three phase conductors within the sheath. The cable extends from a primary station indicated diagrammatically at 12 and which might typically contain a transformer for transforming the voltage down from EHv supply to, for example, 11 kV for transmission over the cable to a plurality of distribution substations. The primary station typically would also include a circuit breaker 13 which would be tripped by passage of excessive current, such as would for example be produced by an earth fault on the feeder. One of the distribution substations is indicated diagrammatically at 14. Typically such a substation would include a switch shown diagrammatically at 15 and known as a section switch, which switch can be operated to isolate a faulty section of the cable. The fault current at a point adjacent the section switch is detected by means of a current transformer 16 extending around the cable. This transformer is coupled also to a bypass connector 17 forming a bypass around an interruption in the sheath of the cable constituted by an insulating gland 18; the current jn the bypass connector is the same as that in the sheath or shield but is coupled in opposition into the current transformer so that the latter only senses the fault current flowing in the core or cores of the cable. This current transformer, in this particular embodiment, is also utilised for the injection of a high frequency signal, typically having a frequency of several kHz, from a transmitter 20 which is illustrated in further detail in Figure 2. Referring to Figure 2 the cable is shown at 25 with the current transformer 16 extending around the cable and also around the aforementioned bypass connection 17. When fault current passes through the conductor cores, a flux at the mains supply frequency is produced in the current transformer and an output signal at this frequency from a secondary winding passes through a low pass filter 26 to energise a charging circuit for an energy store 27. The fault current in a power transmission circuit is of very short duration. Most faults are earth faults and existing earth fault relay systems make use of instrumenttype current transformers with low saturation factors. The larger the magnitude of the fault current, the shorter is the duration of the fault current flow because of the faster protection trip times. Hence the energy storage charging circuit has to be arranged to store sufficient charge for the shortest fault current flow times. A simple capacitor and rectifier circuit may be employed as the energy store 27. A sensing circuit 28 senses the initiation of charging ofthe store 27 and includes a timer or delay unit arranged to close an electronic switch 29 at a predetermined delay time after the cessation of the fault current. This switch 29 couples the energy store 27 to an oscillator and output stage 30 to generate a single pulse of a high frequency carrier, typically at a frequency of several kHz. This carrier pulse is injected by a lead 31 into the current transformer 16 so giving a voltage signal on all the phase conductors, this voltage signal being the same on all three phases with respect to earth. It is alternatively possible to use a separate current transformer for injection of the high frequency signal into the cable instead of using the same current transformer 16 as is provided for sensing the fault current. The voltage signal is injected in series with the cable and the signal effectively is impressed across an impedance made up of the input impedance towards the primary substation 12 plus the transmission line impedance away from the point of injection towards the next section switch along the transmission line. The magnitude of the signal voltage appearing in each direction will depend on the relative network length in each direction and the impedance of the fault in the direction away from the distribution substation at which the signal is injected. It is readily possible to inject sufficient signal energy into the cable to enable signals to be received at the primary substation 12 from any of the further distribution substations along the transmission line The circuit breaker 13 at the primary substation 12 may be open during the signalling period and hence it is necessary, at that point, to receive the signals by detecting a voltage on the cable. This is done by making use of a.capacitive connection as shown in Figure 1 where the capacitance is provided by the screen capacitance of a current transformer, the screen being indicated diagrammatically at 32. For this purpose, it is convenient to use the high voltage discharge-preventing earth screen between the bushing insulation and the windings of this current transformer. The voltage signal on this screen is applied. to a primary winding 33 on a further transformer 34. A secondary winding 36 of this transformer is coupled to a receiver 37 feeding a display or recorder 38. Capacitive coupling to provide a voltage signal for the receiver however may readily be obtained in other ways. For example, a small section of the cable-lead sheath may be isolated by cutting two circumferential gaps in the sheath. An earth continuity connection would be provided across. the isolated section. The isolated section. of the sheath. however may- then. be utilised as a capacitance couplIng with. the cable core conductors to detect the voltage signals with respect to earth on these conductors. The receiver 37 is arranged to indicate the various received signals which are injected at different times at the different distribution substations for each of the substations at which the section switches are passing fault current. Thus, when an earth fault occurs on a feeder, for each section which is passing fault current, the current transformer in that section produces an output which charges the capacitor power store in the associated transmitter 20. The current transforn7ers 16 for the sections which. are not passing fault current remain dormant and the transmitters inactive. For the sections, passing the fault current, the energy stores 27 are charged but the transmitters remain inactive until fault current ceases due to the tripping of the circuit breaker 13 at the primary substation 12. The cessation of the fault current initiates the timer operation and so causes each transmitter along the feeder line to transmit a signal back to the primary substation, provided the fault current has passed through the current transformer at that transmitter to energise the transmitter. Different transmitters have different delay times so that the transmissions from different transmitters are received back at different times. All the transmitters and the receiver are synchronised by the cessation of the fault current. If the fault current should occur in bursts, the system will re-synchronise and reset at the end of each burst. The various time delays for the different transmitters may be in any sequence so long as the order is known at the receiver. The receiver looks in each time slot successively and conveniently includes a storage register which records the particular time slots in which transmissions are detected. The storage register is controlled by a clock 39 to operate in a cycle initiated by a sensor 40 responsive to the cessation of the fault current, i.e. the opening of the circuit breaker 13, and, following the completion of a clock cycle, the receiver is inhibited. The storage register thus indicates the location of the various transmitters which have sent in signals. It is thus possible to determine precisely which section of the network has carried the fault current. Using tris information, the network may be switched manually for example to remove the fault and restore supply from an alternative route without any inspection of individual substations 14 along the feeder.

Claims: CLAIMS: 1. A method of transmitting information from each section of a cable or line in an alternating current power transmission system having a plurality of sections comprising the steps of sensing the passage of a fault current at a point in each section, utilising the sensed fault current at said point to charge capacitance means, energising a transmitter with the stored energy to transmit a signal or signals at a predetermined time after the fault current into the cable or line, the transmitters at the different points being arranged to transmit the signal or signals at different predetermined time intervals after the cessation of the fault current, and receiving the various signals transmitted along said cable or line and recording the received signals to indicate in which sections the fault current has been sensed. 2. A method as claimed in claim 1wherein the fault current is sensed and the capacitance means charged using a current transformer to extract power from the fault current. 3. A method as claimed in either of the preceding claims wherein said signal is transmitted as a pulse. or pulses of a high frequency carrier. 4. A method as claimed in claim 3 wherein the same carrier frequency is used for all the signals. 5. A method as claimed in either claim 3 or claim 4 wherein said high frequency carrier is injected into the cable or line through a current transformer. 6. A method as claimed in either- claim 3 or claim 4 as appendant to claim 2 wherein said carrier frequency signal is injected into the cable or line through the same current transformer as is used for extracting power from the fault current. 7. Apparatus for detecting the location of a faulty section of a power transmission line or cable which has a plurality of sections which apparatus comprises, for each section, means responsive to a fault current arranged to charge capacitance means forming an energy storage, a transmitter arranged for energisation with the stored energy and to transmit into said transmission line or cable a signal at a predetermined time after the cessation of the fault current, said predetermined times differing for the different transmitters whereby the signals may be received in succession at a receiver responsive to signals on-the line. Apparatus as claimed in claim 7 wherein said line is a cable and wherein, for each section, said transmitter is arranged to feed said signal into a current transformer coupled to the cable, which current transformer constitutes part of said means responsive to the fault current. 9. Apparatus as claimed in either claim 7 or claim 8 wherein each transmitter is arranged to produce a pulse or pulses of a high frequency carrier. 10. Apparatus as claimed in any of claims 7 to 9 and having a receiver coupled to said line to be responsive to said signals, the receiver having means for indicating the relative times of reception of the signals whereby the signals received from the various transmitters are identifiable. 11. Apparatus as claimed in claim 10 wherein the indicating means includes means providing a record or indication identifying each of the received signals. 12. Apparatus as claimed in either claim 10 or claim 11 wherein the receiver is capacitively coupled to the line to detect the signals as voltage signals. 13. Apparatus for transmitting a signal representative of a fault current flowing in a conductor or conductors of a power transmission cable having a metal sheath or shield which apparatus comprises a current transformer around the cable, the shielding of the cable being interrupted adjacent the transformer with a bypass circuit to provide electrical continuity extending around the interruption, said bypass circuit being coupled into the current transformer to provide coupling of the bypass current in opposition to the direct coupling from the current in the sheath or shield, which sheath or shield passes through the current transformer, and a transmitter responsive to the flow of fault current and arranged for injecting high frequency signals into said current transformer for coupling to a conductor or conductors of the cable,

Inventors: FORMBY, JOSEPH RICHARD, ROE, GERALD
Applicants: THE ELECTRICITY COUNCIL
Application year: 1979
Main classification: G08B25
IPC codes: H02H3, H02H1