GB1571802A

GB1571802A - Fault location in electric distribution cables

Info

Publication number: GB1571802A
Application number: GB279676A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1977-01-24
Filing date: 1977-01-24
Publication date: 1980-07-16

Description

(54) IMPROVEMENTS IN AND RELATING TO FAULT LOCATION IN ELECTRIC DISTRIBUTION CABLES (71) We, BICC LIMITED of 21 Bloomsbury Street, London, WC1B 30N, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of and apparatus for indicating the existence and condition of a fault and locating the fault in an electric distribution cable.

More particularly, the invention relates to a system of fault indicating and locating on distribution cables to consumers wherein difficulties in disconnecting consumers make existing fault location methods impractical.

Test equipment is presently available for fault location on high voltage cables using a capacitor discharge surge generator. The purpose of the capacitor discharge surge generator is to apply a voltage impulse of sufficient magnitude to the cable in order to create a breakdown at the fault position so that a shock wave is caused to travel from the point of fault through the ground to the surface where it may be detected acoustically.

Generally, however, the voltage at which this equipment must operate excludes its use on distribution cables feeding consumers, especially where the requirement to keep consumers connected is considered.

It is an object of the present invention to provide an improved method of indicating and locating a fault in an electric distribution cable feeding consumers, which method can be effected without disconnecting consumers.

It is a further object of the invention to provide improved apparatus for carrying out the aforesaid method.

According to the invention, the improved method comlprises the steps of: applying a number of pulses derived from cycles of an a.c. voltage normally applied to the cable by the selected operation of a switching device; passing said pulses along said cable; determining the current produced by said pulses in said cable to indicate the existence and condition of a fault; and detecting the energy dissipated by said pulses on reaching the fault in the cable thereby to determine the location of the fault.

It will be appreciated that the level of current flowing through the switching device will serve to determine whether or not a fault condition exists and that the location of a fault can be determined by detection of the mechanical shock created at the fault point by the passage of current through the fault.

Each pulse of the number of pulses may be a positive or negative half cycle or may be a fraction of a half cycle but, preferably, each pulse of the number of pulses is an integral number of consecutive half cycles.

The improved apparatus for effecting the improved method as hereinbefore described comprises a switching device connectable in series with said cable; means for selectively operating said switching device to produce, from an a.c. voltage normally applied to said cable, a number of pulses derived from cycles of said a.c. voltage; current display means to determine the current produced by said pulses in the cable; and means for detecting the energy dissipated by said pulses on reaching the fault in the cable.

The switching device may be a single thyristor but preferably it comprises a pair of thyristors connected in inverse parallel.

Where the switching device is a single thyristor, the operating means of the thyristor is preferably a control circuit adapted to render the thyristor conducting during selected positive or negative half cycles of the a.c.

voltage or during fractions of selected half cycles of the a.c. voltage.

Where the switching device is a pair of thyristors connected in inverse parallel, the operating means of the pair of thyristors is preferably a control circuit adapted to fire the thyristors for a predetermined number of cycles so that each pulse of the number of pulses is an integral number of consecutive half cycles.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram showing the preferred apparatus according to the present invention, and Figure 2 is a circuit diagram showing an alternative embodiment.

Referring to Figure 1 a pair of inverseparallel connected thyristors 1, 2 are connectable between a busbar 10 and a cable 11 under test through connecting links 13 and 14 respectively.

A measurement current transformer 5 is arranged between the thyristors and link 13.

A fuse 3 which replaces the fuse which normally protects the cable and a fuse 4 to protect the thyristors of the apparatus, are connected between the thyristor pair and the links 13 and 14 respectively. The current transformer 5 is connected to the control circuits through a terminal box 15 and an input transformer 16. The current measured by the transformer 5 is passed to a current display unit 17 through terminal box 15 to indicate the existence and condition of a fault.

Power for the control circuits and the voltage reference are taken from the busbar 10 with neutral connection 12 providing the return path through a connecting link 18.

Referring to the alternative embodiment shown in Figure 2, apparatus 21 for indicating and locating a fault in a cable includes a switching device in the form of a single thyristor 22 connected in series with a cable 30 under test through a fuse 31 which protects thyristor 22. In addition a fuse 24 is incorporated in the apparatus 21 to replace the fuse which normally protects the cable 30 and is located as close to the supply busbar as possible. The ratings of the fuses 24 and 31 are so selected to allow a pulse of a desired magnitude to pass but to protect the- apparatus and cable in the event of excessive pulse energy. The apparatus 21 also includes in series with thyristor 22, a main switch 23.

The firing of thyristor 22 is controlled by inhibit control 25 and firing angle control 26, an input to the inhibit control 25 being derived from the busbar voltage. Firing angle control 26 allows the energy content of the pulses to be regulated. A measurement current transformer 33 provides an input to a current display unit 38 through a terminal box 35. The energy dissipated at fault 32 on cable 30 is detected using a suitable acoustic detector. In this way the location of the fault may be readily determined.

ln operation, the thyristor pair of the apparatus shown in Figure 1 and the single thyristor of the apparatus shown in Figure 2 are pulsed at suitable intervals, typically a few seconds. The amplitude of the current pulse obtained depends on the fault level at the point of fault, on th impedance of the fault itself and on the firing angle. Thus the displayed current level provides an indication of the existence and condition of the fault.

WHAT WE CLAIM IS:- 1. A method of indicating and locating a fault in an electric distribution cable which comprises the steps of: applying a number of pulses derived from cycles of an a.c. voltage normally applied to the cable by the selected operation of a switching device; passing said pulses along said cable; determining the current produced by said pulses in said cable to indicate the existence and condition of a fault; and detecting the energy dissipated by said pulses on reaching the fault in the cable thereby to determine the location of the fault.

2. A method as claimed in Claim 1, wherein the acoustic and/or seismic energy dissipated by said pulses is detected.

3. A method as claimed in Claim 1 or 2, wherein each pulse of the number of pulses is a positive or negative half cycle.

4. A method as claimed in Claim 1 or 2, wherein each pulse of the number of pulses is a fraction of a half cycle.

5. A method as claimed in Claim 1 or 2, wherein each pulse of the number of pulses is an integral number of consecutive half cycles.

6. Apparatus for indicating and locating a fault in an electric distribution cable comr prising a switching device connectable in series with said cable, means for selectively operating said switching device to produce, from a.c.

voltage normally applied to said cable, a number of pulses derived from cycles of said a.c. voltage, current display means to determine the current produced by said pulses in the cable; and means for- detecting the energy dissipated by said pulses on reaching the fault in the cable.

7. Apparatus as claimed in Claim 6, wherein the switching device is a thyristor.

8. Apparatus as claimed in Claim 7, wherein the operating means of the thyristor is a control circuit adapted to render said thyristor conducting during selected positive or negative half cycles of the a.c. voltage.

9. Apparatus as claimed in Claim 7, where- in the operating means of the thyristor is a control circuit adapted to render said thyristor conducting during fractions of selected half cycles of the a.c. voltage.

10. Apparatus for indicating and locating a fault in an electric distribution cable comprising a pair of thyristors, connected in inverse-parallel, connectable in series with said cable, means for selectively operating said thyristors to produce, from an a.c. voltage normally applied to said cable, a number of pulses derived from cycles of said a.c. voltage; current display means to determine the current produced by said pulses in the cable; and means for detecting the energy dissipated by said pulses on reaching the fault in the cable.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram showing the preferred apparatus according to the present invention, and Figure 2 is a circuit diagram showing an alternative embodiment.

Referring to Figure 1 a pair of inverseparallel connected thyristors 1, 2 are connectable between a busbar 10 and a cable 11 under test through connecting links 13 and 14 respectively.

A measurement current transformer 5 is arranged between the thyristors and link 13.

A fuse 3 which replaces the fuse which normally protects the cable and a fuse 4 to protect the thyristors of the apparatus, are connected between the thyristor pair and the links 13 and 14 respectively. The current transformer 5 is connected to the control circuits through a terminal box 15 and an input transformer 16. The current measured by the transformer 5 is passed to a current display unit 17 through terminal box 15 to indicate the existence and condition of a fault.

Power for the control circuits and the voltage reference are taken from the busbar 10 with neutral connection 12 providing the return path through a connecting link 18.

Referring to the alternative embodiment shown in Figure 2, apparatus 21 for indicating and locating a fault in a cable includes a switching device in the form of a single thyristor 22 connected in series with a cable 30 under test through a fuse 31 which protects thyristor 22. In addition a fuse 24 is incorporated in the apparatus 21 to replace the fuse which normally protects the cable 30 and is located as close to the supply busbar as possible. The ratings of the fuses 24 and

31 are so selected to allow a pulse of a desired magnitude to pass but to protect the- apparatus and cable in the event of excessive pulse energy. The apparatus 21 also includes in series with thyristor 22, a main switch 23.

The firing of thyristor 22 is controlled by inhibit control 25 and firing angle control 26, an input to the inhibit control 25 being derived from the busbar voltage. Firing angle control 26 allows the energy content of the pulses to be regulated. A measurement current transformer 33 provides an input to a current display unit 38 through a terminal box 35. The energy dissipated at fault 32 on cable 30 is detected using a suitable acoustic detector. In this way the location of the fault may be readily determined.

ln operation, the thyristor pair of the apparatus shown in Figure 1 and the single thyristor of the apparatus shown in Figure 2 are pulsed at suitable intervals, typically a few seconds. The amplitude of the current pulse obtained depends on the fault level at the point of fault, on th impedance of the fault itself and on the firing angle. Thus the displayed current level provides an indication of the existence and condition of the fault.

WHAT WE CLAIM IS:- 1. A method of indicating and locating a fault in an electric distribution cable which comprises the steps of: applying a number of pulses derived from cycles of an a.c. voltage normally applied to the cable by the selected operation of a switching device; passing said pulses along said cable; determining the current produced by said pulses in said cable to indicate the existence and condition of a fault; and detecting the energy dissipated by said pulses on reaching the fault in the cable thereby to determine the location of the fault.
2. A method as claimed in Claim 1, wherein the acoustic and/or seismic energy dissipated by said pulses is detected.
3. A method as claimed in Claim 1 or 2, wherein each pulse of the number of pulses is a positive or negative half cycle.
4. A method as claimed in Claim 1 or 2, wherein each pulse of the number of pulses is a fraction of a half cycle.
5. A method as claimed in Claim 1 or 2, wherein each pulse of the number of pulses is an integral number of consecutive half cycles.
6. Apparatus for indicating and locating a fault in an electric distribution cable comr prising a switching device connectable in series with said cable, means for selectively operating said switching device to produce, from a.c.

voltage normally applied to said cable, a number of pulses derived from cycles of said a.c. voltage, current display means to determine the current produced by said pulses in the cable; and means for- detecting the energy dissipated by said pulses on reaching the fault in the cable.
7. Apparatus as claimed in Claim 6, wherein the switching device is a thyristor.
8. Apparatus as claimed in Claim 7, wherein the operating means of the thyristor is a control circuit adapted to render said thyristor conducting during selected positive or negative half cycles of the a.c. voltage.
9. Apparatus as claimed in Claim 7, where- in the operating means of the thyristor is a control circuit adapted to render said thyristor conducting during fractions of selected half cycles of the a.c. voltage.
10. Apparatus for indicating and locating a fault in an electric distribution cable comprising a pair of thyristors, connected in inverse-parallel, connectable in series with said cable, means for selectively operating said thyristors to produce, from an a.c. voltage normally applied to said cable, a number of pulses derived from cycles of said a.c. voltage; current display means to determine the current produced by said pulses in the cable; and means for detecting the energy dissipated by said pulses on reaching the fault in the cable.
11. Apparatus as claimed in Claim 10,

wherein operating means of the pair of thyristors is a control circuit adapted to fire the thyristors for a predetermined number of cycles so that each pulse of the number of pulses is an integral number of consecutive half cycles.
12. A method of indicating and locating a fault in an electric distribution cable substantially as hereinbefore described with reference to the accompanying drawings.
13. Apparatus for indicating and locating a fault in an electric distribution cable sub stantially as hereinbefore described with reference to and as shown in Figure 1 of the accompanying drawings.
14. Apparatus for indicating and locating a fault in an electric distribution cable substantially as hereinbefore described with reference to and as shown in Figure 2 of the accompanying drawings.