APPARATUS FOR DETECTING DEGRADATION OF AN ARRESTER

Abstract: An apparatus for detecting degradation of a grounded ar­ rester (2), including a wave conversion circuit (3) for producing an output signal corresponding to the total leakage current flowing through the arrester, and a phase shifter circuit (7) cou­ pled to the output signal (10) for providing a capacitive-­ component leakage current signal, which is cancelled from the total leakage current signal by means of a differential amplifier (10) to produce a resistive-component leakage current signal. According to one embodiment, this latter signal is applied to a synchronous rectifier circuit (11) for synchronous rectification of the output signal from the differential amplifier circuit. A real resistive-component leakage current signal is then detected by adjusting the phase shift amount in the phase shifter circuit (7) so that the output signal from the synchronous rectifier circuit (11) may become zero, thereby bringing the phase of the can­ celling wave signal in agreement with the phase of the capacitive-component leakage current signal. In another em­ bodiment, the resistive-component current leakage signal is applied to a multiplier and a time averaging circuit, the output of which is likewise nulled by means of the adjustable phase shifter.

Description: This invention relates to an apparatus for detecting degradation of an electrical arrester, for example, of the zinc oxide type, and more particularly to an apparatus for easily and precisely detecting degradation of an electrical arrester which is connected to a high tension a.c. power transmission line. Electrical arresters employing resistors having an excellent non-linear characteristic, e.g. arresters employing zinc oxide elements, have been widely used for protecting items of electric equipment used in electric power systems from lightning discharges or the like. An important part of the maintenance of such an arrester is to detect degradation by measuring the leakage current through its resistive component. An apparatus to measure such resistive component leakage current of an arrester is disclosed in JP-B-60-7356. However, since this apparatus can only measure the fundamental frequency portion of the resistive component leakage current of the arrester, there is a problem in accuracy in its measurement. Another degradation detecting apparatus is described in JP-A-55-32439 which is shown in Figure 6 of the accompanying drawings. This apparatus extracts the total leakage current flowing through an arrester 2 connected between a power transmission line 1 and ground using a detector 3 constituted of a resistor, current transformer, or the like, and the same is converted into a signal Io corresponding thereto. This signal Io is applied to a degradation detecting apparatus 4. The zinc oxide element constituting the arrester 2 can be described by means of the equivalent circuit shown in Figure 7, in which the total leakage current Io of the arrester is given by a vector sum of a capacitive component Ic which does not vary with degradation of the arrester and the resistive component Ir which varies with the degradation. Since normally Ic > Ir, "peak value of Io = peak value of Ic" holds as shown in Figure 8. Making use of this fact, the degradation detecting apparatus 4 automatically derives a signal for the capacitive component leakage current Ic from the above-mentioned signal for the entire leakage current Io, by means of a Schmitt circuit 5 converting an input into a rectangular wave, a low pass filter 6 to convert the rectangular wave to a sine wave, a phase shifter circuit 7 compensating for a phase delay in an input, a peak value detector circuit 8 detecting the peak value of an input, and an automatic gain control AGC circuit 9 and, further, the signal for the capacitive component leakage current Ic is applied, as a cancelling wave signal, to the above-mentioned signal for the total leakage current Io by means of a differential amplifier 10, and thereby a signal for the resistive component leakage current Ir is provided. The above-described arrester degradation detecting apparatus has an advantage over the previously-mentioned apparatus described in JP-B-60-7356 in that the waveform of the resistive component leakage current can be faithfully reproduced, and is not limited to the basic frequency component, but there is a problem in its phase shifter circuit 7. That is, while the phase shifter circuit 7 is used for compensating for the delay of the phase of the signal for the total leakage current Io from the phase of the signal for the capacitive component current Ic, the mentioned phase different is, in fact, varied by the level of the signal for the resistive component leakage current Ir, and from this fact is produced a measurement error making precise measurement impossible. If it is desired to solve such a problem, a potential transformer PT or the like may be used to extract a voltage signal, for example, as described in JP-B-57-10651, but, if this approach is followed, the apparatus becomes larger in size and also becomes complex and, further, measurement itself becomes impossible in the case where the voltage signal cannot be extracted. Besides, in the measurement with the apparatus of Figure 6, if the phase adjustment should be made each time to improve accuracy of the measurement, it would become necessary to observe waveforms using an oscilloscope or the like, and this would be unfit for the initial purpose to make simple measurement in the field possible. An object of this invention is to solve the above-mentioned problems and to provide an improved degradation detecting apparatus for an arrester which is capable of easily and precisely detecting the degradation of the arrester in the field, ie. while the arrester is connected to a high tension a.c. power transmission line. The invention consists of :- Apparatus for detecting degradation of an electrical arrester which is connected between an electrical supply (1) and ground, said apparatus comprising: means (3) for producing a total leakage current signal (Io) corresponding to the total leakage current flowing through the arrester, said total leakage current signal including a resistive component and a capacitive component; detecting means (4) for receiving the total leakage current signal (Io) and producing a clean sine-wave signal (Ic), said means including a phase shift circuit (7) for shifting the phase of the clean sine-wave signal (Ic); a differential amplifier (10) for adding the clean sine-wave signal (Ic) as a cancelling signal to the total leakage current signal (Io) to thereby obtain a resistive component leakage current signal (Ir); characterised in the provision of a multiplier for multiplying the resistive component leakage current signal (Ir) from the differential amplifier according to the clean signal-wave signal (Ic) and means for adjusting the phase shift circuit to produce a 90° phase difference between the resistive component leakage signal (Ir) and the clean sine-wave signal (Ic) so that the mean output signal from the multiplier becomes zero thereby bringing the phase of the clean sine-wave signal (Ic) into agreement with the phase of the capacitive component of the total leakage current to detect at the output of the differential amplifier the resistive component of the total leakage current signal indicative of degradation of the arrester. In order that the invention may be more readily understood, it will not be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a block diagram showing a first embodiment of the invention; Figure 2 is a block diagram showing a second embodiment of the invention; Figure 3 is a chart showing dependency on the phase difference of the output from the multiplier on the low-pass filter in Figure 2; Figure 4 is a block diagram showing a variation of the first embodiment of the invention; Figure 5 is a block diagram showing a variation of the second embodiment of the invention; Figure 6 is a block diagram showing an example of the prior art degradation detecting apparatus; Figure 7 is a drawing showing an equivalent circuit to a zinc oxide element; and Figure 8 is a waveform chart showing the leakage current in an arrester. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to Figure 1 thereof, numeral 11 denotes a synchronous rectifier circuit for making synchronous rectification of the output signal Ir from the differential amplifier 10 according to the cancelling wave signal Ic as the output signal from the automatic gain control circuit 9, numeral 12 denotes a meter for indicating the magnitude of the output signal from the synchronous rectifier circuit 11, and numeral 13 denotes a meter for indicating the magnitude of the output signal Ir from the differential amplifier 10. In the degradation detecting apparatus of the described structure, the output signal from the synchronous rectifier circuit 11 is varied by adjusting the phase shift amount in the phase shifter circuit 7 and, since this output signal is proportioned to cos  when the phase difference between the cancelling wave signal Ic and the output signal Ir of the differential amplifier 10 is represented by , the output signal becomes zero when  = 90°. Therefore, in a measurement of the leakage current in the field, the phase shift amount in the phase shifter circuit 7 may be adjusted so that the reading of the meter 12 may become zero, and then, the reading of the meter 13 will give the value of the real resistive-component leakage current Ir. On the above occasion, the value of the output signal from the synchronous rectifier circuit 11 becomes zero also when  = -90°. In order not to select this point, it may be checked by increasing the angle  in the vicinity of  = 90° or  = -90° and observing the variation of the output signal, which will vary from positive to negative in one case and from negative to positive in the other case. The apparatus for detecting degradation of arresters according to the present invention as described in the foregoing includes the arrangement of the prior art degradation detecting apparatus, wherein the signal corresponding to the total leakage current Io flowing through the arrester 2 connected between the power transmission line 1 and ground is subjected to a waveform conversion and, then, passed through the phase shifter circuit 7 and the automatic gain control circuit 9, whereby the signal for the capacitive-component leakage current Ic is derived. As in the prior art, this signal Ic is added as the cancelling wave signal to the signal for the total leakage current Io by means of the differential amplifier 10, whereby the resistive-component leakage current Ir is provided, and thereby, the degradation of the arrester 2 is detected. The apparatus of the invention further includes the synchronous rectifier circuit 11 for performing synchronous rectification of the output signal Ir from the differential amplifier 10 according to the signal for the cancelling wave Ic, the meter 12 for indicating the magnitude of the output signal from the synchronous rectifier circuit 11, and the meter 13 for indicating the magnitude of the output signal Ir from the differential amplifier 10, and is adapted to detect the signal for the real resistive-component leakage current by adjustment of the phase shift amount in the phase shifter circuit 7 so that the phase of the cancelling wave signal may be brought in agreement with the phase of the capacitive-component leakage current signal Ic. Thus, the apparatus of the invention enables an easy to conduct and precise in-the-field detection of degradation of the arrester 2 in the state as connected to the high tension a.c. power transmission line 1, without the need for taking a voltage signal from a potential transformer or the like as in the case of the prior art. As a result, there is attained an improvement in efficiency and precision of the maintenance work of the arresters, earlier detection of symptoms of malfunction in the power transmission systems, and improvement in reliability of electric power supplies. Figure 2 is a block diagram of a second embodiment of the invention. Referring to Figure 2, numeral 14 denotes a multiplier for multiplying the output signal Ir from the differential amplifier 10 by the cancelling wave signal Ic as the output signal from the automatic gain control circuit 9, numeral 15 denotes a low-pass filter serving as a time-averaging circuit for averaging the output signal from the multiplier 14 over time, and numeral 16 denotes a meter for indicating the magnitude of the output signal F() from the low-pass filter 15. In the degradation detecting apparatus of the described structure, the output signal from the low-pass filter 15 is varied by adjustment of the phase shift amount in the phase shifter circuit 7 and this output signal becomes zero when the phase difference between the cancelling wave signal Ic and the output signal Ir from the differential amplifier 10 is 90°. Therefore, in the measurement of the leakage current in the field, the phase shift amount in the phase shifter circuit 7 may be adjusted so that the reading of the meter 16 may become zero, and then the indication of the meter 13 will give the actual value of the resistive-component leakage current Ir. In the following, description will be given of the fact that the phase difference between the cancelling wave signal Ic and the output signal Ir from the differential amplifier 10 is 90° when the indicated value by the meter 13 is zero. First, the total leakage current waveform io(t) and the cancelling waveform ic(t) are given respectively, by io(t) = Ic cosωt + [Ir₁ sinωt + Ir₃ sin3ωt + Ir₅ sin5ωt + ... ] ic(t) = Ic cos(ωt - ) where  is the phase lag of the cancelling wave. Then, the output signal waveform ir(t) from the differential amplifier 10 is expressed by Therefore, the reading F() of the meter 16 is given by Therefore, F() takes such a waveform as shown in Figure 3, and F() = 0 holds for  = 0 ± 2nπ and  = 2β ± 2nπ. When  = 0 ± 2nπ, the signal for the cancelling wave ic(t) and the output signal ir(t) from the differential amplifier 10 are in an orthogonal relationship, and the peak value Ir of the ir(t) at this time becomes the true value of the resistive-component leakage current. However, when  = 2β ± 2nπ, the above orthogonal condition is not satisfied. Therefore, to discriminate between these two 's (₁ = 0 ± 2nπ, ₂ = 2β ± 2nπ), the difference in the sign of df/d may be utilized, that is, the  that causes F() to decrease (vary from positive value to negative value) when the same is increased can be taken as the right . The first and second embodiments of the invention each are not limited to the detailed descriptions above described, but can also be embodied in the following ways. Figure 4 is a drawing showing in a block diagram another structural example of the degradation detecting apparatus of arresters according to the first embodiment of the invention, wherein the output signal from the synchronous rectifier circuit 11 is fed back to the phase shifter circuit 7 through a control circuit 17 so that the phase difference  between the cancelling wave signal Ic and the output signal Ir from the differential amplifier 10 may be automatically controlled to become 90°. Therefore, in the measurement of the leakage current on the field with this embodiment, it is only needed to read the indication of the meter 13. Figure 5 is a drawing showing in a block diagram another structural example of the degradation detecting apparatus of arresters according to the second embodiment of the invention, wherein the output signal F() from the low-pass filter 15 is fed back to the phase shifter circuit 7 through a control circuit 18 so that the output signal F() averaged for time may be automatically controlled to become zero. In the present case, the control circuit 18 is adapted, as described previously, to function in the direction to increase  when F() > 0 and in the direction to decrease  when f() < 0. According to the present invention as described so far, an extremely reliable arrester degradation detecting apparatus capable of a simple and precise in-the-field detection of degradation of the arrester in the state as connected to a high tension power transmission line, without the need for taking a voltage signal from a potential transformer or the like, can be provided. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims: Apparatus for detecting degradation of an electrical arrester which is connected between an electrical supply (1) and ground, said apparatus comprising: means (3) for producing a total leakage current signal (Io) corresponding to the total leakage current flowing through the arrester, said total leakage current signal including a resistive component and a capacitive component; detecting means (4) for receiving the total leakage current signal (Io) and producing a clean sine-wave signal (Ic), said means including a phase shift circuit (7) for shifting the phase of the clean sine-wave signal (Ic); a differential amplifier (10) for adding the clean sine-wave signal (Ic) as a cancelling signal to the total leakage current signal (Io) to thereby obtain a resistive component leakage current signal (Ir); characterised in the provision of a multiplier for multiplying the resistive component leakage current signal (Ir) from the differential amplifier according to the clean sine-wave signal (Ic) and means for adjusting the phase shift circuit to produce a 90 degree phase difference between the resistive component leakage signal (Ir) and the clean sine wave signal (Ic) so that the mean output signal from the multiplier becomes zero thereby bringing the phase of the clean sine wave signal (Ic) into agreement with the phase of the capacitive component of the total leakage current to detect at the output of the differential amplifier the resistive component of the total leakage current signal indicative of degradation of the arrester. Apparatus as claimed in claim 1, characterised in that said multiplier comprises a synchronous rectifier circuit arranged to rectify the output signal of the differential amplifier according to the cancelling signal (Ic). Apparatus as claimed in claim 2, characterised in the provision of a control circuit (17) interposed between said synchronous rectifier circuit and said phase shifter circuit with the output signal from said synchronous rectifier circuit fed back to said phase shifter circuit through said control circuit so that the mean output signal from said synchronous rectifier circuit may automatically become zero. Apparatus as claimed in claim 1, characterised in that the multiplier serves to multiply the output signal of the differential amplifier by said cancelling signal, and includes a time averaging circuit for averaging the output signal from said multiplier over a time period. Apparatus as claimed in claim 4, characterised in the further provision of a control circuit interposed between said time averaging circuit and said phase shifter with the output signal from said time averaging circuit fed back to said phase shifter circuit through said control circuit so that the output signal from said time averaging circuit may automatically become zero.

Inventors: KAN, MASAHIRO C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: G01R31
IPC codes: G01R31, H02H7, G01R27, H01T15