JP2735120B2 - Apparatus for discriminating external noise in partial discharge measurement - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Technical Field The present invention relates to a partial discharge and external noise discriminating apparatus which distinguishes between partial discharge and external noise and enables accurate detection of partial discharge. Partial discharge is a discharge phenomenon that occurs partially inside a transformer, capacitor, gas-insulated switchgear, or the like. In high voltage power receiving equipment, large transformers and gas insulated switchgear (GIS) are used. These devices deal with large voltages and large currents, so if an accident occurs, the damage will be great. It has been managed to check for abnormalities through regular inspections. However, between the time of the inspection and the time of the next inspection, an abnormality occurs, which progresses, which may lead to an accident. Therefore, it is strongly desired to predict the occurrence of an abnormality. If something unusual occurs in these devices, it does not immediately cause an accident. An accident occurs only after a certain set of abnormal conditions. Precursor phenomena leading to an accident include phenomena such as partial discharge, generation of decomposition gas, increase in operation time, pressure drop, abnormal temperature rise, and increase in leakage current. Partial discharge is also called corona. Insufficiency occurs in some parts, where weak discharges occur intermittently. Abnormalities can be predicted by measuring the frequency of discharge, the magnitude of current, and the like. (B) Conventional technology A circuit as shown in FIG. 4 is described in JEC for partial discharge measurement. Power supply Vac, impedance Z, specimen C
a, Detection impedance Zd is connected in series. Since a voltage due to the discharge current is generated in the detection impedance Zd, the voltage is measured. FIG. 5 shows the case where the current transformer 3 is used as the detection impedance. When a partial discharge occurs in the sample 1, a current caused by the partial discharge flows to the ground line 2. This is detected by the current transformer 3 and amplified by the amplifier 4. Further, the waveform is converted into a rectangular wave by the waveform converter 5 and the number of pulses is counted by the counter 6. Thus, the frequency of occurrence of partial discharge can be measured. (C) Problems to be solved by the invention The test samples are GIS, transformer, capacitor, etc., which handle large voltage and large current. In addition to the current flowing due to partial discharge, there is a pulse-like change in current and voltage due to external noise. In FIGS. 4 and 5, the specimen is a target device for which the occurrence of partial discharge is to be detected. It is used for substation equipment that handles high voltage and large current, such as transformers, capacitors, and GIS. In the circuit of FIG. 5, it is assumed that a partial discharge has occurred in the test sample. At this moment, a partial discharge current flows to the ground line 2. When this current is detected by the current transformer 3 and amplified by the amplifier 4, a damped oscillation waveform よ う な as shown in FIG. 6 is obtained. Here, the time Ts during which the damped oscillation lasts is referred to as a convergence time. However, external noise from a power supply or a ground line may enter the measurement circuit. A current flows through the ground line 2 due to external noise. When this is detected by the current transformer 3 and amplified by the amplifier 4, a damped oscillation waveform as shown in FIG. 7 is obtained.
This damped vibration Π is similarly converted into a single pulse by the waveform conversion, and is counted by the counter 6. Originally, not due to discharge, but due to external noise, this is also counted by the counter 6, causing a counting error. Both the external noise waveform Π and the partial discharge waveform で are damped oscillations. The time that persists is referred to herein as the convergence time. Both convergence times are short. Convergence time Tn of external noise
The number of periods in which vibration is included and the length of convergence time vary depending on the cause of noise. Unless the external noise Π and the partial discharge Σ are distinguished, highly accurate partial discharge measurement cannot be performed. (D) Purpose It is an object of the present invention to provide a device capable of accurately determining and removing external noise and removing the external noise and capable of performing internal discharge measurement with high accuracy. (E) Configuration The present inventors have examined the waveforms of many examples of the internal discharge waveform Σ and the external noise waveform Π. As a result, as shown in FIG. 6, the partial discharge only changes the voltage once to positive or negative. It was understood that it was one cycle of damped oscillation. Further, it has been found that the external noise does not end in one cycle as shown in FIG. 7, but is generally a damped oscillation of two or more cycles. As always, there is external noise such as thyristor noise coming from a line or noise caused by an external radio wave. The noise coming from the line, even if it is a sharp and short pulse at first, spreads along the transmission line, resulting in a dull waveform. Whenever extraneous noise persists for more than one period, it is not critical. Some end in one cycle. By performing many measurements, it was found that the convergence time Ts of the partial discharge was shorter than the convergence time Tn of the external noise. And it was found that the boundary time was about 70 μsec. In other words, the boundary value is T _0, the partial discharge convergence time Ts, that a statistical rules, such as _{Ts <T 0 <Tn (1} ) between the external noise convergence time Tn has divide. The present invention uses such knowledge to discriminate and remove extraneous noise. FIG. 1 shows a circuit diagram of the device of the present invention. It is almost the same as FIG. 5, but a discriminator 7 is newly added. A power supply Vac, an impedance Z, a specimen 1, and a ground line 2 are connected in series. A current transformer 3 is provided on the ground line 2 and detects a current flowing therethrough. This is amplified by an amplifier 4 and further converted into a rectangular wave by a waveform converter 5. Instead of making the attenuation waveform a square wave as it is, a group of attenuation waveforms is made into one rectangular wave. FIG. 2 shows the inside of the waveform converter 5. The detection circuit 8 rectifies the vibration waveform to form a positive waveform, which is then changed to an envelope waveform. This can be constituted by a half-wave rectifier circuit composed of a diode and a capacitor resistor. The comparator 9 compares a certain reference voltage with an input signal. When the input signal is higher than the reference voltage, the comparator 9 outputs “H”, and when the input signal is lower, “L”.
Output. In this way, the deformation converter 5 converts one damped oscillation waveform into one rectangular wave. Without the detection circuit 8, the damped oscillation becomes several rectangular waves.
, A single rectangular wave is produced. The pulse width of the rectangular wave is the accommodation time T of the original attenuation waveform. The discriminator 7 determines whether the input rectangular wave is due to extraneous noise,
This is a circuit for determining whether the discharge is caused by partial discharge. Boundary time
The duration of the square wave than T ₀ (convergence) time is by go-between discrimination on whether long or short. If T ₀ > T, this is due to partial discharge. If T ₀ <T, this is due to extraneous noise. FIG. 2 shows an example of the discriminator 7. In this example, an inverter 10 and a retriggerable one-shot multivibrator 11 are arranged in parallel, and a signal from the waveform converter 5 is input to both. The retriggerable one-shot multivibrator 11
By the rising edge of the input pulse go-between, a pulse width results in a one-to bracts pulse of T _0. The inverter 10 changes positive logic to negative logic. The AND circuit 12 performs a product operation of the output of the retriggerable one-shot multivibrator 11 and the output of the inverter 10. If the pulse duration T is T <T _0, the NyuTsuta time to discriminator 7 is a rectangular pulse as t = 0, until t = 0 to T _0,
The output of the multivibrator is "H". From t = 0 to T, the output of the inverter is "L". However At a t = T~T _0, of the multivibrator output, the output of the inverter is both "H". The output of the AND circuit 12 becomes “H” during this time. T <T ₀ means that the waveform is due to partial discharge. This can be detected when the output of the AND circuit 12 becomes “H”. The connection time T of the rectangular pulse input to the discriminator 7 is T>
T and Atsuta at _0, multi-vibrator output at t = 0~T ₀ is "H". Thereafter, this output becomes “L”. Inverter
The output of 10 becomes "H" after t = T, but at this time, the output of the multivibrator returns to "L". Therefore, AND circuit 1
The output of 2 does not rise to “H”. It remains at “L”. The fact that T> T ₀ is the fact that external noise. AND
It can be seen from the output of the circuit 12 that external noise has been removed. One attempt bract multivibrator is synchronized with the rise or fall of the pulse, in which produce one-to bract pulse T _0. When the next input pulse is given while the one-shot pulse is sustained, the output does not fall to “L” and stays at “H”. Maintaining “H” by ₀ means “retriggerable”. The reason for using the retriggerable one-shot multivibrator is to distinguish between two partial discharges that are close to each other. The discriminator 7 Can be determined. What has been described above is an example of an analog circuit. Alternatively, the pulse width T may be measured using a clock pulse Cl. This will be a digital circuit. For example, the length of an input rectangular wave is measured using a clock pulse of 1 μsec. This can be constituted by an AND gate, an oscillator and a counter. Thus, when the length of the rectangular wave is known, it is T ₀ = 70
Compared to μsec, long or short is detected by a digital comparator. In addition, using a computer and software
Measuring the convergence time of the voltage waveform, which was compared with T _0, it is also possible to perform the external noise is removed. (F) Operation The operation of the circuit example of FIG. 2 will be described. FIG. 3 shows a partial discharge, an external noise, and two close partial discharges input to the waveform converter 5. (A) Input waveform of the detection circuit (b) Output waveform of the detection circuit and input waveform of the comparator (C) Output waveform of comparator (d) Output waveform of one-shot multivibrator (e) Output waveform of inverter (f) Output waveform of AND circuit. The detection circuit 8 of the waveform converter 5 amplified by the amplifier 4 includes:
It is assumed that a waveform 31 due to partial discharge, an external noise 32, and two adjacent partial discharge waveforms 33 and 34 are input. The detection circuit 8 detects this by diode detection and smoothes it with a capacitor. Therefore, the partial discharge waveform 31 becomes a positive voltage waveform 35 one by one. External noise was a damped oscillation containing several periods, but this is also a rather long positive voltage waveform.
It becomes 36. The two partial discharges result in two positive voltage waveforms 37,38. To achieve this, the interval between partial discharges must be longer than the time constant of the detection circuit. The time constant of the detection circuit may be longer than the period of the damped oscillation. If partial discharges occur at intervals shorter than the time constant of the detection circuit, they cannot be distinguished and are counted as one partial discharge. However, the probability that partial discharges are repeated and occur at such short intervals is low. These are analog waveforms, but are converted into rectangular waves 39, 40, 41, and 42 by passing through the comparator 9. The duration of the rectangular wave 39,41,42 by partial discharge is shorter than T _0. The duration of the square wave 40 due to external noise is longer than T _0. The retriggerable one-shot multivibrator 11
It is triggered by the rising edge of the input pulse results in a one-to bract pulse width T _0. These are pulses 43 and 44 having a constant width. By two partial discharge 41 in proximity, so that the succession in a time shorter than T _0, relative to 41 and 42, one pulse 45 is generated. This is before the one-to bract pulse 45 had it occurred excited 41 ends, because the retrigger is performed by 42, it is of a width of a pulse of T ₀ from 46. The output of the inverter 10 is simply the inverted output of the comparator (c). The rising edge of the rectangular pulse 39 causes a falling edge 47 and a negative pulse 49. When the rectangular pulse 39 ends, the level changes to the H level 48. Negative pulses (L level) 50, 51, 52
The positive pulses 40, 41, and 42 in (c) are inverted. L
Between levels 51 and 52, an H level 53 occurs. Since the output of the AND circuit 12 is the product operation of (d) and (e), a positive pulse 60 is generated from the product of the one-shot pulse 43 and the H level 48 of (e). This corresponds to the partial discharge 31. For a foreign nozzle, the negative pulse 50 is too long (T>
T ₀ ), the product of the one-shot pulse 44 does not become “1”. That is, no pulse is generated. This means that the foreign nozzle could be removed. It remains at L level 61. Since the one-shot pulse 45 of the two partial waveforms is sufficiently long, one positive pulse 62 is generated by the product of the one-shot pulse 45 and the H level 53 generated in between. This is the first partial discharge 33
Is a short pulse corresponding to. Since the duration of the negative pulse 52 in (e) is shorter than T ₀ , H
Level 54 occurs earlier than the end of pulse 45. Therefore, another positive pulse 63 is generated. This corresponds to the subsequent partial discharge 34. In this way, detection pulses 60, 62, 63 are generated corresponding to the partial discharge of (f). The foreign nozzle has been removed. Although the pulse width is different, this is no problem. The counter 6 counts the pulse in synchronization with the rising or falling of the pulse, so that the width does not matter. (G) Effect It is possible to accurately measure the frequency of partial discharges generated inside transformers, capacitors, GIS (gas insulated switchgear), etc. This is because external noise can be removed. The occurrence of partial discharge means that some abnormality has occurred inside the device. The fact that the frequency of the partial discharge has increased means that the state of the device abnormality is progressing. By the partial discharge measurement, it is possible to predict the future occurrence of a failure or accident.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic configuration diagram of a partial discharge measuring device according to the present invention. FIG. 2 is a configuration diagram showing an example of a waveform converter and a discriminator. FIG. 3 is a waveform diagram showing a change in a voltage waveform due to external noise and partial discharge at connection points (a) to (f) of the circuit of FIG. FIG. 4 is a partial discharge measurement circuit diagram defined by JEC. FIG. 5 is a conventional partial discharge measurement circuit diagram. FIG. 6 is a partial discharge waveform diagram. FIG. 7 is an external noise waveform diagram. 1 ... Test sample 2 ... Ground wire 3 ... Current transformer 4 ... Amplifier 5 ... Waveform converter 6 ... Counter 7 ... Discriminator 8 ... Detection circuit 9 ... Comparator 10 ... Inverter 11 ... Retriggerable one-shot multivibrator 12 ... AND circuit

Claims (1)

(57) [Claims] A device to measure the frequency of partial discharge by removing the external noise from the output waveform of the current transformer, in which the sample to measure the occurrence of internal discharge, the power supply, and the current transformer are connected in series. An amplifier 4 for amplifying the output voltage on the secondary side of the current transformer,
A waveform converter 5 for converting a continuous attenuation waveform included in the amplified voltage signal into a rectangular wave having a pulse width T substantially equal to its convergence time;
And the pulse width T of the more becomes a rectangular wave and an AND circuit for calculating a logical product of the output of retriggerable one-shot multivibrator and the inverted output and the one-shot multivibrator and inverter for inverting the square wave for generating a pulse of T ₀ Boundary value T
Discriminator 7 which compares ₀ and outputs only a square wave of T <T ₀
And a counter for counting the rectangular waves output from the discriminator 7.