Insulation degradation diagnostic device

An insulation degradation diagnostic device (1a) comprises a current transformer (7a), a first amplifier (15), a first high-pass filter (17), a low-pass filter (19), a second amplifier (20), a second high-pass filter (21), and a discharge judgement section (30). The current transformer (7a) has a filtering function, with an amount of attenuation of −60 dB or less and a slope characteristic of −5 dB/oct or less at a commercial frequency and detects a current flowing through a grounding conductor (5). The first amplifier (15) amplifies a current signal from the current transformer (7a). The first high-pass filter (17) removes a low frequency component from an amplified current signal. The low-pass filter (19) removes a high frequency component from a current signal from which the low frequency component has been removed. The second amplifier (20) amplifies a current signal from the low-pass filter (19) to a predetermined level. The second high-pass filter (21) extracts a signal corresponding to a discharge current caused by a partial discharge from the current signal amplified in the second amplifier (20). The discharge judgement section (30) judges whether or not a partial discharge has occurred in a cable (2) based on the signal extracted in the second high-pass filter (21).

This application claims priority from PCT Application No. PCT/JP2004/015631 filed Oct. 21, 2004, from Japanese Patent Application No. 2003-362135filed Oct. 22, 2003, and from Japanese Patent Application No. 2004-048148 filed Feb. 24, 2004, which applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a device for diagnosing insulation degradation of a high voltage power transmission apparatus (power cable etc.) connected to a high voltage system and, more particularly, to a diagnostic device for measuring partial discharge that has occurred within a high voltage power transmission apparatus based on discharge current flowing through a grounding conductor of the high voltage power transmission apparatus.

BACKGROUND ART

A method for diagnosing insulation degradation of a power cable is disclosed in Japanese Examined Patent Publication No. H 06-7146. The method diagnoses insulation degradation of a power cable using discharge current caused by a partial discharge. As shown inFIG. 1A, a diagnostic device101comprises a cable102, an intermediate connecting section103, terminal joints104aand104b, a high frequency blocking coil105, a high voltage power source106, a coupling capacitor107, a detecting impedance108, and a partial discharge measurement device109. Here,FIG. 1AandFIG. 1Bare a wiring diagram and an equivalent circuit diagram of the diagnostic device101, respectively.

The cable102is configured by connecting two cables using the intermediate connecting section103. Further, the cable102has a first end portion connected to a first end portion of the terminal joint104aand a second end portion connected to a first end portion of the terminal joint104b. The terminal joint104ahas a second end portion connected to the high voltage power source106via the high frequency blocking coil105. The terminal joint104bhas a second end portion connected to the detecting impedance108via the coupling capacitor107. The partial discharge measurement device109has both ends respectively connected to both ends of the detecting impedance108and detects a potential difference that is generated across both ends of the detecting impedance108. A capacitor102ainFIG. 1Bhas an electrostatic capacitance equal to the electrostatic capacitance of the cable102.

Next, an insulation degradation diagnostic method of the diagnostic device101will be described. After the operation of the cable102is stopped, a test voltage is applied to the cable102from the high voltage power source106. By this operation, a partial discharge occurs in an insulator of the cable102and discharge current is induced in a conductor of the cable102. Incidentally, the discharge current has a high frequency pulse waveform. The discharge current is output to the detecting impedance108via the coupling capacitor107. The partial discharge measurement device109detects a pulse voltage that is generated across both ends of the detecting impedance108and generates data. After performing predetermined processing to the generated data, the insulation degradation of the cable102is diagnosed.

The partial discharge measurement device109is, for example, a tuning type partial discharge measurement device. The partial discharge measurement device109comprises a tuning detector, a wide band attenuator, a tuning amplifier, a detector, etc. (none of them is shown). The tuning detector detects a pulse voltage as a waveform of constant frequency attenuation oscillation. The wide band attenuator attenuates an output waveform of the tuning detector to a proper level. The tuning amplifier tunes and amplifies the output waveform of the wide band attenuator at a tuning frequency with “400” kHz being as its center in order to avoid the radio broadcast band. The detector detects the output waveform of the tuning amplifier.

A method for diagnosing insulation degradation of a branch joint of a power distribution high voltage overhead cable is disclosed in Japanese Patent Application Publication No. 2000-2743. The method diagnoses insulation degradation of a branch joint by utilizing a high frequency current flowing through a lead-in cable.

As shown inFIG. 2, a lead-in cable202is branched from an overhead cable208by a joint203. A diagnostic device201comprises a current transformer204, an amplifier205, a spectrum analyzer206, and a computer207. The current transformer204is attached to the lead-in cable202. The amplifier205amplifies the waveform of a high frequency current detected by the current transformer204. The spectrum analyzer206measures the frequency spectrum of the amplified high frequency current. The computer207stores the waveform pattern and the frequency spectrum of a high frequency current in a memory.

Next, an insulation degradation diagnosing method of the diagnostic device201will be explained. The reference waveform pattern and the reference frequency spectrum of a high frequency current in the normal joint203are stored in advance in the memory of the computer207. Next, the waveform pattern and the frequency spectrum of a high frequency current in the joint203to be diagnosed is stored in the memory of the computer207and compared with the reference waveform pattern and the reference frequency spectrum, respectively. By this comparison, the degree of partial discharge, that is, the degree of insulation degradation in the joint203is diagnosed.

When a partial discharge has occurred in the joint203, in bands of 2 to 6 MHz and 6 to 10 MHz, the frequency spectrum of the high frequency current takes a large value. The frequency spectrum with general environmental noises takes a small value in the bands of 2 to 6 MHz and 6 to 10 MHz, therefore, the occurrence of partial discharge is easily discriminated from the occurrence of environmental noises by the above-mentioned method.

However, with the former diagnostic method, it is necessary to attach the coupling capacitor107to the cable102and stop the operation of the cable102. Therefore, this diagnostic method is used only for inspection on shipping of the product or the characteristic evaluation at the time of development of the product. Further, since a single frequency (400 kHz) is extracted from the pulse voltage detected with the detecting impedance108and used for the partial discharge measurement, this method is not practical. Furthermore, the partial discharge measurement device109is a tuning type partial discharge measurement device etc., therefore, the circuit configuration becomes more complex.

With the latter diagnostic method, since a spectrum analyzer is used to obtain the frequency spectrum of a high frequency current, the device becomes more expensive.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an insulation degradation diagnostic device that realizes simplification and reduction in cost of the device configuration.

Also, another object of the present invention is to provide an insulation degradation diagnostic device for precisely measuring a partial discharge that occurs in a high voltage power transmission apparatus in operation.

In order to attain the above-mentioned objects, the present invention provides an insulation degradation diagnostic device comprising: a current detector configured to have a filtering function with an amount of attenuation of −60 dB or less and a slope characteristic of −5 dB/oct or less at a commercial frequency, and detect a current flowing through a line to be measured; a first high-pass filter configured to remove a low frequency component from a current signal based on a current detected by the current detector; a low-pass filter configured to remove a high frequency component from a current signal from the first high-pass filter; an amplifier configured to amplify a current signal from the low-pass filter to a predetermined level; a second high-pass filter configured to extract a signal corresponding to a discharge current caused by a partial discharge that has occurred in the line to be measured, from a current signal amplified by the amplifier; and a discharge judgment section configured to judge whether or not a partial discharge has occurred in the line to be measured based on a signal extracted by the second high-pass filter.

According to the present invention, since the current detector is configured to have the filtering function with an amount of attenuation of −60 dB or less and a slope characteristic of −5 dB/oct or less at a commercial frequency, the filter for removing the charging current at a commercial frequency flowing through the line to be measured is no longer necessary. Therefore, with a simple configuration, it is possible to measure a partial discharge in a line to be measured in a live state at a low cost and with precision.

Further, in order to attain the above-mentioned objects, the present invention provides an insulation degradation diagnostic device comprising: a first current detector configured to have a filtering function with an amount of attenuation of −60 dB or less and a slope characteristic of −5 dB/oct or less at a commercial frequency, and detect a current flowing through a line to be measured; a first amplifier configured to amplify a signal based on a current detected by the first current detector to a predetermined level; a low band pass filter configured to allow a frequency component belonging to a first frequency band to pass through, from a signal amplified by the first amplifier; a first high band pass filter configured to allow a frequency component belonging to a second frequency band to pass through, from a signal amplified by the first amplifier; a low frequency discharge judgment section configured to judge whether or not a partial discharge has occurred in the line to be measured based on a first signal from the low band pass filter; a second current detector configured to have a filtering function with an amount of attenuation of −60 dB or less and a slope characteristic of −5 dB/oct or less at a commercial frequency, and detect a current flowing through another line connected to the line to be measured; a second amplifier configured to amplify a signal based on a current detected by the second current detector to a predetermined level; a second high band pass filter configured to allow a frequency component belonging to a second frequency band to pass through, from a signal amplified by the second amplifier; a polarity judgment section configured to judge whether or not the opposite polarity is possessed by comparing the polarity of a second signal from the first high band pass filter with the polarity of a third signal from the second high band pass filter; a canceling circuit configured to remove noises from the second signal by applying an arithmetic operation to the second signal and the third signal; a high frequency discharge judgment section configured to judge whether or not a partial discharge has occurred in the line to be measured based on a fourth signal from the canceling circuit; a ratio comparison section configured to judge whether or not a partial discharge has occurred in the line to be measured based on a ratio between the first signal and the fourth signal; and a final discharge judgment section configured to finally judge whether or not a partial discharge has occurred in the line to be measured based on the judgment result by the low frequency discharge judgment section, the judgment result by the high frequency discharge judgment section, and the judgment result by the ratio comparison section.

According to the present invention, whether or not a partial discharge has occurred in the line to be measured is judged after setting the first frequency band and the second frequency band and using the three judgment results by the low frequency discharge judgment section, the high frequency discharge judgment section, and the ratio comparison section. Therefore, it is possible to improve the accuracy of the judgment of a partial discharge in a line to be measured.

BEST MODE FOR CARRYING OUT THE INVENTION

The first to fifth embodiments of the present invention will be described in detail with reference toFIGS. 3 to 25.

In the present invention, as one example of a high voltage power transmission apparatus, a power cable (hereinafter, referred to as a cable) is dealt with.

We have found the following fact. First, an amount of charge of a partial discharge detected in the cable is about 500 pC or less and in particular, about 10 to 200 pC is dominant. Second, the low frequency band of a discharge current (pulse waveform) detected in the cable is in the range of 100 to 500 KHz and in particular, the low frequency in the range of 100 to 400 kHz is dominant. In addition, the low frequency has several peaks and converges in a short time. Third, the high frequency band of a discharge current detected in the cable is within the range of 1.5 to 5 MHz. In addition, the high frequency has several peaks and converges in a short time. Note that if a partial discharge occurs in an insulator of a cable, a discharge current is induced in a conductor of the cable as a pulse waveform.

In the first to fourth embodiments of the present invention, it is assumed that a discharge current having a frequency in a low frequency band of 100 to 400 kHz as its main component is an object to be detected. By this setting, the diagnostic device is capable of extracting a waveform substantially corresponding to the original waveform of a discharge current without being affected by the influence of such as harmonics by removing noises, it is therefore possible to discriminate between the waveform of a discharge current and the waveform of a noise and at the same time, to diagnose the degree of insulation degradation of a cable.

In the fifth embodiment of the present invention, it is assumed that a discharge current having, frequencies in a low frequency band of 100 to 500 kHz and in a high frequency band of 1.5 MHz to 5 MHz as its main components is an object to be detected. By this setting, it is possible for a diagnostic device to discriminate whether a detected partial discharge has occurred from a cable or has occurred from a power apparatus (motor, transformer, etc.) connected to a cable, in addition to the advantage of the setting only in the low frequency band.

FIRST EMBODIMENT

As shown inFIG. 3, a cable2has an end portion to which a terminal joint3is attached. A grounding conductor5has one end connected to a shielding layer on the outer circumferential surface of the end portion of the cable2and the other end grounded. The grounding conductor5is composed of, for example, an IV wire. The IV wire is composed of PVC with a thickness of 0.8 mm enclosing the outer circumference of a copper twisted wire. The shielding layer of the grounding conductor5is composed of a copper tape etc. Here, the grounding conductor5corresponds to a line to be measured of the present invention.

A diagnostic device1ais attached to the grounding conductor5. The diagnostic device1acomprises a current transformer (CT)7a, a load resistor13, a resistor14, a first amplifier15, a first high-pass filter (first HPF)17, a low-pass filter (LPF)19, a second amplifier20, a second high-pass filter (second HPF)21, a delay circuit22, and a discharge judgment section30.

The current transformer7ais formed into a clamp-shape and includes a core8and an output winding (secondary winding)9a. The core8is formed into an annular shape and has a hollow portion through which the grounding conductor5penetrates. The core8is, for example, a ferrite core. The output winding9ais wound around the core8several times (for example, ten times) and has a first end portion grounded. When a current flowing through the grounding conductor5changes, an induced current flows through the output winding9a. Therefore, when a discharge current flows through the grounding conductor5, an induced current occurs in the output winding9ain accordance with the pulse waveform of the discharge current. Here, the current transformer7acorresponds to the current detector of the present invention.

As shown inFIG. 4, the current transformer7ahas a frequency characteristic of a characteristic line T1. The characteristic line T1has an amount of attenuation of −60 dB or less and a slope characteristic of −5 dB/oct or less at a commercial frequency (50 Hz or 60 Hz), therefore, the current transformer7afunctions as a high-pass filter. The load resistor13is connected to both ends of the output winding9aand has, for example, a resistance of 100 O.

The resistor14has a first end portion connected to the second end portion of the output winding9aand a second end portion grounded. The first amplifier15has a first end portion connected to the first end portion of the resistor14. When an induced current flows through the resistor14, the first amplifier15amplifies a voltage that develops across both ends of the resistor14by a factor of about 10 and outputs a first amplification signal. The first amplification signal is output to the first high-pass filter17from the first amplifier15.

The first high-pass filter17has a first end portion connected to the second end portion of the first amplifier15. The first high-pass filter17removes a low frequency component equal to or lower than a first cutoff frequency from the first amplification signal. The first cutoff frequency is about 10 kHz. The first amplification signal from which the low frequency component has been removed is output to the low-pass filter19from the first high-pass filter17.

The low-pass filter19has a first end portion connected to the second end portion of the first high-pass filter17. The low-pass filter19removes a high frequency component equal to or higher than a second cutoff frequency from the first amplification signal from which the low frequency component has been removed. The second cutoff frequency is about 300 to 500 kHz. In the present embodiment, 500 kHz is applied as the second cutoff frequency. The first amplification signal from which the high frequency component has been removed is output to the second amplifier20from the low-pass filter19.

The second amplifier20has a first end portion connected to the second end portion of the low-pass filter19. The second amplifier20amplifies the first amplification signal from which the high frequency component has been removed by a factor of about 100 and outputs a second amplification signal. The second amplification signal is output from the second amplifier20to the second high-pass filter21.

The first stage filter includes the current transformer7a, the first high-pass filter17, and the low-pass filter19and has the frequency characteristic shown inFIG. 5. The shielding frequency of the first high-pass filter17and the low-pass filter19in the first stage filter is about 10 kHz and 300 to 500 kHz, respectively. Therefore, the amount of attenuation in the first stage filter takes a value between −10 and 0 dB for a frequency between 10 and 500 kHz.

The second high-pass filter21has a first end portion connected to the second end portion of the second amplifier19. The second high-pass filter21removes a low frequency component equal to or lower than a third cutoff frequency from the second amplification signal. The third cutoff frequency is about 100 to 200 kHz. In the present embodiment, 100 kHz is applied as the third cutoff frequency. The second amplification signal from which the low frequency component has been removed is output from the second high-pass filter21to the discharge judgment section30.

The delay circuit22has a first end portion connected to the second end portion of the second high-pass filter21and a second end portion connected to the first end portion of the discharge judgment section30. The delay circuit22is composed of an inversion amplifier23and a delay element (DLY)24. The delay circuit22has the function of an active filter that cancels harmonics from the second amplification signal from the second high-pass filter21. Here, the delay circuit22may be omitted.

The inversion amplifier23inverts the second amplification signal from the second high-pass filter21and outputs the inverted signal to the delay element24. The delay element24delays the inverted signal by a minute time (for example, 0.5 μs) and outputs the delayed signal. The delayed signal is combined with the second amplification signal from the second high-pass filter21. The combined signal is output to the discharge judgment section30. Due to this, even when the second amplification signal includes harmonics, the harmonics are canceled nearly completely by the delay circuit22, therefore, a signal with less distorted waveform is obtained.

The second stage filter is composed of the second high-pass filter21and the delay circuit22and has the frequency characteristic shown inFIG. 6. The second stage filter has a slope characteristic of −18 dB/oct. In the second stage filter, the cutoff frequency of the second high-pass filter21is 100 to 200 kHz. Therefore, the amount of attenuation in the second stage filter takes a value between −10 to 10 dB at a frequency between 100 to 500 KHz.

The discharge judgment section30judges whether or not a partial discharge has occurred in the cable2based on the combined signal. In the discharge judgment processing, whether or not the combined signal is a signal caused by the partial discharge in the cable2is judged by detecting the p-p value, the p-p time, and the number of peaks of the input signal. Here, the p-p value is defined as an amplitude value between the peak of a certain waveform and the peak of a waveform adjoining the waveform. The p-p time is defined as a time between the first peak and the last peak of the waveform. Here, the discharge judgment section30corresponds to the discharge judgment section of the present invention.

Next, the operation of the diagnostic device1awill be described.

Through the cable1in operation, a charging current flows. The charging current is a current having a commercial frequency (50 Hz or 60 Hz). In this state, also through the grounding conductor5, the charging current flows.

If a partial discharge occurs in the cable1in operation, a discharge current is overlapped to the charging current and flows through the grounding conductor5. When the discharge current flows through the grounding conductor5, an induced current occurs in the output winding9ain accordance with the pulse waveform of the discharge current. At this time, the charging current having a commercial frequency is removed by the filtering function of the current transformer7a.

The first amplifier15amplifies a current signal based on the induced current from the current transformer7aby a factor of about 10 and outputs the first amplification signal to the first high-pass filter17. The first high-pass filter17removes a low frequency component equal to or lower than about 10 kHz from the first amplification signal and outputs it to the low-pass filter19. The low-pass filter19removes a high frequency component equal to or higher than about 500 kHz from the first amplification signal from which the low frequency component has been removed and outputs it to the second amplifier20. The second amplifier20amplifies the first amplification signal from which the high frequency component has been removed by a factor of about 100 and outputs the second amplification signal to the second bypass filter21. The second high-pass filter21removes a low frequency component equal to or lower than about 100 kHz from the second amplification signal and outputs it to the discharge judgment section30.

If a frequency analysis is performed with a spectrum analyzer on the first amplification signal from which the low frequency component has been removed to be input to the low-pass filter19, the frequency characteristics as shown inFIG. 7are obtained. Referring to the frequency characteristics, the frequency component of the input first amplification signal is about 100 to 400 kHz. Therefore, it is preferable to set the cutoff frequency of the low-pass filter19to about 500 kHz.

The signal waveform at TP1of the second amplification signal output from the second amplifier20has, for example, a waveform as shown inFIG. 8AandFIG. 8C. Here,FIG. 8Cshows a part ofFIG. 8Ain which the time scale is enlarged. The signal waveform at TP1has a frequency component of about tens of kHz. The concave portion present in the vicinity of the top portion of the waveform shown by a circle mark A is a part generated by the pulse waveform of the discharge current. Therefore, in order to detect the signal waveform caused by the discharge current, it is necessary to remove a frequency component of tens of kHz from the signal waveform at TP1.

The signal waveform at TP2of the second amplification signal from the second high-pass filter21has, for example, a waveform as shown inFIG. 8BandFIG. 8D. Here,FIG. 8Dshows a part ofFIG. 8Bin which the time scale is enlarged. The part denoted by the circle mark B corresponds to the part denoted by the circle mark A of the signal waveform at TP1. Since the frequency component of tens of kHz has been removed from the signal waveform at TP1by the second high-pass filter21, the signal waveform at TP2becomes substantially flat before and after the signal waveform caused by the pulse waveform of the discharge current. Therefore, it facilitates discharge judgment processing in the discharge judgment section30.

Note that it may also be possible to further connect a second low-pass filter, a third high-pass filter, and a third low-pass filter in this order to the second end portion of the second high-pass filter21to remove noises. The second low-pass filter has a cutoff frequency of 300 to 500 kHz. The third high-pass filter has a cutoff frequency of 100 to 200 kHz. The third low-pass filter has a cutoff frequency of 300 to 500 kHz.

The discharge judgment section30samples an input signal (analog signal) for, for example, 0.1 to 0.2 μs and converts it to a digital signal. The discharge judgment section30performs discharge judgment processing on this digital signal.

Next, the discharge judgment processing by the discharge judgment section30will be described.

The discharge judgment section30first judges whether or not the p-p value of the waveform of an input signal is equal to or less than the absolute value (reference value) of the difference between the upper limit value and the lower limit value set in advance. The reference value is, for example, 20 to 200 mV.

When the p-p value is not equal to or less than the reference value, the input signal is not a signal caused by a partial discharge in the cable2but a noise signal having a large amplitude, therefore, the discharge judgment section30ends the discharge judgment processing.

When the p-p value is equal to or less than the reference value, the input signal is a signal caused by a partial discharge in the cable2, therefore, the discharge judgment section30performs the following processing. In the above-mentioned judgment, the noise signal having a large amplitude is removed.

Next, the discharge judgment section30judges whether or not the p-p time of the waveform of the input signal is equal to or less than a predetermined time (for example, 20 μs). When the p-p time is not equal to or less than the predetermined time, the input signal is not a signal caused by a partial discharge in the cable2, therefore, the discharge judgment section30ends the discharge judgment processing. When the p-p time is equal to or less than the predetermined time, the input signal is a signal caused by a partial discharge in the cable2, that is, a signal having a waveform that converges in a short time, therefore, the discharge judgment section30performs the following processing.

Here, the pulse waveform of the discharge current has several peaks and converges in a short time, therefore, when the p-p time is greater than the predetermined time, the input signal is judged to be a signal caused by noises.

Next, the discharge judgment section30counts the number of times of discharge judgment processing performed in a unit time using a counter. When the number of times is equal to or more than a predetermined value (for example, five times), the discharge judgment section30judges that the input signal is a signal caused by the discharge current and outputs the information to the outside. When the number of times is less than a predetermined value, the discharge judgment section30judges that the input signal is a signal caused by noises and ends the discharge judgment processing.

The diagnostic device1ahas the following advantageous features.

Since the current transformer having the frequency characteristics that the amount of attenuation and the slope characteristic at a commercial frequency is −60 dB or less and −5 dB/oct or less, respectively, is used as a current detector and used to provide a filtering function, it is not necessary to provide a filter for removing the charging current at a commercial frequency flowing through the cable2.

Since it is not necessary to provide a coupling capacitor, a spectrum analyzer, etc., simplification and reduction in cost can be realized for the device configuration.

Since a partial discharge in a cable is measured without stopping the operation of the grounding conductor5, it is possible to measure a partial discharge that occurs in the cable in operation.

The partial discharge in the cable is a discharge with a pulse wavelength having a frequency of about 100 to 400 kHz as its main frequency component. Therefore, the noise component is removed by setting the cutoff frequency of the low-pass filter19and the second high-pass filter21to about 500 kHz and 100 kHz, respectively. As a result, it is possible to extract the signal waveform that has occurred by the pulse waveform of the discharge current as a waveform substantially corresponding to the original waveform of the discharge current without influence of harmonics etc.

The discharge judgment section30judges that a signal is a signal based on a discharge current caused by a partial discharge only when the p-p value of an input signal is equal to or less than the absolute value of the difference between the upper limit value and the lower limit value set in advance, the p-p time is equal to or less than a predetermined time, and the number of times of discharge judgment processing is equal to or more than a predetermined value. Therefore, a signal caused by a discharge current is discriminated from a signal caused by noises without fail.

Once a partial discharge occurs, it continues for a certain period of time, therefore, by counting the number of times of partial discharge that has occurred in a predetermined time (unit time), that is, by counting the number of times of discharge judgment processing performed, a partial discharge can be discriminated from sudden noises such as opening/closing surges.

The signal waveform based on a discharge current caused by a partial discharge is amplified by a factor of 1000 or more by the first amplifier15and the second amplifier20. Therefore, even if a partial discharge is a minute amount of discharge charge (1 pC) or less, the above-mentioned signal waveform is detected with high sensitivity.

Since a discharge current is measured by attaching a current transformer to the grounding conductor of a power transmission apparatus, therefore, it is not necessary to modify the power transmission apparatus for diagnosis of insulation degradation.

Incidentally, the diagnostic device1acan be applied to the insulation performance evaluation, the insulation diagnosis, and the monitoring of insulation of various power transmission apparatuses with high voltage or with particularly high voltage. Further, the diagnostic device la can be used for the characteristic evaluation test at the time of development, the inspection of the product on shipping, and the insulation diagnosis/monitoring for an apparatus in operation.

In the discharge judgment processing, it is also possible to identify a cause of occurrence of a partial discharge by comparing the signal waveform input to the discharge judgment section30with: the signal waveform of the next partial discharge stored in advance; the signal waveform of the partial discharge before discharge degradation begins; the signal waveform of the partial discharge that occurs in various degraded portions; the signal waveform of the partial discharge that occurs at a poorly worked defective portion at which discharge degradation is advancing; and the signal waveform of the partial discharge when a copper tape is broken; and the like.

SECOND EMBODIMENT

As shown inFIG. 9, a diagnostic device1bhas the same configuration as that of the diagnostic device1aexcept in that a current transformer7bis used as a current detector and a canceling amplifier25is added. Here, the same symbols are attached to the same members as those in the first embodiment and their detailed description is omitted.

The current transformer7ahas the frequency characteristic that the amount of attenuation and the slope characteristic at a commercial frequency are −60 dB or less and −5 dB/oct or less, respectively, as a current detector and a filtering function. In the present embodiment, this function is realized using a current transformer generally used currently.

When a current transformer generally used currently is applied as a current detector of the diagnostic device1a, if a large current to which a discharge current has been overlapped flows through the grounding conductor5, the core of the current transformer or the amplifier saturates. Accompanying this, even if low frequency and high frequency components are removed from an input signal using the first high-pass filter17, the low-pass filter20, and the second high-pass filter21, it is not possible to detect a discharge current successfully.

Therefore, by causing a current transformer to have the filtering function of removing a low frequency component (about 10 kHz) from a commercial frequency, it is possible to correctly detect a discharge current flowing through a grounding conductor without causing the core of the current transformer or the amplifier to saturate.

The diagnostic device1buses the current transformer7binstead of the current transformer7a. The current transformer7bincludes the core8, the output winding (secondary winding)9aand a tertiary winding9b. The core8is formed into an annular shape and has a hollow portion through which the grounding conductor5penetrates. The core8is, for example, a permalloy core. The output winding9ais wound around the core8200 times and has a first end portion grounded. When a discharge current flows through the grounding conductor5, an induced current occurs in the output winding9ain accordance with the pulse waveform of the discharge current. To both ends of the output winding9a, the load resistor13(for example, 200 O) is connected. The tertiary winding9bis wound around the core8several times and has a first end portion grounded.

The canceling amplifier25has a first end portion connected to the second end portion of the first amplifier15and a second end portion connected to the second end portion of the tertiary winding9b.

The first amplifier15amplifies a current signal based on an induced current by a factor of about 10. The canceling amplifier25amplifies the current signal from the first amplifier15and outputs the amplified current signal to the tertiary winding9bin order to remove a low frequency component of the induced current output from the output winding9a.

The current transformer7bhas a high-pass filtering function of removing a low frequency component by introducing the tertiary winding9band the canceling amplifier25to the current transformer7a. The cutoff frequency of the current transformer7bis a commercial frequency to 10 kHz.

As shown inFIG. 4, the current transformer7bhas the frequency characteristic of the characteristic line T2. The characteristic line T2has an amount of attenuation of −60 dB or less and a slope characteristic of −60 dB/oct or less at a commercial frequency (50 Hz or 60 Hz). Therefore, the current transformer7bfunctions as a high-pass filter without fail.

Next, the operation of the diagnostic device1bwill be described.

When a discharge current flows through the grounding conductor5, an induced current occurs in the output winding9aand the tertiary winding9bin accordance with the pulse waveform of the discharge current. The first amplifier15amplifies the induced current from the output winding9aand outputs the first amplification signal. The first amplification signal is further amplified in the canceling amplifier25and input to the tertiary winding9b. Due to this, the first amplification signal from the canceling amplifier25removes a low frequency component (a commercial frequency to 10 kHz) from the induced current output from the output winding9a.

FIG. 10shows a frequency characteristic T11of the first stage filter in the diagnostic device1a, a frequency characteristic T12of the first stage filter in the diagnostic device1b, and a frequency characteristic T13of the second stage filter in the diagnostic devices1aand1b. The frequency characteristic T11has substantially the same characteristic as that of the frequency characteristic T12.

The diagnostic device1bhas the following advantageous features.

By winding the tertiary winding9baround a current transformer generally used currently and adding the canceling amplifier25for inputting a current signal to the tertiary winding9b, it is possible to removed a low frequency component (a commercial frequency to equal to or less than 10 kHz) from an induced current from the tertiary winding9b. Therefore, it is possible for the diagnostic device1bto realize a function equivalent to that of the diagnostic device1a.

THIRD EMBODIMENT

A diagnostic device1cidentifies a cable in which a partial discharge has occurred using a plurality of current transformers. The diagnostic device1cis configured by a plurality of diagnostic devices1aor diagnostic devices1b, however, the processing performed in the discharge judgment section30of the diagnostic device1aor the diagnosis device1bis different.

As shown inFIG. 11, in the diagnostic device1c, at least three cables2a,2b, and2care used. The grounding conductors5,5, and5of the cables2a,2b, and2care commonly grounded and the current transformers7,7, and7are attached to the grounding conductors5,5, and5. Here, the current transformer7is the current transformer7aor the current transformer7b. The diagnostic device1cincludes the discharge judgment sections30,30, and30(not shown) in accordance with the cables2a,2b, and2c. The discharge judgment sections30,30, and30have respective second end portions connected to a microcomputer, a personal computer, an oscilloscope (current direction judgment section), etc.

If a partial discharge occurs in the cable2a, a discharge current in the cable2aflows on the grounding conductor5in the direction of the arrow A. At this time, since the grounding conductors5,5, and5of the cables2a,2b, and2care commonly grounded, the discharge currents in the cables2band2cflow in the direction of the arrows B and C opposite to the direction of the arrow A with substantially the same timing.

When signals are input from the discharge judgment sections30,30, and30, respectively, the current direction judgment section judges whether or not the signals occur with the same timing. When they occur with the same timing, the phases of the three signals are analyzed. When they do not occur with the same timing, the processing ends. In the phase analysis, when the phase of one signal is opposite to the phases of the other two signals, it is judged that a partial discharge has occurred in the cable (inFIG. 11, the cable2a) connected to the current transformer that has produced the one of the signals. When all of the phases of the three signals are the same, it is judged that a partial discharge has occurred in a cable other than the cables2a,2b, and2c.

The diagnostic device1chas the following advantageous features.

When there exist a plurality of cables, it is possible to easily identify a cable in which a partial discharge has occurred.

Note that in the above-mentioned first to third embodiments, the insulation degradation diagnostic device is configured so as to detect a discharge current flowing through the grounding conductor5of the cable2with a current transformer. However, it is also possible to configure so as to detect a current flowing through the cable2with a current transformer. In this case also, the same function and effect as those in the above-mentioned first to third embodiments can be obtained.

FOURTH EMBODIMENT

A diagnostic device1dmeasures a partial discharge after removing (reducing) noises that have occurred on the cable.

As shown inFIG. 12, when grounding conductors5a,5b, and5care connected to the common grounding bus line, if noises invade the grounding bus line through a certain point, a noise current having substantially the same waveform flows through the grounding conductors5a,5b, and5cin the direction shown by the arrow. Here, the grounding conductors5a,5b, and5care connected to one end of the shielding layer (copper tape etc.) formed on the outer circumferential surface of the terminal end of the plurality of cables2a,2b, and2c.

For example, when the cables2a,2b, and2care laid in a factory, as shown inFIG. 13, noise currents having substantially the same waveform and phase flows through the cables2a,2b, and2c.

As shown inFIG. 14, the diagnostic device1dis composed of a first diagnostic section A, a second diagnostic section B, a noise removal section40, a second discharge judgment section50, and a final discharge judgment section60. The cable2ais an object to be measured and the cable2bis used for taking in a noise signal.

The first diagnostic section A is connected to the grounding conductor5aof the cable2a, which is an object to be measured. The configuration and operation of the first diagnostic section A are the same as those of the diagnostic device1a.

The second diagnostic section B is connected to the grounding conductor5bof the cable2bfor taking in a noise signal. The configuration and operation of the second diagnostic section B are the same as those of the diagnostic device1aexcluding the discharge judgment section30.

The noise removal section40includes a first detection section41, a second detection section42, a positive/negative inversion section43, an amplifier44, and a combination section45.

The first detection section41has a first end portion connected to TP2of the first diagnostic section A. The first detection section41performs an envelope detection of a current signal at the TP2of the diagnostic section A. Specifically, first, a current signal at the TP2of the first diagnostic section A is input to the first detection section41(refer toFIG. 15A). Next, the first detection section41performs full wave rectification of the input current signal and forms a full wave rectification signal (refer toFIG. 15B). Then, the full wave rectification signal is smoothed and the first detection signal is output (refer toFIG. 15C). The first detection section41outputs the first detection signal obtained by the envelope detection to the combination section45.

The second detection section42has a first end portion connected to TP2of the second diagnostic section B. The second detection section42performs envelope detection of a current signal at the TP2of the second diagnostic section B. The operation of the envelope detection in the second detection section42is the same as that of the envelope detection in the first detection section41. The second detection section42outputs the second detection signal obtained by the envelope detection to the positive/negative inversion section43.

The positive/negative inversion section43generates an inverted signal by inverting the sign of the second detection signal output from the second detection section42. Then, the positive/negative inversion section43outputs the inverted signal to the amplifier44.

The amplifier44amplifies the inverted signal so that the level of the inverted signal is substantially the same as the level of the first detection signal. The amplified inversed signal is output to the combination section45as an inversion detection signal. Incidentally, it may also be possible to omit the amplifier44and amplify the first amplification signal from the low-pass filter19with the second amplifier20so that the level of the inverted signal is substantially the same as the level of the first detection signal.

The combination section45generates a combined signal by adding the first detection signal from the first detection section41and the inversion detection signal from the amplifier44to combine the two signals. Here, the combination method is not limited to addition and other arithmetic operations will do. Then, the combined signal is output to the second discharge judgment section50.

A noise current signal and the combined signal are shown inFIG. 16AandFIG. 16B, respectively, in a state in which no partial discharge has occurred in the cable2a. Here, the noise current signal has a noise waveform flowing through the cable2a. Referring toFIG. 16AandFIG. 16B, the combination section45outputs a substantially flat combined signal by removing noises from the noise current signal of the cable2a.

A noise-discharge current signal and the combined signal are shown inFIG. 17AandFIG. 17B, respectively, in a state in which a partial discharge has occurred in the cable2a. The noise-discharge current signal has a waveform in which a discharge current signal caused by a partial discharge is overlapped with the noise current signal flowing through the cable2a. Here, inFIG. 17A, the part caused by a discharge current corresponds to the peak P. Referring toFIG. 17AandFIG. 17B, in the combination section45, the part caused by a discharge current has a large peak P and in other parts, noises are removed from the noise current signal of the cable2aand a substantially flat combined signal is output.

The second discharge judgment section50has a first end portion connected to the second end portion of the noise removal section40. The second discharge judgment section50judges whether or not a partial discharge has occurred in the cable2abased on a combined signal. In the discharge judgment processing, first, the second discharge judgment section50samples a combined signal for, for example, 0.1 to 0.2 μs and converts it to a digital signal. The second discharge judgment section50performs discharge judgment processing on the digital signal. In the discharge judgment processing, the second discharge judgment section50judges whether or not the peak value of the combined signal is equal to or more than a predetermined value. When it is equal to or more than the predetermined value, it is judged that a partial discharge has occurred in the cable2aand the information is output to the final discharge judgment section60.

The final discharge judgment section60has a first end portion connected to the second end portion of the second discharge judgment section50. When both the judgment result by the discharge judgment section30of the first diagnostic section A and the judgment result by the second discharge judgment section50are that a partial discharge has occurred in the cable2a, the final discharge judgment section60output the information to the outside.

The diagnostic device1dhas the following advantageous features

In addition to the judgment result by the first diagnostic section A, noises are removed by the noise removal section40and only a waveform caused by a partial discharge is extracted and whether or not a partial discharge has occurred is judged in the second discharge judgment section50. In the final discharge judgment section60, whether or not a partial discharge has occurred in the cable2a, the object to be measured, is finally judged using both the judgment results. Therefore, it is possible to improve the judgment accuracy of a partial discharge in the cable2a.

In the present embodiment, when both the judgment result by the discharge judgment section30of the first diagnostic section A and the judgment result by the second discharge judgment section50are that a partial discharge has occurred in the cable2a, the information is output to the outside. However, this is not limited and it may also be possible to judge that a partial discharge has occurred in the cable2ausing only the judgment result by the second discharge judgment section50.

As shown inFIG. 18, the noise removal section40can be modified so that a high-pass filter (HPF)46is provided between the first detection section41and the combination section45and a high-pass filter (HPF)47is provided between the second detection section42and the positive/negative inversion section43. The cutoff frequencies of the high-pass filter46and the high-pass filter47each is set to a value that allows noises that appear in the detection signal as shown inFIG. 15Cas a peak portion to pass through.

According to the configuration, it is possible to remove a low frequency component included in the first detection signal and the second detection signal. Therefore, the signal waveform other than the peak portions that appear in the first detection signal and the second detection signal becomes flat and if the first detection signal and the second detection signal are combined in the combination section45, the waveform irreguralities that occur in the combined signal caused by the low frequency component can be canceled. As a result, it is possible to more securely detect a current signal caused by a partial discharge.

Further, as shown inFIG. 19, the noise removal section40can be modified so that a subtraction circuit48is provided instead of the positive/negative inversion section43, the amplifier44, and the combination section45. The subtraction circuit48calculates the difference between the first detection signal and the second detection signal and outputs the differential signal to the second discharge judgment section50as a combined signal. Other operation is the same as that of the diagnostic device1d.

According to the configuration, the number of parts constituting the noise removal section40can be reduced, therefore, it is possible to configure the diagnostic device1deasily and at a low cost.

In the present embodiment, an example in which diagnosis of insulation degradation is performed using the two single-core cables (cables2aand2b). However, this is not limited and it is possible to use various cables for diagnosis of insulation degradation, such as a multi-core cable, which is a multi-core covered with an insulator, and a triplex type cable composed of three twisted cores.

FIFTH EMBODIMENT

A diagnostic device1emeasures a partial discharge after removing (reducing) noises that have occurred on a cable by setting a low frequency band and a high frequency band.

As shown inFIG. 20, the diagnostic device1eis composed of a first detection section C, a second detection section D, a low band pass filter (low BPF)71, a first high band pass filter (first high BPF)73, a second high band pass filter (second high BPF)75, a polarity judgment section81, a canceling circuit83, a low frequency discharge judgment section91, a high frequency discharge judgment section93, a ratio comparison section95, and a final discharge judgment section97. The cable2ais an object to be measured and the cable2bis used for taking in a noise signal.

The first detection section C is connected to the grounding conductor5aof the cable2a, an object to be measured. The configuration and operation of the first detection section C are the same as those from the current transformer7ato the amplifier15of the diagnostic device1a.

The second detection section D is connected to the grounding conductor5bof the cable2bfor taking in a noise signal. The configuration and operation of the second detection section D are the same as those of the first detection section C.

The low band pass filter71has a first end portion connected to the second end portion of the first amplifier15of the first detection section C. The low band pass filter71allows a frequency belonging to a low frequency band of 100 to 500 KHz to pass through.

The first high band pass filter73has a first end portion connected to the second end portion of the first amplifier15of the first detection section C. The first high band pass filter73allows a frequency belonging to a high frequency band of 1.5 to 5 MHz to pass through.

The second high band pass filter75has a first end portion connected to the second end portion of the first amplifier15of the second detection section D. The second high band pass filter75allows a frequency belonging to a high frequency band of 1.5 to 5 MHz to pass through.

The polarity judgment section81has a first end portion connected to the second end portion of the first high band pass filter73and the second end portion of the second high band pass filter75. The polarity judgment section81judges whether or not the opposite polarity is possessed by comparing the polarity of a first signal from the cable2aand the polarity of a second signal from the cable2b.

When the grounding conductors5aand5bare connected to the common grounding bus line, if noises invade the grounding bus line through a certain point, a noise current having substantially the same waveform flows on the grounding conductors5aand5bfrom the grounding bus line toward the cable in the same direction. Therefore, when no partial discharge has occurred in the cable2a, the polarity judgment section81judges that the two signals do not have opposite polarity and ends the diagnosis of insulation degradation. When a partial discharge occurs in the cable2a, discharge current flows on the grounding conductor5ain the direction opposite to the noise current, therefore, the polarity judgment section81judges that the two signals have opposite polarities.

The canceling circuit83has a first end portion connected to the second end portion of the polarity judgment section81and receives the first signal and the second signal. The canceling circuit83adds the first signal and the second signal and removes noise from the first signal.

The low frequency discharge judgment section91has a first end portion connected to the second end portion of the low band pass filter71. The low frequency discharge judgment section91judges whether or not a partial discharge has occurred in the cable2aby finding the p-p value, the p-p time, and the period for the low frequency output signal that has passed through the low band pass filter71. Here, the period is a duration time of one wave having a peak.

As shown inFIG. 21AandFIG. 22A, when a partial discharge occurs in the cable2a, the low frequency output signal exhibits the attenuated oscillation waveform that converges within 15 μs with both the short cable (150 m) and the long cable (1,630 m). In addition, the low frequency output signal has a period of 2 to 5 μs. Incidentally, the noise waveforms of the short cable and the long cable when no partial discharge occurs in the cable2aexhibit waveforms as shown inFIG. 23AandFIG. 24A, respectively.

The high frequency discharge judgment section93has a first end portion connected to the second end portion of the canceling circuit83. The high frequency discharge judgment section93judges whether or not a partial discharge has occurred in the cable2aby finding the p-p value and the p-p time for the high frequency output signal that has passed through the canceling circuit83.

As shown inFIG. 21BandFIG. 21C, when a partial discharge occurs in the cable2a, the high frequency output signal exhibits an attenuated oscillation waveform in which reflected waves continue several times at an interval corresponding to the propagation speed of the pulse waveform of the discharge current in the short cable. The duration time (wavelength) of one wave is less than 2 μs. Further, as shown inFIG. 22BandFIG. 22C, in the long cable, the reflected waves rapidly attenuate and only one wave appears. The duration time (wavelength) of one wave is less than 2 μs. Incidentally, the noise waveforms of the short cable and the long cable when no partial discharge occurs in the cable2aexhibit waveforms as shown inFIG. 23B(orFIG. 23C) andFIG. 24B(orFIG. 24C), respectively.

The ratio comparison section95has a first end portion connected to the second end portion of the canceling circuit83. The ratio comparison section95judges whether or not a partial discharge has occurred in the cable2aby finding the ratio between the low frequency output signal that has passed through the low band pass filter and the high frequency output signal that has passed through the cancelling circuit83. Specifically, as shown inFIG. 25, when the high frequency output signal value/the low frequency output signal value is in the range of 1.2 to 3.0, it is judged that a partial discharge has occurred in the cable2a.

The final discharge judgment section97has a first end portion connected to the second end portion of the low frequency discharge judgment section91, the second end portion of the high frequency discharge judgment section93, and the second end portion of the ratio comparison section95. When all of the judgment result by the low frequency discharge judgment section91, the judgment result by the high frequency discharge judgment section93, and the judgment result by the ratio comparison section95are that a partial discharge has occurred in the cable2a, the final discharge judgment section97outputs the information to the outside.

The diagnostic device1ehas the following advantageous features.

Whether or not a partial discharge has occurred in the cable2ais judged using the three judgment results by the low frequency discharge judgment section91, the high frequency discharge judgment section93, and the ratio comparison section95after a low frequency band and a high frequency band are set. Therefore, it is possible to improve the judgment accuracy of a partial discharge in the cable2a.

It may also possible to set the low band pass filter71so as to allow a frequency belonging to a frequency band of 1.5 to 4 MHz to pass through and set the first and second high band pass filters73and75so as to allow a frequency belonging to a frequency band of 5 to 8 MHz to pass through.

INDUSTRIAL APPLICABILITY

A diagnostic device of the present invention can be applied to an insulation performance evaluation, an insulation diagnosis, and an insulation monitoring of the cable used in various apparatuses from high voltage to particularly high voltage. Further, the diagnostic device can be used for a characteristic evaluation test at the time of development, a production inspection on shipping, and an insulation diagnosis and monitoring of an apparatus in operation.