Apparatus for assessing insulation conditions

Abstract: An apparatus for judging whether a signal of insulating deterioration generated due to deterioration in insulation of an object of measurement is superimposed on the current flowing through this object. An apparatus equipped with a first coil composed of two primary windings wound with the same number of turns and in the same direction and a secondary winding, the primary and secondary windings being wound on a core whose magnetomotive force and magnetic flux density are roughly proportional to each other; a second coil composed of a primary winding and a secondary winding both wound as a core whose magnetomotive force and magnetic flux density are roughly proportional to each other, and an impedance circuit.

Description: This invention relates to an apparatus for assessing the insulation condition in electrical equipment, cables, etc. The present invention is to be utilized for detecting any deterioration in insulation in electrical equipment and cables by detecting the corona discharge or the partial discharge generated when such a deterioration occurs. Generally, various factors can locally cause a poor insulation in an underground cable and the electrical equipment connected thereto. At least 80 percent of serious accidents in such cables and their equipment are attributable to such a deterioration in insulation which can be caused by external mechanical forces, chemical changes in the insulators, tree-branch-like deterioration of the insulator known as "water tree", etc. In view of this, various methods of assessing insulation conditions have been proposed. According to one of these methods, a power supply system is periodically tested for the insulation condition thereof by temporarily stopping the power supply. In this method, the test may be conducted, for example, by applying DC voltage to the power line. What is to be measured to assess the insulating performance of the system is: firstly, partial discharge; secondly, residual voltage, discharge current or residual charge, which shows reduction in dielectric strength; or potential damping and leakage current. On the other hand, there is a method of assessing the insulating performance of a power line by applying AC voltage to the power line concerned. What is to be measured is: partial discharge; or dielectric relaxation which can be assessed through dielectric tangent. Apart from this, there is a method wherein a power supply system is examined in a hot-line condition using a portable measuring apparatus. In the above-described method of assessing the insulataing performance of a power supply system by periodically stopping the power supply, the power lines forming the system have to be examined one by one, so that it takes a lot of time to conduct the measurement. Further, there is a limitation in the number of places that can be measured during a single stopping of power supply, so that if the insulation of some part has gradually deteriorated, there is no knowing the trend thereof. Accordingly, no preventive measures can be taken against a deterioration of this type. On the other hand, the problem with the method in which a power supply system is examined in a hot-line condition using a portable measuring apparatus is that only a skilled engineer can perform the measurement because of the difficulty in securing the operational safety, to say nothing of the labour intensiveness in the preparatory operations and the measurement. US 4,356,443 discloses an apparatus for detecting arcing faults in a polyphase apparatus by considering the absolute and relative magnitudes of currents flowing in the different phases. This apparatus employs a plurality of sensors to detect a progressive wave generated by arcing in a section of one phase where deterioration in insulation has occurred. Each of said sensors has first and second windings and an annular core with an approximately linear BH characteristic, is associated with a respective phase and generates an output signal upon detection of said progressive wave. Moreover, this apparatus uses a measurement section responsive to said output signal from said sensors. Document FR-A-2 400 247 discloses a current sensor having a first winding whose ends can be short-circuited by a switch and a second winding, both windings wound on an annular core. Accordingly, it is an aim of this invention to provide an apparatus for examining insulation condition of electric equipment and power cables enabling the insulation condition thereof to be continuously monitored in a hot-line condition. According to this invention, judgement is made as to whether a deterioration signal generated by any deterioration in the insulation of equipment or cables is superimposed on the current flowing through the object of measurement. According to the invention there is provided apparatus for assessing the insulating condition of a power supply path comprising a plurality of power supply lines connected to common buses said apparatus comprising: a plurality of first sensors fitted to said power supply lines so as each to surround a respective said power supply line, said first sensors detecting a progressive wave current generated by a partial discharge or a corona discharge in a section of said power supply path where deterioration in insulation has occurred; capacitors each disposed between a respective one of said common buses and the ground; second sensors fitted to ground lines at ground sides of said capacitors, each of said second sensors detecting a progressive wave current passing through respective ground lines as a reference signal for judging the direction of progression of said progressive wave current detected by said first sensors; and a measurement section for receiving a signal from each of said first sensors upon detection of said progressive wave current and said reference signal from each of said second sensors and, when a signal from one of said first sensors is almost opposite in phase to said reference signal, for identifying a line, to which said one of said first sensors is fixed, as a line which is connected to said section of said power supply in which the insulating condition is deteriorated; wherein each of said first and second sensors comprises a first winding with short-circuited ends and a second winding, both windings wound on an annular core having an approximately linear BH characteristic in which magnetomotive force and magnetic flux density are roughly proportional to each other, and a magnetic permeability which is approximately constant over a frequency range from low to high regions. The present invention will be further described hereinafter with reference to the following description of exemplary embodiments and the accompanying drawings, in which: Fig. 1 is a block diagram showing the overall arrangement in accordance with an embodiment of this invention; Fig. 2 is a schematic diagram illustrating the circuits in the sensor section; Figs. 3A and 3B are front views illustrating the sensor; Fig. 4 is a graph showing the BH-characteristic curve of the core used in the sensor; Fig. 5 is a graph showing the frequency characteristic of the core used in the sensor; Fig. 6 is a graph showing a detection result of the progressive wave; Fig. 7 is a block diagram showing the signal processing circuit; and Figs. 8 and 9 are graphs showing a measurement result. An exemplary embodiment of this invention will now be specifically described with reference to the attached drawings. A method of and an apparatus for ascertaining the power line whose insulating performance has deteriorated when a power supply system with two or more power lines is suffering from a degraded insulation by detecting a progressive wave generated by the corona discharge or the partial discharge in the section concerned will now be described. When an insulation defect occurs in a power line a corona discharge or a partial discharge is generated in the section concerned. This discharge causes a progressive wave to be generated, which travels in both directions of a line from the section concerned. Accordingly, it is possible to ascertain the power line which has deteriorated in insulation out of a plurality of lines forming a power supply system by detecting this progressive wave. Further, it is possible to locate the point at which the abnormality has occurred by sensing the travel direction of said progressive wave. This is made possible by comparing the phase of the progressive wave at a specific (reference) point in a common bus which is an electrical reference point with the phase thereof in each power supply line branching off from the common bus. In other words, the point at which the progressive wave has generated (deteriorated point) can be located by measuring the travel direction of the progressive wave at a plurality of positions. This method will now be explained with reference to Fig. 1. First, the progressive wave generated by the deterioration in insulation at the point P travels through all the sensors. Assuming that the direction of the progressive wave travelling through the sensor SF in the vicinity of the capacitor C provided in the first common bus LF is to be regarded as the reference, it will be appreciated that the travel direction of the progressive wave detected by the sensor S1 of the cable L1 whose insulation has deteriorated is opposed to the reference, i.e., the travel direction of the progressive wave detected by the sensors S of the other cables L. Similarly, assuming that the travel direction of the progressive wave travelling through the sensor SG placed between the second common bus LG and the ground GND is to be regarded as the reference, the travel direction of the progressive wave detected by the sensor S4 of the cable L1 whose insulation has deteriorated is opposed to the direction of the progressive wave detected by the sensors S provided in the other cables L. Consequently, it is possible to locate the place where the deterioration in insulation has originated by measuring in a measurement section 53 the signal detected by the sensors S. As the sensor for detecting the progressive wave, one described below may be employed: This sensor (signal discriminator) is composed of a first winding M1 with short-circuited ends and a second winding M2 for detecting signals, said first and second windings M1 and M2 being wound on an annular core K which has an approximately linear BH characteristic, i.e., whose magnetomotive force and magnetic flux density are roughly proportional to each other, and whose magnetic permeability is approximately constant over the entire frequency range from low to high regions. Said annular core K may be, for example, made of an amorphous metal containing cobalt as the main component. As shown in Fig. 3A, the cable L, the line from which signals are to be detected, is wound on the annular core K. A low frequency current and a high frequency current flow through the cable L, thereby generating a magnetomotive fcrce in the core K. The first and second windings M1 and M2 act as a secondary coil with respect to the cable L (primary coil), so that an electromotive force is generated in the first winding M1. Since the ends of the first winding M1 are short-circuited, a current which cancels the magnetic flux change in the annular core K flows through this first winding M1. Here, the annular core K has a high magnetic permeability which is approximately constant over the entire frequency range from low to high regions. Further, its residual magnetism and coercive force are both small, and it presents an approximately linear BH characteristic, i.e., its magnetomotive force and magnetic flux density are roughly proportional to each other. Accordingly, the inductive reactance of the first winding M1 is low for a low frequency and high for a high frequency. On account of this, only the high frequency component can be obtained from the second winding M2, the low frequency component being cancelled. In practice, the line L from which signals are to be detected can be set merely by passing it through the core K, as shown in Fig. 3B. Said core may be, for example, made of a cobalt based amorphous alloy which is composed of cobalt (Co), iron (Fe), silicon (Si), boron (B), molybdenum (Mo), and nickel (Ni) and which can be formulated as: (Co) a (Fe) b (Si) c (B) d (Mo) e (Ni) f where a to f represent the atomic percentage of the component elements. Here, a = 50 to 90, b = 1 to 10, c = 5 to 20, d = 0 to 20, e = 0 to 20, and f = 1 to 5, the sum of a through f being 100. The core K may be formed, for example, as a toroidal core made of a ribbon of a cobalt based amorphous alloy. After being formed, this toroidal core is provided with a desired magnetic permeability by a heat treatment under a temperature of 150 to 450 degrees C for 5 to 180 minutes. It is desirable that the heat treatment should be effected in a DC or AC magnetic field so that a uniform performance may be obtained. A more stable performance will be attained by conducting the heat treatment in a nitrogen atmosphere. Although the first and second windings M1 and M2 may be wound separately, they may also share a part. As for the material of the core K, the 6025F which is a ribbon of an amorphous alloy manufactured by Vacuumschmelze GmbH may be employed. When formed into a toroidal core, this material can provide a desired magnetic permeability. An embodiment of this invention will now be described with reference to Figures 1 through 9. First, the Applicant has verified that any deterioration in insulation of a power supply line causes a corona discharge or a partial discharge in the section concerned and that such a discharge generates a progressive wave in the power supply line. An apparatus which utilizes said progressive wave in order to locate the section where the deterioration in insulation is generated will be described first. In a substation 51 which is supplied with power by an AC power source A, a transformer T1 and a circuit breaker B1 are provided in the transmission line, to form a first common bus LF, which is connected to the ground GND through the intermediary of a capacitor C. Provided in the section between the capacitor C and the ground is an annular sensor SF surrounding the line. The output signal from this sensor SF acts as the signal of the reference point provided in the common bus. Transmission cables L1, L2 and L3 are connected to said common bus LF through the circuit breakers B2, B3 and B4, respectively. Annular sensors S1, S2 and S3 are attached to these cables in such a manner as to surround them. Said cable L1 extends to a place 52 where the power is to be consumed. In said place 52, a sensor S4 is attached to the cable L1, which is connected to a second common bus LG through the intermediary of a circuit breaker B5. Said second common bus LG is connected to the ground GND through the intermediary of a capacitor C. An annular sensor SG is attached to the line at the position between the capacitor C and the ground GND in such a manner as to surround the line. The output signal from this sensor SG acts as the signal at the reference point in the second common bus LG. Power transmission cables L4 and L5 are connected to said second common bus LG through the intermediary of circuit breakers B6 and B7, respectively. Annular sensors S5 and S6 are attached to these cables L4 and L5, respectively, in such a manner as to surround them. Said cable L4 is connected to a motor M, and said cable L5 is connected to a transformer T2. The output signals of said sensors S1, S2 and S3 are transmitted to a scanning circuit 60 where they undergo time series division. They are then transmitted to a direction comparator circuit 61 and are compared with the signal from said sensor SF. The result of this comparison is transmitted to a data transmission circuit 62. On the other hand, the output signals of said sensors S5 and S6 are transmitted to a scanning circuit 70 where they undergo time series division. They are then input to a direction comparator circuit 71 and are compared with the signal from said sensor SG. The result of this comparison is transmitted to a data transmission circuit 72. The signals from said data transmission circuits 62 and 72 are input to a measurement section 53. They are first input to a scanning circuit 81 where they undergo time series division. They are then input to a high-speed data storage circuit 82 as well as to an alarm indication circuit 83. The high-speed data storage circuit 82 is connected to a personal computer 84 so that data can be exchanged. A CRT 86 and a printer 87 are connected to the personal computer 84 so that the examination result can be displayed. Fig. 7 shows a specific example of the hardware of said high-speed data storage circuit 82. This hardware comprises a sensor S, a buffer BU connected to the output of this sensor S, an amplifier AP adapted to amplify the signal from the buffer BU, a peak sensor PS connected to the output of the amplifier AP and adapted to detect the maximum value of the output signal, an A/D converter AD of 20 MHz connected parallel to this peak sensor PS, a memory board MB adapted to store the respective output signals from the peak sensor PS and the A/D converter AD and having a capacity of 2kB, a personal computer 84 adapted to exchange signals with this memory board MB, and a printer 85 as the output device. The operational principle of said sensor S and that of the circuits will now be explained. Said sensor S is composed of a core K and coils wound thereon. Said core K is made of a cobalt based amorphous metal whose magnetic permeability is approximately constant from the low frequency region to the high frequency region, as shown in Fig. 5. Its residual magnetism and coercive force are both small, and its BH characteristic is approximately linear, as shown in Fig. 4. On this core K are wound a first winding M1 with short-circuited ends and a second winding M2 with open ends as shown in Fig. 3. The core K has a width of 10 mm, an inner diameter of 150 mm and a height of 3 mm. The number of turns is three for said first winding M1 and ten for said second winding M2. This construction enables said progressive wave generated by a corona discharge or a partial discharge to be discriminated from the frequency of the power source and the low frequency current which is a higher harmonic thereof. A corona discharge amount of 20 pC was detected in an experiment of the sensitivity of the sensor S with the above construction. Fig. 2 shows this construction as applied to a power supply line for three-phase AC current. The travel velocity V of the progressive wave may be expressed as: V = [(magnetic permeability x dielectric constant) 1/2 ] -1 Here, the dielectric constant of a polyethylene is four times as large as that of air, so that the propagation velocity in the transmission line is about one half of the velocity of light. Consequently, V is ca. 150m/µs. The progressive wave thus travels through the core at a very high speed, resulting in a sharp pulse magnetomotive force. A low frequency current which is power source and higher harmonic thereof and a progressive wave current due to said corona discharge or partial discharge are generated in each winding, and the inductive reactance of the first winding M1 is low to a low frequency and high to a pulse. Accordingly, the magnetic flux change due to the magnetomotive force of the low frequency current i E can be neglected almost completely, but the magnetic flux change caused by the magnetomotive force of the pulse current generated by the passing of the progressive wave remains uncancelled. Consequently, only the signal caused by the passing of the progressive wave is obtained at the terminals of the second winding M2. Further, it is possible to know through which phase of the capacitors CT the progressive wave travels by means of sensors SR provided in each of the phases, thereby obtaining a discrimination signal for ascertaining the deteriorated phase. Furthermore, it is possible to obtain by means of a sensor SF provided in the common line of the capacitors CT connected to the bus LF, a signal which serves as the reference for the travel direction of the progressive wave since the progressive wave passes through the sensor in the same direction, no matter which phase or which section of the system may have deteriorated. Although the signals from these detection coils may be transmitted on a time-series basis as in the above-described embodiment, it is also possible, if there is some allowance in the signal transmission capacity, to transmit them in parallel. Said progressive wave presents a frequency spectrum which is similar to that of a discharge noise and possesses energy over a wide frequency range. However, it can sometimes present a frequency distribution which is peculiar to the corona discharge caused by deterioration in insulation and improves its S/N ratio against external noises by limiting the unnecessary bands, so that a band pass filter may be efficiently employed for restricting the band. In an experiment using a band pass filter, a satisfactory result was obtained when the frequency range to be sensed was set to a range of 20kHz to 200MHz, more preferably, 300kHz to 50MHz, and still more preferably, 300kHz to 5MHz. The actual passing frequency has to be adapted to the individual cases by means of a spectrum analyzer, etc. As for this filter, a tandem-compound filter of the multipoint tuning type may naturally be employed, instead of the above-described single tuning type. Next, the result of an experiment in which the progressive wave generated by the corona discharge due to deterioration of the insulator of a cable was detected using the circuit shown in Fig. 1 employing the sensor of Fig. 3, will be described with reference to Fig. 6. In the graph, J denotes the signal characteristic curve of the sensor S1 provided in the cable, and Q the signal characteristic curve of the sensor SF provided in the bus. If there is some insulation defect in the cable, the progressive wave travels in both directions in the cable. Since the directions of the progressive wave currents passing through the sensors SF and S1 are opposed to each other, the phases of J and Q are approximately reverse to each other. This indicates the existence of the progressive wave, i.e. the existence of the defect in the cable clearly. The actually measured waveform will now be described with reference to Figs. 8 and 9. Fig. 8 shows the waveform when the progressive wave is generated at a place distant from the point of measurement. A progressive wave reflected at the terminal load such as a motor appears periodically by decreasing its strength. The pulse due to the corona discharge can be observed as it is. However, it is generated within a period of very short duration, so that it is sometimes difficult to catch. In view of this, a resonance circuit may be provided in the pulse detection circuit so that the pulse may be caught with ease. Fig. 9 shows the waveform when such a circuit is employed. In this figure, J1 denotes the pulse due to the corona discharge. After that, the pulse excites the resonance circuit and presents an attenuated waveform J2 with a specific frequency. The size, configuration and material of the core K are not to be restricted to those of the above-described embodiments. They can naturally be modified according to the detecting conditions. Thus, the present invention makes it possible to constantly monitor the insulating condition of electrical equipment and cables in a hot-line condition. Further, if any abnormality has occurred in insulation of electrical equipment or cables, the position thereof can be located. Accordingly, any insulating defect can be detected before it becomes serious, thereby making it possible to prevent accidents arising from poor insuiation.

Claims: Apparatus for assessing the insulating condition of a power supply path comprising a plurality of power supply lines (L1 to L5) connected to common buses (LF, LG), said apparatus comprising: a plurality of first sensors (S1-S5) fitted to said power supply lines so as each to surround a respective said power supply line, said first sensors detecting a progressive wave current generated by a partial discharge or a corona discharge in a section of said power supply path where deterioration in insulation has occurred; capacitors (C) each disposed between a respective one of said common buses (LF, LG) and the ground; second sensors (SF, SG) fitted to ground lines at ground sides of said capacitors, each of said second sensors detecting a progressive wave current passing through respective ground lines as a reference signal for judging the direction of progression of said progressive wave current detected by said first sensors; and a measurement section (53) for receiving a signal from each of said first sensors upon detection of said progressive wave current and said reference signal from each of said second sensors and, when a signal from one of said first sensors is almost opposite in phase to said reference signal, for identifying a line, to which said one of said first sensors is fixed, as a line which is connected to said section of said power supply in which the insulating condition is deteriorated; wherein each of said first and second sensors comprises a first winding (M1) with short-circuited ends and a second winding (M2), both windings wound on an annular core (K) having an approximately linear BH characteristic in which magnetomotive force and magnetic flux density are roughly proportional to each other, and a magnetic permeability which is approximately constant over a frequency range from low to high regions. Apparatus according to claim 1, wherein the annular cores of each of said first sensors and second sensors are made of an amorphous metal alloy containing cobalt as a main component.

Inventors: ISHIKURA KEIZABURO, KAGEYAMA TOMOAKI, MAEZAWA SHIGENARI, SAIGO AKIRA, SERA AKIO, SUEHIRO JUNJI, ISHIKURA, KEIZABURO, KAGEYAMA, TOMOAKI, MAEZAWA, SHIGENARI, SAIGO, AKIRA, SERA, AKIO, SUEHIRO, JUNJI
Applicants: MITSUI CHEMICALS INC, MITSUI CHEMICALS, INC.
Application year: 1988
Main classification: G01R31
IPC codes: G01R31