Use of detecting electrode to measure partial discharge in a wire

Abstract: In an electric apparatus such as an electric power cable system, a high frequency pulse is induced on a conductive member such as a metal sheath, where partial discharge occurs in an insulation. The high frequency pulse is detected by means of a bypass lead wire, a metal foil electrode, a detecting coil, and the like provided on the electric apparatus without interrupting a live operation of the electric apparatus.

Description: FIELD OF INVENTION This invention relates to a method for detecting partial discharge in an insulation of an electric power apparatus, and more particularly to a method for the use of a magnetic core to measure the partial discharge. BACKGROUND OF THE INVENTION An electric power cable system comprises a plurality of electric power cables which are connected to provide power cable lines by normal joints, insulating joints, etc., wherein the power cable lines are connected through terminal joints at both ends to high voltage conductors. Each of the power cables comprises, for instance, an electric conductor, an inner semi-conductive layer provided around the electric conductor, an insulation provided around the semi-conductive layer to insulate the electric conductor, an outer semi-conductive layer provided around the insulation, a metal sheath provided around the outer semi-conductive layer, and an anti-corrosion layer of poly-vinyl chrolide provided around the metal sheath. In this power cable system, the metal sheath is connected, typically, at the normal joints, the terminal joints, etc. to the ground by lead wires. In detecting partial discharge in the insulation of the power cable, a high voltage applied to the power cable system is turned off to stop operation thereof, and a detecting impedance is inserted into a selected one of the lead wires connecting the metal sheaths to the ground. Furthermore, an apparatus for detecting the partial discharge is connected to both terminals of the detecting impedance. Then, a predetermined high voltage is applied to the electric conductor of the power cable, so that the partial discharge occurs at deteriorating or defect portions of the insulation, thereby generating a high frequency pulse therein. This high frequency pulse induces a high frequency current flowing through the lead wire, so that an electric potential difference is generated across the both terminals of the detecting impedance. The electric potential difference is detected in the partial discharge detecting apparatus to diagnose the deterioration of the insulation. In the other method for detecting the partial discharge in the insulation of the power cable, the detecting impedance is inserted between a coupling capacitor, which is connected to the high voltage conductor connected through the terminal joint to the electric conductor of the power cable, and the ground, and the apparatus for detecting the partial discharge is connected to the both terminals of the detecting impedance. The partial discharge is detected in the same manner as described in the former method. However, the conventional method for detecting partial discharge in an insulation of an electric power cable has a first disadvantage in that the reliance of the electric power cable system is lowered, because the selected lead wire is removed to insert the detecting impedance between the metal sheath and the ground, and an original state is restored, after the partial discharge detecting procedure is over. Furthermore, second and third disadvantages are observed in that operation of the power cable system is interrupted as described before, and in that the detecting operation is troublesome, because the insertion of the coupling capacitor is inevitable. In addition, there is a fourth disadvantage in that the high frequency pulse is attenuated to be propagated to the coupling capacitor in the latter method, and the coupling capacitor must withstand a high voltage. More additionally, fifth to seventh disadvantages are found in that a high detecting sensitivity of the high frequency pulse is not obtained, because the power cable is exposed to external noise, and an overall capacitance of the power cable is large, in that a S/N ratio is lowered, where a measuring frequency is coincident with a frequency at which a noise level is high, and in that a calibration of the apparatus for detecting the partial discharge is difficult in operation, where the partial discharge is detected at the normal joints, the insulating joints, the cable insulation, etc. which are remote from the terminal joints, because a calibration pulse is applied to the high voltage conductor connected to the electric conductor of the power cable. In DE-A1 26 05 418 an apparatus for detecting insulation defects in cables is disclosed, wherein a high frequency pulse is applied to one or more electrodes, whereby said electrodes have a tubular shape and the cable to be tested is continuously passed through said electrodes during testing. JP-A-57 189 074 discloses an apparatus for eliminating external noise to a corona signal by connecting an impedance element with a shielding layer and a junction capacitor lead in series between conductors of power cables. Accordingly, it is a first object of the invention to provide a method for detecting partial discharge in an insulation of an electric power apparatus, by which the reliance of the electric power cable apparatus is not deteriorated. It is a second object of the invention to provide a method for detecting partial discharge in an insulation of an electric power apparatus which is carried out without interrupting operation of the electric power apparatus. It is a third object of the invention to provide a method for detecting partial discharge in an insulation of an electric power apparatus in which the detecting operation is simple. It is a fourth object of the invention to provide a method for detecting partial discharge in an insulation of an electric power apparatus in which the attenuation of a high frequency pulse is avoided, and an additional device such as a coupling capacitor is not required to be provided. It is a fifth object of the invention to provide a method for detecting partial discharge in an insulation of an electric power apparatus in which a detecting sensitivity of a high frequency pulse is high. It is a sixth object of the invention to provide a method for detecting partial discharge in an insulation of an electric power apparatus in which a S/N ratio is improved. It is a seventh object of the invention to provide a method for detecting partial discharge in an insulation of an electric power apparatus in a calibration of an apparatus for detecting the partial discharge is easy. These objects are solved by a method according to claim 1. Additional preferred features are subject of the dependent claims. These and other features of the invention will be apparent in following descriptions of preferred embodiments. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A to 1C are explanatory diagrams showing conventional methods for detecting partial discharge in an insulation of an electric apparatus, Figs. 2A and 2B are an explanatory diagram and an equivalent circuit for calibrating operation in a conventional method for detecting partial discharge in an insulation of an electric apparatus, Figs. 3A to 3C are a frequency spectrum of noise obtained from an electric power cable, Fig. 4 is an explanatory diagram showing a method for detecting partial discharge in an insulation of an electric apparatus in a first embodiment according to the invention, Fig. 5 is an explanatory view showing a normal joint in the first preferred embodiment, Fig. 6 is an explanatory view showing an insulating joint in a method for detecting partial discharge in an insulation of an electric apparatus in a second preferred embodiment according to the invention, Figs. 7 and 8 show methods for detecting partial discharge in an insulation of an electric apparatus in third to sixth preferred embodiments according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Before explaining a method for detecting partial discharge an insulation of an electric power apparatus in a preferred embodiment according to the invention, the aforementioned conventional methods for detecting partial discharge will be explained. Figs. 1A to 1C show an electric power cable line, to which the conventional methods for detecting partial discharge in an insulation of a power cable, comprising power cables 1 which are connected to each other by a normal joint 2, and each of which is connected to a high voltage conductor 4 by a terminal joint 3. Metal sheaths (not shown) of the power cables 1 are connected at the normal joint 2 and the terminal joint 3 to the ground by lead wires 5 and 6, respectively. In Fig. 1A, a detecting impedance 9 is inserted between the metal sheath and the ground by use of the lead wire 6 connected to the terminal joint 3, and an apparatus 10 for detecting partial discharge is connected to both terminals of the detecting impedance 9. In addition, a coupling capacitor 8 is connected between the high voltage conductor 4 and the ground, thereby providing a closed circuit for the partial discharge detection. In Fig. 1B, a coupling capacitor 8 is provided be connected at one electrode to the high voltage conductor 4 and at the other electrode to the detecting impedance 9, and a choke coil 7 is provided to stop a high frequency pulse to be passed therethrough and allow the high frequency pulse to be passed through the coupling capacitor 8. As a matter of course, the partial discharge detecting apparatus 10 is connected to the both terminals of the detecting impedance. In Fig. 1C, the detecting impedance 9 is inserted between the metal sheath and the ground by use of the lead wire 5 connected to the normal joint 2, and is connected at the both terminals to the partial discharge detecting apparatus 10. In operation, a predetermined high voltage is applied to the high voltage conductor 4. Where partial discharge occurs in the insulation of the power cable 1, especially at the normal and terminal joints 5 and 3, a high frequency pulse is induced between the electric conductor and the metal sheath, so that an electric potential difference is generated across the both terminals of the detecting impedance 9 in accordance with the flowing of a high frequency current through the detecting impedance 9 from the metal sheath to the ground. The electric potential difference is detected by the apparatus 10, so that the deterioration of the insulation is diagnosed in the apparatus 10. Next, the calibration of the partial discharge detecting apparatus 10 will be explained in Fig. 2A, wherein like parts are indicated by like reference numerals as used in Fig. 1C, except that a calibration signal generator 11 is connected to the high voltage conductor 4 and the metal sheath of the terminal joint 3, and a coupling capacitor 8 of a capacitance C K is connected between the high voltage conductor 4 and the ground. Fig. 2B shows an equivalent circuit corresponding to the construction of Fig. 2A. In Fig. 2B, the calibration signal generator 11 supplies a high frequency pulse having a voltage V Q through a capacitor of a capacitance C Q across the conductor 12 and the metal sheath 14 of the power cable 1, between which a capacitor 13 of a capacitance C C is formed. Thus, the high frequency pulse flowing through the capacitor 13 is partially shunted into the detecting impedance 9, so that an electric potential difference is generated across both terminals of the detecting impedance 9. Then, the sensitivity calibration of the partial discharge detecting apparatus 10 is carried out. Here, if it is assumed that an apparent charge of discharge which is carried out by the capacitor 13 is Q, and an electric potential difference across the capacitor 13 is V 1 , the equation (1) obtained Q = V Q · C Q = V 1 · C C From the equation (1), the equation (2) is obtained. V 1 = C Q C C · V Q In an actual power cable system, a length of a power cable line is, for instance, several Km, so that the capacitance C C is much greater than the capacitance C Q (C C > C Q ). Consequently, the voltage V 1 becomes very small, so that the calibration of the partial discharge detecting apparatus 10 is difficult in operation to be carried out. In other words, the high frequency pulse V Q is attenuated to reach the normal joint 2, at which the partial discharge detecting apparatus 10 is located to be connected across the detecting impedance 9. In the explanation of the methods for detecting partial discharge in the insulation of the power cable system, frequency dependency of noise which has been confirmed by the inventors will be finally explained. In the detection of the noise frequency dependency, the method as shown in Fig. 1B is used, wherein a length of the power cables 1 is 10 Km, and the partial discharge detecting apparatus 10 is of a frequency-sweep type signal intensity detector. Figs. 3A to 3C show a frequency spectrum of noise which is received by the apparatus 10, where no voltage is applied to the conductors of the power cables 1. As clearly illustrated, Fig. 3A shows the frequency spectrum ranging up to 10 MHz which is characterized by having a high level of noise in the vicinity of 4 MHz, and Figs. 3B and 3C show enlarged frequency spectrums which cover ranges of 3 to 5 MHz and 3.8 to 4.2 MHz, respectively. As shown, especially, in Fig. 3C, noise is high in its level at 3.82 MHz, and 3.92 to 3.95 MHz, and low at 3.88 MHz, and 4.0 to 4.14 MHz. It should be noted that the noise level is lower at 3.88 MHz than at 3.82 MHz by approximately 35 dB. The inventors have confirmed that partial discharge according in an insulation of an electric power cable is also of a frequency spectrum similar to that as described above, because: (1) a circuit structure of an electric power cable system is complicated to have a number of inductance and capacitance components, by which resonance and anti-resonance are generated at various frequencies, so that an amplitude of a signal is complicatedly varied dependent on a frequency of the signal; and (2) a high frequency pulse is reflected to go forward and back along an electric power cable line in accordance with the mismatching of impedance, thereby generating a standing wave, so that a high detecting sensitivity is obtained, where a peak point of the standing wave is positioned at a detecting location, while a low detecting sensitivity is obtained, where a nodal point of the standing wave is positioned at the detecting location. Fig. 4 shows a method for detecting partial discharge in an insulation of an electric power apparatus in a first preferred embodiment according to the invention, wherein like parts are indicated by like reference numerals as used in Figs. 1A to 1C, except that detecting capacitors 40 are connected at one electrode to metal sheaths of the normal joint 2 and the terminal joint 3 and at the other electrode to the detecting impedances 9. Each one of the detecting impedances 9 and the detecting apparatus 10 may be provided at one of the terminal joint 3 and the normal joint 2. In more detail, the detecting capacitor 40 is provided around the normal joint 2, such that a planar electrode 41 of, for instance, copper, aluminum, etc. surrounds the outer surface of a poly-vinyl chloride sheath 42 of the normal joint 2, as shown in Fig. 5. The planar electrode 41 is provided with a connecting terminal 43, to which the detecting impedance 9 is connected by use of a lead wire. As explained before, the metal sheath of the normal joint 2 is connected to the ground by the lead wire 5, and the both terminals of the detecting impedance 9 is connected to the partial discharge detecting apparatus 10. A size (area) of the planar electrode 41 is determined dependent on a frequency band of a high frequency pulse, a value of the detecting impedance 9, etc. by considering a reactance value of the detecting capacitor 40 not to be large. There is a residual inductance of a very small value between the grounding wire 5 and the detecting impedance 9, so that a voltage drop across the residual inductance can be detected. Fig. 6 shows a method for detecting partial discharge in an insulation of an electric power apparatus in a second preferred embodiment according to the invention, wherein a pair of planar electrodes 41 are provided on a poly-vinyl chloride sheath 42 of the insulating joint 20 having an insulating cylindrical member 41. The planar electrodes 41 are provided with connecting terminals 43, respectively, to which the detecting impedance 9 is connected by use of two lead wires. In the detecting impedance 9, the two lead wires are differentially connected to provide a middle point which is connected to the ground. The grounding may be omitted. In operation, an electric potential generated in accordance with a difference of electric potentials on the metal sheaths on the both sides of the insulating cylindrical member 44 is applied to the detecting impedance 9, to which the partial discharge detecting apparatus 10 is connected. The detecting impedance 9 may be replaced by a pair of detecting impedances each connected to a corresponding one of the planar electrodes 41. In the first and second preferred embodiments, modifications may be made as stated below. (1) The planar electrode may be provided around the grounding lead wire. For instance, if the grounding lead wire is of a coaxial cable, an inner conductor can be a grounding lead wire, and an outer conductor can be a planar electrode. (2) The planar electrode may be provided in a ring-shape around a grounding metal member of a terminal joint. (3) The planar electrode may be replaced by an insulated metal member provided on a grounding side of a bushing for a terminal joint. (4) The planar electrode may be reinforced on its outer and/or inner surfaces electrically and mechanically by adhering a vinyl tape, etc. thereto. (5) The planar electrode may be composed of a metallized paper, etc. (6) The planar electrode may be provided permanently, or temporarily, only when the partial discharge detection is carried out. Fig. 7 shows a method for detecting partial discharge in an insulation of an electric power apparatus in a third preferred embodiment according to the invention. In figure 7, an electric power cable 1 comprises a copper electric conductor 91, an inner semi-conductive layer 92 including conductive carbon, a cross-linked polyethylen-insulation 93, an outer semi-conductive layer 94 including conductive carbon, and an aluminium metal sheath 95. The detecting impedance 9 and the partial discharge detecting apparatus 10 are connected across the metal sheath 95 separated at the insulating joint 20 including a connecting sleeve 99 for connecting the conductors 91 and an auxiliary electrode 100 provided on a poly-vinyl chloride sheath 101 covering the metal sheath 95. At the insulating joint 20, generally, a high voltage happens to be applied to the metal sheath 95, where an accident such as ground fault, etc. occurs in the power cable line. Even if such a high voltage is generated thereon, a high voltage proof capacitor is formed between the auxiliary electrode 100 and one of the separated sheaths 95, so that the partial discharge can be detected at the insulating joint 20. Fig. 8 shows a method for detecting partial discharge in an insulation of an electric power apparatus, wherein like parts are indicated by like reference numerals as used in Fig. 7, except that a gapless arrester 102 is connected between the metal sheath 95 in the vicinity of the sheath removed portion 98 and the detecting impedance 9a, and a coupling capacitor 103 is connected between the metal sheath 95 and the detecting impedance 9b. The gapless arrester 102 protects the detecting impedance 9a and the partial discharge detecting apparatus 10a from a surge voltage, and provides a coupling capacitance in the partial discharge detecting system. The separated metal sheath 95 functions as an auxiliary electrode. Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.

Claims: A method for detecting partial discharge in an insulation of an electric power cable, comprising the steps of: providing a detecting electrode (41, 95, 100), for capacitive coupling on a conductive member, on which a high frequency pulse is induced by said partial discharge, to provide a capacitor (40) at a joint (2, 20) of said electric power cable; and introducing said high frequency pulse through said detecting electrode (41, 95, 100) to a partial discharge detecting apparatus (10, 10a, 10b), in which said partial discharge is detected to diagnose a degradation of said insulation. A method for detecting partial discharge in an insulation of an electric power cable, according to claim 1, characterized in that: said joint is a normal joint (2); and said detecting electrode is an electrode (41) provided on a plastic sheath (42) of said normal joint. A method for detecting partial discharge in an insulation of an electric power cable, according to claim 1, characterized in that: said joint is an insulating joint (20) at which metal sheaths of said electric power cable are electrically separated from each other; and said detecting electrode is a pair of electrodes (41), said pair of electrodes being provided on a plastic sheath (42) of said insulating joint (20). A method for detecting partial discharge in an insulation of an electric power cable, according to claim 1, characterized in that: said joint is an insulating joint (20); and said detecting electrode is an isolated metal sheath section (95) which is formed by removing a metal sheath on both sides of said isolated sheath section. A method for detecting partial discharge in an insulation of an electric power cable, according to claim 1, characterized in that: said joint is an insulating joint (20); and said detecting electrode is an auxiliary electrode (100) provided on a plastic sheath of said insulating joint.

Inventors: ENDOH TAKESHI, HAGIYA MIKIO, IKEDA CHUKI, IMAI TOMOAKI, ENDOH, TAKESHI, HAGIYA, MIKIO, IKEDA, CHUKI, IMAI, TOMOAKI, Imai, Tomoaki, Hitachi Cable, Ltd.
Applicants: HITACHI CABLE, HITACHI CABLE, LTD.
Application year: 1990
Main classification: G01R31
IPC codes: G01R31