Method for comparing frequency spectrums

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 THE INVENTION This invention relates to a method for determining a frequency to detect partial discharge in an insolation of an electric apparatus. 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 chloride 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. The document DE 31 48 735 A1 discloses a method of frequency analysis in which in the harmonic parts of the spectrum are determined. The signal spectrum is compared or correlated, respectively, to a reference spectrum, the spectral values thereof beeing associated to the logarithm of frequencies. From the shift in the correlation maximum, the basic frequency can be determined. This method a applicable in particular for the analysis of machine noises. EP 0 180 322 A2 describes a partial discharge measuring device which intends to remove shot noise or periodic noise by specifically processing a signal, in particular delaying the same and calculating differences between the delayed and undelayed signal. A signal comprising a partial discharge pulse signal. is obtained which is substantially without any noise signal. 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 which a calibration of an apparatus for detecting the partial discharge is easy. These objects are solved by a method as defined in claim 1. The features of the invention will be apparent in following descriptions of preferred embodiments. BRIEF DESCRIPTION OF THE DRAWINGS The 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 a circuitry diagram showing calibration operation in a method for detecting partial discharge in an insulation of an electric apparatus in a first embodiment according to the invention, Figs. 5A and 5B are frequency spectrums of noise and calibration pulse signal in the first preferred embodiment, Fig. 6 is an explanatory diagram showing calibration operation in a method for detecting partial discharge in an insulation of an electric apparatus in a second preferred embodiment according to the invention, Figs. 7A to 7C are explanatory diagrams showing modification circuits for calibrating operation in the second preferred embodiment, Fig. 8 is an explanatory view showing a method for detecting partial discharge in an insulation of an electric apparatus in a third preferred embodiment according to the invention, Fig. 9 is a cross-sectional view showing a wire shielded electric power cable in the third preferred embodiment, Fig. 10 is an explanatory diagram showing a whole circuitry structure in the third preferred embodiment, Figs. 11A and 11B are frequency spectrums showing signal intensities based on noise and calibration in the third preferred embodiment, Fig. 12 is a cross-sectional view showing another wire shielded electric power cable in the third preferred embodiment, Figs. 13A and 13B are frequency spectrums showing signal intensities in use of the electric power cable of Fig. 12 in the third preferred embodiment, Figs. 14 and 15 are explanatory diagrams showing methods for detecting partial discharge in an insulation of an electric apparatus in fourth and fifth preferred embodiments according to the invention, Figs. 16A and 16B are explanatory views showing a modification in the fourth and fifth embodiments, Fig. 17 is an explanatory diagram showing a method for detecting partial discharge in an insulation of an electric apparatus in a sixth preferred embodiment according to the invention, Figs. 18A and 18B are frequency spectrums showing signal intensities based on noise and calibration pulse in the sixth preferred embodiment, 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 9 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. Now, a method for detecting partial discharge in an insulation of an electric power apparatus in a first preferred embodiment according to the invention will be explained. Fig. 4 shows a simplified circuit for carrying out a method for detecting partial discharge in an insulation of an electric power cable in a first preferred embodiment according to the invention, which comprises a calibrating signal generator 11 for supplying a pulse to a power cable 1 having a capacitance Cx connected to a high voltage conductor 4, and a detecting impedance 9, connected at one terminal through a coupling capacitor 8 to the high voltage conductor 4 and at the other terminal to the ground, to which a partial discharge detecting apparatus 10 for detecting a high frequency pulse at a predetermined frequency in a predetermined frequency range is connected. The power cable 1 is a cross-linked polyethylene insulated cable of a rated voltage 66 KV, to which a high voltage is applied through the high voltage conductor 4. In operation, no voltage is applied to the power cable 1, and the calibration signal generator 11 is controlled to stop operating. Thus, external noise is detected by the partial discharge detecting apparatus 10. A frequency spectrum of the detected noise is shown in Fig. 5A. Then, a calibration pulse of 100 pC (pico-coulomb) is supplied between a conductor of the power cable 1 and a metal sheath thereof from the calibration signal generator 11, so that the calibration pulse is detected through the detecting impedance 9 by the partial discharge detecting apparatus 10, in which a frequency spectrum of the detected calibration pulse is obtained as shown in Fig. 5B. In comparing the frequency spectrums of Figs. 5A and 5B, it is understood that the calibration pulse which is supplied from the calibration signal generator 11 to the power cable 1 does not contribute to the increase of a signal intensity resulted from the external noise in a frequency range less than approximately 3.5 MHz, and the signal intensity is kept to be approximately -50 dB in the frequency range more than that frequency by the calibration pulse, while the signal intensity of the external noise is decreased to be less than -60 dB in that frequency range except for the vicinity of 4MHz, especially remarkably decreased in the frequency range from 4.5 MHz to 5.5 MHz, and to be less than -70 dB in the frequency range more than 5.5 MHz. As apparent from the frequency spectrums, a S/N ratio becomes high in the frequency range from 4.8 MHz to 10 MHz. Especially, the noise level is remarkably low at the frequencies of 5.5 MHz, 6.2 MHz and 9.2 MHz, so that a high S/N ratio is obtained at these frequencies. For instance, where a detecting frequency of 5.5 MHz is selected, a S/N ratio of 34 dB is obtained, because levels of the external noise and the calibration pulse are -82 dB and -48 dB, respectively. In this state, a detecting limitation is one fiftieth of 100 pC which is 2 pC. For comparison with a conventional method, a low frequency detecting method is carried out by using a detecting impedance, a bandpass filter, and an amplifier, wherein an amplified frequency is continuously varied in the range of 10 KHz to 150 KHz, and of 10 KHz to 1000 KHz, thereby detecting a S/N ratio. In this conventional method, a detecting limitation is approximately 70 pC. This means that a high sensitivity is not obtained in the low frequency detecting method. Furthermore, a tuning type partial discharge detecting apparatus which is manufactured and sold by the Nippon Keisokuki Manufacturing, Inc. of Japan is adopted to detect partial discharge by using a tuning frequency of 400 KHz. As a result, a S/N ratio of 10 dB, and a detecting limitation of 30 pC is obtained. This means that the sensitivity is one fifteenth of that obtained in the method of Fig. 19, in which a frequency of 5.5 MHz is selected. Fig. 6 shows a method for detecting partial discharge in an insulation of an electric apparatus in a second preferred embodiment according to the invention, wherein like parts are indicated by like reference numerals as used in Fig. 4, except that a calibration signal generator 11 is connected to metal foil electrodes 120 provided on the insulating joint 20. Operation is carried out in the same manner as in the first preferred embodiment, and the same result is obtained. Figs. 7A to 7C show modifications of the method for detecting partial discharge in an insulation of an electric apparatus in the second preferred embodiment. In Fig. 7A, the calibration signal generator 11 is connected to an insulating joint 20, and a detecting apparatus including the detecting impedance 9 and the partial discharge detecting apparatus 10 is connected to another insulating joint communicated to the former insulating joint 20 by the power cable 1. In Fig. 7B, the calibration signal generator 11 is connected to an insulating joint 20, and the detecting impedance 9, etc. are connected to a normal joint 2. In Fig. 7C, the calibration signal generator 11 is connected to an insulating joint 20 of a power cable 1A, and the detecting impedance 9 is connected between non-ground type normal joints 2 of the power cable 1A and a power cable 1B of a different phase. Fig. 8 shows a method for detecting partial discharge in an insulation of an electric apparatus in a third preferred embodiment according to the invention, which comprises a 66 KV wire shielded cable 1 having a length of 10 m, a detecting coil 150 having 20 turns provided around the cable 1, a detecting impedance 9 connected to terminals of the detecting coil 150, and a partial discharge detecting apparatus 10 connected to the detecting impedance 9. Fig. 9 shows the wire shielded cable 1 which has a cross-sectional area of 100 mm 2 and comprises an electric conductor 151 of copper or aluminum wires, an inner semi-conductive layer 152 of, for instance, cross-linked polyethylene pregnant with conductive carbon surrounding the conductor 151, an insulation 153 of, for instance, cross-linked polyethylene provided around the inner semi-conductive layer 152, an outer semi-conductive layer 154 provided around the insulation 153, a wire shielding layer 155 of spiralled copper wires, and an anti-corrosion layer 156 of, for instance, poly-vinyl chloride. Fig. 10 shows a whole circuitry structure in the third preferred embodiment, wherein like parts are indicated by like reference numerals as used before. In operation, a predetermined line voltage is applied to the wire shielded cable 1, and a frequency spectrum of external noise is obtained as shown in Fig. 11A by use of the detecting coil 150, the detecting impedance 9, and the detecting apparatus. Next, a calibration pulse of 20 pC is supplied to the high voltage conductor 4 by the calibration signal generator 11, and a frequency spectrum of the calibration pulse including the external noise propagated through the wire shielded cable 1 is also obtained as shown in Fig. 11B by use of the detecting coil 150, the detecting impedance 9, and the detecting apparatus 10. As apparent from the comparison between the frequency spectrums of Figs. 11A and 11B, the highest sensitivity is obtained at a frequency of approximately 11 MHz, and the effect of supplying the calibration pulse is not observed in the frequency range less than approximately 7 MHz. At a frequency of 11 MHz, a noise level is -75 dB, and a calibration pulse level including the noise level is -55 dB, so that a S/N ratio of 20 dB is obtained in the state that a detecting limitation is approximately 3 pC. At a frequency of 7 MHz, the noise and calibration pulse levels are both approximately -80 dB, so that a S/N ratio becomes 0 dB, thereby making it impossible to detect the calibration pulse. It is considered that the noise is mainly caused by electro-magnetic wave in the air, noise current flowing through the shielding wire layer 155, and the floating of the ground potential due to noise current flowing through the ground. Furthermore, the reason why the detecting sensitivity is decreased at lower than a frequency of 7 MHz is that a voltage induced on the detecting coil 150 is proportional to a frequency, and the magnetic coupling between the detecting coil 150 and the wire shielded cable 1 is not dense to result in mismatching with an impedance of the cable 1 at a low frequency, and the reason why the detecting sensitivity is decreased at more than a frequency of 14 MHz is that the calibration pulse is attenuated by itself and impedance matching is not good between the detecting coil 150 and the cable 1. In the modification of the third embodiment, a wire shielded cable 1 as shown in Fig. 12 is used in place of the wire shielded cable 1 as shown in Fig. 9, wherein like parts are indicated by like references in the both cables 1, except that a stainless steel sheath 157 is provided between the wire shielding layer 155 and the anti-corrosion layer 156 in Fig. 12. The detecting coil 150 having 5 turns is provided around the cable 1 having a length of 5 m which is in the state of short-circuit at a remote end thereof. In operation, a calibration pulse of 2,500 pC having a rise time of 30 ns is supplied between the conductor 151 and the stainless steel sheath 157 at a near end of the cable 1, and the calibration pulse is detected by use of the detecting coil 150, the detecting impedance 9, and the detecting apparatus 10. The detected result is shown in Fig. 13A, from which it is understood that the calibration pulse is detected to be -13 dB at a frequency of 20 MHz, and -31 dB at a frequency of 100 MHz. Furthermore, the calibration pulse is detected to be less than a noise level of an amplifier in the detecting apparatus 10 at a frequency of more than 120 MHz. In a further modification of the third embodiment, the stainless steel sheath 157 is removed in an axial direction of the cable 1 by 100 mm, and the detecting coil 150 is provided in the center of the stainless steel sheath removed portion by 5 turns. The calibration pulse, which is the same as in the aforementioned modified third preferred embodiment, is supplied to the cable 1. The detected result is shown in Fig. 13B, in which the calibration pulse is detected to be -11 dB at a frequency of 20 MHz, and -22 dB at a frequency of 10 MHz. In the frequency rang of more than 170 MHz, a detected signal level is less than a noise level of the amplifier of the detecting apparatus 10. In comparison between the results of Figs. 13A and 13B, the decrease of the detected signal is approximately 2 dB at a frequency of 20 MHz, and 9 dB at a frequency of 100 MHz in accordance with a provision of the stainless steel sheath 157. This means that partial discharge can be detected in a wire shielded cable without removing an outer metal sheath in the invention, although a detecting sensitivity is lowered to some extent. Fig. 14 shows a method for detecting partial discharge in an insulation of an electric apparatus in a fourth preferred embodiment according to the invention. In this method, the detecting coil 150 is covered by a magnetic core 201 and an aluminum shielding box 202, such that the detecting coil 150 is connected through a coaxial cable 203, which is passed through a wall of the box 202, to the detecting apparatus 10. As a result, the detecting coil 150 is protected from picking external noise up. Fig. 15 shows a method for detecting partial discharge in an insulation of an electric apparatus in a fifth preferred embodiment according to the invention, wherein the detecting coil 150 is provided around the wire shielded cable 1 in the vicinity of the normal joint 2, and the detecting impedance 9 is connected to the detecting coil 150, thereby supplying a partial discharge signal to the detecting apparatus 10. In operation, partial discharge occurring in the normal joint 2 is detected with a high sensitivity, because the partial discharge signal is not attenuated substantially. Figs. 16A and 16B show two detecting coils 150 each wound on a semi-circular magnetic core 210, and positioned around the wire shielded cable 1 to face each other, such that the two detecting coils 150 are connected in series to provide a single detecting coil. Fig. 17 shows a method for detecting partial discharge in an insulation of an electric apparatus in a sixth preferred embodiment according to the invention, wherein like parts are indicated by like reference numerals as used before, except that the pulse injecting coil 150 is used to be connected to the pulse generator 130, and the detecting impedance 9 is connected between the stainless steel sheath 157 of the wire shielded cable i and the ground. In operation, external noise is detected as shown in Fig. 18A, by the detecting apparatus 10, in the state that a predetermined line voltage is applied to the wire shielded cable 1, and no calibration signal is injected into the wire shielded cable 1 from the pulse generator 130. Next, a calibration signal which is injected through the pulse injecting coil 150 from the pulse generator 130 into the wire shielded cable 1 is detected as shown in Fig. 18B by the detecting apparatus 10, in the state that the line voltage is applied to the wire shielded cable 1. As apparent from the frequency spectrums of Figs. 18A and 18B, a S/N ratio of 57 dB is obtained at a frequency of 5 MHz in accordance with a noise level of approximately -110 dB and a calibration pulse level of approximately -53 dB, and a S/N ratio of 23 dB is obtained at a frequency of 6.4 MHz in accordance with a noise level of approximately -53 dB and a calibration signal level of approximately -83 dB. In this sixth preferred embodiment, the pulse injecting coil 150 is positioned at a place such as a normal joint 2, in which partial discharge tends to occur.

Claims: Method for determining a frequency to detect partial discharge in an insulation of an electric apparatus, comprising: providing one or more electrodes on said electric apparatus (1) for detecting said partial discharge; detecting a frequency spectrum of noise obtained from said one or more electrodes; detecting a frequency spectrum of an output signal obtained from said one or more electrodes, when a calibration pulse is supplied to said electric apparatus (1); comparing said frequency spectrums of said noise and said output signal to provide said frequency, at which a predetermined signal to noise ratio is obtained; and detecting said partial discharge of said insolation of said electric apparatus (1) at said frequency. Method of claim 1, characterized in that said electric apparatus is an electric power cable (1), and said calibration pulse is supplied between a conductor (151) and a metal sheath (157) of said electric power cable. The method of claim 2, characterized in that said calibration pulse is supplied to said electric power cable (1) at an appropiate position which is selected from a terminal joint (3), an insulation joint, a normal joint, and a longitudinal selected portion of said electric power cable (1).

Inventors: ENDOH TAKESHI, HAGIYA MIKIO, IKEDA CHUKI, IMAI TOMOAKI, ENDOH, TAKESHI, HAGIYA, MIKIO, IKEDA, CHUKI, IMAI, TOMOAKI, ENDOH, TAKESHI, C/O CABLE LAB., HITACHI CABLE, LTD, HAGIYA, MIKIO, C/O CABLE LAB., HITACHI CABLE, LTD, IKEDA, CHUKI, C/O CABLE LAB., HITACHI CABLE, LTD, IMAI, TOMOAKI, C/O CABLE LAB., HITACHI CABLE, LTD
Applicants: HITACHI CABLE, HITACHI CABLE, LTD.
Application year: 1990
Main classification: G01R23
IPC codes: G01R31