JP3333725B2 - Partial discharge measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring partial discharge characteristics of a power cable having an open terminal, such as a drum-wound cable, with high sensitivity and indirect calibration using a test power supply unit having a minimum capacity. The present invention relates to a partial discharge measuring method.

[0002]

2. Description of the Related Art Conventionally, a partial discharge test for a drum-wound power cable has been performed by detecting a frequency of about several hundred kHz, such as a low frequency method. For signal detection, a coupling capacitor is connected to a sample cable. The charge calibration is also performed by connecting a pulse generator to the end of the cable.

[0003]

However, since the detection frequency used in this case is low, the signal attenuation inside the cable is small, and the measurement sensitivity in the longitudinal direction is oscillated due to the influence of the superposition of reflection. When the shield room is not used, several tens of
A sensitivity of about 0 pC is generally used, and sufficient sensitivity cannot be obtained.

SUMMARY OF THE INVENTION It is an object of the present invention to measure a discharge characteristic of a power cable having an open distal end irrespective of the length of the cable, with good uniformity of sensitivity, to use a power supply device having a minimum capacity, An object of the present invention is to provide a partial discharge measurement method which can be used in combination with charge calibration by indirect calibration with sensitivity.

[0005]

According to the present invention, there is provided a method for measuring a partial discharge, comprising the steps of: measuring a cable to be measured for a partial discharge and a dummy cable substantially equivalent to the cable to be measured; The ends of the first and second conductors of the parallel conductor having a length at which the line impedance becomes zero at the measurement frequency at which the partial discharge sensitivity characteristic becomes flat over the entire length of the cable to be measured are removed. Connected to the near ends of both cables, a test transformer was connected to the other end of the first conductor, and a simulation circuit equivalent to the test transformer was connected to the other end of the second conductor. Connected, a detection foil electrode is attached to the anticorrosion layer at the near end of the cable to be measured and the dummy cable, and a measurement circuit is connected to these detection foil electrodes, respectively, in the vicinity of the detection foil electrode. After attaching the calibration foil electrode, The calibration frequency electrode is connected to a calibration circuit, a test voltage is applied to the cable under test from the test transformer, and the measurement frequency component including a partial discharge of the cable under test obtained by the detection foil electrode. , And the measurement signal is calibrated using the calibration foil electrode and the calibration circuit to measure the magnitude of the partial discharge of the cable under test. It is characterized by doing.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the illustrated embodiment. FIG. 1 is a configuration diagram of an embodiment for implementing a method according to the present invention. The detection foil electrodes 3a, 3b are respectively provided on the outer sides of the anticorrosion layers 2a, 2b at the near ends of the cable under test 1a which is open at the far end and is drum-wound, and the dummy cable 1b which is also drum-wound and has the same structure.
b, and is connected to a measuring instrument 5 via a signal detection circuit 4 for differentially detecting the outputs of the detection foil electrodes 3a and 3b. The calibration foil electrodes 6a and 6b are attached near the detection electrodes 3a and 3b.
The calibration circuit 7 is connected to a and 6b.

[0007] The conductors 8a and 8b of the parallel conductor 8 having the length L are connected to the conductors at the near ends of the cable under test 1a and the dummy cable 1b, respectively. A blocking coil 9 is connected to the other end of the conductor 8a connected to the cable under test 1a.
The test transformer 10 is connected via a, and a simulation circuit 11 equivalent to the test transformer 10 is connected to the other end of the conductor 8b via a blocking coil 9b. The other ends of the test transformer 10 and the simulation circuit 11 are grounded.

The blocking coils 9a and 9b are inserted so that the load on the parallel conductor 8 becomes an open-end load, and may be omitted by modifying the transformer 10 and its simulation circuit 11. In addition, by arranging the ring-shaped magnetic bodies 12a and 12b on the grounding wires of the insulating layers of the cables 1a and 1b, the impedance becomes high at a high frequency used as a measurement frequency in the present invention, and it is as if the insulating connection portion is reproduced. On the other hand, the ground state is ensured at the commercial frequency.

Prior to measurement, a measurement frequency unique to the cable under test 1a is selected so that uniform sensitivity characteristics to partial discharge can be obtained over the entire length of the cable under test 1a. The applicant of the present invention has filed a method of obtaining the measurement frequency as Japanese Patent Application No. 5-204674. In this method, the measurement sensitivity is obtained from the output response ratio of the indirect calibration to the direct calibration and the response rate of the input signal, and a measurement frequency at which the measurement sensitivity becomes flat in the length direction is selected. By selecting this measurement frequency, the signal is attenuated and the reflection is reduced compared to the conventional method.
Approximately flat characteristics are realized.

[0010] The measurement frequency is the length of the cable,
Although it depends on the structure and the like, it does not necessarily need to be obtained each time the measurement according to the present invention is performed, and may be selected from a pickup table or the like that has been manufactured in advance.

Following selection of the measurement frequency, the length L of the parallel conductor 8 is determined. This is to make the impedance seen from one end between the conductors 8a and 8b of the parallel conductor 8 zero at the selected measurement frequency. When the far end is open, that is, when the far end which is the connection end of the parallel conductor 8 is open, the reactance is known to alternately become zero and infinity every wavelength / 4 of the cable length. Have been. Therefore, when the frequency is f and the pulse propagation velocity in the parallel conductor 8 is v, if the length L is an odd multiple of v / (4f), the line impedance becomes zero. Here, assuming that the inductance of the parallel conductor 8 is L ₀ and the capacitance is C ₀ , the propagation speed v is obtained by calculation at v = 1 / (L ₀ · C ₀ ) ^1/2 . If there is data in advance, the length L of the parallel conductor 8 need not be determined by this calculation.

As described above, when the measurement frequency and the length L of the parallel conductor 8 are selected, the line impedance of the parallel conductor 8 becomes zero at the selected measurement frequency, and
This is equivalent to a short circuit between the near ends of a and 1b. Although depending on the structure of the parallel conductor 8, when the measured common wave number is 10 MHz, for example, the length L is about 10 m.

In this state, when a test voltage is applied from the test transformer 10 through the blocking coil 9a and the conductor 8a of the parallel conductor 8, the voltage is changed to the cable under test 1a.
And a symmetrical balance circuit for noise is formed without being added to the dummy cable 1b. As described above, since the test voltage only needs to be applied to only the conductor under test 1a and the conductor 8a of the parallel conductor 8, for example, the power supply is compared with the method described in Japanese Patent Application No. 6-191971 previously proposed by the present applicant. Requires less capacity, halving the cost of expensive power supplies.

In such a configuration, when a partial discharge occurs in the cable under test 1a due to the application of a voltage, a portion corresponding to the measurement frequency is passed through the detection foil electrodes 3a and 3b, which can be regarded as capacitors, and the signal detection circuit 4. The discharge signal is detected by the measuring device 5. Even if noise is mixed into the two cables 1a and 1b from outside, these noises are easily canceled by the differential connection of the balance circuit in the signal detection circuit 4 due to the symmetry of the cables 1a and 1b. In the measuring device 5, only the signal due to the discharge of the cable under measurement 1a can be obtained with high sensitivity.

In addition, by injecting a known charge from the power supply of the calibration circuit 7 through the calibration foil electrodes 6a and 6b, indirect calibration is performed, that is, a discharge generated inside the cable is injected into an external detection unit. Perform calibration. With this calibration,
The magnitude of the discharge signal obtained by the measuring device 5 can be properly evaluated.

Further, the parallel conductor 8 needs to have a structure in which the capacitance C ₀ per unit length and the inductance L ₀ are uniform in the longitudinal direction. If the length is adjusted in the middle part, handling is easy.

Further, the parallel conductor 8 is formed of GIL (Gas Insulate).
d line) and may be diverted. If a shielding conductor is provided outside, handling is easy, and it is also a measure against noise. The parallel conductor 8 may have a cable structure, and the appearance with respect to the length L can be reduced by forming the inner conductor into a coil shape.

As described above, the present invention is a method which can be handled in the same manner as the partial discharge measurement of the insulated connection portion, and a new detection method which does not have any other problems as long as the measurement frequency and the length of the parallel conductor 8 are properly selected. Becomes

Further, it is not necessary to insert the cable drum into the shield drum, and accurate measurement can be performed only by symmetrically disposing the dummy cable 1b with respect to the cable 1a to be measured.

[0020]

As described above, in the partial discharge measuring method according to the present invention, the detecting foil electrode is attached to the near end of both the cable to be measured and the dummy cable having the same specifications, and the line-to-line Using a parallel conductor with zero impedance, replace the discharge detection operation of the terminal part with the operation of the insulated connection part, and measure the partial discharge generated in the cable under test with high sensitivity regardless of the length position To make things possible.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment.

[Explanation of symbols]

 1a Cable to be measured 1b Dummy cable 2a, 2b Corrosion protection layer 3a, 3b Foil electrode for detection 4 Signal detection circuit 5 Measuring instrument 6a, 6b Foil electrode for calibration 7 Calibration circuit 8 Parallel conductor 8a, 8b Conductor 9a, 9b Blocking coil 10 Test transformer 11 Simulation circuit

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-178997 (JP, A) JP-A-7-128390 (JP, A) JP-A-6-109801 (JP, A) JP-A-2- 309269 (JP, A) JP-A-8-36018 (JP, A) JP-A-7-280873 (JP, A) JP-B-3-15707 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01R 31/12

Claims (2)

(57) [Claims] 1. A cable to be measured for a partial discharge and a far end of a dummy cable substantially equivalent to the cable to be measured are released, and a partial discharge sensitivity characteristic becomes flat over the entire length of the cable to be measured. The ends of the first and second conductors of the parallel conductor having a length at which the line impedance becomes zero at the measurement frequency are respectively connected to the near ends of the two cables, and a test is performed on the other end of the first conductor. Connect the transformer for the second
A simulation circuit equivalent to the test transformer is connected to the other end of the conductor, and a foil electrode for detection is attached to the anticorrosion layer at the near end of the cable under test and the dummy cable, respectively. A measuring circuit is connected to the foil electrode, a calibration foil electrode is attached in the vicinity of the detection foil electrode, a calibration circuit is connected to these calibration foil electrodes, and the cable under test is connected from the test transformer. A test voltage is applied to the detection foil electrode, and a signal based on the measurement frequency component including the partial discharge of the cable to be measured obtained by the detection foil electrode is extracted from the two cables by the measurement circuit in a balanced manner. A partial discharge measuring method, comprising: performing calibration using the calibration foil electrode and the calibration circuit to measure the magnitude of the partial discharge of the cable to be measured. 2. The length L of the parallel conductor is L = v / (4f) or an odd multiple thereof, where f is the measurement frequency and v is the pulse propagation velocity in the parallel conductor. Partial discharge measurement method described.