Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Patent
1995-06-12
1998-06-16
Karlsen, Ernest F.
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
324127, 324523, G01R 3108
Patent
active
057676844
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for measuring partial discharges in cables.
Partial discharges are discharges which occur in the dielectric material situated between the inner conductor and the earth shield of a high-voltage cable, wherein no complete breakdown occurs.
The occurrence of such partial discharges causes deterioration in the quality of a cable. Partial discharges are also often the initiator of complete discharges (breakdown).
Known from JP-4084779 and JP-317077 is a method for detecting partial discharges occurring in a cable, wherein the cable has an earth screen with a helical structure, comprising the following steps of: arranging at least one detection coil round the cable for detecting electromagnetic pulses travelling along the cable; applying a potential difference between the core and the earth shield of the cable; and analyzing pulses generated by possible partial discharges in the at least one detection coil in order to determine the location and quality of the partial discharge having caused the pulse.
This known method makes use of the fact that the electromagnetic pulse caused by a partial discharge travels to either side in the cable. In the case of a cable provided with a solid earth screen such pulses on the outside of the cable are not detectable in this manner without local destruction.
In cables provided with an earth shield consisting of separate conductors with a helical structure, travelling pulses cause currents in both axial and tangential direction in the conductors of the earth shield. It should be noted here that, also as a result of the high frequency of the above described phenomena, the thus induced currents continue to follow the original conductor and do not cross over to a following conductor.
These tangentially flowing currents cause an axial magnetic field that is detectable outside the cable.
With regard to the theoretical basis of the above stated phenomena, reference is made to the graduate thesis "Study of the detection of partial discharge systems over an XPLE cable" by E. J. Bresser, Eindhoven, January 1984, and the doctoral thesis "The development of high-voltage measuring techniques" by G. G. Wolzak, Eindhoven, December 1983.
This axially travelling magnetic field comprises components in a wide frequency range, namely to several GHz. It is herein noted that the high frequency components are damped within a short distance and that the low frequency components can travel greater distances.
The method and apparatus known from JP-4084779 and JP-317077 relate to delivery inspection of cables. The cable in which possible partial discharges occur moves in relation to the measuring coil. For test purposes a voltage is applied to the cable. Any partial discharges will occur repeatedly and always at the same location or locations. An a location where a partial discharge generally occurs repeatedly moves past the coil, the voltage detected in the coil will be at a maximum. Localization is thus performed by repeatedly measuring at different relative positions of the measuring coil relative to the cable.
This method cannot be used for in situ measurements and is less suitable for measurements in laboratory situations.
In this known method the detection coil otherwise provides a signal deformed such that it is a representation dispersed through time of the pulse travelling along the cable. This makes it difficult to study the physical phenomena occurring during partial discharges and makes localization of the partial discharge inaccurate. These deficiencies are caused by the comparatively low frequency at which the measurements are performed.
The object of the invention is to provide a method and an apparatus applicable in as many situations as possible, by providing a greater accuracy in the localization and a greater insensitivity to external noise sources and a more faithful representation of the original signal form.
This object is achieved in that the detection coil is adapted to process signals with a frequency higher than 100 MHz.
The fact that th
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Patent Abstracts of Japan, vol. 16, No. 308, 7 Jul. 1992 (JP 4084779, 18 Mar. 1992).
Patent Abstracts of Japan, vol. 15, No. 418, 23 Oct. 1991 (Jp 4170077, 23 Jul. 1991).
IEEE Transactions on Power Delivery, vol. 5 No. 2, Apr. 1990; Mashikian et al., Location and Characterization of Partial Discharge Sites in Shielded Power Cables.
Karlsen Ernest F.
N.V. Kema
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