Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Patent
1997-05-12
1999-08-03
Ballato, Josie A.
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
324529, 324547, 324536, G01R 3102
Patent
active
059330120
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a device for sensing of electric discharges in an object to be tested (hereinafter referred to as a "test object").
BACKGROUND OF THE INVENTION
The presence of local electric discharges, so-called partial discharges, in the insulation system for electric installation components (hereinafter referred to as "components" only) or electric high-voltage apparatus, such as, for example, instrument transformers, switching devices, generators, power transformers, cables, etc., constitutes an important parameter when assessing the quality, condition, and expected service life of the component. Therefore, the acceptance tests of components for an electric high-voltage installation normally comprise a testing with respect to the occurrence of partial discharges under well defined conditions, for example according to IEC Publ. 270 "Partial Discharge Measurements". In addition to this, different methods and devices have been developed for testing at the site of the installation for the purpose of estimating, after some time in operation, the condition of components and hence, for example, creating a more reliable basis for decision on preventive maintenance. For power generators, for example, measurement equipment is also available which allows a continuous monitoring during operation.
In installations and/or components which comprise several electric circuits, it is also important, in addition to being able to distinguish partial discharges from external noise, to be able to locate a detected partial discharge to a definite electric circuit, for example a certain generator coil in a power generator.
A partial discharge in a component gives rise to current pulses through the conductors by which the component is connected to the surroundings and these current pulses can be sensed either by measuring the voltage caused by the current pulses when passing through a measuring impedance connected to one of the conductors, or by a direct or indirect current measurement. During voltage measurement, the measurement equipment comprises a coupling capacitor, which must be free from partial discharges under the conditions wherein the measurement is carried out, and the test object, the coupling capacitor and the measuring impedance are galvanically interconnected, in a known manner, and connected to the surrounding components and testing equipment, respectively.
One problem when measuring partial discharges is to distinguish measurement signals emanating from partial discharges in the test object from disturbances emanating from partial discharges in the surrounding components or in the testing equipment or which have another origin but a frequency and amplitude spectrum resembling the spectrum of the partial discharges. Such disturbances may be passed to the test object through the conductors by which they are galvanically connected to the surroundings but may also, in the frequency ranges used, be captured by the measurement device by electromagnetic radiation from the surroundings.
During testing in a test room environment, the noise level may be reduced by screening the test room and filtering the voltage supply, but this is normally not possible when testing at an installation site. Disturbances of the kind mentioned may also be suppressed by so-called balanced test circuits where the component to be tested is balanced against a reference impedance, in which case disturbances occur as cophasal signals across measuring impedances series-connected to the test object and to the reference impedance. The reference impedance may comprise a component similar to the test object or of an impedance imitating this, usually a reference capacitor, which is then to be free from partial discharges during the testing. The balanced test circuits may also be designed as complete bridge connections. The above-mentioned coupling capacitors and the reference impedances and measuring impedances, respectively, must be galvanically connected to the component during testing and also to the high-vol
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Bengtsson Tord
Dahlberg Lars-Gunnar
Eriksson Thomas
Kheirmand Akbar
Leijon Mats
ABB Research Ltd.
Ballato Josie A.
Solis Jose M.
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