This paper handles the practical application of the ultra high frequency (UHF) partial discharge (PD) measuring technique in on-site tests using a resonant test set-up. Application during the service life of gasinsulated switchgear (GIS) is also addressed (on-line monitoring). Tests made during commissioning proved that even small PD defects can be detected. Even at a test voltage frequency of 122 Hz (produced by a resonant test system), the type of PD defect can be recognised on the basis of PRPD patterns from 50 Hz laboratory measurements. Furthermore, the application of PD expert system diagnosis is discussed in terms of reliability. Results based on more than a thousand measurements made at a 420/550 kV GIs laboratory set-up are presented. tntroduction Partial discharges iinside SF6-insulated switchgear show impulse-shapecl signals with a front time of less than 1 ns. Electromagnetic waves occur within a frequency range of up to several GHz, and propagate at the speed of light within the GIs as TEM-, TE-and TM-waves. The propagation of the electromagnetic waves is scarcely infiluenced by open disconnectors or closed earthing switches. However, reflection and refraction take place at discontinuities (spacers, angular modules, circuit breakers, flanges, etc.). This causes electromagnelic resonance frequencies within the switchgear, which can then be detected by special sensors. These sensors are disc-shaped and act as antenna for the ultra high fiequency signals.The intensity of the lJHF signals depends on the rise time of the PD pulse, therefore the UHF method enables the detection of a PD, but does not register its apparent charge. Extensive investigations have confirmed that the detection of PD using the UHF method results in higher or at least the same sensitivity as detection by PD measurement as set out in IEC270 (Table 1). A Cigr6 working group (TF 1S33.03.05) is currently proposing an on-site sensitivity verification for the UHF method. Table 1. Test results of comparative measurements [ 11 I I I ~~m e a s~e m e n t 1 1PC I 2PC Needle position 1 UHF '1 1 As a result of the discontinuities mentioned above, attenuation of approx. 2 dB/m occurs in an actual GIs. Therefore as a general rule the distance between two UHF sensors should not exceed 20 m. This also makes it easier to locate a PD source by means of travelling time measurement. 0-7803-4927-X/98/$10:00 0 1998 IEEE 61
This paper handles the experience with the ultra high frequency (UHF) partial discharge (PD) detection for CIS. After a short description of a typical PD monitoring system the CigrC scnsiti\ity verification for the UHF method is csplained. Examples of PD events rrom on-site testing and scnice monitoring are given and discussed in terms of risk assessment. A new procedure for risk assessment o€ inoving particles by detection of PD bursts is introduced.
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