92_pd_distribution_liqd - IEEE Transactions on Electrical...

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IEEE Transactions on Electrical Insulation Vol. 27 No. 1, February 1902 QQ Partial Discharge Distrubutions in Liquid Dielectrics M. Pompili, C. Mazzetti Electrical Engineering Department, University of Roma , Italy and E. 0. Forster Department of Physics, Rutgers University of New Jersey, Newark, NJ ABSTRACT Different dielectric fluids used in HV electrical components were subjected to ac stresses and the partial discharges (PD) spectra produced under non-uniform and uniform field condi- tions were recorded with the help of an electrical detector and a multichannel analyzer (MCA) as a function of applied stress. The analysis of the collected data confirms that PD activity in a liquid is a random phenomenon as well as a breakdown process. The presence of PD may be related with the presence in the liquid of bubbles near the electrode interface and with a low density region (vapor phase), that is the first step in the breakdown development. This confirms that PD occur only in a gas and not in a solid or liquid. In this paper, which is a part of an ongoing research project it is shown that the use of a MCA may be an helpful tool in PD measurements and analy- sis and this allows to show how the partial discharge inception voltage (PDIV) significantly change according to the adopted inception criteria. INTRODUCTION inception [l-41, as well as electrical breakdown are trical field and on the presence in the gap of impurities, free charges, etc. It is well known that continuous or in- termittent PD activity can readily lead to breakdown be- cause PD cause degradation of the oil. To define in a more or less reproducible manner the voltage at which the first PD can be detected in an electric system, the partial dis- charge inception voltage (PDIV), was introduced. The detection of a PD depends in addition to its amplitude on the sensitivity of the detecting circuit used [2]. For this PD random phenomena that depend on the applied elec- reason the PDIV has been usually defined in terms of the voltage at which PD of a given intensity are detected [5, 6,7,l. Because of the random PD appearance it is also of great importance to know the PD amplitude distribu- tion (PDAD) in order to follow the growth of PD activity with time and test voltage level [4,5,8]. To achieve this knowledge, it appears desirable to utilize a multichannel analyzer (MCA) that provides information on the PDAD from which it is possible to evaluate the PDIV for any intensity threshold adopted as inception criterion. 0018-9367 $1.00 @ 1992 IEEE Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY KANPUR. Downloaded on August 15,2010 at 17:03:54 UTC from IEEE Xplore. Restrictions apply.
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100 Massetti: Partial Discharge Distrubutions in Liquid Dielectrics In the present paper, the research is extended to sev- eral synthetic fluids which include a new perfluorinatad liquid. It is shown that the PDAD changes both with increasing voltage and test duration. The MCA yields additional information that allows a more complete eval-
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92_pd_distribution_liqd - IEEE Transactions on Electrical...

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