pd_rapid-voltage - IEEE Transactions on Dielectrics and...

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Measurement of Partial Discharges at Rapidly Changing Voltages Elisabeth Lindell, Tord Bengtsson 1 , Jörgen Blennow and Stanislaw M. Gubanski Department of Materials and Manufacturing Technology Chalmers University of Technology, SE 412 96 Göteborg, Sweden 1 Also with ABB Corporate Research, SE 721 78 Västerås, Sweden ABSTRACT This work presents a measuring system for PDs at repetitive voltages with short rise times, based on a coupling device adapted for this kind of voltages and a method for removing contributions from the applied voltage. The potential of the system has been demonstrated through measurements of PDs around a needle in air at voltages of 5.5 kV – 9.5 kV with rise times down to 10 μ s. The observations obtained with this PD source prove that the measuring technique described enables detailed studies of PD properties under repetitive rapidly changing voltage waveforms in the investigated range of voltages and rise times. Index Terms Partial discharges, pulse measurements, measurement system data handling, pulse shaping circuits. 1 INTRODUCTION S TUDIES of partial discharges (PDs) under ac and dc voltages have been performed for long time [1, 2]. The conventional technique for measuring PDs according to IEC 60270, [3], is based on the assumption that a very large difference in frequency contents between the PDs and the applied voltage exists, which can then be used for separation between the PDs and the applied voltage. However, insulation systems are today more and more exposed to voltages with considerably higher frequency contents. For example, numerous publications report on problems with motors fed by PWM (pulse width modulated)-inverters, e.g. [4-7], where PDs can strongly contribute to insulation degradation. Effects can also be observed in systems operating at much higher voltage levels, e.g. in cable terminations and joints [8]. Measuring of PDs at these kinds of stresses is difficult since the frequency contents of the PDs and the applied voltage are not significantly separated. In such cases the conventional PD measuring method [3] cannot be used since PDs also appear during the fast voltage rise [9, 10]. This requires a new solution as regards the measuring system used. The new standard IEC/TS 61934 [11] offers three main ways of detecting PDs at voltages with short rise times; capacitive decoupling, inductive decoupling and electromagnetic decoupling, to be used in conjunction with filtering or gating techniques. The capacitive decoupling provides possibilities to calibrate the system in a conventional way to obtain the apparent charge and is applicable in many different kinds of equipment. Different ways to handle the contributions from the applied voltage in the signal used for PD detection have been presented in literature. One way is to use a high pass filter that efficiently suppresses the applied voltage without affecting the PDs [4, 12]. This solution requires however that the frequency contents of the PDs are still substantially higher than those of the applied voltage,
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