Abstract-We studied the propagation of partial discharge pulses in a stator winding by means of measurements on a dismantled 35-MW generator and found that a stator winding acts as a transmission line. Therefore, a partial-discharge (PD) signal manifests itself at the generator terminals after a transit time that depends on the location of the discharge. Due to capacitive and inductive couplings in the end-winding region, a second propagation mode is present for higher frequencies. This "fast mode" manifests itself at the terminals without appreciable time delay. The capacitive and inductive couplings also cause crosstalk between the phases. As a consequence, a signal measured in one phase does not necessarily originate from a discharge in that phase. The amplitudes of the fast mode and, to a lesser extent, the slow mode (or transmission-line mode) are heavily damped when the discharge occurs further away from the measuring terminal. The consequence is that only discharges close to the measuring terminal can be observed. The propagation of a PD signal is not only influenced by the construction of the generator but also by its external connections.
Based on field experiences on a number of different turbo-generators in the Netherlands, several fundamental and applied aspects of on-line very high frequency (VHF) partial discharge (PD) diagnostics are discussed. In particular the systematic results presented in this paper contribute to: 1. the characteristics of different on-line detection using suitable VHF sensors, 2. the methods to suppress disturbances and to distinguish between different types of discharges and 3. the interpretation of measured results to obtain knowledge about the insulation condition.
Abstraef: This paper " k s the results of a preliminary study on the influence of generator operating conditions on its partial discharge (PD) activity. This study covers results from the field measurements as well as from laboratory investigations. The turbo generator discussed in t h i s paper showed an extra PD source at high temperature. This extra PD source was activated at high temperatures and vanished at lower temperatures. The activation and the deactivation process had a clear hysteresis character.Both the measurements on the turbo generator and in the laboratory showed that there is certain dependence between the internal PD and the actual temperature of the stator coil. Due to the fact that in practice the moment at which a periodic measurement is performed does not always represent the PD activity of a generator under all its operating conditions, it is of practical interest to investigate the dependences between the power, temperature and the actual PD activity. A full investigation into the relation between generator operating conditions and its PD activity is justified.
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