Cecilia Forssén

Partial discharges in a cavity at variable applied frequency part 2: measurements and modeling

In this paper partial discharges (PD) in a disc-shaped cavity are measured at variable frequency (0.01 - 100 Hz) of the applied voltage. The measured PD phase and magnitude distributions, as well as the number of PDs per voltage cycle, changed with the varying frequency. A charge consistent model is presented and used to dynamically simulate the sequence of PDs in the cavity. The simulation results show that the properties of the cavity surface, mainly the surface conductivity and the surface emission of electrons, change with the varying applied frequency. This is interpreted as an effect of the difference in time between consecutive PDs at different applied frequencies. This is the second of two papers addressing the frequency dependence of PD in a cavity. The first paper described how the PD frequency dependence changes with the applied voltage amplitude, the cavity size and the cavity location.

Partial discharges in a cavity at variable applied frequency part 1: measurements

In this paper partial discharges (PD) in disc-shaped cavities in polycarbonate are measured at variable frequency (0.01-100 Hz) of the applied voltage. The advantage of PD measurements at variable frequency is that more information about the insulation system may be extracted than from traditional PD measurements at a single frequency (usually 50/60 Hz). The PD activity in the cavity is seen to depend on the applied frequency. Moreover, the PD frequency dependence changes with the applied voltage amplitude, the cavity diameter, and the cavity location (insulated or electrode bounded). It is suggested that the PD frequency dependence is governed by the statistical time lag of PD and the surface charge decay in the cavity. This is the first of two papers addressing the frequency dependence of PD in a cavity. In the second paper a physical model of PD in a cavity at variable applied frequency is presented.

Modeling partial discharges in a cavity at different applied frequencies

A model of partial discharges (PD) in an insulated disc-shaped cavity is presented. The flat cavity walls are covered with copper foil and each PD is assumed to affect the whole cavity. The discharge process in the cavity is simulated dynamically and the model is charge consistent. The model is used to simulate the sequence of PDs in the cavity at two different applied frequencies: 0.02 Hz and 100 Hz. The simulation results are compared with measurements and good agreement is found. As the applied frequency is increased from 0.02 Hz to 100 Hz, the mean apparent charge increases and the number of PDs per cycle of the applied voltage reduces. This is interpreted as an effect of the statistical time lag of PD.

Modelling of partial discharge spectra measured with variable applied frequency

Variable Frequency Phase Resolved Partial Discharge Analysis (VF-PRPDA) is a newly developed method for partial discharge measurements. In this method the frequency of the applied voltage is variable and not fixed at the power frequency as in the standard PRPDA implementation. In this paper, simulations illuminate the frequency dependence of PD of a discharging cavity embedded in an otherwise homogeneous insulation. It is shown how the frequency dependence is influenced by the conductivity of the bulk insulation, the surface conductivity of the cavity walls and the statistical time lag. The results of simulations are compared with result of measurements on an asphalt-mica insulated machine stator bar.

Influence of cavity size and cavity location on partial discharge frequency dependence [polymer insulation]

Variable frequency phase resolved partial discharge analysis (VF-PRPDA) is a partial discharge measurement technique where the frequency of the applied voltage is variable. In this paper, the VF-PRPDA technique is applied to a test object containing a cylindrical cavity in an otherwise homogeneous insulation of polycarbonate. The frequency is varied in the range 10 mHz-100 Hz. It is shown how the frequency dependence of PD is influenced by the cavity diameter and the cavity location in relation to the electrodes. The PD frequency dependence is also numerically modeled for an isolated cylindrical cavity with diameter 1.5 mm using a field model.

Test cell for electric strength of rubber-epoxy interfaces

Interfaces between solid insulating materials are critical in high voltage apparatus. Here a new test cell for electric strength of rubber-epoxy interfaces was designed and evaluated experimentally. The electrically active part of the interface was about 10 mm long and the interface endpoints were screened. The rubber in the test cell was compressed and the pressure at the rubber-epoxy interface could be varied and measured. The test cell was designed for AC breakdown testing and in 36 out of 39 tested samples the breakdown occurred at the interface.

Evolution of the electric field due to forced flow in oil pressboard ducts

The electrical insulation system of power transformers is a combination of mineral oil and pressboard materials. We have measured the influence of forced oil flow on the electric field stress in oil pressboard insulation system with applied DC potential using Kerr electro-optic effect. The data show a variation in the electric field strength in flow direction, depending on velocity, time and applied voltage. The observations are explained by variation in space charge distribution on the pressboard.