Methods for Reducing Electromagnetic Disturbances in Partial Discharge Measurement Circuits for Insulation Assessment of High Voltage Equipment

Conventional electrical partial discharge measurements have been proven as an indispensable tool to trace dielectric imperfections in high voltage apparatus which may lead to an inevitable breakdown. Therefore, the increased quality requirements for modern insulation systems as well as the enhancement of the electrical field strength and last but not least the desired enlargement of the lifetime of high voltage apparatus requires an early detection of severe partial discharge defects due to design failures and/or poor assembling work. These discharges can be caused by insulation inhomogeneity, thermal processes, unpotentialized components, thermally and electrically aged oil. The measurements are influenced by disturbances that should be small enough to allow sufficiently sensitive and accurate measurements of the PD magnitude to be monitored. Since disturbances can coincide with PD pulses and since they often overlap with the measured magnitudes, it is preferable that the background noise level is below 50% of the specified allowable amplitude of the partial discharge. The article presents both the sources of disturbances that can occur in the partial discharge measurement circuits, how they appear in the measurement system recordings and methods of reducing these disturbances. It considers methods of signal filtering, the use of balanced circuits, the use of time-domain windows to eliminate disturbances occurring at constant intervals with respect to the phase of the power-frequency voltage. The results obtained at the partial discharges measurement in the insulation of a instrument transformer under simulated conditions of certain types of disturbances and the methods used to reduce these disturbances are presented.