Abstract
In order to include more renewable power sources in the grid, with as little power loss as possible, an increasing number of power electronics are being integrated as well. The switching of the power electronics leads to more and larger transients, which might harm the electrical insulation. Partial discharges (PDs) can lead to degradation and ageing of the insulation system. Studying the PDs occurring in the insulation can therefore give a good indication to the state of the insulation. The objective of this project has been to study the partial discharges in two different transformer insulation systems, in order to gain a better understanding of how transient voltage stresses affect the insulation compared to sinusoidal voltage stresses. Nytro 10XN, a mineral oil that is commonly used in transformer insulation, was used as the insulation liquid in one of the systems. Midel 7131, a biodegradable synthetic ester that is becoming a popular alternative to Nytro for use in transformer insulation, was used as the insulation liquid in the other system. The insulation systems were stressed by a sinusoidal voltage in order to see how the systems behave under ''normal'' conditions, and by a bipolar voltage pulse in order to see how they behave when stressed by fast repetitive voltage pulses. The PDs have been detected and measured using photomultiplier tubes (PMTs) and a current transformer for both voltage stresses. For the measurements with a sinusoidal voltage stress, Omicron MPD 600 and a high-speed video camera was used as well. Finding the partial discharge inception voltage (PDIV) for the two insulation systems under the two different voltage stresses has been the main focus. The patterns in the PD-plots and the visible ageing on the insulation caused by the PDs have also been studied. The PDIV was found to be much lower, for both insulation systems, when the systems were stressed by a bipolar voltage pulse. The PDIV for void discharges was approximately three times higher when the systems were stressed by a sinusoidal voltage. The PDIV for surface discharges was approximately twice as high when the insulation systems were stressed by a sinusoidal voltage. Some slight differences were observed between the behaviour of the two insulation systems. The difference in PDIV suggests that PDs, which might lead to deterioration and ageing of the insulation systems, occur at lower voltages if the insulation system is stressed by fast repetitive voltage pulses compared to if it is stressed by a sinusoidal voltage. The results have therefore reaffirmed the hypothesis that fast repeating voltage pulses are more harmful to transformer insulation than a sinusoidal voltage.