Partial discharge inception voltage investigation of mineral oil: effect of electrode configurations and oil conditions

The aim of this paper is to describe the Partial Discharge Inception Voltage (PDIV) and Partial Discharge (PD) characteristic of mineral oil tested by various electrode configurations. Furthermore, mineral oil with different conditions was investigated under AC voltage. The PDIV and PD of the mineral oil, Nynas Nytro 4000x, with water content about 4 ppm was investigated by using various electrode configurations of needle - plane and needle - sphere electrodes under room temperature. The PDIV test circuit was set up according to IEC 60270. Then, two PDIV measurements were applied for the investigation. The first PDIV test procedure (IEC method) was performed in accordance with IEC 61294. The second PDIV test procedure so-called combined PDIV method was adapted from IEC standard. Moreover, the electric field distributions of the electrode systems were also calculated. According to the test results, PDIV depended clearly on the electric field stress of the electrode system and the test method. Compared with other electrode configurations, the 10 μm tip radius needle - 75 mm dia. plane electrode gave the highest electric field stress and inversely provided the lowest PDIV value tested by both test methods. The combined PDIV test method generated the PDIV, charge quantity, and charge repetition significantly lower than measuring by IEC method. To investigate the effect of oil conditions on PDIV and PD characteristics, the combined conditions of temperature and water content of the mineral oil were controlled as the parameters for investigation. It was found that the water content in the mineral oil influenced approximately on PDIV value when the oil temperature was at room temperature. However, at the oil temperature of 40-90 °C, the PDIV values of the mineral oil with water content 4, 20 and 40 ppm were relatively the same. From the PD pulse current test results, PD pulse current recorded at the PDIV value from both test methods was about 0.15-1.3 mA. Pulse duration was in the range of 0.7-9 μs and the rise time was between 40-570 ns.

1 Introduction

Mineral oil is one of the most important dielectric liquids used for high voltage equipment, and has been used since the first commercial liquid - insulated transformer was produced [1]. It has been playing an important role for an insulation system especially for the transformers which are expected to reliably operate with service lives of 40 years or more. Mineral oil provides not only an excellent electrical insulation function but also an outstanding cooling property function. To accomplish these multifunction, IEC 60422 recommends that the mineral oil needs to possess certain properties such as high dielectric strength, sufficiently low viscosity, resistance to oxidation and so on [2].