Study on Partial Discharge Characteristics in Transformer based on Oscillating Lightning Impulse Voltage Waveforms

Power transformers are known as one of the most important equipment in power system. The experiment was carried out on a 35kV transformer, which is an actual structural scale model for simulating large oil-immersed power transformers. The turn-to-turn insulation defect was set in its high voltage winding to study the PD characteristics under lightning impulse voltages, including standard lightning impulse (SLI) voltage and oscillating lightning impulse (OLI) voltages with different oscillating frequencies. The results show that there are two types of partial discharge, the main discharge and the reverse discharge. The pulse current method was more sensitive to the main discharge detection. In addition to detecting a large amplitude main discharge, the UHF method can detect many reverse discharge signals as well. The PD number under OLI is significantly larger than that under SLI. The 50% probability PD inception voltage (PDIV50) is analyzed based on the measurement results. The PDIV50 increases with the increasing of oscillating frequency of the OLI voltage.

I. Introduction

Power transformers are known as one of the most important equipment in power system. However, the unavoidable insulation defects in transformers occurring during manufacturing, transportation, assembly, or operation can lead to insulation degradation or even breaking down. The on-site standard lightning impulse (SLI) voltage withstand test is a necessary means to ensure its safe operation. However, due to the large size of conventional impulse voltage generators, SLI voltage withstand tests are rarely performed on site. The oscillating lightning impulse (OLI) voltage is recommended for the field test by IEC60060-3 standard because of its high generation efficiency. The OLI voltage rises rapidly to a peak value and then falls to zero with damped oscillations of a frequency between 15 kHz to 400kHz with or without polarity changes. It is characterized by its front time and time-to-half-value of the enveloping curve and the frequency of the oscillations [1], [2].