Analysis of Causes for Faults Arising in the AC Withstand Voltage Test of 35kV Power Transformer

Introduction: The AC withstand voltage test of power transformer aims to detect the main insulation strength of winding to ground or between windings, which is an important method of avoiding insulation faults. It plays a decisive role in whether power transformer can be put into operation. Therefore, Preventive Test Code for Electric Power Equipment stipulates that AC withstand voltage test must be conducted for 66kV and above power transformer if overhauled. The test falls into the category of destructive test because test voltage is far higher than operation voltage. Follow the instructions very strictly when conducting the test. Otherwise, it could lead to broken equipment.   

Faults Briefing 

The AC withstand voltage test was carried out for a set of overhauled 35kV power transformer on July 28, 2006. The test wiring is shown in Fig.1 The model of power transformer is SF9-10000/35 and rated voltage is (35±2.5%)/10.5kV. After the maintenance, all data of destructive test (dielectric resistance) are qualified and stewing for over 24h. The AC withstand voltage value for high voltage side of power transformer to low side and the ground is 85kV. According to Preventive Test Code for Electric Power Equipment, test voltage shall be 85% of delivery test voltage and actual applied voltage is 72kV. The capacitance of HV side to LV side and ground is 5569pF. If AC withstand voltage value is 72kV, output current of transformer HV winding is 0.126A and LV side current is 47.25A. The power transformer fully meets the requirement of field test.