Analysis of the Excessive Partial Discharge at Valve Side of ±800kV Converter Transformer during the AC Withstand Voltage Test

Introduction: Excessive discharge has been found in the 2.1 bushing at the valve side of HY12 converter transformer in Yunan-Guangdong UHVDC project during the AC withstand voltage test and partial discharge test. Based on the field distribution of valve-side winding in the insulation test and test data of voltage raising process, the paper analyzes the partial discharge circuit and concludes that the excessive discharge is caused by internal partial discharge of 2.1 bushing. After the replacement of 2.1 bushing, the tests are successfully carried out. 

Key words: converter transformer, valve-side winding, partial discharge, bushing 

Introduction  

The model of ±800kV high-end converter transformer HY12 in the   Yunan-Guangdong DCUHV transmission project is ZZDFPSZ-250000/800.  The model of valve-side bushing is GSETF 2090/844-3600. The delivery test began on 4th March 2010. The valve-side AC withstand voltage test and partial discharge measurement is carried out according to test procedure following successful conduction of the valve-side winding DC dielectric test, partial discharge measurement and DC polarity reversal test. When the test voltage rises to 750kV, quantity of partial discharge exceeds 1000pC. Under the influence of AC voltage, electric field distribution of oil-paper insulation is determined by permittivity; its distribution is capacitive; material permittivity  does not change as factors such as temperature and field strength vary; electric field distribution is linear. However, under the influence of DC voltage, electric field distribution of oil-paper insulation is mainly dependent on the resistivity of material; the distribution is resistive; the resistivity of material changes according to the temperature and field strength; the distribution of DC electric field is non-linear.