500 kV series transformer is the key equipment in Suzhou 500 kV unified power flow controller (UPFC) project,which undertakes the regulation of voltage and power between the converter and the line.Because of the special connection mode and operating condition of 500 kV series transformers,there are many differences from conventional transformers.First,the main insulation of the net side winding is matched with the line voltage grade,and the longitudinal insulation of the end is determined by the overvoltage,which brings about the complex characteristics of the insulation level of the net side windings.Secondly,the lightning full wave shock test and the external AC withstand voltage test are also different from the conventional transformers in the insulation test.Finally,the special working condition of series transformer has caused the high requirement of resisting short-circuit ability and over excitation capability.In Suzhou UPFC Project,a large number of technical researches have been carried out for 500 kV oil-immersed series transformers,which overcomes the difficult problems of the complex insulation of the windings,the special test technology,the short circuit ability and the high excitation ability.In this paper,the technical analysis of 500 kV series transformers in Suzhou UPFC project is carried out from various aspects,such as operating conditions,technical characteristics and test requirements.