中文摘要：

检测局部放电是监测换流变压器绝缘状态的有效手段，但是针对不同电压类型（交流、直流以及极性反转电压）对油纸绝缘起始放电特性影响的研究很少。为此，通过球板电极对油纸绝缘施加交流、直流以及交直流复合电压，借助常规脉冲电流法和宽带脉冲电流法2种检测手段，研究了油纸绝缘在不同电压类型下发生起始放电时的起始放电电压、视在放电量、放电相位以及放电波形等的差异。研究结果表明：不同电压类型时，起始放电电压按照电压升序排列为交流、正极性直流＋交流、正极性直流、负极性直流＋交流、负极性直流；直流下的起始放电电压比交流下高4倍；球板电极下油纸绝缘起始放电均多发生在1、3象限，且2个象限的放点次数不同。此外交流和直流电压下发生起始放电时，其放电脉冲的波形特性各不相同；正极性直流下起始放电脉冲波形最陡（脉宽为数ns），中心频率最高（42MHz），而负极性直流下起始放电时脉冲最平坦，中心频率最低。

英文摘要：

Observing partial discharges （PDs） is an effective means to monitor the oil-paper insulation in converter transformers, but the impact of voltage type on PD＇s inception characteristic is hardly focused on. In order to study the varying PD＇s inception voltage（PDIV）, apparent charge, pulse phase, and pulse waveform, etc., under different voltage types, we applied AC, DC, and AC-DC composite voltages to the oil-paper insulation with ball-plate electrode, and used the normal pulse current method and broadband pulse current method to detect PDs. It is found the order of voltage types with ascending PDIV is AC, positive polarity DC ＋ AC, positive polarity DC, negative polarity DC ＋ AC, and negative polarity DC. The PDIV under DC voltage is 4 times higher than that under AC voltage. The inception PD of the tested model occurs in the first and the third quadrants, while the discharge numbers are different between the two quadrants. In addition, the discharge pulse waveform varies under AC and DC voltages when the inception discharge happens： the pulse waveform under positive polarity DC is the steepest （with width of nanoseconds）, and it has the center frequency （42 MHz）; on the contrary, the pulse waveform under negative polarity DC is the flattest with the lowest center frequency.