中文摘要：

在高压直流（high voltage direct current,HVDC）输电系统中，受铁心饱和及交直流场各种滤波器影响，相比常规变压器，换流变压器空载合闸时差动保护更容易误动，由于换流变压器工作环境的特殊性，在发生不对称故障时，可能在差动电流中存在较大的2次谐波，从而导致差动保护误制动。对换流变空载合闸和区内故障进行仿真分析，验证了2次谐波制动在HVDC系统变压器差动保护中确实有较大的局限性，同时具有误动和拒动的可能。经分析，由于变压器的铁心进入饱和需要一定时间，励磁涌流引起的虚假差动电流突变量出现时刻要比相电压突变量的出现时刻滞后一个时间差。据此，提出基于相电压突变量和差动电流突变量出现时差的变压器差动保护判据，运用该判据对各种换流变压器区内故障和励磁涌流进行判别。通过EMTDC进行了大量的仿真试验，结果验证了该判据的可行性和有效性。

英文摘要：

Due to the impacts of the core saturation and the filters in AC and DC fields in high voltage direct current （HVDC） systems, the differential protection of the converter transformer may real-operate during the unloaded transformer energization. On the other hand, in virtue of the particularity of the operating environment of the converter transformer, as the differential current may contain the 2nd order harmonic component with quite high amplitude, the transformer differential protection may fail to trip in the case of asymmetric internal faults. The energizations and internal faults of the converter transformer are simulated and analyzed in this paper. It is disclosed that the 2nd order harmonic restraint criterion is not completely appropriate when it is applied to the differential protection for the converter transformer. According to the investigation of the characteristic of the transformer core, there exists a time difference between the sudden change of phase voltage and the emergence of differential current. Therefore, a novel criterion utilizing the time difference between the superimposed phase voltage and differential current to distinguish between the internal faults and energizations of the converter transformer is proposed. The feasibility and effectiveness of the proposed criterion are validated with numerous EMTDC based simulation tests.