电流互感器中性线电阻增大对线路差动保护的影响

针对电流互感器(TA)中性线电阻增大导致线路差动保护误动的情况,进一步分析了此类误动产生的原理和机理。TA中性线电阻大于一定数值时,故障相二次回路电流在中性线上产生的电压降会使非故障相TA在二次侧出现类似励磁涌流的效应,从而产生波形偏向一侧的异常电流,其本质原因是TA磁链的累积。此外,通过改变二次回路方程的参数研究了中性线电阻大小、非周期分量大小、剩磁水平,以及TA容量对异常电流幅值的影响。在研究、归纳了此类误动的各种特征后,提出将三相最大电流引入制动判据中,结合该相二次谐波基波比,识别TA中性线异常后短时闭锁分相差动保护的动作逻辑。经仿真与录波图回放验证了所提保护方案的正确性和有效性。

Influence of Increase of Neutral Line Resistance in Current Transformer on Line Differential Protection

The increase of neutral line resistance in current transformer may induce misoperation of line differential protections. On the basis of previous research, this paper further analyzes its mechanism: when neutral line resistance in current transformer is greater than a certain value, if a fault happens, the voltage on neutral line generated by fault phase current will produce false current on non-fault phase by the excitation effect, resulting in the asymmetrical waveform of false current. The root cause of the false current is the accumulation of flux linkage. By changing the parameters of the secondary circuit equation, the effect of different neutral line resistance, non-periodic current, remnant level, and volume of current transformer on the value of false current is analyzed. Finally, the characteristics of this kind of misoperation are concluded. The ratio of second harmonic to first harmonic and the ratio of phase differential current to the three phase maximum current is chosen to be criteria. If both criteria are satisfied, the differential protection shall be locked for a short time to avoid this kind of misoperation. The correctness and effectiveness of the proposed scheme are verified by simulation and recorded diagrams.