Research On The Influence Of Mixed Use Of Electromagnetic And Electro-Optical Current Transformer In Line Differential Protection

In recent years, the electromagnetic current transformer has been widely used in power system. The current transformer has the similar working principle with the transformer. At present time, with the continuous increasing capacity of the power system, the voltage level increased. Due to the limit of the transmission principle, a lot of problems have emerged in the use of electromagnetic current transformer. However, there is no need to replace all of the traditional current transformer during the intellectualized reconstruction of the conventional substation. Thus, two kinds of current transformer would both use in one transmission line differential protection. When vast higher harmonic and magnetizing inrush current flow into the line due to external faults and breaker reclosure after faults removed, the differential protection would miss-action since the different transfer characteristics between TA and ETA.B-Spline wavelet was proposed in this paper to detect the saturation point of the current on secondary side of electromagnetic current transformer. The electromagnetic current transformer was mixed use with electro-optical current transformer in a same transmission line differential protection.The electromagnetic current transformer is widely used and there is many research upon problem of core saturation. The TA is easily saturated after vast higher harmonic and magnetizing inrush current flow into the transformer in line between Jiang Xian and Dong Huashan, Yun Cheng. The saturation of TA affect the correct action of protection seriously especially in the transmission line that both use TA and HETA. Two kinds of current model based on Lucas theory and J-A theory was discussed in this paper. The mathematical model based on Lucas theory is chosen since it is more suitable for the simulation experiment.The ETA has a better transmission performance than the TA and doesn’t need to consider the problem of saturation. This paper analyses each part of the ETA with Matlab and establishes the mathematical model which has the similar transmission characteristics with practical transformer produced by Nanjing Xin Ning Power Technology Co., Ltd. According to the simulation experiment, the transient characteristics of the model was good. The model could also reflect the primary current and recurrent the characteristics of the practical transformer.In this paper, the transmission line model was built in PSCAD/EMTDC according to the field transmission line in Jiang Xian-Dong Huashan. A serial of simulation experiment were designed in order to restore the fault circumstance in the field in the transmission line using above mentioned TAs. It is the saturation of the TA that causes the miss-action of the differential protectionAccording to the problem caused the false action of differential protection, this paper proposes using method of B-Spline wavelet transform to detect the saturation of the TA. The wavelet transform is widely used in the power system and it has a better real-time and accuracy performance than the other kinds of signal detection method especially in the detection of Singular point. It is proved that with the use of saturation detection method coordinate with other determine conditions could effectively avoid the false action of differential protection.