Study Of Making Zero Sequence Current In Distribution Networks With Two Phases' Current Transformers

In the power distribution networks of 66KV level or lower levels in China, the style of neutral insulated or neutral grounded with Petersen Coil is widely used. 80% of the network fault is single phase grounding fault. The line-selecting methods adopted nowadays are all based on the zero-sequence current, except for signal-tracing methods. There are 2 ways to acquire zero-sequence current: One is to use zero-sequence TA, the other is to connect the 3-phase TAs to the zero-sequence filter style. In China, only A-phase TA and C-phase TA are installed in most distribution networks. Therefore, in order to select the faulty line, B-phase TA or zero-sequence TA would be installed additionally, which would increase expense and bring more difficulty to the implementing work. There is no practical instrument that can select faulty line with 2-phase currents home or abroad and all of the related methods are in the process of theory research. How to detect and isolate the faulty line has been a technical problem long disturbing the practical operation.In this paper, with the analysis on the zero-sequence current in frequency domain and in time domain, the transient current of each phase when single phase grounding fault occurs is analyzed with symmetrical component method. In addition, based on the zero-sequence network, the reference from A-phase and B-phase transient currents to B-phase transient current is founded out, and the way to construct B-phase transient current exactly is obtained. The method of constructing zero-sequence is not influenced by the existence of Petersen Coil or the fault resistance. To prove the correctness of the method in all kinds of single phase grounding fault, thorough emulating research work has been done with the electro-magnetic transient program (ATP) and MATLAB/Simulink which is widely used in dynamic modeling and simulation. Constructing zero-sequence current successfully provides a base and some new thoughts for implementing line-selecting methods for single phase grounding fault in distribution network with 2-phase TA, which is good for the development of line-selecting methods.