Abstract：Through the practical case of branch zero sequence over-current protection of start up/standby transformer and HV station transformer refuse operation when 6 kV ground fault happening, this article points out that the setting calculation not only needs to comply with the relative regulations and standardized methods, but also should combine with the status quo and the primary connecting mode. It puts forward three different situations to calculate fault current, using Multisim software. In comparison with actual figures, it summarizes distribution characteristics of fault current, chooses the best proposal, which ensures the correctness of protection, optimizes the protection setting value, and improves the reliability of protection．
2013, Vol. 14 
