A new selective transient protection for the ground fault of large unit-connected generators

Based on the multi-loop circuit method of AC machine a mathematic model is established for the large unit-connected generators, namely the outlet terminal of generators connected to a common busbar, and the transient zero-sequence current and voltage of these generators are simulated and analyzed at different grounding location. Further, making use of the wavelet transform sensitively detecting the singularity of signal, the properties of their value of wavelet transform are analyzed, and a selective transient protection is proposed according to the sign and value of the modulus maxima of wavelet transform. The results of simulation and experiment verify the new protection can correctly detect the generator with ground fault, and discriminate between the internal and external faults.