Fault Detection in Ring-bus Multi-terminal DC System Based on Modified S Transform

The pole-to-pole (PP) dc fault normally causes dramatic increase of dc current in dc grid, which may result in disastrous consequences in a very short time. This severe challenge has created a very high requirement on the detection time of dc fault detection method. In this paper, a S transform (ST) based fault detection method is developed with 0.4 ms detection time which is quite low comparing with existing methods. Basically, the method is composed by the high frequency detection and the low frequency screening algorithm. The high frequency detection algorithm is responsible for providing fast response to various transients, while the low frequency screening is used to pick up dc faults among them. The principle of this method and the concrete computation method is illustrated in detail and its validity is verified by applying it on the 4-bus ring multi-terminal dc (MTDC) system in MATLAB/Simulink. This method shows a great performance in terms of detection time and correct identification of the fault line. A comparison of this method between wavelet transform (WT) and short time Fourier transform (STFT) is also made to show its uniqueness.