Application of Digital Current Selective Tripping Protection in Subway Medium Voltage Ring Network Power Supply

Objective Compared with traditional protection mechanism of medium-voltage ring network in power supply system, digital current selective tripping protection has quicker motion performance and better selectivity. However, while providing the possibility of infinite extension of protection range by selective tripping, the impact of cable capacitance current gets prominent and becomes a factor that cannot be ignored. Therefore, it is necessary to conduct in-depth research on digital current selective tripping protection and its setting design under the large partition of medium voltage network in subway power supply system. Method In view of the fact that digital current selective tripping protection is widely adopted by many urban rail power supply systems in China, a specific protection tripping case of Guangzhou Metro is selected to analyze in detail the waveform data recorded by the protection device, and further analyze the root causes of tripping phenomena in and out the fault zone. Result & Conclusion The wave recording data in the fault zone exhibits typical single-phase grounding fault characteristics, which is consistent with the actual cable fault situation. However, outside the fault zone, the distortion of the current waveform is shown to be caused by the sudden change of voltage. Due to the basic balance of the three-phase load in subway 33 kV ring network, and in the case of phase C voltage loss, although the voltage of phases A and B experiences a brief distortion (about 3 cycles), the change in load current during this period can be ignored. Therefore, the distortion of the three-phase current is mainly caused by the capacitance current of the cable. In view of this situation, the setting design principle of the digital current selective tripping protection should be improved in a timely manner.