| With the increasing power load and the rapid development of urban load density,the 20 kV voltage distribution network will become the inevitable trend of the development of my country’s medium voltage distribution network.Compared with the 10 kV urban distribution network,the 20 kV medium voltage distribution system is usually based on all-cable power transmission.A larger power supply radius leads to a larger capacitance current to the ground of the system.The traditional phased zero-sequence overcurrent protection The scheme will fail in the case of single-phase high-resistance ground faults.At the same time,single-phase grounding faults under different neutral grounding methods have different fault characteristics,especially for zero sequence components.The existing research on relay protection of medium voltage distribution network is mainly carried out for the 10 kV voltage level.With the increase of the capacitance current to the ground and the fault impedance,the adaptability of these protection methods to 20 kV is still to be discussed.Therefore,how to effectively identify the high-resistance ground fault of the 20 kV distribution network is of great significance to the safety and reliability of the distribution network.For this reason,this article has carried out the following research work for the ground fault detection of 20 kV distribution network:(1)Based on the symmetrical component method,the ground fault characteristics of the distribution network under different neutral point grounding methods are analyzed,including the neutral point ungrounded method,the neutral point resonance grounding method,the neutral point grounding method through a small resistance,and the neutral point grounding method.The zero-sequence voltage offset characteristics and the distribution characteristics of the zerosequence current in the steady state of the fault under the neutral point flexible grounding mode,and the constraint relationship between the zero-sequence current and the zero-sequence voltage under each neutral point grounding mode is summarized;based on PSCAD/ The simulation calculation of EMTDC verifies the correctness of the theoretical analysis.(2)In view of the characteristics of the 20 kV neutral point grounded system with small resistance,the constraint relationship between the zero sequence current of the line and the zero sequence voltage of the bus is derived from the perspective of the time domain,and the zero sequence current corresponding to the faulty line and the sound line is analyzed Based on the time-domain characteristics of the zero-sequence voltage of the bus,a highly sensitive ground fault detection method based on the average value of the differential value of the zero-sequence current and the zero-sequence voltage of the bus is proposed.This method can realize accurate and reliable identification of different types of single-phase ground faults.The method is verified based on PSCAD/EMTDC simulation.The simulation results show that this method can effectively detect high-resistance ground faults and protect against transition resistance.The ability is increased to more than 1000Ω.(3)Analyze the fault characteristics of the flexible grounding system in which the arc suppression coil is connected in parallel with a small resistance,and the time domain equations of the zero sequence current and the zero sequence voltage before and after the parallel small resistance are put into use are deduced,and the constraint relationship between the two before and after the small resistance is put into use is studied.Change the law,and introduce the concept of waveform similarity,and then propose a flexible grounding system fault line selection method based on the change of zero-sequence current and voltage waveform similarity.Simulation results based on PSCAD/EMTDC show that this method avoids frequent line trips under transient faults,and also improves the success rate of fault line selection under permanent faults. |
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