Research On Neutral Grounding Mode And Fault Location Technology Of Medium Voltage Island Microgrid

In recent years,in order to make full use of green energy such as solar energy and wind energy,microgrid has been widely developed,and its access to 10～35kV Power Grid is very common.Medium voltage microgrid will become an important direction of power grid development in the future.Microgrid is different from traditional distribution network because of its flexible circuit composition,short length line,more measuring equipment,flexible power access and flexible operation mode.For the traditional medium voltage distribution network,the grounding methods of neutral point are divided into:ungrounded neutral point,grounded by arc suppression coil and grounded by small resistance.At present,there is no clear regulation on the neutral grounding mode of medium voltage microgrid under the planned island operation mode.The operation mode of microgrid can be divided into grid connected operation and island operation.When short circuit fault occurs in microgrid,the fault current of microgrid under different operation modes is significantly different,so the fault location strategies adopted under different operation modes are also very different.At present,most of the researches focus on the short-circuit model of distributed generation and fault location under grid connected operation,while there are few researches on fault location technology of medium voltage microgrid under island operation mode.Since distributed generation is connected to medium voltage power grid,low voltage ride through control is usually configured to support system voltage,and the difference of output current characteristics caused by low voltage ride through makes fault location more difficult.Therefore,it is necessary to select the neutral grounding mode of medium voltage island microgrid and study the fault location technology on this basis.The main research work of this paper is as follows:1.For the planned island operation mode,the neutral grounding mode of medium voltage island microgrid is determined.The factors that affect the neutral grounding mode include power supply reliability,overvoltage,relay protection setting,insulation level,personal safety and so on.Due to the existence of multiple PT parallel operation in microgrid,when microgrid enters island operation mode and loses the grounding neutral point of main grid,multi PT ferroresonance will occur.For multi PT ferroresonance,ferroresonance cannot be reliably eliminated by using a series resistance in primary or secondary winding.This paper proposes a method to eliminate the ferroresonance:put in a neutral point grounded by a small resistance.Due to the short circuit and small capacitance to ground in microgrid,it is difficult to locate the single-phase ground fault when the neutral point is not grounded or grounded through arc suppression coil.The neutral point of medium voltage island microgrid is grounded through small resistance.2.In the planned islanded operation mode,for the medium voltage island microgrid with master-slave control,fault location technology is proposed.In islanded microgrid,distributed generation under constant power control mode usually has low voltage ride through control and can output certain reactive power to support system voltage.It is found that the amplitude of fundamental frequency component of system short-circuit current is small,and the phase change is complex,so it is difficult to use it for fault location.Through the analysis of different kinds of short circuit faults,the fault location strategy is proposed.For three-phase and two-phase short circuit,the transient process of two kinds of short circuit is analyzed.The fault location is realized by using the transient characteristic that the instantaneous value of fault phase current of two adjacent detection points on both sides of the short circuit point has opposite mutation direction at the time of short circuit,and the fault phase current is identified by wavelet transform.For single-phase to ground and two-phase to ground short-circuit faults,based on the neutral point grounded by small resistance proposed in research content 1,the distribution characteristics of zero sequence current in the system are analyzed,and the fault location is realized based on this.3.According to the actual situation,the software and hardware development and design are carried out for the neutral point small resistance automatic input and fault location technology of medium voltage island microgrid,which realizes the accurate fault location of medium voltage island microgrid,ensures the safe operation of microgrid,and is conducive to the long-term development of microgrid.