Grounding Fault Analysis And Protection Technology Research Of Unipolar DC Microgrid

As an important networking method of smart grid,DC microgrid has received wide attention,and protection technology plays a key role in ensuring the reliable operation of microgrid.At present,the research on the protection of the inter-pole fault is relatively sufficient,but the research on the protection of the ground fault is relatively few.With the promotion and implementation of related demonstration projects,a protection system with practical value under various failures needs to be developed.Therefore,it is of great significance to analyze the characteristics of ground faults and propose ground fault protection schemes.Firstly,the key issues that need to be solved in the selection of the DC microgrid grounding method are explained.Among them,the selection of transformer connection method,filter grounding method,capacitor grounding method and user-side grounding method are analyzed in detail from the four aspects of the microgrid’s AC network side,AC valve side,DC system side and DC user side.At the same time,it summarizes the selection of the number of grounding points of the DC microgrid,the selection of the grounding electrode,and the grounding when the operation mode changes.Secondly,the ground fault characteristics of the symmetrical unipolar connection DC microgrid are analyzed.This paper divides the small transition resistance ground fault characteristics of the single-ended system into the capacitor discharge phase,the initial stage of diode conduction,and the alternate conduction phase of two diodes and three diodes.Ground fault characteristics of microgrid with large transition resistance.Thirdly,for the symmetrical unipolar DC microgrid,a ground fault detection and protection scheme is proposed.The differential of the outgoing current of the converter at the grounding point is used as the criterion to detect ground faults with small transition resistance,and the unbalance of capacitor voltage is used as the criterion to detect ground faults with large transition resistance.A protection scheme is proposed to cut off the fault circuit first and then isolate the fault line.This scheme reduces the protection requirements for DC circuit breakers and communications,guarantees absolute power supply reliability without affecting selectivity,and meets economic requirements.Finally,the ground fault characteristics and protection schemes of DC microgrid with asymmetric unipolar connection are described.The differential and amplitude of the ground wire current are used as the ground fault detection criterion for the positive pole small transition resistance,and the ground wire current and the VSC negative pole outgoing current are used as the positive pole large transition resistance ground fault detection criterion.The grounding protection of the asymmetric unipolar DC microgrid is realized by cutting off the fault circuit first and then isolating the fault line.The thesis verifies the correctness of the above theoretical analysis by building a simulation model in PSCAD.