Valve-Side Grounding Fault Analysis And Line Protection Strategies Of MMC HVDC

With its unique advantages,modular multilevel converter(MMC)DC transmission technology has become one of the effective ways to solve energy transmission problems,e.g.,large-scale new energy grid connection,DC distribution network and long-distance high-capacity transmission.The fault characteristics and their evolvement of the MMC DC transmission system is quite different from those of AC system.Therefore,the valve-side grounding fault characteristics in different grounding modes and their influence on line protection are analyzed in this Ph D thesis.Furthermore,multiple line protection strategies of symmetrical monopole system and MMC bipole network system are proposed respectively,which can resist interference from valve-side faults.The main research contents are summarized as follows:1.The characteristics of valve-side grounding fault are analyzed under four different grounding modes: DC bus via clamping resistance grounding,DC neutral bus grounding,converter transformer neutral via large resistance grounding and star reactor via large resistance grounding.Based on the deep analysis of the valve-side grounding faults,it can be concluded that the ground mode serious interferes line protection.2.According to the common characteristics of valve-side grounding fault,the valve-side grounding fault identification methods based on multi-port network are proposed.The essential differences between the valve-side and DC-side grounding modes are analyzed to present the impact of the different grounding modes.The proposed method requires a short sampling time window,which solves the problem of identification of high resistance grounding fault in converter station.It can be used as a protective auxiliary component to resist the interference of valve-side grounding faults in line protection.3.The fault characteristics of the single pole-to-ground in symmetrical monopole transmission are similar to that of the low-current grounding system.The fault current is small,but the over-voltage of the non-fault pole is serious.According to the similar characteristics of positive and negative pole voltage waveform,a protection scheme based on voltage correlation is proposed.As the characteristics of single-phase grounding fault at valve-side are similar,the multi-port network DC outlet current can be used to resist interference.The protection scheme needs short time window,which solves the problem of difficult identification of high resistance grounding fault.4.Based on the transient process of MMC DC transmission system after the occurrence of single pole-to-ground fault,which is the discharge process of line distributed capacitance,the capacitance characteristic coefficient is proposed.The capacitance characteristic coefficient is used to identify the single pole-to-ground fault and complete the fault pole selection.In view of the inadequate applicability of capacitance characteristic coefficient,a method of single pole-to-ground fault protection based on inductance characteristic coefficient is proposed according to the correlation coefficient between current change rate and voltage at observation points.This method has stronger ability and applicability to withstand transition resistance.5.The single pole-to-ground fault and pole-to-pole ground fault in MMC bipole system are analyzed,which are all characterized by the rapid discharge of the converter sub-module capacitance,resulting in the rapid rise of the fault current.According to the characteristics of these faults,an inverse time protection method based on current rising rate is proposed.In this Ph D thesis,the essential differences between faulty and non-faulty lines are analyzed and compared by calculating the initial values of fault currents and their rising rates in MMC network system.Using the inverse time method,by setting a lower starting value,the protection can be reliably delayed action in the event of a high resistance fault.Moreover,setting a suitable instantaneous action value ensures that the fault can be quickly removed in the case of a lower transition resistance.The inverse time characteristics of the method are able to resist effect from non-faulty lines and non-faulty pole.This Ph D thesis takes the grounding mode of MMC DC transmission systems as the main line.It deeply analyzes the essential characteristics of valve-side grounding faults,symmetrical monopole DC line faults and MMC bipole DC line faults.The grounding faults of the valve-side are greatly affected by the grounding modes.The grounding fault current of the DC neutral bus grounding mode is mainly the sub-module capacitance discharge current of the lower bridge arm.Although the single-phase fault grounding current of other grounding modes are very small,their positive and negative pole-to-ground voltage turn into the bridge arm voltages of fault phase.All of them cause serious interference to DC line protection.To solve this problem,a port characteristic identification method based on multi-port network theory is proposed.According to the characteristics of small current grounding of single pole-to-ground fault of symmetrical monopole system,the protection scheme of voltage correlation and the protection method of characteristic coefficient are proposed.According to the characteristics of rapid discharge of sub-module capacitance caused by fault of MMC bipole DC line,an inverse time limit protection based on current rising rate is proposed.These protection methods can resist the interference of valve-side fault,and the data time window is short.The algorithm speed is fast,and the sensitivity is high.The selectivity is strong.These protection methods can solve the problem of high resistance fault identification difficulty and have a strong robustness.