Research On Fault Ride?through Strategy For Converters And Relay Protection Strategy In Medium?voltage?level Multi?terminal Distribution Networks

Recently,to achieve the closed loop operations of distribution networks,the academia and engineering have drawn attentions on introducing the technologies of voltage source converter(VSC)based high-voltage direct current(VSC-HVDC)transmission systems to the medium-voltage distribution networks(DNs),in order to realize the interconnection between AC networks and DC networks.The key equipments of the multi-terminal DNs are VSCs,which use the power electronic technologies,and can take place of tie switches.VSCs can interconnect the different networks and regulate power flow.What's more,VSCs can provide power supply to loads after faults are cleared.As the power electronic devices,VSCs have the limited ability of current limitation.In the high voltage-level networks,to deduce the cost,the maximum currents of VSCs are selected over 1.5 p.u..When faults occur,the amplitude of fault currents are several dozen times of normal currents,which results in the shut-down of VSCs,if current limitation strategies are not used.The voltages of point of common coupling(PCC)influence the time that VSCs can keep connecting to grid.It takes networks dozens of milliseconds to hundreds of milliseconds to clear faults.If VSCs cannot keep connecting to networks longer than the time that relay protection needs,VSCs fail to supply power to loads.To guarantee the safety operation of VSCs during faults,and to achieve the goal that VSCs can provide power supply to loads,VSCs should adopt fault ride-through(FRT)strategies,limiting fault currents while injecting currents to support the voltages of PCC,so that the VSCs can keep connecting to networks longer.However,due to the VSCs,the distribution of fault currents are changed,and the sensitivity of relay protection is influenced.The protection range is decreased,and override trip may occur.To solve the problems above,this paper has built a three-terminal distribution network simulation model according to the project in Hangzhou.The fault characteristics of all kinds of faults including single-phase grounding(SPG)fault,phase-to-phase fault,double phase-to-earth fault,and three-phase fault under different neutral grounding types have been anlyzed through symmetrical component method.Particularly,voltages of nonfaulted phases will increase.Based on the analysis,the fault current limitation strategy of VSC,the fault voltage support strategy of VSC,and the strategy of relay protection based on VSC have been studied.To avoid the overcurrent of VSCs,this paper has studied the fault current limitation strategy of VSCs.As the SPG faults occur,which are the most common faults in distribution networks,the zero sequence currents can flow through the transformerless VSCs into the non-faulted networks.This results in the zero sequence voltage imbalances higher than 4%,which does not meet the requirements of IEEE.To solve this problem,the strategy of zero sequence current limitation for mulit-terminal transformerless VSCs is proposed: When SPG fault occurs at lower voltage-level network,VSC in this network implements zero sequence current limitation control;when SPG fault occurs at higher voltage-level network,and if VSC in this network cannot limit fault currents,the VSC in lower voltage-level network can help to eliminate the zero sequence currents.The zero sequence currents are limited to 0 A,which means zero sequence currents are isolated.Meanwhile,the zero sequence voltage imbalances are nearly 0%,which meets the requirements of IEEE.The strategy is also suitable for two-phase grounding faults.For the phase-to-phase faults,the strategy of negative current limitation is introduced,and the negative currents are eliminated.For the phase-to-phase faults,negative sequence currents are eliminated by negative sequence current controller,while positive sequence currents are injected to support the voltages of gird.To make the VSCs keep connecting to networks longer than the time that relay protection needs,so that the VSCs can supply power to loads after the faults are cleared,the fault voltage support strategy of VSCs has been studied.As the traditional strategy of reactive current injection is not suitable for distribution networks whose line resistance-reactance(R/X)ratios are much larger,based on the fault current limitation strategy,the current reference setting is studied.The influences of line parameters on fault voltage support through current injection are analyzed,and the conclusion that current injection ratio which equals to the line R/X ratio can support voltages of PCC most effectively is drawn.A two-stage fault voltage support strategy by injecting active-reactive currents is put forward: At the beginging of fault period,VSCs inject currents according to the traditional grid codes,while the line R/X ratio can be obtained.Then,if the active power is enough,active-reactive currents whose ratios equal to the line R/X ratios can be injected,otherwise active currents are injected as much as possible while injecting reactive currents.The severer the fault is,the better effect the strategy can achieve.When the three-phase fault occurs,positive sequence voltages drop to 0.1 p.u..After using the proposed strategy,the voltages of PCC can at most increase 4% compared with the tradional strategy,and the time that VSCs remain connecting to grid increases from 0.15 s to0.625 s.As the connection of VSCs influences the fault current distribution in networks,and the sensitivity and reliability of relay protection are affected,this paper has studied the strategy of relay protection based on VSCs.SPG faults are taken as the examples,which are the most common faults in the DNs.The sensitivity of zero sequence current differential protection is analyzed considering the stragey of zero sequence current limitation proposed in this paper,and the conclusion can be drawn that the VSCs which totally eliminate zero sequence currents have negative influences on the sensitivity of protection.Then the strategy of zero sequence current differential protection based on the transformerless VSCs is proposed.The zero sequence currents that flow through VSCs are partly suppressed.Under the same protection parameters and line impedances,compared with the situation that VSCs eliminate zero sequence currents,the performance of protection under high fault resistance conditions can be improved by 70% after using the proposed strategy.