Research On Characteristics And Control Of PQ Controlled Three-phase Four-wire Converter Under Grid Asymmetric Fault

In distributed generation systems that connected to new energy sources and energy storage devices such as wind power and photovoltaic,grid-connected converters are key components for energy exchange between distributed sources and AC power grids.The distribution network is usually located at the end of the power system,and the operating environment is very complicated.The asymmetric faults such as single-phase grounding and the switching of high-power loads cause the bus voltage to fall unbalanced frequently.Therefore,the safety ride through capability of the new energy grid-connected converter needs to be improved.At present,three-phase three-wire grid-connected converters are widely used in new energy grid-connected generation,power quality control and other fields,with superior control performance and high market prospects.In contrast,a three-phase four-wire grid-connected converter with split-capacitor or four-legged arm structure has higher control performance due to the addition of a zero sequence current path.However,in the case of asymmetric faults in the distribution network,the output currents of the three-phase four-wire grid-connected converters have positive,negative and zero sequence components,and their operating characteristics will be more complicated.How to use mathematical methods to accurately describe the maximum phase current and power fluctuation under various control targets,and providing guidance for subsequent fault current limiting and coordinated control,is of great significance.The research work in this paper includes the following:Firstly,for the fault operation characteristics of three-phase four-wire converter,the three-phase four-wire converters with split-capacitor on the DC side is taken as the research object,and the instantaneous power expression of the three-phase four-wire converter is derived.The mathematical model of the target is established,and different reference current expressions are calculated to analyze and calculate the fault operation characteristics of the four-wire converter in the case of asymmetric faults.Secondly,a new overcurrent protection method based on maximum phase current amplitude estimation is proposed.In this chapter,the output current characteristics of the three-phase four-wire converter are analyzed to suppress the power fluctuation.The amplitude of the three-phase unbalanced current are calculated first,and then the amplitude and grid voltage drop depth and power factor are analyzed in turn.The relationship between the fault current phase and the fault type are estimated and the maximum phase current amplitude is estimated to adjust the reference current that ca n simultaneously suppress the fault overcurrent and power oscillation.Since the maximum phase current amplitude are obtained only by calculating one of the phases,the calculation process can be greatly simplified.Finally,the simulation model of the three-wire four-wire system was built in PSCAD,and the simulation was verified.At the same time,a three-phase four-wire converter RT-LAB experimental platform with 2000 V DC side and 2200 V AC side was built and verified by experiments.The comparison and si mulation results show that the feasibility and superiority of the three-phase four-wire system are connected to the grid.