Research On Control Strategy Of PV Micro-grid Inverter Under Unbalanced Three-phase Condition

As a small power distribution system with the ability to effectively integrate distributed energy,micro-grid has the advantages of flexible operation,green environmental protection and high energy conversion rate.Photovoltaic power generation is an important power source for micro-grids.PV inverters are the key devices for converting DC inverters into AC power.Under the condition of non-ideal voltage of micro-grid,the inverter adopts conventional proportional integral(Proportional-Integral,PI)control strategy,the output three-phase current unbalance,active power,reactive power will produce double frequency fluctuations,does not meet the requirements of the grid.Therefore,it is very important to study the unbalanced control strategy of PV inverter under the three-phase unbalanced micro-grid.This paper analyzes the PI control strategy of PV inverter based on dq rotation coordinate system under the ideal voltage of micro-grid.The simulation and experiment show that the PI control strategy can meet the requirements of the grid under the ideal voltage condition.Micro-grid three-phase unbalance,the voltage component contains negative sequence components.In this paper,the traditional PI control strategy is proved by theoretical derivation,simulation and experiment,respectively,and the control ability of negative sequence components is lacking.In this paper,the positive and negative sequence separation systems based on the double second-order generalized integrator frequency locked loop(DSOGI-FLL)are used to separate the positive and negative sequences.The power of the grid-connected inverter under the condition of three-phase unbalanced micro-grid is analyzed.Combined with the positive and negative sequence voltage components and the active power obtained by the DC voltage outer ring,the reactive power is given,the inverter control target three different reference current command value.The improved proportional resonant(Proportional-Resonant,PR)regulator is used to control the current of the inverter under three-phase unbalance.In this paper,the design and implementation of the micro-grid PV branch are carried out.The circuit breaker,multi-function meter and current transformer of the PV branch are selected an,the measurement and metering circuit,sub-closing operation circuit and status indication circuit are made.The capacitance of the DC side and the filter capacitor of the AC side of the inverter are calculated.Based on the inverter's power,input and output voltage,current,the voltage and current sensor and the IPM module and other components are selected.The simulation and experimental results show that the unbalanced control strategy mentioned in this paper can realize different control targets such as constant active power or constant reactive power or three-phase current balance.