Research On PQ Control Of Three-Phase Four-Leg Converter Under Unbalanced Grid Fault

In order to cope with global warming,improve the ecological environment of the earth and reach carbon peaking and carbon neutrality,a large number of renewable energy resources are connected to the grid in the form of distributed power sources,promoting the transformation of traditional power system to new power system.With the boosting capacity of distributed generation,the operating condition of the power grid was deteriorated due to the collective cutting of the inverter power supply.So it’s urgent to study the low voltage ride through methods of the inverter power supply.Firstly,the 3P4L(three-phase four-legs)inverter is taken as the research object,and a voltage/current double closed-loop controller is designed.An improved method is proposed to solve the instability problem of the unbalance phasors detection technology under severe faults.The improved amplitude and phase detection technology is proved to have higher anti-interference ability from theoretical analysis and simulation.Secondly,a voltage controlled LVRT(low voltage ride through)control method is introduced.According to the orthogonal complex power model of the 3P4 L inverter,three kinds of reference current are given,and the contradiction between the power fluctuation control and the output current balancing is coordinated,and a reference calculation for minimum the sum of squares of three phase current.According to the seven current references given,the output current limit is considered,voltage reference of capacitance is calculated,so that the voltage/current double closed-loop control is achieved.In order to simplify the calculation of three-phase current amplitude,this paper presents a LVRT control strategy with the unbalance coordinate transformation without zero sequence current,which can limit current amplitude by equivalent current amplitude reference.Firstly,the improved amplitude-phase detection technique is used to obtain the grid voltage unbalance phasors.The reference of three phase current phase and d-axis current is calculated in the unbalanced coordinate system secondly.Then the current amplitude and each phase transmission power under different faults are analyzed theoretically.The LVRT control strategy based on unbalanced coordinate is simulated and verified.Finally,the grid connected experiment platform was built with three phase programmable power supply and 3P4 L inverter for experimental verification.The experimental results show that the stability of the amplitude phase detection technique is improved,and the proposed LVRT control method has achieved the control goal.