Research On Control Strategy Of Off-Grid Parallel Inverters Under Mixed Load

As the basic interface unit of the microgrid,the control strategy of the inverter has a significant impact on the safe and stable operation of the system.Droop control can achieve reasonable power distribution in parallel inverter systems without communication,and is widely used in microgrid inverter control.However,the power distribution between inverters is unbalanced and the voltage is severely distorted due to the line impedance mismatch and unbalanced as well as the nonlinear mixed loads are connected to the system.This paper studies the power sharing and power quality problems of off-grid parallel inverters under mixed loads.Firstly,the mathematical models of the inverters are established in three coordinate systems,the resistive droop control equation was deduced,and the conditions that parallel inverters needed to achieve power sharing are analyzed.The principle of using the virtual impedance to compensate the line impedance and realizing the power sharing of the parallel inverters is explained.Secondly,under the mixed load,the effect of the sub-sequence virtual impedance and the realization method in the αβ two-phase static coordinate system are analyzed according to the equivalent circuit of the inverter at different frequencies and sequences.The equivalent line impedance under the fundamental negative sequence and harmonic networks is reduced by the sub-sequence virtual impedance,which satisfies the power sharing condition of the parallel inverters,so as to achieve the equal sharing of the fundamental negative sequence power and harmonic power and reduce the common coupling point(PCC)for unbalanced voltages and harmonic voltages.Since the realization of the sub-sequence virtual impedance requires each sequence component of inverter output current,the three-phase power grid current model and the satisfied dynamic system are established according to whether there is harmonic or not.The Luenberger observers of three-phase current of power grid are designed respectively for the two dynamic systems,and their stability are analyzed.Improved active droop control to compensate for PCC voltage drop due to equivalent line impedance voltage.Adopt PI and quasi proportional resonance parallel voltage controller to realize the static-free control of DC and AC.Based on the sub-sequence virtual impedance,improved active power droop control and parallel voltage controller,a control strategy for unbalanced load and a control strategy for mixed load are designed,which improves the power sharing accuracy and power quality.Finally,the parallel inverter simulation model is built on the Matlab/Simulink platform.An experimental prototype of parallel inverter was built with TMS320F28335 as the control core.The effectiveness of the designed control strategies for unbalanced load and mixed load in this paper are verified by simulation and experiment respectively.