Parallel connection of inverters can improve the capacity and reliability of power supply system,and is applied in the fields of new energy power generation,large-scale energy storage and micro-grid.When the parallel system adopts the droop control,there is no communication line between the inverters,the reliability is high,the cost is low,and the application prospect is good.However,the power distribution accuracy of the traditional frequency droop control needs to be improved,and the dynamic effect needs to be improved.This paper conducts the following research on the droop control technology.In this paper,the mathematical model of the combined three-phase inverter rotating coordinate system is studied.The capacitor voltage outer loop and the inductor current inner loop controller are designed directly in the digital domain.According to the output impedance model of the three-phase inverter and the Thevenin equivalent circuit The output impedance of the inverter under closed-loop control is much smaller than the output impedance of the inverter under open-loop control,and the influence of the coupling and the leakage reactance of the transformer on the output impedance is studied.This paper also establishes a mathematical model of the system when multiple machines are connected in parallel.The circulating current can be reduced by reducing the inverter output voltage difference and increasing the output impedance.This paper analyzes the basic principle of traditional frequency droop control,establishes the small signal model of frequency droop control,analyzes the influence of droop coefficient change on the characteristic equation by root locus,and the effect of the droop coefficient on the active power and reactive power distribution of the inverter at steady state is studied.This paper studies the principle and small-signal model of three improved droop control strategies for introducing virtual impedance,introducing power differential term and introducing angle droop control.The purpose of introducing virtual impedance is to increase the output impedance of the inverter to reduce circulating current.The paper also studies a simplified virtual impedance implementation method,but the virtual impedance will affect the phase and amplitude of the output voltage and affect the parameter design of the droop control.This part of the influence needs to be considered in the parameter design.Angle droop can increase system damping and improve system dynamics.If there is no common frequency reference,only the angle droop strategy is used,and the parallel system cannot be stabilized.In this paper,we also study a simple method of controlling the angle droop control by using the lag-modulated wave.Considering the dynamic and steady-state effects of power distribution for different control methods,we finally introduce virtual impedance control and angle droop on the basis of traditional frequency droop.Based on the control technology,a detailed design method for the parameters of the droop control system is proposed.The improved droop control strategy runs on a parallel system simulation platform and experimental platform consisting of two 75 kVA inverters.The system can work stably and the power sharing effect is good.This verifies the effectiveness of the improved droop control strategy proposed in this paper. |