Droop Control Strategy of Parallel Inverters Based on Port Voltage Integration and Variable Droop Coefficient

When inverters are paralleled using the traditional droop method in the isolated island microgrid, the difference between inverter output impedance and line impedance leads to the problem of unequal reactive power distribution and over-migration of output voltage. It is necessary to rationally configure the output impedance of the inverter and reconstruct the droop controller to meet the requirements of the inverter parallel index. The equivalent model of two paralleled inverters was established. The power distribution characteristics of the parallel system and droop control equations corresponding to different impedance angles were analyzed. Active power frequency droop and reactive power voltage droop were mainly realized based on inductive output impedance in this paper. In steady-state, the frequency of each inverter in the parallel system was consistent, so the active power can be evenly divided. There was no integration link in voltage droop, so the reactive power was unequal when the line impedance was not consistent. The causes of voltage drop in traditional droop control were revealed, including the drop caused by droop control and output impedance. Aiming at the problem of uneven reactive power distribution caused by line impedance differences and output voltage drop exceeding the standard caused by output impedance and droop control, a droop control strategy based on port voltage integration and variable droop coefficient was proposed. Firstly, the active power and reactive power were decoupling by designing the output inductor parameter and the voltage and current double-loop control parameters, so that the P-ω/Q-V droop control equation was applicable to the inverter. Secondly, the power droop controller was redesigned. The integral term of the difference between the output reactive power and the average reactive power was added to the conventional output voltage-reactive power droop equation. Two inverter output port voltage, current and its reactive power was calculated, at the same time the use of digital communication its reactive power was sent to the other inverter, reactive power real-time calculated average, through the average error of the reactive power and reactive power regulation droop coefficient, compensation inverter due to inconsistent line impedance between reactive power differences. The integral term of the difference between the output port voltage and the rated voltage was added to the droop equation, which was used to suppress the port voltage drop and stabilize the port voltage within the allowable offset range. Finally, the traditional droop control, virtual impedance droop control and the droop control strategy proposed in this paper were compared by simulation and experiment. The results show that the reactive power cannot be evenly divided due to the difference of line impedance in the traditional droop control, and the port output voltage drop was serious. Although the virtual impedance method can achieve equal division of reactive power, the output voltage still drops seriously, exceeding the ±5% requirement. The inverter parallel control method proposed in this paper can ensure equal active power division and control the difference of reactive power distribution within 5%, improve the equal division of reactive power, and maintain the inverter output voltage offset rate within ±5% of the rated output voltage. © 2023 Chinese Machine Press. All rights reserved.