| At present,as energy and environmental issues become increasingly prominent,distributed power generation technology has attracted widespread attention.Due to the limitation of hardware conditions,distributed energy is often connected to the grid or microgrid through the inverter parallel system.Therefore,it is necessary to adopt a effective parallel control strategy for the inverter.Droop control is one of the most commonly used noninterconnected power loop control strategies.It has the advantages of simple design,flexible control,easy expansion,and high reliability,but it also has some disadvantages like sensitive line impedance,poor reactive power distribution accuracy,insufficient inertia and damping and so on.Aiming at the deficiencies of the droop control,this paper modeled and analyzed the droop control system,and then proposed corresponding improved control strategies for the droop control problems such as power coupling,reactive power distribution error,and insufficient inertia.Aiming at the power coupling problem of the droop control strategy,the second chapter of this paper established the system model of the droop controller,and analyzed the influence of the droop control parameters on the control effect and system stability based on the model.According to the analysis,it is explained that when the output line impedance is resistiveinductive,the output power of the inverter will be coupled,which will have a great harm to the control performance and stability of inductive droop control.On this basis,an improved virtual negative impedance scheme and corresponding online impedance acquisition scheme are proposed,which can realize power decoupling to improve the power distribution accuracy of droop control and the stability of the system.Finally,an experimental verification platform is built to show that the proposed scheme had an excellent power decoupling effect,and can maintain the stable operation of the system under the condition of poor line impedance sampling accuracy.Aiming at the problem of the reactive power distribution error of the parallel inverter system,the third chapter of this paper establishes the inverter parallel system model based on droop control,and analyzes the influence of the droop control control parameters on the reactive power distribution error.According to the parallel system model,when the output impedance of each inverter does not match,there exists an error in the reactive power distribution of the parallel system.Therefore,this paper proposes a reactive power distribution strategy for the parallel system of microgrid inverters based on multi-dimensional adaptive droop control to improve the reactive power distribution accuracy of droop control.The simulation verification of the proposed strategy is carried out to show that the proposed multi-dimensional adaptive droop control strategy has a better reactive power distribution error suppression effect.Aiming at the problems of inertia and insufficient damping in droop control,the fourth chapter of this paper first introduces the virtual synchronous machine(VSG)control strategy.This paper compares the control characteristics of the virtual synchronous machine control strategy and the droop control strategy.It is found that although the virtual synchronous machine control can improve the inertia and insufficient damping of the droop controller,its active power transient characteristics are relatively poorer than the droop control.Therefore,in this paper,combining virtual synchronous machine control and droop control,a generalized droop control strategy is proposed,and the simplest structure and parameter range of the generalized droop control system are given.The experimental verification of the proposed generalized droop control is carried out through the inverter prototype system,which shows that the proposed generalized droop control can effectively compensate for the lack of inertia and damping links of droop control,and has good optimization of the transient characteristics of the active powerfrequency control loop.The effect can effectively increase the adjustment speed and reduce the overshoot. |
