Research On Hiercarchical Control And Experimental Validations For Islanded Multi-inverter AC Microgrid

As non-renewable energy sources continue to dry up and the scale of distributed generation(DG)of new energy sources gradually expands,distributed generation technology has also received increasing attention.In particular,many remote areas rich in solar and wind energy can achieve self-sufficiency through new energy generation technology.In this context,the islanded multi-converter power generation system plays a huge role.Therefore,the stability of voltage and frequency of islanded multi-converter system,and the coordination of power allocation among multi-converter system become the current research hotspot.In this paper,the multi-converter system in islanding mode is studied from the following four aspects:(1)The voltage and current control,parameter design and control strategy design of single DC/AC converter are studied.Firstly,the main circuit structure and corresponding mathematical model of LC single converter system are established,then the open-loop transfer function of voltage and current double closed-loop with DQ decoupling is derived,and the corresponding bode graph is drawn by transfer function.According to the Nyquist stability criterion,a tradeoff between the dynamic performance and the stability margin of the system is analyzed,then the corresponding current inner loop proportion coefficient and voltage outer loop proportion integral coefficient are proposed.In addition,the corresponding voltage compensation control strategy is designed for unbalanced and nonlinear load conditions,the derivation process of the positive and negative sequence separation of the fundamental component and the extraction technology of the harmonic component of the nonlinear load are given.Finally,the correctness of control parameter design and the effectiveness of unbalanced voltage compensation control and harmonic voltage compensation control strategy are verified by the PLESC simulation software,and the above research lays the foundation for the research of upper-layer coordinated control of the multi-inverter system.(2)The parallel system of two DC/AC converters based on hierarchical control mode is studied.Hierarchical control mode is mainly composed of primary control and secondary control,it is found out that the traditional droop control in primary control has voltage and frequency deviations,and the classical hierarchical control mechanism in secondary control may be affected by communication delay.Aiming at the defects of traditional droop control and traditional secondary control,the enhanced droop control based on band-pass filter is introduced,then the mechanism that the enhanced droop control can be equivalent to the secondary control is derived.The small signal model of the system under the enhanced droop control is established,and the influence of the control parameters on the stability and dynamic characteristics of the system is analyzed by the eigenvalue method.Finally,the effectiveness of the enhanced droop control and the correctness of the theoretical analysis are verified by the PLESC simulation software.(3)Aiming at the problem of reactive power sharing of different lines impedance and loads disturbance in multi-converter system,a consensus algorithm is introduced based on the strategy of the enhanced droop control.The convergence mechanism of the algorithm is analyzed,and the influence of convergence weight factor and node connection topology on the convergence performance of the algorithm is also analyzed.Then combined with the enhanced droop control,a consensus-based enhanced droop control scheme is proposed,which can achieve the power sharing,voltage and frequency stability.Furthermore,the steady-state analysis of the consensus-based enhanced droop control scheme is carried out,and the control mechanism of the proposed control strategy is analyzed.Finally,a multi-inverter system simulation model composed of 6 inverters is built in PLECS to verify the control strategy mentioned in this chapter.(4)The hardware experiment platform of parallel system composed of two three-phase DC / AC converters is built.Firstly,the hardware design part is introduced with one of the converters,including DC bus circuit design,three-phase full bridge inverter and filter circuit design,current and voltage sampling circuit design,isolation drive circuit design,etc.Then the software design of the experimental platform is introduced briefly,including ADC sampling module and ePWM module.Finally,the correctness and effectiveness of the voltage and current control strategy,the fundamental positive and negative sequence voltage and current control,and the harmonic elimination control are verified by a single converter hardware experimental platform,the validity of the multi-level coordinated control strategy is verified by the parallel hardware experiment platform of two converters.