Research On The Parallel-operation Control Technique Of Inverters In Islanding Mode Of Smart Microgrid System

Parallel operation of voltage-source inverters in the islanding mode of microgrid has excellent redundancy, stability and reliability. However, the output-voltage differences of inverters will cause the circulating-current and the imbalance of output active and reactive power of inverters in the parallel system. In the worse case, it will cause the overload of inverters and the failure of parallel system.Therefore, it is a huge challenge for the design of the energy management system in the smart microgrid, which should make all parallel inverters share the load accurately according to the power capacity of every inverter, and eliminate the circulating-current effectively for avoiding the damage due to over-load or over-current.Accordingly, the topic of this thesis is the research of parallel-operation control technique of inverters in islanding mode of smart microgrid system, which is to design an effective control system based on the mathematical modeling analysis of voltage-source-inverters parallel system (VSIPS) in order to realize the technique requirements of energy management system of the smart microgrid.Firstly, this thesis introduces the definition, system topology, operating mode and control system of smart microgrid, analyzes the application of power electronics in smart microgrid, and summarizes the work mode of parallel inverters in smart microgrid. Subsequently, this thesis analyzes and summarizes the traditional parallel-operation control technique of inverters, and proposes the research direction that combines the advantages of wired and wireless parallel operation control techniques.Secondly, this thesis analyzes the VSIPS by mathematical modeling, and proposes a general mathematical model of VSIPS, in which the parallel inverters share the users’ load in any proportion. Subsequently, after the optimized design of the inverters’ wire-impedance, this thesis proposes an optimized mathematical model of VSIPS, which comprises of the steady-state model of the optimized VSIPS, the model of the circulating-current in the optimized VSIPS, and the circulating-current small-signal model of the optimized VSIPS. Moreover, this thesis analyzes the dynamic performance of the optimized VSIPS based on the proposed mathematical model.Thirdly, on the basis of the optimized VSIPS mathematical model, this thesis analysis the role of inverters’ wire-impedance in VSIPS, which consists of the influence of wire-impedance on the circulating-current of VSIPS, the influence of wire-impedance on the power quality of common AC bus, and the influence of wire-impedance on the power loss of VSIPS. Subsequently, this thesis summarizes the design principles of wire-impedance in VSIPS based on the above analysis.Fourthly, based on the optimized VSIPS mathematical model, this thesis proposes the circulating-current power mathematical model, which describes the functional relationship of the amplitude/phase difference of inverters’ output voltage and the circulating-current power with the mathematical model.According to the proposed circulating-current power mathematical model, this thesis builds and analyses the mathematical model of the circulating-current-eliminating mechanism of the traditional droop control, and summarizes its advantages and disadvantages. Subsequently, this thesis proposes an improved parallel-operation control method (ω-PCir and V-QCir control) based on the circulating-current power mathematical model, and proves that the improved method has better performance of load-power sharing by analyzing its mathematical model comparing to the traditional droop control.Fifthly, the implementation of the proposed parallel-operation control method is analyzed and designed after considering the advantages of wired parallel operation and the application background of microgrid. Subsequently, this thesis proposes a control strategy in the islanding mode of the smart microgrid based on the proposed parallel-operation control method, which includes the steady-state voltage controller and re-synchronization controller.Sixthly, according to the above research results, this thesis discusses the implementation of the control system in islanding mode of smart microgrid, then analyzes and designs the parameters of the discrete-signal control system and the power circuit of inverters. In this process, this thesis also proposes an improved power-calculating method based on the p-q instantaneous power theory, which is used to improve the performance of the traditional power-calculating method.Finally, simulation and experiment results are presented which validate the performance of the research results in this thesis:(1) the optimized VSIPS mathematical model, the circulating-current power mathematical model, the proposed parallel-operation control method and the control system in islanding mode of microgrid are validated by PLECS simulation platform; (2) three 110V/50Hz single-phase inverter prototypes are built to make up a parallel inverter system, which is used to simulate the islanding mode of microgrid, and the proposed control system in islanding mode of microgrid is validated on this experiment platform.According to the simulation and experiment results, the proposed parallel-operation control method in this thesis can decrease the active and reactive circulating-current of parallel system effectively, and it have the excellent dynamic performance and better static performance comparing to the traditional droop control menthod. Therefore, the proposed parallel-operation control method is an excellent solution for the application of VSIPS in the islanding mode of smart microgrid.