Research On Control Technology For Parallel Three Phase Photovoltaic Grid Connected Inverter

Photovoltaic energy has grown under the dual crisis of both the energy and the environment after the industrial revolution,and is quickly becoming an important part of the energy mix in some regions and power systems.Until recently,paralleling operation was not a common practice in three-phase power conversion because of interactions that cause additional overhead requirements,such as transformers.To reach this goal,a number of issues need to be addressed.Among these issues,parallel ability is one of the key components because it is desirable and inevitable to use a parallel architecture with standardized modules.The use of parallel converters provides many advantages.Since the converters are usually designed individually,when they operate in parallel,potential interactions may occur.One distinguishing feature of parallel three-phase converters is a zero-sequence circulating current in the implementations without isolation.The research object of this paper is three-phase photovoltaic grid power conversion system.Through literature review,this paper first introduces the research status of photovoltaic power generation,a directly parallel three-phase converter structure is presented.Summarizes the characteristics of three-phase photovoltaic grid power conversion topologies.And then introduces the research on the parallel current,loop suppression and switching loss suppression of the PV grid-connected inverter under the centralized structure are introduced in detail in detail,Which will lay a theoretical foundation for the study of modular parallel operation control of two-level inverter.Secondly,based on the single-arm switch cycle average model,the common model components in the reserve model are used to model the basic power module.Two parallel photovoltaic grid-connected inverters are used as the main circuit,and the vector of the grid voltage.The control theory is designed for the controller of the tricyclic structure,the inner ring is the voltage/current double closed-loop controller,and the outer ring is the power loop to realize the MPPT.Again,On the basis of establishing the parallel system model,the mechanism of circulation generation and the basic principle of circulation suppression are analyzed by enumeration analysis method.The relationship between switch state and zero sequence circulation is summarized.For the space vector PWM(SVM)modulation method of alternating zero vector,the zero sequence component duty cycle is analyzed and the change of zero vector distribution does not affect the duty cycle duty cycle and the input DC voltage and output AC current control objectives of the important conclusions,and its zero sequence circulation controller was designed.However,the zero-sequence loop suppression method with zero vector adjustment factor k can only be used for alternating zero vector SVM modulation method.For the minimum switching loss SVM is not applicable,the limitation of the method means that it is not suitable for large-scale promotion and application,But it provides theoretical guidance for the proposed new algorithm.This thesis also focuses on the problem of switching loss at the same time for circulation suppression.First,the switching loss and system efficiency issues are reviewed.A compound control scheme of zero sequence circulation is proposed,which is the zero sequence circulation suppression method using SVM modulation.The SVM method is implemented by adjusting the switching on-time.The adjusted turn-on time is calculated by simply adding or subtracting the minimum turn-on time in the three-phase on-time.Due to the decrease in the switching frequency,the method can reduce the switching losses.The RT-L AB real-time simulation platform is used to build the simulation model of the three-phase photovoltaic grid-connected inverter system.The real-time simulation results show the feasibility and reliability of the method.