| In distributed photovoltaic generation system,energy storage components are usually not considered due to cost and other reasons.However,with the increasing scale of the photovoltaic power supply,the fluctuation of photovoltaic power generation will have a great impact on the power grid,so it is also very important to suppress the power fluctuation of the photovoltaic system.The existing PV inverter structure with energy storage components has many problems such as high DC side voltage and low output level,and requires large volume output filter.Therefore,the key point of this paper is to put forward a topology structure that concatenates the energy storage unit and several photovoltaic cells in a mixed cascade,and uses the cooperative modulation method of specific harmonic elimination(SHE)and carrier phase shift(PS-PWM)method for this structure.The work completed in this paper includes the following aspects.(1)In view of the current development status,background and existing problems of cascaded photovoltaic inverter system at home and abroad,the development of H bridge cascaded inverter is emphasized,and the principles and advantages and disadvantages of various modulation methods are compared.(2)In view of the disadvantages of the poor output waveform of the traditional cascaded photovoltaic inverter,a hybrid cascaded H bridge inverter topology with energy storage is designed in this paper,and its structure and working principle are explained in detail.(3)All kinds of output waveforms suitable for cascaded inverter modulation methods are studied.The shortcomings of high harmonic content and poor output waveform can be seen through the analysis of the results of various traditional modulation methods by Matlab.To solve this problem,a cooperative modulation method based on specific harmonic elimination and carrier phase shifting is proposed in this paper.The energy storage unit uses SHE to eliminate the specific low order harmonic in the output voltage.In the photovoltaic unit,the PS-PWM modulation method is used,and the output of the photovoltaic unit is used to compensate the other low subharmonics that are not eliminated in the output of the energy storage unit,so that the harmonic content in the total output voltage is lower and the output waveform is better.(4)The simulation model of a hybrid cascaded H bridge inverter with energy storage is constructed by using Matlab.The co modulation method and the carrier phase shift unified modulation method are simulated.Through comparison and analysis,it is found that the total harmonic distortion(THD)proposed in this paper is 2.12% lower than that of the unified modulation method,and the co modulated THD can be stable when the simulation parameters change.It is fixed at about 6.5%,while the THD of the unified modulation is very large.It shows that the cooperative modulation method is feasible and has strong applicability to different operation conditions.(5)The hardware in loop(HIL)test platform of a real time emulator OP5600 produced by OPAL-RT and a digital controller composed of DSP(TMS320F2812)and FPGA(EP4CE115)is set up.The system parameters and the hardware and software structure of the simulation model and the experimental platform are introduced in detail.The experimental results show that the THD of the output waveform of the single PS-PWM modulation method and the co modulation method are 16.55% and 11.87% respectively,which further illustrates the correctness and feasibility of the co modulation method. |
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