Research On High Frequency Isolated Grid-connected PV Inverter Without Intermediate DC Link

As environmental and energy issues have become the focus in our global scope, solar photovoltaic power generation has gotten more and more attention with its advantages of clarity, safety and convenience. Photovoltaic grid connected inverter is the key equipment of energy conversion between photovoltaic cell and power grid, which also has been a research hotspot in field of photovoltaic. This paper focuses on a new type of high frequency isolated grid-connected PV inverter without intermediate DC link and a novel hybrid SPWPM control strategy, which is suitable for any load of four quadrants operation and single-phase PV grid-connected system.First of all, this paper makes a classification research on a variety of high frequency isolated PV grid-connected inverters and deliberately analyses all kinds of working characteristics of the inverter as well as their respective advantages and disadvantages. In view of shortcomings of traditional inverter, this paper studies a high frequency isolated inverter without intermediate DC link. The operation mode of the output AC current and AC voltage in four quadrant is analyzed in detail under the control of SPWPM, and novel non-intermediate DC link equivalent circuit is obtained by analysis of SPWPM demodulation process.Secondly, combined with characteristics of the novel high frequency isolated inverter without intermediate DC link under high frequency control and power frequency control strategy, a novel hybrid SPWPM control strategy is proposed. The new control strategy combined with advantage of power frequency switching and high frequency demodulation, the new strategy could reduce switch loss and acheieve smooth AC current while crossing zero. SPWPM control is used primary stage of transformer switches. The secondary stage adopts hybrid control of power frequency and high frequency pulse to achieve inverting function of DC input and AC output. At the same time, the paper analyses working principle and closed-loop control strategy of high frequency isolated grid-connected PV inverter without intermediate DC link system based on hybrid SPWPM control.Moreover, this paper analyzes principle and output characteristics of photovoltaic cells, as well as comparative analysis of several photovoltaic maximum power point tracking (MPPT), which gains each advantages and disadvantages. In this paper, a single-stage photovoltaic grid connected inverter with high frequency isolated DC link inverter and PV array is studied. The process of control strategy and MPPT tracking the system are analyzed by using MPPT algorithm of variable step disturbance observation and the three closed loops control.At last, based on the theoretical analysis above, this paper designs hardware and software circuit of high frequency isolated grid-connected PV inverter without intermediate DC link. It has completed the structural design of the main circuit parameter design and digital control circuits, and implemented software system design including main program and interrupt subroutines. Finally, experiment is verified and its platform is established, and experimental waveform is also obtained. Through analysis of experimental waveforms of output AC current and AC voltage, voltage of switch and voltage of switch tube as well as SPWPM modulation and demodulation, correctness of the proposed theory and effectiveness of control strategy is verified.