Research On Software Phase-locked Loop And Grid-connected Control Of High Frequency Link Inverter

Nowadays, high frequency link matrix converter(MC) topology is researched popularly, which is combined high frequency inverter with matrix converter. It can achieve the trend of high frequency, bidirectional power flow, isolation transform and so on. Moreover, the volume and weight of the system are reduced. Because of special structure, the topology is being applied gradually in photovoltaic grid-connected system and motor control application fields. The paper mainly researches the topology of three-phase high frequency link matrix converter, and applies the topology in photovoltaic grid-connected system. As grid-connected control system needs extract synchronization signals of grid voltage, the software phase-locked loop(PLL) control system is studied in-depth.Modulation is the foundation of applying high frequency link matrix converter. So firstly, the paper introduces a new modulation which called de-re-couple SPWM modulation. There will be a peak voltage when the topology is converting. In order to solve that issue, the paper introduces another modulation, called integration de-re-couple SPWM modulation. The simulation verifies the feasibility of these two modulation strategies, as well as the latter one inhibits the peak voltage.Secondly, the PLL control method, which also called grid voltage synchronization detection technology, is studied. Analyses DSOGI-PLL and T/4 delay methods under unbalanced grid voltage. Based on the discussions, a double fundamental frequency PLL(DFF-PLL) method is proposed. Focusing on restraining harmonic components of grid voltage, based on dual band pass filter structure DFF-PLL is introduced. Comparing these methods by simulation, the advantages of later DFF-PLL are highlighted.Then, the mathematical model of high frequency link matrix converter is established, and the validity of de-re-couple modulation is demonstrated. Based on de-re-couple modulation, grid-connected closed-loop control methord is designed. The simulation results show the feasibility of grid-connected operation using the topology. While the photovoltaic grid-connected system is operating, the grid voltage will always in uabalance state. The paper introduces a balanced current control method to adapt that condition, applied DFF-PLL and DSOGI-PLL in control method. It is proved that the performance of PLL is important to affect grid-connected control.Finally, the designing of hardware and devices selection are introduced in details, and an experience platform is built. Using digital control, which is composed of DSP and CPLD chips, the paper completes the experiments. The experiment results demonstrate the feasibility and effectiveness of two modulations, PLL methods, and with load, grid-connected control method.