Research On Digital Three-phase Phase-locked Loop In The Photovoltaic Grid System

With the development of the global economy, energy shortages, ecological deteriorationof the environment and other issues have become increasingly prominent. Many countriesfocus their eyes on renewable energy such as wind and solar energy.The use of solar photovoltaic power generation technology to solve the energy problem hasbecome one of the principal means China. The advantage of grid-connected PV systems isthat they do not need high energy storage and have a high energy efficiency. In thephotovoltaic grid systems, there is a need to detect the phase and frequency of the gridvoltage in real-time to control the Grid-connected Inverter, so that the phase and frequencyof the output current is synchronous with the grid voltage. Wether the phase is trackedquickly and accurately is the key technology and also a difficult problem.In this paper, the working principle and basic structure of the phase-locked loop areanalyzed deeply include the principle and mathematical model of phase detector, loop filterand voltage-controlled oscillator.And phase-locked loop working condition and the mainparameters are described,also,the principle of coordinate transformation of three-phase digitalphase-locked loop has been given.A design is made about three-phase digital phase-locked loop based on the aboveprinciple and there is a detailed description of each module.Three frequency divider,fourfrequency divider and five frequency divider are concluded in the subtraction circuit.If a leadpulse or lag pulse is detected, the four frequency divider will make phase be forward or bebackward to achieve synchronization by adding or reducing pulse equivalently. Theadjustment of the phase is controlled by the waveform output of driver without analog circuit,besides the ability of anti-jamming is improved by the full digital designation and thedynamic characteristics is better. The coordinate conversion algorithm is used in the programto avoided the impact of noise on the accuracy of circuit in zero-crossing comparator. TheRAM two-dimensional look-up table is used to avoid complex data calculations. Structure ismore conducive to the full hardware implementation,and it can be fully realized using FPGA.The above advantages improved circuit efficiency and reduced hardware resource usage. Thesystem is simulated with MATLAB/Simulink and the given results show that this methodgreatly shorten the time needed for adjusting the phase difference. Obviously the precision ofthe system has been improved and then the effectiveness of the proposed method was verified.