Research On Double Closed-loop Control And Grid-connected Stability Of The Single-phase Inverter

The situation of energy shortage and environmental pollution is not optimistic.It is one of the effective ways developing photovoltaic(PV)inverter technology vigorously to ease the crisis.At present,the phase-locked loop(PLL)or frequency-locked loop(FLL)is used widely to realize the synchronous control of the grid-connected in the single-phase PV inverter with small and medium-sized capacity.However,the above-mentioned methods cause the problem that the inverter's power factor becomes low under the distorted grid.In addition,the recent research is found that the PV inverter is lack of stable state when it is switched from the strong grid to the weak grid,bringing the serious consequences always.This paper presents a method to improve the power factor of the inverter by using the mixed synchronization technology included the PLL and the FLL on the basis of the control of double closed-loop.The advantages and disadvantages of the PLL and FLL are analyzed theoretically.In the context of the distorted grid such as the decline of the amplitude of the voltage and the fluctuation of the frequency and phase as well as the increase of harmonic suddenly,the simulation based on Matlab/simulink shows that the influence between the traditional synchronous and the compound control on the output characteristics of the PV inverter.Aiming at the problem that the PV inverter is easy to be unstable under the weak grid,the strategy of voltage feedforward is added based on the control of the compound synchronization.Which can improve the output impedance of the inverter so that enhance the stability.The block diagram of the internal current-loop is constructed firstly.After contacting the inverter output impedance and the grid-line impedance,the criterion of the stability in the weak network is unfolded.The simulation results show that the control strategy of the voltage feedforward based on the complex grid synchronization technology can reduce the error in the process of grid connected and improve the stability under the weak grid.It also can avoid operation with low power factor and stopping working.