An Impedance Reshaping Control Strategy to Enhance Adaptability to Grid for Grid-Connected Inverters

With the distributed generation accessing to power grid in high proportion, multi-level transformers and long-distance transmission lines lead to larger equivalent impedance at the grid side, so the grid gradually shows the weak grid characteristics. Under this condition, grid-connected inverters prone to resonance or instability. This paper investigates the influence of grid impedance on the stability of grid-connected system based on impedance analysis method, and it is revealed that the essential reason for the resonance of the grid-connected system is the lack of phase margin, which easily triggers that the harmonic component at the impedance crossover frequency is amplified sharply and output current has a serious distortion. Aiming at this issue, an impedance reshaping control strategy is proposed to compensate the phase margin of the system, thereby improving the robustness of the grid-connected inverter to a wide range of varying grid impedance and ensuring the inverter operate stably. This approach relies on the grid voltage feed-forward control, which can achieve series correction to output impedance by choosing a appropriate feedforward function. Due to the lead correction with leading phase characteristics, it can increase the phase of the inverter impedance, and then increasing the stability margin of grid-connected system. In order to maximize the compensation of lead correction to the phase, the minimum phase margin of the grid-connected system with minimum short-circuit ratio is calculated, and specific design process and parameter selection method are given to make the phase margin meet the requirements of stable operation. Unlike the existing approach, the proposed scheme does not need to detect the real-time grid impedance to complete the phase margin compensation, therefore, it overcomes the shortcomings that the traditional solution is difficult to achieve the expected compensation effect owing to the inaccurate measurement result of the grid impedance. Moreover, the proposed controller has the advantage of facile realization and low cost. Finally, a 10-kW inverter simulation model is built in the Matlab / Simulink. Simulation results show that the inverter before adoping impedance reshaping control strategy lose instability in the weak grid, while the inverter after adoping impedance reshaping control kept stable and has a good current wave. Simulation results verify the validity and effectiveness of the proposed control strategy.