In the condition of a weak grid

the interaction between the grid-connected inverters and the grid might directly lead to the instability of inverters especially for those inverters with grid-voltage feedforward controllers. A serial-parallel impedance reshaping controller is proposed to improve the phase margin at the intersected frequency between the inverter and the grid after analyzing the output impedance characteristics. Moreover

a self-adjusting differential coefficient design method is presented as well after discussing the sensitivity of the differential coefficient to the phase enhancement. With this proposed controller

the system stability and the robustness against grid-voltage-harmonics are improved. The experimental results verify the effectiveness of the proposed control method.