The Control Strategy for the Grid-Connected Inverter Through Impedance Reshaping in q-Axis and its Stability Analysis Under a Weak Grid

The grid-connected inverter is the vital energy conversion device in renewable energy power generation. With the increasing installed capacity of renewable energy, the grid presents characteristics of weak grids with large grid impedance. In general, the inverter often obtains grid synchronization information by the phase-locked loop (PLL) and to suppress the background harmonic and amplitude disturbance of grid voltage, the grid voltage feedforward (GVF) control is also needed. However, previous researches revealed that the PLL and the GVF would shape the quadrature-axis (
q
-axis) output impedance of the inverter into a negative resistance in the low-frequency band, and this would be responsible for the instability of the inverter under a weak grid. To resolve this instability issue, this article proposes an impedance controller for reshaping the
q
-axis impedance into a positive resistance in the low-frequency band, which eliminates the negative effect introduced by the PLL and the GVF; therefore, the inverter will operate under very weak grids. Moreover, the proposed control strategy is only for
q
-axis impedance reshaping, and it does not affect the output characteristics of
d
-axis, thus ensuring the ability of the inverter to suppress background harmonics and amplitude disturbance of the grid voltage. The conclusions are verified by experimental results.