Symmetrical Control Method for Grid-Connected Inverters in Weak Grid

In the control system of a three-phase grid-connected inverter, phase-locked loop and power loop are usually used for control. Due to its control structure being asymmetrical in dq frame, it is necessary to determine whether the eigenvalues of the system return ratio matrix satisfy the generalized Nyquist criterion when analyzing stability. However, the form of the eigenvalues is relatively complicated. Therefore, this article analyzes the form of eigenvalues, studies ways to simplify eigenvalues, and explains that symmetrical control can simplify the eigenvalues of the return ratio matrix. On this basis, a symmetrical phase-locked loop is adopted to make the control structure of the phase-locked loop symmetrical in dq frame, and it is proposed to make the control of the power loop symmetrical in dq frame by q-axis voltage feedforward. The theoretical analysis and the simulation results show that the simplified stability analysis method proposed is accurate.