Impedance Characteristics Comparison of Different Grid-Forming Control Strategies

This paper firstly establishes the self-impedance and accompany-impedance models of Droop control, VSG control, and VSG control with virtual impedance in grid-forming control, and further derives their equivalent impedances. Subsequently, a detailed comparative analysis of these three control strategies is conducted. It is found that Droop control can essentially be regarded as a special case of VSG control. By comparing the output impedance, it can be found that the equivalent impedance of the three control strategies has little difference in high frequency characteristics, but the difference is significant in the middle and low frequency range. Particularly noteworthy is the sub-super synchronous frequency band (within 100Hz) , where both the magnitude and phase of VSG output impedance are lower than those of Droop control, indicating a weaker damping characteristic. At the same time, VSG controls have a larger phase jump near 50Hz, indicating a greater risk of synchronous frequency oscillations. In addition, the introduction of a positive virtual resistance leads to an increase in the resistance component of the equivalent impedance and a decrease in the inductance component, which increases the stability margin by raising the intersection of the impedance curve, indicating that a positive virtual impedance reduces the risk of oscillation generated by interaction with the grid by enhancing the positive damping of the system.