Analysis and Comparison of Two Virtual Synchronous Control Strategies with and without Current Inner Loop of Doubly-Fed Induction Generator to Power Grid Strength Adaptability

This paper takes the two-machine and two-zone DFIG system as the research object, and studies two virtual synchronous control strategies of the "voltage and current double loop control strategy" with current inner loop and the "direct voltage control strategy" without current inner loop to power grid strength adaptability. Then two systems using the two differernt virtual synchronous control strategies are analyzed by "Modal analysis method" and "Participatory factor analysis method". And an electromechanical time-scale mode with low damping and high participation only in virtual synchronous control and terminal voltage control exists in both systems, which has a great impact on the dynamic performance of the system. Then the frequency and damping of this mode change with the power grid strength and the terminal voltage control bandwidth are studied in both two systems. The comparative analysis shows that the mode found in both systems change with the terminal voltage control bandwidth rules are the same, but change with the power grid strength rules are the opposite. With the increasing of the power grid strength, the stability of the two-machine system using the control strategy with current inner loop becomes worse and using the control strategy without current inner loop becomes better. Electromagnetic physical models of the two research systems verify the rules and the results show that the wind turbine using these two virtual synchronous control strategies have some differences in adaptability to power grid strength.