A power decoupling control strategy for droop controlled inverters and virtual synchronous generators

Droop control and virtual synchronous generator (VSG) are two promising effective control strategies to integrate the renewable energy to the grid, which both mimic the frequency and voltage regulating functions of synchronous generators through droop method. The foundation of the droop method is that, assuming a small power angle, the output active power and reactive power are approximately decoupled and can be controlled by the frequency and voltage independently for a voltage source with inductive output impedance connected to an ac bus. However, the condition of a small power angle is not always satisfied. As a result, the active power and reactive power are coupled and cannot be controlled separately any more. To tackle this issue, this paper proposes a novel accurate power decoupling method by introducing current compensation into the current loop, which relaxes the assumption of small power angle. The proposed method can dramatically eliminate the power dynamic oscillation and steady-state error of a droop controlled or VSG controlled grid-tied inverter caused by the power coupling. Furthermore, the proposed method can make the paralleled inverters to share the load power by the predefined ratio accurately. The simulation results validate the effectiveness of the proposed method.