Additional Kinetic Energy Injection and Piecewise Damping Based Postfault Antiwindup and Transient Stability Enhanced Control for Grid-Forming Inverter

The transient instability mechanism and enhanced control for grid-forming controlled voltage source converter (GFM-VSC) during voltage sags has been widely investigated. However, the postfault performance are overlooked in the previous studies. In this article, the postfault transient instability and windup problem of GFM-VSC with virtual inertia is first revealed based on established piecewise nonlinear model and power angle curve. Second, the impact of key parameters on postfault transient stability and windup phenomenon and the impact mechanism is analyzed, revealing that the conventional deceleration effect enhancement method increases the postfault transient stability, but increases the risk of windup problem. Third, the torque error and q-axis voltage feedforward based hybrid synchronization method is adopted to achieve transient stability enhancement and antiwindup. Different from the conventional global deceleration effect enhancement, the adopted control method can inject extra kinetic energy during the first current saturation (CS) mode and extra damping during the current unsaturation state, therefore, the global deceleration effect can be enhanced and the local deceleration effect in the first CS period can be deteriorated. Then, the antiwindup and transient stability enhancement is achieved. Finally, the analysis and effectiveness of the designed control strategy have been verified on a 500-W experimental prototype.