Prediction and avoidance of grid-connected converter´s instability caused by wind park typical, load-varying grid resonance

Instability of current control loops in grid-connected converters can cause problems in wind power systems. This phenomenon has gained increasing attention in recent literature. Often, the current control loop is designed with conventional methods neglecting the presence of grid resonances. If this approach would bear the risk of instability when grid resonances occur, it could mean a large risk for wind turbine operation. This paper investigates under which circumstances such an control loop can become instable. It evaluates the influence of a grid voltage feedforward (GVFF) on stability of the current control by transfer function analysis and simulation. The results show that with GVFF, such a simple design approach yields stable current control loops, while without GVFF, instability can exist. The simulation and analysis are validated by experimental results.