Analysis of grid current control in consideration of voltage feedforward and cable capacitance demonstrated on a fully sized wind turbine installed in a wind park

Before commissioning grid side converters of wind turbines, their behaviour is often validated by computer simulation. In simulation models, the grid coupling is usually modelled as inductance, resistance and ideal voltage source. The current controller is typically designed assuming that the grid influence is perfectly compensated by a voltage feedforward. In this paper it is shown that these two assumptions are under some conditions too rough and can hide oscillatory behaviour due to the following reasons. In reality the grid impedance is the result of the layout of the grid, having multiple resonances and varying with load and time. Furthermore, the voltage feedforward of the grid side current control is filtered and discrete (converter switching frequency) and does therefore not guarantee perfect decoupling from the grid situation. In this paper, the effect of imperfect grid voltage compensation is analysed for a 2 MW wind turbine installed in a wind park by using a more complex representation of the grid impedance. It is shown by transfer function analysis and time domain simulation that resonances with the cable capacitance can lead to oscillatory behaviour, although the current control loop shows excellent results when no capacitances are present.