For inner Voltage Controller (VC), the following properties are desirable: 1) High loop gain at dc to control the magnitude of the fundamental frequency voltage with low error, 2) High loop gains at fundamental and twice at the fundamental frequency in order to suppress imbalance, 3) High bandwidth to reject harmonic distortions, 4) Robust for uncertainties of the plant model 5) Maximum resonance damping, 6) fast response with smooth and minimum overshoot, 7) Negligible interaction (coupling) between real and reactive powers and 8) Highly stable to any nonlinear behavior toward instability [22]-[23].
For inner Current Controller (CC), the following properties are desirable: 1) Fast transient response with minimum overshoot, 2) Precise current control, 3) Zero steady-state error, 4) Low distortion and current ripple, 6) High bandwidth (this will be limited by the switching frequency and the controller-sampling period), and 7) Less sensitivity to the system uncertain parameters. The response speed of inner current controllers must be much more than that of inner voltage controllers because current dynamics are much faster. The current high frequency modes made by any transient is more common and wider than voltage high frequency modes; thus, the bandwidth of current controller must be much higher to be shown the desired performance.
These properties might be achieved by different techniques for different MGs. For example, those MGs with high uncertainties need a more robust control technique to show the desired performance.
