Active voltage and frequency regulator design for a wind-driven induction generator to alleviate transient impacts on power grid

Grid-connected induction generators (IGs) suffer from voltage and frequency fluctuations when stator circuits are disconnected from the power grid. A typical strategy to prevent this problem is to remove the shunt capacitor to cease self-excitation. However, when paralleling the IG with the power grid, another problem of severe voltage drop caused by excitation current inrush occurs. This paper presents an active voltage and frequency regulator (AVFR) to alleviate the undesired transients for the IG in grid-connection and grid-disconnection cases. The proposed AVFR is realized using a voltage-source inverter (VSI) with a controllable resistor at the dc link. The active power at the ac side is rectified to maintain a constant dc-link voltage using a VSI that also supplies excitation current to the machine. To achieve quick ac current response for the VSI, a stationary α-β-axis-based direct current controller is proposed. A new algorithm based on the normalized voltage vector, which features a linear relation between the α-axis voltage and phase angle, leads directly to a rule that will detect the frequency deviation and provide effective frequency control. The experimental results from a 2.5-kVA digital-signal-processor-based laboratory prototype verify the effectiveness of the proposed AVFR.