A cage rotor induction generator capable of supplying reactive power

This paper describes the operation and control of a cage induction machine generator developed to enable control of the reactive power delivered to the utility grid. The topology consists of an open winding induction machine connected to a 3-phase voltage source converter bridge using a floating dc-link capacitor. Reactive VAR control is achieved by injecting a voltage in series with the machine windings (series voltage injection). This technique contrasts with many existing schemes that inject current into the grid (parallel current injection); e.g. STACOMS, capacitors or synchronous machines. Two essential control elements are described: the regulation of the floating bridge capacitor voltage; the nature of the voltage injection required to control the grid reactive power flow as the induction machine generator power fluctuates. Operation of the proposed system is demonstrated by dynamic simulation. The system is shown to be controllable and stable during changes to the mechanical torque.