A constant frequency constant voltage variable speed stand alone wound rotor induction generator

Remote villages and communities isolated from utility power have to generate their own electric power. When the prime mover of the generator has variable speed, as is the case when wind power is used, the electric generator must be able to generate constant voltage and frequency at the terminals regardless of the shaft speed. A doubly-fed induction generator (DFIG) may be used in this situation because it can generate constant voltage and frequency at variable speed. The DFIG has also the characteristic of generating power from both the stator and the rotor when it operates at supersynchronous speed. This characteristic can only be exploited if bidirectional power flow capability for the rotor excitation is provided. In this paper, an experimental investigation into using a DFIG to supply an isolated load is presented. A schematic representation of the system is shown. The system uses a Scherbius scheme with two voltage-fed current-regulated PWM power converters, namely the rotor converter and the front-end converter. This allows bidirectional power flow between the stator and the rotor through the DC link and hence operation at sub and supersynchronous speed with low distortion currents is achieved. The stator voltage is indirectly regulated by controlling the stator flux magnitude. The stator frequency is kept constant as the rotor speed varies by imposing slip-frequency rotor currents in the machine