A simplified state switching control strategy for survivable variable-speed induction generators

Induction machines are one of the most widely used electric machines in wind power applications. However, the control of an induction generator is complicated if advanced control strategy is employed in order to maximize the performance. Also, the complexity of advanced control strategies and their high demand on hardware and sensors can lower the overall reliability of the generator system and potentially result in system malfunction, leading to unscheduled maintenance and power yield reduction. In this paper, an effective, yet simple and low-cost state switching control technique is proposed and implemented as a backup control strategy in case of control or sensor failure. This state switching control system is only allowed to operate at two different duty cycles of PWM which produces phase voltages with different magnitudes. At the same time, the frequency of the phase voltages is derived from the reference speed. By switching between these two states, precise speed regulation can be achieved. Owing to its simplicity, the control strategy can be implemented on a low cost FPGA or embedded into existing controllers for use as a backup strategy. In addition since it does not require the use of current sensors, it is very suitable for survivable operation. Simulation and experimental results are included to validate our claims.