A Novel Doubly-fed Induction Wind Generator Control Scheme for Reactive Power Control and Torque Pulsation Compensation Under Unbalanced Grid Voltage Conditions

Wind energy is often installed in rural, remote areas characterized by weak, unbalanced power transmission grids. In induction wind generators, unbalanced three phase stator voltages cause a number of problems, including overheating and stress on the mechanical components from torque pulsations. Therefore, beyond a certain amount of unbalance (for example 6%), induction wind generators are switched out of the network. In doublyfed induction generators, control of rotor currents allows for adjustable speed operation and reactive power control. In addition, it is possible to control the rotor currents to correct for the problems caused by unbalanced stator voltages. This paper presents a novel controller design for a doublpfed induction generator that provides adjustable speed and reactive power control while greatly reducing torque pulsations. I. GOALS AND OBJECTIVES The goal of the research is to develop a control method for doubly -fed induction wind generators that addresses issues associated with weak rural grids. The control method presented improves the robustness and all around performance of doubly -fed induction wind generators. A controller is designed and tested in simulation and a hardware model will be built and tested. Research and simulation results are presented below. 11. INDUCTION GENERATOR UNBALANCE FFECTS Wind energy generation equipment is most often installed in remote, rural areas. These remote areas usually have weak grids, often with voltage unbalances and undedover-voltage conditions [ 1,2]. Under such abnormal conditions, wind turbines are disconnected from the grid for their own protection, thus significantly impacting their energy production and the capacity credit that they are otherwise due. As an example, the turbines in the southwestern part of Minnesota are removed from the grid beyond an unbalance of 6% in phase-phase voltages, an under-voltage of lo%, or an over-voltage of5%. Induction generators are the predominant type of wind turbine. When the stator phase voltages, supplied by the grid, are unbalanced, the torque produced by the induction generator is not constant. Instead the torque has periodic pulsations, which can result in acoustic noise at low levels and at high levels can actually destroy the rotor shaft, gearbox, or blade assembly [3]. A special type of induction generator called a doubly-fed induction generator (DFIG), shown in Fig. 1, is becoming the most popular choice for wind turbines. The ability to control the rotor currents allows for variable speed operation, so that a DFIG can operate at maximum efficiency over a wide range of wind speeds.