Emulation of wind turbine and sensorless control of doubly-fed induction generator for wind energy application

Abstract

Wind energy utilization has been growing at a rapid rate, fuelling research and development in wind turbine – generator systems. Doubly fed induction generator (DFIG) driven by a wind turbine is a commonly used wind energy conversion system. To advance research and education in wind energy conversion systems, a controlled test bed is necessary that does not depend on wind availability. Hence this thesis deals with emulation of wind turbine using a power electronic controlled squirrel cage induction motor (SCIM). The thesis also concerns improved control techniques for the DFIG to enhance the performance of the wind energy system. Wind turbine emulation involves controlling the SCIM drive such that the motor exhibits the characteristics of a wind turbine. The inertia of the motor being much lower than the inertia of the emulated turbine is an important challenge in such wind turbine emulation schemes. This thesis proposes one degree of freedom (1-DOF) and two degree of freedom (2-DOF) control structures for wind turbine emulation, overcoming this challenge. Regarding control of DFIG in wind energy applications, position sensorless operation of DFIG is desirable from considerations of cost, maintenance, cabling and reliability. This thesis proposes two stator flux based model reference adaptive observers (SF-MRAOs) for estimation of rotor speed and position in stand-alone DFIG. One of the proposed SF-MRAO is shown to work with good dynamic performance in vector control of stand-alone DFIG. The thesis also proposes a PLL based MRAO (PLL-MRAO) which does not require integration of sensed quantities, unlike other existing MRAOs. The linearized SF-MRAO is further utilized to propose a modified direct voltage control (DVC) of stand-alone DFIG with simplified design of controllers compared to existing DVC. Grid integration of DFIG system requires synchronization of stator induced voltages with grid voltages before grid connection, and active and reactive power (PQ) control after grid connection. This thesis also proposes a unified control algorithm for synchronization and power control, enabling a seamless transfer from synchronization mode to PQ control mode. Synchronization requires initial rotor position information, obtained through either a novel rotor parking scheme or PLL-MRAO, which are proposed in the thesis. It is shown that the unified control has negligible transients during grid integration of DFIG. All the proposed observers and control algorithms are validated through simulations and experiments, performed on a 7.5 kW doubly-fed induction generator coupled to a 5.5 kW squirrel cage induction motor, available in the laboratory.