Tlie control of a doubly-fed induction machine (DFM) with aid without tlie use of a rotor position encoder is examined. First, a stator fliix linkage oriented control scheme for power and speed control, with a position encoder, shows the high performance control capabilities of this control arrangement and serves as foundation for the sensorless scheme. The power-control method is then applied to a sensorless method, based on a previous control arrangement. This method is then exteiideci to derive a sensorless speed-control mechanism for the DFM. All control schemes are iniplenicntcd and pertbimcd on an experimental test system, comprising two 8OC167 microcontrollers and a 2.251tW wound rotor induction machine.

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This paper examines the performance of a vector-controlled doubly-fed induction generator for wind turbine use. The scheme is considered for variable-speed-range applications to produce a greater wind energy capture from varying wind velocity. A major advantage of the doubly-fed scheme over the squirrel-cage generator is the reduced volt-ampere rating of the power converter used to control the rotor current. The singly-fed scheme requires that the power converter is able to carry the full generator output. In order to produce decoupled regulation of active and reactive output power, the rotor current is controlled using field orientation principles. The measurement of rotor position and the grid voltage vector position allows the rotor current to be controlled in a reference frame closely associated with the stator magnetising current. The stator active and reactive power are also measured and fed back into outer Ps and Qs control loops. The Ps and Qs controllers produce d and q axis rotor current demands for the inner rotor current vector controller. By control of the reactive power loop the generator can be made to produce power at unity power factor. The vector control of rotor current is carried out using a commercial PWM inverter equipped with vector control hardware as standard. The inverter also produces a unity power factor sinusoidal current interface to the grid network. The performance of the scheme is examined using both simulation and experimental results. The decoupling control of active and reactive power is demonstrated and shows the ability to generate power at unity power factor.

A cascaded doubly-fed induction machine (CDFM) is a connection of two wound rotor induction machines. The rotors of the two individual machines are electrically and mechanically coupled so that brushes for the rotor sliprings are no longer required. This paper describes the steady state behaviour of such a cascaded machine arrangement. Frequency and slip values are defined. A per-phase equivalent circuit, based on the common induction machine equivalent circuit, helps to gain understanding of the machine quantities. Measurements on a CDFM confirm simulations with the equivalent circuit experimentally. Saturation effects are discussed to show the limitation of the simple equivalent circuit model. By studying the power flow within the CDFM and taking machine utilisation into consideration allows to suggest that a CDFM has to consist of two identical machines.