Doubly fed induction generators (DFIGs), often organized in wind parks, are the most important generators used for variable-speed wind energy generation. This paper reviews the control systems for the operation of DFIGs and brushless DFIGs in wind energy applications. Control systems for stand-alone operation, connection to balanced or unbalanced grids, sensorless control, and frequency support from DFIGs and low-voltage ride-through issues are discussed.

Overview of Control Systems for the Operation of DFIGs in Wind Energy Applications

The paper describes a new type of variable speed doubly-fed reluctance machine and presents the model for the analysis of the machine. The stator of the machine is equipped with two sets of sinusoidally distributed windings having pole numbers P1 and P2. The rotor consists of simple saliencies of pole number Pr, which is shown to be constrained to particular values by P1 and P2. A d-q-n model and equivalent circuit is developed for this machine. Simulation results using the new model show that this machine can have the advantages of doubly-fed wound-rotor machine while eliminating the complicated wound rotor, brushes and slip rings normally associated with doubly fed machines.

d-q ANALYSIS OF A VARIABLE SPEED DOUBLY AC EXCITED RELUCTANCE MOTOR

In this paper, the concept and implementation of field-orientation control (FOC) of a doubly excited brushless reluctance machine (DEBRM) for variable-speed drives and generating systems are presented. A stator-flux-orientation scheme is employed to achieve decoupled control of torque and reactive power. A 2 HP DEBRM prototype system with a digital signal processor (DSP)-based controller and a bidirectional power converter is built to verify the theoretical analysis. Computer simulation and laboratory experimental results are shown in excellent agreement.

Field orientation control of a doubly excited brushless reluctance machine

The paper describes a technique for controlling a doubly fed induction generator driven by a windmill, or other form of variable-speed prime mover, to provide power generation into the national grid system. The secondary circuit of the generator is supplied at a variable frequency from a current source inverter which for test purposes is rated to allow energy recovery, from a simulated windmill, from maximum speed to standstill. To overcome the stability problems normally associated with doubly fed machines a novel signal generator, which is locked in phase with the rotor EMF, controls the secondary power to provide operation over a wide range of subsynchronous and supersynchronous speeds. Consideration of power flow enables the VA rating of the secondary power source to be determined as a function of the gear ratio and online operating range of the system. A simple current source model is used to predict performance which is compared with experimental results. The results indicate a viable system, and suggestions for further work are proposed

Wind-energy recovery by a static Scherbius induction generator

The paper describes a control system in which a cycloconvertor is connected between the secondary winding of a 3-phase induction motor and the a.c. mains supply to give variable-speed subsynchronous and supersynchronous operation. Considerations of power flow in the secondary circuit show that for stable operation the cycloconvertor must be controlled from a frequency signal source synchronised to the secondary e.m.f. A secondary e.m.f. signal generator is described which produces the necessary control signal and provides a means of power-factor control. A simple model is proposed in which the cycloconvertor appears as a current source in the secondary circuit of the machine. Experimental results are presented showing smooth operation below, through and above synchronous speed with either driving or braking torques.

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Static Scherbius system of induction-motor speed control

A control system is described in which a current-source inverter is connected between the secondary winding of a three-phase slip-ring induction motor and the ac mains supply to give a variable speed sub- and supersynchronous operation. Control techniques developed in an earlier work were used to ensure fully stable operation over a very wide speed range with either driving or braking torque. Means are provided for power-factor control to compensate for the lagging power-factor effect of the inverter when operating into the large secondary electromotive forces (EMF) at high slip values. Experimental results are presented for a speed range from standstill to approaching twice synchronous speed for both driving and braking torque. There are no instabilities and the developed torque even at low frequencies near synchronous speed is steady with motor derating due only to the quasi-square wave current in the secondary circuit.

A Current-Source Inverter in the Secondary Circuit of a Wound Rotor Induction Motor Provides Sub- and Supersynchronous Operation

The paper describes a control system for 4-quadrant operation of induction motors with characteristics suitable for crane hoist duty. The control system is based on a combination of stator-voltage control and rotor chopping. A thyristor stator-voltage regulator is used to provide a means of motor-speed closed-loop control. The regulator automatically reverses the phase rotation for plug braking or provides d.c. to the stator for injection braking as determined by load conditions. The rotor chopper operates with a duty cycle that is a predetermined function of rotor speed. Analysis of the chopper action in the rotor is used to determine the optimum function required to develop the maximum possible torque per stator ampere over the operating speed range. Different control functions are found to be necessary for motoring and d.c.-injection modes of operation. Analysis of the system and experimental results are presented showing successful 4-quadrant operation.

Gordon Arthur Smith

Papers

Static Scherbius system of induction-motor speed control

A Current-Source Inverter in the Secondary Circuit of a Wound Rotor Induction Motor Provides Sub- and Supersynchronous Operation

Induction Motors for Crane Applications