Due to its many advantages such as the improved power quality, high energy efficiency and controllability, etc. the variable speed wind turbine using a doubly fed induction generator (DFIG) is becoming a popular concept and thus the modeling of the DFIG based wind turbine becomes an interesting research topic. Fundamental frequency models have been presented but these models are often complex with significant numerical overhead as the power converter block consisting of power control, rotor side and grid side converter control and DC link are often simulated in detail. This paper develops a simple DFIG wind turbine model in which the power converter is simulated as a controlled voltage source, regulating the rotor current to meet the command of real and reactive power production. This model has the form of traditional generator model and hence is easy to integrate into the power system simulation tool such as PSS/E. As an example, the interaction between the Arklow Bank Wind Farm and the Irish National Grid was simulated using the proposed model. The model performance and accuracy was also compared with the detailed model developed by DIgSILENT. Considering the simplification adopted for the model development, the limitation and applicability of the model were also discussed in this paper.

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Modeling of the wind turbine with a doubly fed induction generator for grid integration studies

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

A power control system for a doubly fed wound rotor induction generator has been developed. This power control system applies a control method using a rotating reference frame fixed on the gap flux of the generator, and can control active and reactive power independently and stably. The characteristics of the control system have been proved by experiment. Harmonic currents fed to the rotor windings are transmitted to the stator winding changing its frequency. The transmitting characteristics of the harmonic currents have been analyzed and verified by experiments. >

Active and reactive power control for doubly-fed wound rotor induction generator

Variable-speed constant-frequency generating systems are used in wind power, hydroelectric power, aerospace, and naval power generation applications to enhance efficiency and reduce friction. In these applications, an attractive candidate is the slip power recovery system comprising a doubly excited induction machine or doubly excited brushless reluctance machine and PWM power converters with a DC link. In this paper, a flexible active and reactive power control strategy is developed, such that the optimal torque-speed profile of the turbine can be followed and overall reactive power can be controlled, while the machine copper losses have been minimized. At the same time, harmonics injected into the power network have also been minimized. In this manner, the system can function as both a highly-efficient power generator and a flexible reactive power compensator. >

A flexible active and reactive power control strategy for a variable speed constant frequency generating system

A detailed analysis of the performance of the doubly-fed induction generator (DFG) under synchronous operating condition in the steady state is presented. The rotor is excited by a voltage phasor, the magnitude and the frequency of which can vary independently. The frequency supplied always has such a value that, if superimposed on the rotor speed, a synchronous rotating field results. This produces variable-speed constant-frequency characteristics of the DFG over a wide speed range. By varying the excitation voltage magnitude the stator and rotor power factor can be controlled so that maximum efficiency can be achieved. By varying the angle between the stator and the rotor applied voltages, the DFG active power delivered can be controlled. The wide range of variable-speed constant-frequency operation and the capability of load variation and power factor control makes the DFG attractive for wind power conversion systems as well as variable-speed hydroelectric generators. >

Steady State Analysis of a Doubly-Fed Induction Generator under Synchronous Operation

The paper describes a slip-ring induction machine operating as a doubly-fed generator above and below synchronous speed. A prime mover simulates a wind turbine by producing a torque increasing in proportion to the square of the speed. This torque is balanced in steady-state operation by the doubly-fed generator. The stator, or secondary of the generator is arranged in two electrically separate, magnetically coupled layers connected to a cycloconvertor operating with continuous circulating current. This reduces the harmonics injected into the supply and prevents line-to-line short circuits under shock conditions. The secondary frequency is locked to the difference between actual speed and synchronous speed. A deadband is built into the frequency demand loop to ensure steady-state synchronous rather than asynchronous performance of the generator. Control of the secondary voltage amplitude enables the system to generate power proportional to the cube of the speed over a wide speed range. Three alternative methods of secondary control are considered. These are: (i) secondary current modification, (ii) a voltage-speed function and (iii) constant secondary current feed. The efficiences and peak power capabilities of the methods of systems (ii) and (iii) are contrasted in operation as a quasi-wind-energy transducerss.

P.G. Holmes

Papers

Cycloconvertor-excited divided-winding doubly-fed machine as a wind-power convertor