Control Dynamics of a Doubly Fed Induction Generator Under Sub- and Super-Synchronous Modes of Operation

M. Aktarujjaman, Student Member,
IEEE, M.E. Haque, Member, IEEE, K. M. Muttaqi, Senior
Member, IEEE, M. Negnevitsky, Member,
IEEE and G. Ledwich, Senior Member, IEEE

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Abstract-
Abstract—
Depending on wind speed, a doubly fed induction generator (DFIG) based variable speed wind turbine is capable of operating in sub- or super-synchronous mode of operation using a back to back PWM converter. A smooth transition between these two modes of operation is necessary for reliable operations of the wind turbine under fluctuating wind. This paper presents the analysis and modeling of DFIG based variable speed wind turbine and investigates the control dynamics under two modes of operation. A battery energy storage (BESS) with a bidirectional dc-dc converter is added for a smooth transition between the modes. Mathematical analysis and corresponding modeling results show that the power flow in the rotor circuit under two modes can be controlled by changing current and voltage phase sequence through the rotor side converter (RSC) and line side converter (LSC). A coordinated control among RSC, LSC and DC page link storage system ensure variable speed and maximum power extraction from the fluctuating wind and reduce the possibility of instability around synchronous speed. Extensive simulations have been conducted to investigate control dynamics under the two modes of operation and during transitions.

INTRODUCTION
HE contribution of renewable based distributed generation has been increasing dramatically into the power system for last two decades. A variable speed generator based wind turbine can extract more power from the wind than a fixed speed wind turbine. Doubly fed induction generator (DFIG) is a popular choice for variable speed wind turbine application, as it is able to generate power at constant voltage and frequency while the rotor speed varies. A decoupled control of the real and reactive power is possible.
Moreover, a fraction of total system power needs to be controlled, resulting in the reduction of the power losses and the cost of the converters, filters and EMI filters. Generally, the stator of the DFIG is connected to the grid directly, and the rotor is fed through bi-directional back-to- back PWM converters. These converters are used for exchanging the slip power to and from the grid for variable speed operation . It is possible to control rotor current injection using fully controlled electronic converters to ensure effective operation in both sub- and super-synchronous modes.
In sub-synchronous mode, the RSC works as an inverter and LSC as a rectifier and controls the power flow into the rotor. In the case of super-synchronous mode, RSC acts as a rectifier and LSC as an inverter, the direction of power flow is out of the rotor. In reference, maximum wind power extraction capability by using DFIG has been demonstrated. Active and reactive power control technique using vector controls are presented in. Authors in have described the ability of DFIG to compensate unbalance situation. The effect of commutation angle of the converters during sub- and super-synchronous operations is reported in.
For effective control, it is necessary to understand the control dynamics of DFIG based variable speed wind turbine and converters action under sub- and super-synchronous modes of operation and during the transition of these two modes. This paper investigates the control dynamics of a DFIG based variable speed wind turbine during two modes of operation and subsequent transition period with a battery storage. This paper is organized as follows: In section II, system configuration is presented. The model of the turbine-generator system is given in section III. In section IV, Operation and control issues in sub- and super-synchronous modes are discussed. Simulation and discussion are given in section V. Finally, conclusions are made in section VI.

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