Performance Evaluation of DFIG During Asymmetrical Grid Disturbances Using Internal Model Controller and Resonant Controller

Abstract

In this paper, grid disturbances like asymmetrical sag and swell faults in phase-A are analyzed for the grid connected DFIG. The dynamic behavior of DFIG is compared and tabulated with results using state error compensators like PI, PI with resonant (PIR) and internal model control (IMC) on rotor side converter (RSC). The RSC is designed analytically with enhanced field oriented control (EFOC) technique for better performance during such grid disturbances. In this technique, rotor flux reference changes from synchronous speed to some smaller speed or zero during the fault for injecting current at the rotor slip frequency. In this process, DC-Offset component of flux is controlled in not decomposing into a lower value of faults and maintaining it. This offset decomposition of flux will be oscillatory in conventional FOC, whereas in EFOC with internal model controller, flux can damp quickly not only for single fault but multiple faults. This strategy can regulate stator and rotor current waveform to sinusoidal without much distortion during and after fault. It has better damped torque oscillations, control in rotor speed and generator flux during and after fault. The fluctuations in DC bus voltage across capacitor are also controlled using proposed EFOC technique. The system performance with phase-A over voltage and under-voltage with above controllers and EFOC technique are analyzed using simulation studies.Keywords: Asymmetrical grid disturbance; DFIG; Field oriented control; Internal model control (IMC); PI and resonant controoller (PIR); unbalanced and distorted load. INTRODUCTIONIn the comparison between other wind turbines driven generators, the doubly fed induction generator (DFIG) is having more advantages. The major reasons like better real and reactive power capability, variable speed constant frequency operation etc. make it to stand at the top of all. However, it is sensitive to asymmetrical grid disturbances like single phase or two phase's voltage disturbances as it are directly connected to grid. Based on modern grid rules, DFIG needs to operate effectively on asymmetrical faults and also under unbalanced and distorted loading conditions.The status of research on the LVRT issue for DFIG for symmetrical and asymmetrical faults and comparison of different control strategies is given in [1]. Understanding the capability of RSC to deliver desired reactive power and withstanding capability during fault in [2]. LVRT enhancement based on flux trajectory [3], effect of different types of faults were studied in [4] for DFIG. Controlling DC link current of RSC to smoothen DC voltage fluctuations due to grid faults by using stored Kinetic Energy [5] is proposed in this paper. RSC with static damping resistor placed in series with stator windings to limit torque oscillations and transient response is done in [6]. FFTC scheme with PIR [7] and PI [8] are adopted for symmetrical and asymmetrical faults for improving uninterrupted P, Q supply from wind turbine (WT) to grid. Few control phase sequence cont...