Study Of Multitype Control Of Grid-Connected Doubly Fed Induction Generator (DFIG)

The wind energy is one of the fastest growing renewable-energy resources. The wind energy also has got a lot of support from the governmental and institutional. Therefore, the wind-energy improvement and conversion technology has led to a fast development of wind power generation in recent years. The continuous increasing of the wind-power penetration brings a result that the wind-power gradually becomes a significant component in the power system. So, the study of the wind-power issues and the interactions between the wind turbines and grid are necessary and important.Accordingly, this thesis deals to present the study of multitype control of grid-connected doubly fed induction generator (DFIG). This study considers the DFIG voltage control and frequency coordinated control in a steady state mode and transient operation. Further, the subsynchronous resonance (SSR) and subsynchronous oscillation (SSO) phenomena analysis and mitigation are also presented. The complete grid-connected wind turbine model, including wind speed, aerodynamic and mechanical models are studied in more details. The electrical components of turbine namely the DFIG with back to back converters based on PWM voltage source and current source converters, transformer, transmission line with series compensated capacitor are clearly represented and explained. The two flexible AC transmission systems (FACTS) devices, static var compensator (SVC) and gate controlled series capacitor (GCSC) are installed in the system studied model for enhancing voltage control and mitigating SSR phenomena, respectively. The wind turbine aerodynamic, mechanical models and the control system are built with custom components developed in PSCAD/EMTDC. According to each case study, a certain control scheme is implemented in the developed grid-connected DFIG model.In the control method, all power system parameters, which directly impacted to DFIG operation are considered. All these parameters are functioned in both rotor side converter (RSC) and grid side converter (GSC) to achieve voltage control and improve power system stability performance. Different voltage control methods are investigated with the objective of eliminating the voltage fluctuations in the DFIG bus-bar and the power system grid. The main method used in this study is voltage oriented control (VOC) which is modified from the vector voltage control. The VOC uses the grid reactive power to generate q-axis current and DC link voltage to regulate the d-axis current in the GSC while the rotor mechanical speed and reactive power are employed to generate the dq-axis reference currents in the RSC. This method has an ability to suppress the influence of the back potential voltage without voltage sags occurred. In addition, the method is tested in the system studied model diagram under steady state and dynamic operation during constant and varying wind speeds. It is found that this method also has a better robustness to parameter deviations.Furthermore, the electromagnetic torque is applied as a main parameter in the GSC control to achieve DFIG dynamic voltage control under variable wind speed. The parameter controls which applied in GSC are electromagnetic torque and DC link voltage, whereas in the RSC, the converter uses DFIG rotor speed and reactive power. Both of GSC and RSC are used back to back converters with two-level IGBT devices which the pulses are generated from the PWM. The results show that, this method is a suitable to apply in DFIG with variable wind speed in dynamic operation and steady state mode.Currently, with large-scale DFIG integrates into power systems, the power grid faced frequency stability challenges, because the DFIG provides no contributions to frequency changes. The main reason is that the DFIG rotor is connected to the grid through back to back converter, which decouples the output power from the grid frequency. Another reason is that the DFIG normally operates at maximum power point tracking (MPPT), so there are no ways to increase the output power further than the greatest power level. Thus, in this project, the author solves these problems by designing adequate converter models in both rotor and stator sides, considering frequency problems to achieve complete frequency coordinated control (FCC). This control method is essentially depended on the strong relationship between frequency and generator active power. The FCC method based on the system frequency deviation, DFIG stored kinetic energy associated to the rotor inertia constant and normal system active power. In this context, a new method control of FCC applying in the d-axis of the RSC has been presented. Moreover, the results of this method are validated under steady and dynamic states during constant and varying wind speeds. It is concluded from the analysis that the controller action during abnormal operation increased pitch angle which directly led to decrease the turbine torque, and this helps to recover the voltage after the short-circuit fault clearance.A novel control method for SSR mitigating in the wind power systems based on DFIG using GCSC is presented. The GCSC is composed of three pairs of anti-parallel GTO (Gate Turn- off) thyristors connected in parallel with a fixed capacitor. The GCSC is applied to reduce inrush current in capacitor compensator during the transient operation by executing a proper firing angle control of thyristor gates. In order to realize SSR oscillation when the transient operation occurred, the DFIG turbine is connected through the shaft turbine model. The simulation results shown that the GCSC device is suitable and reasonable for suppressing SSR caused by torsional interaction (TI) and torque amplification (TA) and also damping the SSO as well. The capability of GCSC for mitigating SSR and SSO is demonstrated under steady state and dynamic mode.The last part in this thesis tried to present an example for simplified voltage control of shunt connected DFIG using SVC. The DFIG converters are based on a voltage source converter in the GSC and current source converter in the RSC. The GSC is employed to work like static synchronous compensator (STATCOM) to provide precise control to maintain the DC link voltage under control. The RSC is oriented to control real and reactive powers of the DFIG. Instead of adding two SVCs in every DFIG bus, it connected only one SVC at the point of common coupling (PCC) of the generators to enhance the voltage stability at a steady state and dynamic operation by compensating the reactive power hence to increase the voltage amplitude. The results show that the SVC has improved voltage stability and promote the system transient response by compensating reactive power. Furthermore, this study has been presented the possibility of using one SVC with low capacity based on the SVC characteristics adjusted only by choosing a suitable transformer rating. Moreover, the system simulation model was checked at steady state and dynamic mode during constant and random wind speeds to confirm the results. Using one SVC with low capacity leads to decrease power dissipation, and this directly reduced the power cost.This thesis brings readers to knowledge in details about DFIG control and applications in wind power with new ideas implemented in the converter controllers beside main concepts of DFIG model control.