Stabilization of a Grid-Connected Wind Farm by Using SMES

This thesis deals with different approaches to stabilize a grid-connected wind farm. Due to the environmental and economical concerns, it is expected that a huge number of wind farms are going to be connected with the existing networks in the near future. Therefore, it is essential to analyze both steady state and transient characteristics of the grid connected wind farms. In this study, Superconducting Magnetic Energy Storage (SMES) system is considered to stabilize a grid-connected wind farm composed of fixed-speed wind turbine generator system (WTGS). The SMES has both active and reactive power compensation abilities with high response speed. Therefore, the steady state and transient performance of the wind farm can be enhanced extensively. Wind power fluctuation due to randomly varying wind speed is still a serious problem for power grid companies or transmission system operators (TSO), especially in the case of fixed-speed wind generators. The wind power fluctuation usually occurs in the time scale of few sec to several hours, depending on the wind condition, wind turbine size, topology, etc. Considering the future energy systems with high wind power penetration, it is essential to emphasize the research on wind power smoothing. However, reference power generation is still an unsolved problem for wind generator output power smoothing, because more energy storage capacity is needed in the case of constant reference line power. In this study, wind power fluctuation in the time scale of minute range is focused, where SMES system with variable reference line power is proposed to smooth the output power and to maintain the terminal voltage of the wind farm to the rated value. As a result, the energy storage capacity of SMES can be made comparatively small. Another salient feature of this thesis is the minimization of frequency fluctuation of power system with high wind power penetration. As the output power from wind farm fluctuating due to wind speed variations becomes large, fluctuations of the network frequency and voltage also become large. As the wind turbine dynamics and governor control systems have the significant influence on the system frequency, effects of different governor control system models have been investigated. However, adopting only wind turbine pitch controller and governor control system is not sufficient to maintain network frequency of the power system with high wind power penetration to the desired level. Though the pitch control system can maintain the system frequency if the wind power generation is in a few percentage of the total power capacity, but it would be difficult when the wind power penetration becomes 10% or larger. Fluctuation of power system frequency due to large incorporation of wind farm output power to the grid has some adverse effect on power system operation. Therefore, in this study, SMES is proposed to mitigate the power system frequency oscillation caused by wind farm. It is expected that large SMES capacity give better smoothing performance. However, large capacity will definitely increase the system overall cost. Therefore, the optimum size determination of SMES is one of the key points from the viewpoint of cost-effectiveness. So in this study, an evaluation method of SMES power rating has also been analyzed. Moreover, the minimum energy storage capacity of SMES unit to mitigate the frequency fluctuation is determined. The simulation results show that, using the proposed SMES system, the wind farm output fluctuations can be decreased, and hence the frequency of the grid system can be maintained within an acceptable range. Wind farm grid voltage can also be maintained to the rated value by the proposed system. Between the two types oftrends, the fixed-speed WTGS has inferior fault ride through capability compared to that of variable speed WTGS. Therefore, fixed-speed wind generator that uses the squirrel-cage induction generator needs additional tool to enhance the fault ride through capability. This is because it requires large reactive power to recover the air gap flux when a short circuit fault occurs in the power system. If sufficient reactive power is not supplied, then the electromagnetic torque of wind generator decreases significantly. Then wind generator and turbine speeds increase rapidly due to large difference between mechanical and electromagnetic torques. As a result, the induction generator becomes unstable and it requires to be disconnected from the power system. However, the recent trend is to decrease the shut down operation because a shut down of large wind farm can have a serious effect on power system operation. In this study, a pulse width modulation (PWM) based voltage source converter (VSC) and two-quadrant DC-DC chopper using insulated gate bipolar transistor are proposed for controlling SMES to enhance the fault ride through capability of fixed-speed WTGS. Comprehensive study is carried out to enhance the transient stability of multimachine power system including wind farm by using the proposed SMES. Two-mass drive train model ofWTGS is used in the analysis as the drive train modeling has great influence on the dynamic characteristics of WTGS. Both symmetrical and unsymmetrical faults are considered for transient stability analysis of WTGS as the type of fault that occurs in a power system is quite uncertain. Simulation results clearly show that the proposed SMES can enhance the transient stability of wind generators. It is also observed that SMES helps not only in regulating the voltage, but also in mitigating the rotor speed instability, thus it can improve the stability of entire power system. It is also presented that the value of compensating capacitor in wind farm can be reduced by a certain percentage from the original rated value because the rest of the reactive power drawn by the wind farm at steady state can be supplied by the SMES. Transient stability has also been evaluated considering stability index. From these results, it can be understood that the proposed controlled SMES can improve the transient stability of entire power system. Considering all aspects of proposed SMES system, it is found that the SMES is a good tool to be integrated with fixed-speed wind farm. Therefore, the integration of the proposed SMES system into a wind farm can be an effective means of mitigating the frequency fluctuations of the grid system which consequently improve the stability and reliability of entire power system.北見工業大学,博士(工学),甲第107号,0000545124