Stabilization of a grid-connected wind farm by using SMES 超電導エネルギー貯蔵装置によるウィンドファームの安定化
Stabilization of a grid-connected wind farm by using SMES
Md. Rafiqul Islam Sheikh
Md. ラフィクル イスラム シェイク
This thesis deals with different approaches to stabilize a grid-connected wind farm. Due to theenvironmental and economical concerns, it is expected that a huge number of wind farms are going tobe connected with the existing networks in the near future. Therefore, it is essential to analyze bothsteady 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-connectedwind farm composed of fixed-speed wind turbine generator system (WTGS). The SMES has bothactive and reactive power compensation abilities with high response speed. Therefore, the steady stateand 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 powergrid companies or transmission system operators (TSO), especially in the case of fixed-speed windgenerators. 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 energysystems with high wind power penetration, it is essential to emphasize the research on wind powersmoothing. However, reference power generation is still an unsolved problem for wind generatoroutput power smoothing, because more energy storage capacity is needed in the case of constantreference line power. In this study, wind power fluctuation in the time scale of minute range isfocused, where SMES system with variable reference line power is proposed to smooth the outputpower and to maintain the terminal voltage of the wind farm to the rated value. As a result, the energystorage capacity of SMES can be made comparatively small.Another salient feature of this thesis is the minimization of frequency fluctuation of powersystem with high wind power penetration. As the output power from wind farm fluctuating due towind speed variations becomes large, fluctuations of the network frequency and voltage also becomelarge. As the wind turbine dynamics and governor control systems have the significant influence onthe 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 tomaintain network frequency of the power system with high wind power penetration to the desiredlevel. Though the pitch control system can maintain the system frequency if the wind powergeneration is in a few percentage of the total power capacity, but it would be difficult when the windpower penetration becomes 10% or larger.Fluctuation of power system frequency due to large incorporation of wind farm output power tothe grid has some adverse effect on power system operation. Therefore, in this study, SMES isproposed to mitigate the power system frequency oscillation caused by wind farm. It is expected thatlarge SMES capacity give better smoothing performance. However, large capacity will definitelyincrease 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 SMESpower rating has also been analyzed. Moreover, the minimum energy storage capacity of SMES unitto mitigate the frequency fluctuation is determined. The simulation results show that, using theproposed SMES system, the wind farm output fluctuations can be decreased, and hence the frequencyof the grid system can be maintained within an acceptable range. Wind farm grid voltage can also bemaintained to the rated value by the proposed system.Between the two types oftrends, the fixed-speed WTGS has inferior fault ride through capabilitycompared to that of variable speed WTGS. Therefore, fixed-speed wind generator that uses thesquirrel-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 faultoccurs in the power system. If sufficient reactive power is not supplied, then the electromagnetictorque of wind generator decreases significantly. Then wind generator and turbine speeds increaserapidly due to large difference between mechanical and electromagnetic torques. As a result, theinduction 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 windfarm 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 gatebipolar transistor are proposed for controlling SMES to enhance the fault ride through capability offixed-speed WTGS. Comprehensive study is carried out to enhance the transient stability of multimachinepower system including wind farm by using the proposed SMES. Two-mass drive trainmodel ofWTGS is used in the analysis as the drive train modeling has great influence on the dynamiccharacteristics of WTGS. Both symmetrical and unsymmetrical faults are considered for transientstability 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 windgenerators. It is also observed that SMES helps not only in regulating the voltage, but also inmitigating the rotor speed instability, thus it can improve the stability of entire power system. It is alsopresented that the value of compensating capacitor in wind farm can be reduced by a certainpercentage from the original rated value because the rest of the reactive power drawn by the windfarm at steady state can be supplied by the SMES. Transient stability has also been evaluatedconsidering stability index. From these results, it can be understood that the proposed controlledSMES 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 beintegrated with fixed-speed wind farm. Therefore, the integration of the proposed SMES system into awind farm can be an effective means of mitigating the frequency fluctuations of the grid system whichconsequently improve the stability and reliability of entire power system.