Study Of Control Strategy Of Fault Current Limiter For Improving Power Systems' Stability

The growing demand for electricity has resulted in a corresponding increase in power generated and transmitted. The increase in installed capacity will results in high fault currents. Utilities are faced with fault currents that can exceed the rated capacity of the installed circuit breaker, and other equipment. With the addition of new and larger generating stations and strengthening of the distribution system to meet the growth in the demand, the magnitude of the fault current has also grown. To correct this problem at the distribution level, a fault current limiter (FCL) can be used.The fault-current limiter (FCL) problems and the background of this thesis are summarized in the first chapter, as well as what's this thesis does.In chapter 2, based on the analysis performance and the characteristic of different type of solid fault-current limiter, a new concept for a combined short circuit limiter and dynamic series compensator is presented. It is composed of a fixed capacitor, thyristor switched capacitors that is parallel with a bypass reactor controlled by GTO. The compensation is controlled by the capacitors switching on and off in steps, while the current limitation is achieved by inserting a reactor circuit in transmission line.A distribution system is simulated by the means of MATLAB in chapter 3. The obtained result shows the device can effectively limit the fault current, decrease the sag of voltage, and improve the power quality.Begin with chapter 4, the influence of Fault Current Limiter (FCL) with controllable series compensation on improving transient stability of power systems is described. Based on the FCL transient model, the power characteristics of the system with FCL are analyzed, and the accurate mathematical equation of the critical eliminating angle of power systems with FCL is derived, which show that it is an active way to improve the transient stability by using FCL in power systems.With the development of control theory and technique, intelligent control technology is gradually showing its superiority. Chapter 5 mainly presents a fuzzy stable controller of FCL that is based on the fuzzy logical theory. Compared with conventional controllers, it has the advantages of responding rapidly, improving the dynamic characteristics of power systems effectively and enhancing systems' stability. The results of simulation testify the controller's validity and robustness for improving power system stability.For the large interconnected power systems, chapter 6 presents that fault current limiter (FCL) can be used in it in order to assist power. In this chapter using transient energy function (TEF) as the assessment of system stability and applying the derivative of TEF to determine the control strategy of FCL with controllable series compensation in transient stability process, the transient stability characteristics can be effectively improved and subsequent oscillation can be quickly damped. The presented control strategy is tested and verified by three-phase short circuit fault and the results show that this control strategy is effective and reliable.