Study Of Power System Frequency Measurement And Under Frequency Load Shedding Scheme

Three defenses are set in'Power System Security and Stability Guidance'to guarantee the safe and reliable operation of the electric power system. UFLS is an important part of the last defense. A good scheme of under frequency load shddding (UFLS) can not only avoid power system collapse in serious fault, but also can get frequency back to tolerance range rapidly with little load shedding. So it is of important significance to work out practical UFLS schemes.This paper makes a comprehensive analysis of power system UFLS and the passing study in this field. Based on the summarizing and analyzing of the research presently, this paper present some new viewpoints in frequency measurement and UFLS design which make the system can shed load quickly and try to reduce load shedding amount after occurring active power shortage in the system.The base for the right action of UFLS sets is the precise and rapid measurement of frequency. In allusion to the deviation which is directly proportional to the rate of frequency decline, when full-wave Fourier algorithm applied to frequency transient, this paper amends full-wave Fourier algorithm. Against its response delay, this paper proposed a new forecasting algorithm conforming to the law of the frequency transient change to forecast the system frequency at the end time of last sampling point. The results of simulation show that this forecasting algorithm effectively eliminates the deviation caused by the frequency dynamic changes and the response delay of Fourier algorithm.The UFLS scheme requires getting the frequency back to the tolerance range with little load shedding. In allusion to the traditional UFLS scheme's deficiencies of off-line estimation of the total amount of load shedding, changeless frequency setting value and amount of load shedding of every run, this paper proposes reformative UFLS scheme which can alter the frequency setting value of the first run and the amount of load shedding of every run dynamicly according to the magnitude of frequency change rate, namely the insufficient amount of active power. The simulation results of stand-alone model show that the reformative UFLS scheme can restrain the frequency falling depth effectively, fit speed controller preferably, make full use of system rotating spare capacity, and reduce the amount of load shedding and the possibility of over-cut or owes-cut. On account of the space distribution of frequency in multi-machine system, this paper presents a method of distributing shedding load according to rate of frequency decline. In single area, this method can shed more loads at high sensitive node which is favorable to frequency resume. In multi-area, this method helps balance the shortage of active power locally and reduces the possibility of tie line overload. In order to proof the effectivity of the reformative UFLS scheme in multi-area, this paper structures a five-node muliti-machine system and a three-area system and makes comparisons with traditional UFLS scheme. The simulation results show that the reformative UFLS scheme sheds less load and its recovery frequency meets the requirement of power system.