UNDER FREQUENCY LOAD SHEDDING OF POWER SYSTEM. Pavlov G.M.

In this report is considered under frequency liad shedding that is used in power systems of Russia. It is presented a new method of calculation decreasing of frequency under load shedding. The new method will make possible to arise the frequency to normal value during 7 - 10 sec. In steady state operating conditions the load and generation of power system are balanced and the system operates at normal frequency 50 Hz. There may be a situation when generation is not enough to meetload demands.Under such condition frequency would be declined. The more unbalance between power supply and demands the more declination of frequency. With large decrease of frequency care has to be taken of corresponding decrease load of power system. in this case a part of consumers must be switched of f .under frequency load shedding is very important for big power systems. For example, this problem is under discussion in General Report of group 54 (protection) CIGRE [1]. Here are several phrases from this report: The initial trend and the general shape of this frequency time curve will dictate whether only frequency or only df/dt or a combination of both are useful as triggers to induce load shedding and which form this load shedding should have (how many steps of underfrequency, what load block size at each step, what time delay between steps, etc.) So load shedding is, so to speak, tuned to the transient evolving generation capabilities in the grid. In Russian power Systems automation of Wad shedding, which consists of two parts - UFLS-i and UFLS-li Minder Frequency Load Wedding) is used [2]. Automation UFLS-i has a number of frequency stages. The frequent cyrelay of the first stage picksup when frequency equal f = 48.5 W. The second stage relay picksup at f = 48.5 – (0.1 or 0.2) Hz. There are about 10 - 20 stages. The last stage pickup at 46,5 Hz. Total capacity of consumers which may be switched off equal Pdef, where Pdef-is a maximum deficit of generation that may occur in power sistem. in such case Pi = Pdef/N, N -is the number of stages. The task of UFLS-i is to finish declination of frequency.