This paper presents a simple and practical method to improve the dynamic response of load frequency control for large load variations. The generation companies with different characteristics have various effects on frequency deviation and tie-line power. Moreover, large or very large increases in demand of electrical energy are limited to some time periods. In these periods, the power system operates near its security margin with increased probability of faults. Furthermore, large fluctuations in area frequencies and tie-line power can cause unexpected difficulties in the power system. In order to maintain the system security, this paper suggests that the independent system operator replaces the common combination of generation companies with the combination which has better effect on the frequency and tie-line power deviation
