Increasing the penetration of wind turbine generations, needs more study about controlling frequency impacts of power system. Frequency control is changed with unbalancing real-time system generation and load. Also wind turbine generations have more fluctuations and make system more unbalance. Then Automatic Generation Control (AGC) loop helps to adjust system frequency and the scheduled tie-line powers. The quality of AGC loop is measured by some indices. It is expected a proper measure shows

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... he AGC performance just as it acts (operates). One of well-known measures in literature which was introduced by North American Electric Reliability Corporation (NERC) is Control Performance Standards (CPS). Previously it is claimed that a key factor in CPS index is P K * / σ. This paper focuses on impact of a day ahead wind speed forecast error on this key factor and CPS. The study system is a two area system. One area has only thermal power and other area constitutes of significant wind farm and thermal power. Effects of wind speed standard deviation and also degree of wind farm penetration are analyzed and importance of mentioned factor criticised. After that, influence of mean speed forecast error on this factor is noticed.