Concerns have been raised due to the impact that large-scale wind power integration will have on reserves and secure operation of power systems. Case studies on a simplified model of the UK system with large penetration of wind power in Scotland are presented for illustration. A method called stepwise power flow was used to simulate the impact of wind power generation fluctuation output on the system frequency, reserves and power flow. It was demonstrated that the supply–demand imbalance caused by forecast errors in wind power production and load, and fall out of wind power production would influence the system frequency, system bias and the use of reserves. This method can also be used to estimate the amount of wind power that can be integrated before the system experiences serious problems due to inadequacy of the available reserves. Copyright © 2007 John Wiley & Sons, Ltd.