Forecasting and acquiring of short-term demand for frequency control reserve is a significant driver of cost for transmission system operators (TSO). A method is presented of identifying the significance of different factors by analyzing the correlation between historical data of actual reserve activation and renewable energy feed-ins, planned and unplanned power plant outages, vertical load, renewable energy forecasting errors and balancing energy provision. Application of this method shows that balancing energy has a moderate influence on the activation of secondary reserve. It has also been found that for minute reserves, corresponding activation of secondary reserves shows weak influence. Quarter-hourly prior activation of reserves also has a significant influence on the demand for control power. The analysis shows insignificant correlation between the investigated factors and the demand for secondary control reserves and minute reserves.