In this paper we present an improved probabilistic approach for estimating the additional operating reserve in power systems with installed renewable energy sources. This approach uses a novel method for determining the reliability of power systems, where the variable generation of renewable energy sources and the ageing of generating units are considered. The new approach also takes into account the fact that several generating units can share the same source of failure, which causes them to fail or become unavailable simultaneously. Improvements are achieved by the implementation of common cause failures. The obtained results of the reliability analyses are basis for estimating the additional operating reserve in the power system, so that the system operation is supplemented with the proposed reliability criteria

The presented approach is tested on an IEEE Reliability Test System where the one-day-ahead circumstances are observed. The results show that the required additional operating reserve depends primarily on the power generation of the renewable energy sources and on the unavailabilities of the generating units, which are influenced by ageing and by common cause failures. The results show that the new method is suitable for achieving reliable power systems in the future, where a high penetration of variable renewable energy sources is expected