Modelling and Risk Assessment of Special Protection Schemes in Transmission Systems

According to the N-1 criterion, system security in the event of a failure of a network element (N-1 situation) must still be guaranteed. In transmission systems, this is traditionally realized with different preventive (i. e. pre-fault) actions, e. g. the provision of unexploited transmission capacities and redispatch of power plants. In contrast to this, the curative approach ensures a N-1 security reactively (i. e. post-fault). This approach is based on comprehensive information and communication technologies which allow a highly automated process for identifying critical system states and the determination of subsequent corrective actions, e. g. load shedding or generator rejection. These actions are commonly designated as Special Protection Schemes (SPS). However, with increasing applications of SPS, the reliability of SPS needs to be determined and the impact of SPS on the security of supply needs to be assessed. In this contribution analytical methods are applied to determine the probability of different states (in service, limited operation and outage) for five different SPS. Furthermore, the probability and maximum level of overloads, quantifying risk in situations where curative and preventive actions fail, are compared in certain network areas in the transmission system. Results show that a highly redundant design of SPS (especially communication networks, battery storages) is crucial to reach a similar level of reliability compared to conventional network elements. The risk analysis emphasizes that the probability and level of overloads can be reduced or is on a similar level compared to preventive actions.