Electric power transmission systems transfer large amounts of power (typically hundreds of MW) over long distances (typically hundreds of kilometres) at high voltage (typically hundreds of kilovolts). The operational security of the transmission system has always been high top priority for the transmission system operator (TSO); because of this supply interruptions originating in the transmission grid are very rare. 

To maintain the high reliability of a transmission grid, TSOs deploy the N-1 operational security criterion. The predominant shortcoming of this criterion is that all contingency cases are treated as equal; no differential is made concerning probability and impact of individual contingency cases. 

Operational risk assessment complements the N-1 security assessment method, by incorporating the probability of contingency cases and their impact, the latter in terms of severity factors.  Important elements of operational risk assessment are lead time, instantaneous component outage model, contingency definition, contingency list and filtering, probability of contingency cases, and severity factor. The existing literature on operational risk assessment concentrates on contingency filtration and ranking. Only a limited amount of literature exists on definition of severity factors. 

The main purpose of this thesis is to identify and summarise different existing and required research trends on the fundamental elements of operational risk assessment.  The contributions of the thesis include:

•   Identifying the fundamental elements of operational risk assessment and highlighting potential barriers against the practical implementation of operational risk assessment into the transmission system. Currently, TSOs are not deploying operational risk assessment, among others due to the absence of proper guidance and because of the high reliability resulting from the (N-1) criterion. Potential barriers against implementation of operational risk assessment, that were identified in the work, include absence of acceptable operational risk criteria, lack of a common and standardized set of severity factors, lack of sufficient knowledge on interpretation of operational risk results, and improper guidance on when and which types of measures are required to reduce the operational risk. 
• Introducing multi-state component models, including hidden failures, to operational risk assessment. In the power grid, major blackouts occur due to contingency cases involving protection failures. Including protection and protection failures in operational risk assessment results in several practical and mathematical challenges. Practical challenges include obtaining transition rate data; mathematical challenges include computing the time-dependent state probability of a large Markov model. This thesis addresses these mathematical challenges and provides a way to resolve them.  
• Clarifying the role of severity factor in operational risk assessment and proposing different deterministic and stochastic severity factors. The definition of the severity factor has a big impact on the way in which the results from operational risk assessment should be interpreted. A common set of severity factors is important for the interpretation of operational risk results and for the exchange of information and experience.

An important finding from this work is that operational risk assessment provides additional dimensions to the operational security planning, next to deterministic security criteria. However, several research gaps remain that need to be filled before implementation of operational risk assessment to existing transmission systems is possible.