Incorporating failures of System Protection Schemes into power system operation

- A generic dependability model for System Protection Schemes (SPS) is proposed.
- The model is embedded in a cost-benefit framework for optimal system operation.
- Significant cost savings are possible even with an unreliable SPS.
- Optimal operational strategies are closely linked to the failures modes of the SPS.

The power transfer capability of existing transmission networks can be enhanced through the use of automated system protection schemes (SPS), which rapidly respond to disturbances on the network to keep the system's variables within operational bounds. However, reliance on such schemes may expose the network to large impacts-including blackouts-if the SPS does not respond as designed, so the deployment of SPS should balance risks and benefits. This paper formulates a risk-based cost-benefit framework that allows the operator to strike an optimal balance between constraint costs and risks of demand curtailment due to malfunctioning SPS. It is applied to a simple 4-bus power system inspired by the GB network, for which an exact optimisation problem can be formulated. A component-based dependability model is developed for the SPS to determine its failure modes and associated probabilities. The resulting cost-minimisation problem is solved for a range of operating conditions and SPS reliability levels. The results consistently show cost savings from the use of an SPS, even if it is highly unreliable, when a hedging strategy may be used. The optimal solution is highly sensitive to the problem parameters, but it is demonstrated that optimal operational strategies are associated with particular SPS outcomes. This finding may be used as empirical guidance to develop operational strategies for complex networks with unreliable SPS.