The problem

Currently the transmission system in England and Wales (E&W) must provide security on an operational timescale for up to and including a double circuit event (N-D). Moreover the system is planned to up and including a simultaneous circuit outage of any two circuits (N-2) on the network.

Developing models with the aim of providing a robust argument for adapting these criteria to allow the standards to cater efficiently for high penetrations of variable generation is challenging.

The remit of this project was to develop a mathematical model in order firstly to determine vulnerabilities for the E&W supergrid, then secondly to investigate the impacts of these contingencies from the perspective of how customers value their electricity monetarily if subjected to a loss of supply.

“Working with a company at the forefront of the industry has been a fantastic experience. I will be able to take the knowledge gained during the internship back to Edinburgh and apply it not only to my own PhD, but also to provide useful input to other research projects. I have also been given the opportunity to visit both the NG control room and a large 400kV substation, both of which were very enjoyable experiences,” said intern Dan Eager, University of Edinburgh.

The approach

The approach used was firstly to determine fault rates for equipment on the E&W supergrid by performing a statistical analysis on recent fault data. This included analysing the influence of weather, equipment location and repair times.

Secondly mathematical models were developed to determine risk of secured events for key circuits on the system. Markov modelling techniques were employed to determine the probability of circuits being unavailable during a market settlement period and combinatorial outages of N-x contingencies between two zones were calculated, e.g. see figure of 4-circuit system between two system zones.

Factoring the uncertain value of lost load (VOLL) into a decision process using risk aversion factors [2] was also studied. This has provided useful input into the ongoing debate surrounding VOLL and has resulted in a fuller understanding of the modelling methods available.

In parallel with the main work, a review of loss of supply indices and reliability metrics for power systems was also carried out [3,4]. Detailing these metrics and how they are calculated when applied to transmission systems has been of enormous benefit to the company.

References

[1] GB Security and Quality of Supply Standard, http://www.nationalgrid.com/uk/Electricity/Codes/.
[2] Faber, M. H., Schubert, M.: Decision making subject to aversion of low frequency high consequence events, Workshop on Risk Acceptance and Risk Communication, 2007, Stanford University.
[3] Billinton, R., Allan, R. N.: Reliability Evaluation of Power Systems, Plenum Press, London, 1984.
[4] CIGRE WG C1. 11: Asset Management Performance Benchmarking, Feb 2009.