The impact of tightening margins on plant availability

Dr Amy  O’Mahoney, Ofgem, United Kingdom 

Mr Kristian Marr, Ofgem,United Kingdom 

Shashi van de Graaff, Ofgem, United Kingdom

 Mr David Hall,  Ofgem, United Kingdom

In recent years, the generation mix in most electricity markets has been in transition, resulting in larger proportions of variable generation. In GB, wind and solar accounted for 13% and 5% of installed capacity respectively in 2015 , and 11.5% of generation over the same period.  This intermittent generation is not always capable of delivering in periods of tightness, and thus contributes considerably less to peak demand. This development in the electricity market has led to changes to how the system is operated.


As a result, more flexible thermal plant is now required to run differently than originally intended; wind generation has been shown increase the cycling of existing units (Troy et al., 2010). This relates to additional start-ups, ramping and operation at part load (Denny and O’Malley, 2009; Troy et al., 2010). Analysis of availabilities in other markets has indicated that the availability of CCGTs in GB is below those in comparable markets.  Forced breakdown rates are assumed to be broadly in line with other markets. So, we propose that lower availabilities could be a result of market conditions rather than technical constraints. We would also expect tightening generation margins to lead to an uplift in price, supporting CCGT plant economics. Thus, the aim of this paper is to assess what happens to CCGT availability when low capacity of other fuel types drives tightness, opposed low CCGT availability driving tightness.

We use half-hourly data from April 2013 to November 2015. Over this period, we use a profile of the maximum level at which each Balancing Mechanism Unit (BMU) may be exporting to the GB Transmission System at the Grid Supply Point, defined as Maximum Export Limits (MELs).


Using time series regression analysis and controlling for periods where less flexible plant on the system was unavailable to meet demand, we focus on the impact on CCGT availabilities during times of historic nuclear unavailability on the GB system.


We expect our analysis to show that lower CCGT availability is as a result of market conditions, and when considering tighter periods when less alternative sources of generation were available that CCGT availability was increased. If this is correct, then CCGT potential in GB is currently underestimated. Current availabilities are used in the capacity market in order to determine the necessary volume of capacity to procure; therefore increased availabilities may reduce this requirement in future analysis.



Denny, E., O’Malley, M., 2009. The impact of carbon prices on generation-cycling costs. Energy Policy 37 (4), 1204 – 1212.

Troy, N., Denny, E., O’Malley, M., May 2010. Base-load cycling on a system with significant wind penetration. Power Systems, IEEE Transactions on 25 (2), 1088 –1097.


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