ESO considering buying inertia in next reactive power tender

National Grid Electricity System Operator (ESO) has said it is considering buying inertia as part of its next long-term tender for reactive power, noting that the technologies used to provide one are often capable of providing the other.

ESO head of networks Julian Leslie was speaking to Utility Week after the company recently announced the results of its first such tender, awarding two contracts worth £8.67 million – one to PeakGen to build a shunt reactor and another to Zenobe Energy to build a battery storage system.

Reactive power is produced or absorbed to control voltage on the power grid. Producing reactive power raises the voltage and absorbing it does the opposite.

The ESO is facing an increased need for reactive power absorption in the Merseyside region – the area covered by the tender – due to the growth of distributed generation. This is not only lowering demand on the transmission network and hence the voltage, but also changing the nature of loads.

“We’re seeing reduced demand because it’s netted off with embedded wind or something,” Leslie explained, “but we’re also seeing an impact on voltage because of solar farms connected at the end of cables. During the night time when they’re not producing anything, those cables act as capacitors and increase the voltage on the system”.

Leslie said the tender was mainly spurred by closure of Fiddler’s Ferry coal-fired power station in Cheshire, which was previously used for voltage control: “We wanted to ensure whatever solution was put in place to manage the voltage in that region represented the best value to consumers. Historically, we would have just called the incumbent transmission owner and said: ‘Hey, Fiddler’s Ferry is closing. You need to build a shunt reactor.’

“However, this time we took a different approach and said to the market: ‘Hey, we need some reactive power capability. What capabilities/technologies can you provide?’”

He said the costs were compared directly against those of a competing proposal from the transmission owner (TO) in the area – National Grid Electricity Transmission: “The transmission owner was notified of the need at the same time as the market. The transmission owner didn’t enter the tender process as the market participants did but it did bid as per the normal way that we and transmission owners would have engaged on things like this.”

“We did know the bid price on the same basis for the market participants and for the TO and therefore we were able to do the comparison. And if the TO’s had been a cheaper asset, a cheaper solution, then we obviously would have gone for the TO.”

Leslie said the next long-term tender for reactive power will be for the Pennines region and that the ESO was considering buying inertia at the same time.

Inertia refers to the resistance of the electricity system to sudden changes in frequency. It has traditionally been provided by large coal and gas plants with turbines spinning in harmony with the frequency of the power grid.

In January, the company completed its first ever tender for inertia as part of the trial of a wider stability service incorporating voltage control and short circuit level contribution. It had previously only procured inertia by buying power from generators.

Leslie said one lesson that has emerged from the tenders is that “generally, the technologies that provide your solutions for stability will also provide your solutions for voltage.”

He said, if there proves to be a requirement for inertia in the Pennines, then “we’ll combine them together”.

“What we wouldn’t want to do is go and buy a load of reactive compensation equipment in the Pennines region and then go out three months later with a stability pathfinder that then buys technology that can also do voltage control.

“We’d rather do that once, and if we need stability, we’ll go with a combined stability and voltage process, or if it’s just voltage, then we’ll buy just voltage.”