Nuclear, CCGT, biomass the winners as system operator continues to struggle with low demand

EDF Energy provided 600MW of additional nuclear generation on the weekend of 4-5 July as the Torness plant returned from outage, while National Grid ESO continued to pay the company to reduce generation from its Sizewell B plant by a similar volume to help manage continuing low demand.

True GB electricity demand during the weekend’s overnight minimum demand period had increased from the previous weekend – up from 19.9GW to 21.25GW. But the demand using the transmission system would have hit a new record low – 13.3GW, compared to last week’s record low of 13.4GW – if the system operator had not raised the demand level artificially by contracting providers of its new Optional Downward Flexibility Management (ODFM) product for 1.2GW of additional transmission ‘demand’.

The weekend illustrated the new challenges in operating a more weather-dependent electricity system and one where much more demand is met from resources on the distribution network. NGESO needs to have enough demand on its transmission system to bring on line power generators that help maintain frequency and voltage, but more demand is being met from local resources – raising a management issue as total demand is supressed during  Covid-19 restrictions.

In a review of the weekend, the system operator said that on Friday morning the transmission demand expected had been 13.9GW, but it became clear that windy conditions would hit more of the country than initially forecast. This meant that wind power provided from distribution-connected turbines would rise to 3.1GW instead of 2.2GW, reducing transmission demand. In response, the SO reduced its predicted transmission demand to 13GW and decided to use ODFM.

On the previous weekend interconnectors raised demand by exporting power, but last weekend it was uncertain whether that would happen. In fact, the interconnectors exported, but uncertainty remained, because a very low price differential between GB and its neighbours meant there was “quite a lot of risk” as there was a possibility of  ‘countertrades’ unfavourable to the GB system needs.

NGESO exports power against market flows (ie paying to export) when it needs more demand. But the SO may also reduce the power exported, because interconnectors are potentially the largest single infeed loss on the system. If an interconnector ‘trips’ out of service – as happened on Wednesday – the SO has to have reserves to cover the event. At the weekend it ‘bought back’ over 1GW of interconnector export for this reason.

The system operator also raised demand by asking pumped storage plants to pump, but said that would raise problems in the event low demand was expected to last for long periods.

It reduced transmission-level wind generation by 3.8GW to 8.4GW but noted that it retains wind “to keep contingency to come down further if necessary”. The actions allowed it to bring on an extra 3.6GW of gas or biomass-fuelled power plants to maintain stability.

Going into the summer the system operator said it would be difficult to predict demand. There is a “Little bit of hope that we are going back to normality” as lockdown eases and demand is rising slightly. But there has been a growing tendency in recent years for daytime transmission demand to fall on sunny days as solar kicks in and raise stability issues. The potential for local lockdowns and the general uncertainty over the progress of Covid-19 measures added to the uncertainty.