Blackouts: Why does the National Grid issue and then cancel electricity warnings?

Capacity market notices sent automatically in advance whenever power surplus drops below desired level

Night falls behind electricity pylons and a sub station in Manchester (Getty)

The National Grid sent out a surprise alert at 2.33pm on Tuesday afternoon declaring the imminent possibility of an electricity supply shortfall as the colder weather begins to ratchet up British energy consumption, only to then cancel the warning just as swiftly.

The UK network operator issues these types of notification, known as capacity market notices (CMN), whenever it is concerned about having insufficient surplus electricity capacity available.

Tuesday’s caution was designed to warn homeowners of possible shortages during the 7pm period of peak usage, when the majority of people return home from work and begin switching on domestic appliances as they prepare dinner and settle down for the evening.

CMN’s are designed to warn customers of intermittent disruption rather than sustained blackouts, which are rare and most likely to be experienced in isolated areas where residents would expect advanced notice.

Taking to Twitter, National Grid ESO explained that Tuesday’s alert had been triggered automatically: “The ESO is confident that electricity margins are sufficient for this evening. However, a capacity market notice (CMN) has been triggered by the automated system.

“CMN forecasts are issued automatically and are only based on information in the public domain. They do not take into account all the factors which our engineers are working on.”

The service then directed readers to its website, where the National Grid explains that its role is to “balance” Britain’s energy flow by engaging in “continuous communication with the electricity market, where the electricity is being created”.

“This might mean asking generators to turn power up or down. Or we might ask them to increase or decrease demand,” the operator states.

Occasionally, its analysts’ forecasts for Britain’s needs might not match the realities of demand or generation on the ground, leaving the Grid with too little or too much power in the system.

“If we’re not able to match supply and demand through the normal mechanisms, we’ll send a more formal message to the electricity market to let them know,” it continues.

“These messages are sometimes referred to as system warnings or notices. They’re a routine way that we communicate to the market and operate the system, and they don’t mean that electricity supply is at risk. We’re simply telling the market what we need.”

While electricity margin notices (EMN) are issued manually by engineers in the control room “using operational and engineering judgements” to warn against insufficient capacity in reserve, CMNs are based on real-time data and triggered automatically around four hours ahead of time as notification to providers in the capacity market that the Grids’s safety margins have been reduced: specifically, that its forecast surplus has dropped below the preferred cushion of 500MW.

As was the case on Tuesday evening, an improvement in generation availability in the hours after the automatic CMN has been issued – restoring the buffer – will lead to the notice being withdrawn.

Commenting on the apparent strain on the system on Tuesday, Phil Hewitt, director at energy consultancy EnAppSys, told The Daily Telegraph: “This is the first tight day of the winter but it is not super tight.

“It is a small appetiser of tightness, there will be much tighter days ahead.”

While EMNs and CMNs do not actually denote a shortage of electricity available to the Grid, there are alerts for use when those circumstances do arise.

These are known as high risk of demand control or demand control imminent announcements but they are far rarer and much more seldom issued.