ESO says SSE proposals do not solve fault ride through issue

National Grid Electricity System Operator (ESO) has said SSE’s proposed response to the failure of a generator to remain connected during a “run-of-the-mill” network fault would continue to leave the power grid at risk.

Speaking to Utility Week, ESO head of networks Julian Leslie nevertheless welcomed efforts to codify the arrangements.

In a letter to industry in May, Leslie warned that the ESO had seen a growing number of instances where generators had failed to “ride through” a fault on the electricity transmission network that did not push it outside of normal operational limits.

In cases where this happens, Leslie instructed generators to confirm their compliance with the fault ride through requirements in the Grid Code within two hours and provide an explanation for the failure within two days. If a potential compliance issue is identified, Leslie said they should remain out operation until a resolution is implemented.

In an update in June, the ESO revealed there were eight transmission faults between 10 February and 18 April where generation was unexpectedly lost at the same time. It reiterated that if there is a persistent failure of an asset, generators should reduce their Maximum Export Limit to “zero or to a safe level” until a fault ride through issue has been ruled out.

“If you get a transmission fault and there’s any number of generators coming off the system in sympathy and it’s not planned, then obviously you want to understand what operational risk you’re managing,” Leslie told Utility Week.

“And therefore, it’s been really important to us to understand the scale of the issue, so writing a letter to get all generators to reconfirm their compliance with their obligations. This isn’t something that we check or sign off. This is the generators. They know what they need to do and it’s their obligation of their license to maintain compliance.”

Following the update, SSE proposed a modification to the Grid Code that would formalise the response to the failure of a generator to ride through a network fault. It said the “voluntary” interim process set out by the ESO could have a significant commercial impact, whilst putting generators in breach of regulations prohibiting market manipulation.

Leslie acknowledged that compliance with its instruction is reliant on the “goodwill” of generators: “We are asking for a reconfirmation that they are all compliant, but also parallel with that, we’re using the existing frameworks that we have in place to today to just clarify what we expect to happen in those first couple of hours following a fault happening where a generator has tripped off the system in a way that you wouldn’t have expected it to.

“What we’re asking generators to do is, if they haven’t got to the root cause of the problem, they should remain off until they have got to the root cause of the problem. But the way codes are written today we have no rights to do that.”

SSE’s proposed modification, which was recently granted urgent status by Ofgem, would impose less stringent restrictions on generators that failed to ride through a fault and give them more time to respond with an explanation.

Leslie welcomed the effort to move things forward, saying the ESO is planning to raise some modifications of its own to address the current “vagueness” in the Grid Code and establish generators’ exact responsibilities.

At the same time, Leslie said the ESO does not support the specific proposals put forward by SSE: “While we acknowledge SSE’s concerns around market requirements, system security always trumps everything. Keeping the lights on for the nation for the system operator has got to be our number one priority.

“Our challenge to the SSE mod is it still doesn’t solve this immediate response of getting the generation off until we find the root cause, and the SSE response gives them three months. It’s just a continuation of the situation we have today.”

He drew attention to a series of incidents on 6 April when a generator tripped off twice within a few hours in response to faults and came close to triggering Rate of Change of Frequency (RoCoF) protections: “It’s a real good example of why we cannot let generation back onto the system unless we really know why it tripped off in the first place because had it been over the RoCoF on 6 April it could have posed an operational risk, which is just completely unacceptable for anybody.”

It was this same snowball effect, which on 9 August 2019 caused a widespread blackout that brought large parts of the country to a standstill during the Friday evening rush hour.

A lightning strike on the transmission network was quickly followed by outages at RWE’s Little Barford combined-cycle gas turbine plant and then Orsted’s Hornsea offshore windfarm, removing 1.4GW of generation from the power grid.

The resulting drop in frequency triggered the RoCoF protections for hundreds of megawatts of embedded generation, which in turn triggered the automatic disconnection of demand to prevent a complete nationwide blackout.

“I’ve spoken to the generators group at Energy UK and a whole bunch of other people and everybody absolutely agrees that this is an issue that needs careful attention,” said Leslie.

“It’s an issue that every generator should take seriously and they do take it seriously and they can absolutely understand why the incidents that we saw led me to write the letter I wrote and we now need to have a Grid Code modification to help manage the situation.”

He noted that electricity networks’ Accelerated Loss of Mains Change programme to make embedded generators less susceptible to so-called nuisance tripping, which was stepped up a gear following the 2019 blackout, has now seen changes to the protection settings for more than 10GW of generation.

When asked why there was an uptick in incidents of generators failing to ride through faults earlier this year, Leslie responded: “I don’t think it’s any one particular thing.”

He said the electricity network is becoming more complex, with more offshore wind and interconnection, whilst the reduction in conventional fossil generation means that at times inertia is “lower than we’ve ever seen before”.

“We’re getting these new levels of operation and the simulations that the developers use when designing and building these offshore windfarms, they have to make some background assumptions about the network and I think we’re stretching those assumptions they made in their modelling,” he explained.

“There isn’t any one particular thing you can point your finger to and say that is the root cause. I think every one of these is a slightly different scenario.”

Leslie said generators’ willingness to comply with the ESO’s instructions has not yet been put to the test: “We haven’t had any incidents since I wrote the letter.”

He said this is not because there haven’t been any network faults: “We’ve had quite a bit of storms and lightning over recent weeks. We’ve had quite a number of transmission-related faults – we’ve had the Western HVDC faulted and things like that – and we haven’t seen generation tripping off in sympathy.”

Leslie said it could be because there has been more gas generation on the system, making it less susceptible to disturbances, or because generators have “double-checked everything and tweaked things where necessary.”

He continued: “I’m hoping with my letter, the focus on it and the conversations we’ve been having, that either these generators were complying anyway and we were just unlucky in February to April that we just caught the ones that were on the edge of compliance and actually there isn’t a system problem, or that the problem has now been fixed and the generators have reviewed their protections and modes of operation and they are now compliant.

“Either way, I’m not seeing the continuation of the trend between February and April.”

As part of its proposed modification, SSE called for the ESO to publish both historic and immediate post-event fault data as well as the largest potential infeed loss on the power grid at any given moment.

Leslie agreed with the call for greater transparency in principle but said fault data is commercially sensitive and would need to be anonymised: “I think publicising some of the faults that we’ve had and the root cause that we’ve found would be a good thing to do.”

“Whether the working group as it goes through and the stakeholders and generators can get themselves comfortable with that level of transparency remains to be seen,” he remarked.

He said publishing the largest infeed loss “every minute of every day” could also pose a potential security risk, providing “a good signal if you want to cause disruption.”

Earlier this week, the ESO issued an early warning of tight supply margins over the coming winter, partly due to the closure of several nuclear power stations.