Ofgem fast tracks proposal to address fault ride through issue

Ofgem has granted urgent status to a Grid Code modification proposed by SSE to address an issue over the fault ride through requirements for generators and other assets connected to the transmission network.

SSE raised the modification after National Grid Electricity System Operator (ESO) instructed generators to temporarily restrict their output in instances where they did not stay connected and continue operating during a “normal” fault.

In a letter sent to affected parties in May, the ESO said it had seen “a growing number of instances” whereby generators or other connected assets had failed to “ride through” a fault on the electricity transmission network that did not push it outside of normal operational parameters. It said these incidents represent a “serious risk” that it cannot tolerate.

Accordingly, the ESO instructed all obligated parties to confirm within three months that they are complaint with the fault through requirements stipulated in the Grid Code.

In cases where the unexpected loss of load coincided with a normally cleared fault, the ESO said its national control room for electricity would issue a notification to the party that dropped off.

The ESO said they should respond to the notification within two hours to confirm their compliance with the fault ride through requirements and not resume normal operation until they had done so. It said they should also provide an explanation for the failure within two days, and if there is a potential compliance issue, remain out of operation until a resolution is in place.

In a subsequent update in late 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 said theses faults were normal system events with “no direct widespread impact.”

Nevertheless, the ESO said the generation losses were significant, coming close to triggering Rate of Change of Frequency protections and in one case exceeding 1000MW. It said one transmission fault occurred twice three hours apart, with the same generators dropping output on both occasions.

“As prudent system and plant operators, we cannot afford to expose the system to risks where these have been identified as this would be against our licence obligations,” it added.

Elaborating on its previous instructions, the ESO said following the persistent failure of an asset, network users should reduce their Maximum Export Limit to “zero or to a safe level” until a fault ride through issue has been ruled out. “This is to mitigate system risk and not to penalise the user,” it stated.

It said the request to respond to a notification from the national control room is not about finding the root cause or fully explaining events: “It could be as simple as to say that the causes are being investigated and to set out a timeline to give a fuller explanation.”

In proposing the Grid Code modification GC0151, SSE raised a number of concerns over the instructions issued by the ESO. It said the “uncodified” and “voluntary” interim process would potentially put network users in breach of regulations prohibiting market manipulation.

The company said it could also have a “significant commercial impact”, particular during periods when supply margins are tight, with network users being treated as “guilty until proven innocent”. It said in some instances, namely over-voltage on the transmission network, generators could actually be in breach of the Grid Code if they did not disconnect in response to a fault.

Furthermore, SSE said the security of the electricity system could be jeopardised if multiple network users are required to restrict their output at the same, leading to shortages of generation. It said the required timeframes for responding to notifications are not realistic, and citing the specific wording of the ESO’s communications, could also be discriminatory to some users, applying to generators but not interconnectors or network owners.

SSE therefore proposed to codify the process and amend it in the following ways:

If, after being initially notified of a loss coinciding with a fault, a network user subsequently receives an Interim Operational Notification (ION) from the ESO confirming that they are compliant with Grid Code, then the restriction on output should amount to 70 per cent of their transmission entry capacity or the prevailing largest infeed limit, whichever is lower. The constraint should not be reduced below 100MW or apply to generators with a capacity of less than 100MW.

Network users should have three months after receiving relevant waveform data from the ESO to investigate, and if necessary, resolve the cause of any non-compliance. If the network user receives a Final Operational Notification (FON) confirming their compliance, there should be no export limit during this period.

If they receive a Limited Operational Notification stating that they may be, or are, unable to comply with the Grid Code, and the issue relates to equipment changes that could reasonably be expected to affect fault ride through requirements, they should also be subject to the same restrictions as those in receipt of an ION. If the issue relates to anything else, then they should be treated as if they have received an FON and not subjected to any export limit.

If a network uses does not resolve the cause of an instance of non-compliance within three months of receiving relevant waveform data from the ESO, their output should be constrained to 50 per cent of their transmission entry capacity or asset capability until it has been resolved.

SSE said the longer timeframes for investigation would be more reasonable, whilst the loosened limited would strike a balance between the costs of holding additional response to account for potential non-compliance with fault ride through requirements and those of unnecessarily constraining a network user.

It also called for the ESO to be obliged to provide the industry with historic fault data and a dynamic figure for the largest infeed loss, as well as the aforementioned post-fault waveforms to concerned parties.

A spokesperson for the company told Utility Week in a statement: “We are supportive of changes but believe these should be met by official code moderations rather than voluntary requirements to ensure clarity and certainty for compliance with all legal and regulatory frameworks.”

Ofgem has granted urgent status for the modification, acknowledging the concerns of both the ESO and SSE. It said the Grid Code panel should make its recommendation on whether to approve the modification on 7 October but did not give any date for its final decision.