Ofgem codifies response to fault ride through failures

Ofgem has approved a modification to the Grid Code to formalise the response to the failure of a generator to ride through a normal fault on the transmission network.

The modification was originally proposed by SSE after National Grid Electricity System Operator (ESO) raised concerns over a growing number of instances in which generators suffered outages at the same time as a fault on the transmission network that did not push it outside of normal operational limits.

This same snowball effect contributed to the blackout in August 2019 when a lightning strike on the transmission network was quickly followed by outages at RWE’s Little Barford gas plant and then Orsted’s Hornsea offshore windfarm, removing 1.4GW of generation from the power grid.

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

In a letter to the industry in May, the ESO highlighted a series of instances between February and April in which generator outages had coincided with faults on the transmission network and set out an interim process to respond to such failures.

It instructed network users that had not remained online during a normal fault 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, the ESO said they should remain out of operation until a resolution is implemented.

In a subsequent letter in June, the ESO reiterated that if there is a recurrent 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.

ESO head of networks Julian Leslie told Utility Week there were several incidents in April in which generators tripping off in sympathy with network faults came close to triggering RoCoF protections again.

SSE raised the modification GC0151 to codify the response to generator failures and address its concerns over the interim process laid out by the ESO, which it argued would, among other things, put generators in breach of market manipulation rules and treat them as “guilty until proven innocent” with significant financial ramifications.

It proposed that if the ESO considers a network user to have breached fault ride through requirements, it should notify the user, which would then have three months to investigate and explain the breach and resolve any issues that emerge.

During this period, the user would be required to limit its exports to 70% of its unrestricted capacity or the largest potential infeed loss on the electricity system at the time, whichever is smaller. The restriction would not apply to users with a Transmission Entry Capacity of 100MW or less or limit exports to below this level.

If after three months, the ESO and the user had not agreed to an explanation or necessary actions to resolve any issues, then they would be subject to an export limit of 50%.

The ESO would also be required to publish the largest potential infeed loss for each settlement period on a daily basis as well as post-fault voltage waveform data.

Ofgem granted urgent status to the modification in July.

A series of alternatives were also proposed during the modification process, the first of which would require network users to respond to a notification from the ESO of a potential ride through failure within two hours, or longer if agreed between the user and the ESO. On receipt of this notification, the user would be required to limit their output to a level, and for a period, agreed between the user and the ESO. This limit could be as low as zero.

The second alternative would clarify the instances in which users are required to trip in order to a clear a fault on the transmission network; amend requirements to generate maximum reactive current during faults that may be unachievable for many generators; and amend post-fault active power requirements to reflect the fact that low load generators may have greater oscillations than they currently allow.

The third alternative would combine the original proposal and the second alternative, whilst the fourth alternative would combine the first and second.

Ofgem has decided to approve the first of the alternatives.

The regulator said the risks presented by non-compliance with fault ride through requirements depend on the “prevailing system conditions and level and availability of services held by the ESO to control frequency deviations caused by a sudden loss to the system” and said the body should be able exercise its judgement in mitigating these risks.

Unlike the original proposal, which would impose “inflexible pre-determined export reductions,” the first alternative would give the ESO this flexibility, including the ability to limit exports to zero when necessary.

If the ESO and a network user cannot reach an agreement an on appropriate export limit, the ESO stated that users would be able to continue operating as normal, while the issue could be escalated to the regulator. Ofgem said although generation licence holders can refer disputes to itself, “we expect the ESO to use existing provisions, including emergency instructions where necessary, to manage system risks in real time”.

Ofgem said it does not expect such a situation to arise as reconnecting post-fault without understanding why an outage occurred represents “poor asset management practice” on the part of the user and could pose safety risks. It said it expects users’ capabilities to be taken into account when restrictions are sought and that the ESO should “clearly articulate” its justifications.

Although Ofgem ruled out the second alternative on the grounds that it had not been fully scrutinised given the urgency of the modification – and hence also the third and fourth alternatives – the regulator said it “highlights a number of deficiencies” with the current fault ride through requirements that should be addressed through a separate modification.

The approved modification took effect on Saturday (6 November).