New Fault Ride Through compliance arrangements introduced for transmission-connected generators

Ofgem approved Workgroup Alternative Grid Code Modification 1 (WAGCM1) of GC0151 Grid Code Compliance with Fault Ride Through (FRT) Requirements on 5 November. In short, this decision introduces a new, legal process, into the Grid Code – the legal text that governs those connecting to the electricity transmission system – that will allow the Electricity System Operator (ESO) the right to require a user that is suspected of a FRT failure to restrict their output to an agreed level, which could be as low as 0MW.

Fault ride through (FRT) refers to the ability of generation units and High Voltage Direct Current systems to maintain operation during periods of low voltage caused by transmission system faults and disturbances. The Grid Code sets out that assets connected to the onshore transmission network, should remain stable and connected to the system without tripping during short circuit faults and voltage dips at voltage above 200kV (supergrid voltage).

The ESO first made industry aware of its concerns surrounding FRT compliance when it issued an open letter on 6 May 2021 outlining that certain assets had tripped during faults deemed “normal” and within system parameters. From the letter it was clear that the ESO considered that the inability of generation, interconnectors or other transmission-connected plant to ride through such faults was intolerable and a serious risk that should be managed quickly and effectively. On a more factual note, the ESO outlined that between 10 February and 18 April 2021 there were eight transmission faults where generation was unexpectedly lost, which in one case, was more than 1GW. The ESO said that it would consider raising modifications to provide clarity on non-compliances, but in the meantime set out an interim process to manage failures.

In response to the open letter, SSE Generation formally raised the modification GC0151 because it had concerns that the ESO’s interim process was potentially discriminatory, and could result in generators breaching legal requirements. The ESO’s interim process could have resulted in generators reducing their Maximum Export Limit to zero, which SSE considered would risk being deemed to have physically withheld generation capacity, potentially in breach of the EU Regulation on Wholesale Energy Market Integrity and Transparency (REMIT) market manipulation rules. Under SSE’s proposed solution, generators and networks company assets demonstrating fault ride through non-compliance would have initially faced no export limits or be constrained to 70% of Transmission Entry Capacity (TEC) or asset capability, depending on the situation. If after three months the issue was not resolved, this would be reduced to 50% until compliance was achieved.

An alternative developed by the workgroup (WAGCM1) effectively codified the process set out in the ESO’s open letter. This mirrored the interim process where a preliminary report would be required from those failing to comply with the FRT requirements within two hours of an incident, although there is a level of flexibility for longer timescales to be agreed. If those at fault find no evidence of FRT shortcomings in their part they can then self-certify for compliance with FRT prior to their reconnection or re-demonstrate compliance if there are issues found and corrected. Where FRT compliance issues are identified, WAGCM1 provides the ESO with the right to require a user that is suspected of a FRT failure to restrict their output to an agreed level, which could be as low as 0MW, albeit this is likely only to be in the most extreme cases.

The main differences between WAGCM1 and the original were the magnitude and duration of required export restrictions, and the timeframe in which a user is expected to provide an explanation of potential FRT non-compliances. Ofgem approved the alternative akin to the process outlined in the ESO’s open letter, considering that allowing the ESO and those who failed to comply with the FRT requirements to agree a level of output restrictions would mitigate the potential risks of FRT non-compliance. SSE’s proposal would have imposed an inflexible pre-determined export reduction on generators, however, Ofgem was concerned that where they choose to reconnect post-fault, it forces potential FRT risks to remain on the system operating at a level that may not sufficiently reduce the impact of the risk to the transmission system. In addition, while several workgroup members and consultation respondents considered the two hour timeframe proposed under WAGCM1 too short, Ofgem considered generators should respond as quickly as possible in order to return to normal service.

As mentioned before, SSE raised GC0151 to address concerns around commercial implications that could arise from the ESO’s interim solution, including whether reducing output to 0MW would be in breach of EU market transparency obligations. Ofgem noted that from an engineering perspective, it expects export restrictions applied at the request of the ESO as a result of GC0151 to be considered akin to a reduction in the capacity of that unit which was technically available, and thus would not comprise market manipulation.

GC0151 was implemented on 8 November. While the new process could have serious implications for those failing to comply with FRT requirements, material output reductions are likely to only be used as a last resort and in the interests of mitigating impacts on the wider transmission system. In its decision, Ofgem touched on the potential for further modifications with workgroup discussions highlighting a number of deficiencies in the current FRT requirements. The ESO is expected by Ofgem to take these issues into consideration in assessing Grid Code compliance and there is potential for changes to the FRT requirements to be progressed as a separate modification in the future if the industry deems it beneficial.

We will be keeping track of all future FRT changes through our bespoke weekly impact report for generators which sets out the current open energy consultations and industry code modifications you should be aware of. Such changes can impact the technical requirements for assets, costs and revenues, and how parties might trade or participate in balancing services or the balancing mechanism. An impact rating and explanation are provided for each change to help prioritise your resources and understand the effect on the business.

For more information on the Bespoke weekly impact report for generators please contact Tom Faulkner at or by calling 01603 542123.

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