Regulation of distributed generators to be reviewed after blackout

Ofgem has recommended a review of the regulatory compliance and enforcement regime for distributed generators after more than a gigawatt of output was disconnected during the power cut in August.

A significant proportion was lost due to the triggering of overly sensitive loss-of-mains protection systems. According to its final report on the incident, some generators may have been in breach of industry rules mandating that they be recalibrated.

The regulator said the review should consider extending the requirement to hold a generation license to those smaller generators that are currently exempt.

The initial cause of the blackout was a lightning strike on a transmission line in Cambridgeshire. Although it was quickly rectified, the resulting fault is thought to have tripped 150MW of distributed generation with vector shift protection systems.

Faulty voltage control software also caused the Hornsea One offshore windfarm to cut most of its output in response to the disturbance and shortly afterwards the steam turbine at the Little Barford power station was automatically disconnected due a discrepancy in readings from its speed sensors.

The combined losses caused a rapid fall in frequency that tripped between 350MW and 430MW of distributed generation with rate of change of frequency (RoCoF) protections.

The electricity system operator (ESO) attempted to contain imbalance by activating its frequency response reserves and was initially successful, with the frequency briefly plateauing at 49.2Hz.

However, one of the gas turbines at Little Barford was then disconnected due to a build-up of pressure. Around the same time, the ESO recorded an additional 100MW increase in the system imbalance, perhaps due to the further loss of distributed generation. The grid frequency continued to fall, tripping at least another 200MW of distributed generation as it dropped to 49Hz.

When it hit 48.8Hz, the emergency load-shedding mechanism was activated, cutting power supplies to more than a million customers. It is also estimated to have disconnected another 550MW of distributed generation, bringing the estimated distributed generation losses to between 1250MW and 1430 MW.

In recent years, efforts have been made to prevent nuisance tripping by phasing out vector shift protection systems and recalibrating those based on RoCoF to make them more forgiving.

For generators with a capacity of more than 5MW, the latter was required by a series of amendments to the Distribution Code introduced between 2014 and 2018. The code was also amended in 2010 to the lower their trigger point for under-frequency protections from 49Hz to 47Hz.

But given the large volume of distribution generation that was lost, Ofgem said it is “reasonable to assume” that their compliance process “have not been effective in all cases”. It said this is “particularly concerning” as it recently approved an amendment to the Distribution Code requiring generators with a capacity of less than 5MW to alter their RoCoF settings.

In response, Ofgem has proposed a joint review of the regulatory compliance and enforcement framework for distributed generators alongside the Department for Business, Energy and Industrial Strategy, explaining: “This review should explore options for setting and enforcing technical requirements on these generators, including consideration of licensing smaller generators which would require government action.”

The regulator said they should begin consulting with the energy industry in Spring 2020.

Ofgem also recommended a review of the timetable for the Accelerated Loss of Mains Change Programme – an industry-run scheme to help generators adjust their RoCoF settings and maintain compliance with the Distribution Code. It said the Energy Network Association should submit recommendations to the Energy Emergencies Executive Committee (E3C) by April 2020.

And it said E3C, which also published its final report on the power cut last week, should undertake a “fundamental review” of the emergency load-shedding mechanism after its activation failed to achieve to the intended reduction of demand, whilst also causing widespread disruption to services.

The owners of Hornsea One and Little Barford have each agreed to make £4.5 million in redress payments for their roles in the blackout.