Ofgem has approved a modification to the Distribution Code that aims to prevent distributed generators from nuisance tripping in response to disturbances on the power grid.

The regulator said the change will allow the electricity system to be operated with a higher rate of change of frequency (RoCoF), saving millions of pounds each year.

The Distribution Code requires all distributed generators to equip loss of mains protections to prevent the formation of power islands following a fault on the network. Power islands can cause damage to electrical equipment if they are reconnected to the rest of the network whilst out of sync.

These protections often take the form of relays that are tripped if the RoCoF is too high.

But as renewables come to make up an increasing share of generation, the inertia of the power grid – its resistance to sharp changes in frequency – is falling.

Inertia is provided by synchronous generators that store energy in large rotating masses such as gas turbines spinning in harmony with the frequency of the power grid. Most renewable generators are asynchronous.

If left unchecked, this increased volatility could cause generators with RoCoF relays to shut down unnecessarily in instances where the mains power supply has not been lost. This is sometimes referred to as nuisance tripping.

To prevent this happening, the electricity system operator (ESO) at National Grid has spent large amounts of money to manage the RoCoF. The total bill for the 2018/19 financial year was £143.6 million.

The modification approved by Ofgem, DC0079, is the latest in a series attempting to address the underlying issue – that RoCoF relays were and are too sensitive.

The first, given the all-clear in 2014, required distributed generators with a capacity of more than 5MW to alter their relay settings to make them more forgiving.

The second, approved the following the year, extended this requirement to new non-type-tested generators with a capacity of less than 5MW. It also removed the alternative vector shift technique as a suitable form of loss of mains protection for new non-type-tested generators.

The third, approved in 2018, extended these requirements to type-tested generators.

DC0079, the fourth and final in the series, will extend them to all existing generators with a capacity of less than 5MW. The modification specifically states that:

  • RoCoF relays should be set to trigger if the RoCoF exceeds 1 Hertz per second
  • The vector shift technique should be removed where it is being used for loss of mains protection
  • Any existing over-frequency setting relays still set to 50.5 Hertz should, if possible, be changed to 52 Hertz

“Aligning RoCoF settings across new and existing generation such that it is consistent with the previous modifications would allow higher system RoCoF and a reduction in the constraint costs currently incurred to limit system RoCoF,” Ofgem explained in a decision letter.

“We consider that system RoCoF has the potential to increase significantly, due to reduced system inertia and an increase in the largest infeed loss.

“This will increase the cost and complexity of avoiding inadvertent operation of distributed generation protection that could otherwise result in widespread shut down of distributed generation across the system.”

The regulator said the ESO will continue to manage the RoCoF following the change but will have more flexibility to accommodate larger generating units and more distributed and asynchronous generation.

“Alongside this, removing vector shift protection from the system would improve the resilience of the system to spurious tripping and possible wide scale impacts created by vector shift mal-operation,” Ofgem added.

The regulator said the technique has “proven to be ineffective at detecting faults leading to electrically islanded conditions and is highly susceptible to mal-operation for transient faults on the transmission system”.

It added: “There have been a number of documented incidents where large volumes of distributed generation have tripped for transient transmission faults.

“If these had been coincidental with a loss of transmission generation infeed, there is the possibility for significant system impact.”

Ofgem acknowledged that the modification could increase the risks of injury to people and damage to equipment as result of power islands forming. However, the regulator said an assessment by the University of Strathclyde found them to be within acceptable limits.

It also noted that the change is expected to cost around £100 million to implement over a three-year period but said this will be offset a “significant” reduction in RoCoF management costs going forward.

The burden will not fall solely on the affected generators as the costs will be spread across the wider industry through balancing charges.

The regulator welcomed the industry’s efforts to work together to solve the problem but said distribution network operators and the ESO must taking a leading role in implementing the modification given its “unprecedented nature” in terms of “scale and scope”.

The modification will take effect on 1 September 2019.

The decision was published just days before National Grid was forced to cut power to around a million customers to prevent a collapse in frequency.

Speaking to Utility Week, Cornwall Insights senior consultant Thomas Edwards said the ESO was at the time (9 August) running coal generation out of merit in order to maintain system inertia.

He said one way the ESO could prevent this happening again would be to expedite the implementation of the modification.