System stability fears will require changes at 50,000 distributed generation plants

Up to 50,000 distributed generators, who operate small plant less than 5MW in capacity, will have to make changes to plant operation to stop faults propagating across the electricity system and causing local blackouts.

A consultation on how to implement the change is due to be published next week and it will also apply to storage.

The change is required because most such plant automatically disconnects from the electricity grid when the supply frequency moves above or below tight limits around 50Hz. The frequency deviations are themselves caused when power plants disconnect from the network (among other circumstances), so when fleets of small plants trip – National Grid has seen hundreds of MW of plant react in this way – the fault propagates and can cause blackouts.

The issue has been growing in importance over recent years as large rotating plant, which ‘ride through’ the disturbances, have been displaced by distributed generation, including solar and wind farms. It is most acute during summer days when little fossil plant is on the system.

To maintain stability, the System Operator (SO) National Grid responds by paying large rotating plant to operate ‘out of merit’. On 17 May this year (royal wedding day) it spent £1 million taking such action.

The annual cost (paid for by system users in balancing charges) has doubled, rising from around £30 million in 2015 and 2016, to £59 million last year.

In June, concerned that it would see “involuntary disconnections” over the summer, the SO called on some plants to make adjustments. It targeted 72 sites totalling 800MW in the most vulnerable areas and offered compensation to those that could change their settings.

To deal with the issue in the longer term, since 2014 all plant sized at 5MW and above, and new plant less than 5MW in size, has been required to ‘ride through’ the disturbances (which in fact comprise two types, ‘rate of change of frequency’ (ROCOF), and ‘vector shift’) instead of disconnecting. Now the rising level of distributed and ‘non synchronous’ generation means small existing plant must also take action. Single domestic installations will not be affected.

On an individual plant basis the change may be relatively simple – changing the limits at which plant automatically disconnects – while for others it will require additional testing and recommissioning. But the ride-through changes have to be made and verified at 50,000 small plants totalling 15GW. The SO wants to complete the wave of changes by the early 2020s, which it described as a ‘tipping point’ because at that time new interconnectors will come into operation. These are also ‘non-synchronous’ generators and because they are large infeeds (1-3GW) they could have a large ROCOF impact. National Grid modelling suggests annual costs as high as £250 million by the mid 2020s unless action is taken.

What is not clear is how the changes will be implemented and – crucially – who bears the cost. The consultation to be published next week will consider how best to achieve this.

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