Increasing Electricity System Fragility in the UK

Credit: telegraph.co.uk

It was always obvious that replacing on-demand power generation, like coal and gas power stations, with mainly intermittent power from weather-dependent renewables, was going to make reliability of the system an issue sooner rather than later. And here we are.

The UK’s electricity network is likely to become significantly weaker within five years, due to falling Short Circuit Levels that reduce the reliability of protection systems designed to limit the geographical extent of supply loss during a fault, and also make it more likely that asynchronous sources of electricity such as wind, solar and High Voltage Direct Current interconnectors will disconnect during a fault.

Ironically, Short Circuit Levels are falling because of a rising input from asynchronous sources, says The GWPF.

A remedy for this problem is unlikely to be cheap. Who will pay?

Electricity networks of any size are complex systems, with all the advantages and disadvantages that this implies.

The uninitiated believe that the principal threat to such systems is the failure of electricity producers, the generators, to meet the demands of consumers for energy, resulting in a blackout.

This is not completely mistaken, but, in fact, blackouts on a modern and developed electricity system are only rarely directly caused by shortfalls of generation resulting from poor system planning, a power station accident, or an unexpectedly high consumer demand.

System operators are nearly always able, even at short notice, to call on sufficient additional resources either to increase generation or reduce demand, though of course this remedial action comes at a considerable cost.

A rather more probable cause of a system blackout is a transmission system equipment failure, at a transformer for example, or a sudden external event, such as a storm or a vehicle, a plane or a ship perhaps, damaging a transmission line. In a weak or poorly designed system such accidents will overload other transmission lines that then themselves have to shut down to avoid damage, sending a further ripple of overloading through a large part of the network, forcing generators themselves to come offline and resulting in a widespread blackout.

Apart from ensuring a high specification for the components used and a high standard for design and construction, the best protection against such accidents is to ensure that the system is sufficiently stable under stress that it can contain the loss of supply to a small part of the network, this capability usually being automatic since action must be taken in milliseconds to prevent a cascade of faults.

Accidents will happen, but a strong system can prevent a local problem from becoming a regional or even a nationwide disaster.

The strength of the system must be continually monitored to ensure that it will be stable under stress, a precaution that would be necessary at any time, but has particular relevance in the UK at present due to the rapid and dramatic changes in the electricity supply industry being driven by climate change policy.

It is therefore only prudent for National Grid ESO (the Electricity System Operator) to have initiated detailed work on an Operability Strategy with the aim of ensuring that its System Operability Framework is adequate to the task.

The first tranche of documents was published in November and December 2018, and are available at the link above, with updates being provided in the regular Operational Forums, the most recent of which was held on the 26th of March (the presentations are all available via the link below).

National Grid identifies five areas of concern, Frequency Control, Voltage Control, Restoration (i.e. recovery after a blackout), Stability, as discussed above, and Thermal (transmission line temperature).

All are important, but to judge from the volume of commentary devoted to it, it is Stability that is giving most cause for concern. Specifically, Short Circuit Levels (SCL) in Great Britain are predicted to fall considerably over the next decade. The Short Circuit Level is the current that will flow through the system during a fault, an accident affecting a transmission line for example.

It is, as National Grid explains, “a measure of strength”, and a “key parameter for protection systems” in the network itself and also in other equipment attached to it (National Grid, System Operability Framework: Impact of Declining Short Circuit Levels(December 2018), p. 1).

Full article here.