distributed restart events national grid eso

What is Short Circuit Level?

As the Electricity System Operator, we make sure that supply and demand are balanced and power flows across the network safely and reliably. To do this we need to manage key properties of electricity such as frequency or voltage.

Another vital parameter is Short Circuit Level (SCL). But what is it and why is it important?

Short Circuit Level is the amount of current that flows on the system during a fault. These faults can be caused by a lightning strike, weather conditions or equipment failure. During the fault, the system can see a direct connection to the earth and current flows from all sources into it. SCL is vital during such a fault as it helps us to maintain system voltage.

The bigger the current, the stronger the system

If we get a disturbance, then a stronger system will dampen it out quickly. It’s just the same as how a strong bridge with re-enforced joints would wobble but settle quickly when placed under force. If we operate a system with low SCL, it may take longer to recover after a disturbance.

Why is SCL lower than it used to be?

Large Coal and Gas generation create five times more fault current compared to wind and solar. A 100MW generation would provide in the region of 500-700 MVA of Short Circuit Level. Wind farms are limited by the rating their electronic component so the same level of generation may provide only around 100MVA of SCL.

We’ve seen lengthy periods of coal free electricity generation this year due to low demand. A higher proportion of our generation has come from renewable generation which as we’ve stated above, means there is less SCL on the system. In turn, the low amount of SCL can create operability challenges for us.

The reason for this is that when there is a low amount of SCL, any voltage change causes a bigger disturbance which travels further.  If left unmanaged, these disturbances could trip generation or make the whole system go unstable, which would damage equipment.

Similarly, if SCL is too low our network protection that opens circuit to protect the system could miss that a fault is happening and leave the network in an unsafe and unstable condition. That isn’t an issue as we have the expertise to safely manage an energy system with low SCL.

How do we deal with low system SCL?

Traditionally we had the required level of SCL from all the generation that is running to meet the energy needs of the country. As we’re now seeing a shortfall in SCL in some areas we need to look to top up our SCL from other sources and artificially inject SCL into the grid.

At the ESO we’re working to manage this through our NOA Stability Pathfinder project. We have identified areas of the network where additional support is needed and we’re reaching out to Transmission Owners and commercial parties to provide us with additional capability to inject current into the system.

We are also working on a grid forming working group and a future stability market to make sure these needs are met.

Part of the solutions are likely to include Synchronous Condensers. They’re similar to a traditional Coal or Gas generator, but we will just receive the SCL support without the energy and the without the need to burn CO2 emitting fuels.

The other more innovative solution we’re exploring is via the SCL and stability support from wind, solar and batteries through the new grid forming convert to technology. The project is set up to allow all technologies and providers to compete to drive innovation and get the right level of SCL at the lowest cost.

SCL is just one of our future operability challenges, and we’re working hard to identify and solve them as we move towards our 2025 zero carbon ambition.