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Technical terms explained

On this page, you’ll find a list of some of the technical terms used in the ESO’s interim technical report to Ofgem.


What is frequency?

Frequency is a measure of how we manage the energy in and out of the electricity power system.

Here is a short video that explains frequency in more detail.

Here are answers to all the commonly asked questions around frequency.

 View frequency Q&As

What is Delayed Auto Reclose (DAR)?

Most faults on the electricity system are transient. This means they are not “permanent” faults – they happen instantly and then resolve.

If a fault on an overhead powerline is detected and has tripped the circuit breaker, an auto recloser starts timing. After a short time delay it attempts to reclose the breaker so the power line can go back into service. Typically all powerlines have this mechanism in place.

DAR is a mechanism to protect the system – it is a way of ensuring assets return to service as fast as possible without risk of damage.

What are Constraint Payments?

It’s our job to keep the cost of running the system as low as possible, so we can keep consumers’ energy bills down. One of the ways we do this is with constraint payments.

When there are physical constraints on the network (ie the network cannot physically transfer the power from one region to another), we ask generators to reduce their output in order to maintain stability and manage the flows on the network. Generators are then compensated via a constraint payment. The alternative is building more infrastructure at a significant cost, meaning higher bills for consumers.

If we use the analogy of motorways, it is like paying traffic to queue for a few times a year, instead of building more motorways which will rarely be used.

We continuously weigh up the costs of constraint payments versus building more infrastructure. On an annual basis we reassess whether it is better to continue making constraint payments or to trigger the process which leads to an investment. We ensure that whenever we make constraint payments it is the most economic option.

What is Low Frequency Demand Disconnection (LFDD)?

Low Frequency Demand Disconnection is a process designed to limit the fall in frequency of the electricity network during unusual events to ensure critical demand can continue to be met. The process is managed by the distribution network operators (DNOs) and governed by Ofgem.

The DNOs’ LFDD schemes make an assessment of what is critical, and in the event of an excursion will automatically disconnect demand to non-critical infrastructure to ensure the protection of the whole system.

What is inertia?

Inertia is a concept of physics – things that are moving will keep moving unless a force, like friction, causes them to stop. And things that are not moving need a force – like a gust of wind – to get them moving.

Inertia works to keep the electricity system running at the right frequency by using the kinetic energy in spinning parts in power plant generator turbines. When needed, the spinning parts in generator turbines can rotate slightly faster or slower to help balance out supply and demand. The more turbines you have, the more energy there is in the system and the greater the system inertia, which helps to stabilise the frequency.

What is transmission connected generation?

Great Britain’s electricity system is one big connected network. Generators connect onto the network in two ways, either as transmission connected generation or embedded generation.

Transmission connected generation means the generator feeds straight into the transmission network. National Grid ESO has sight of all this generation and around 70% of Great Britain’s electricity connects in this way. An example of this would be a large coal fired power station.

What is embedded generation?

Great Britain’s electricity system is one big connected network. Generators connect onto the network in two ways, either as transmission connected generation or embedded generation.

Embedded generation (also called distributed generation) refers to electricity generation or storage plants connected to a distribution network rather than the transmission network.

It can be helpful to think of Britain’s electricity network in the same way as our roads; the transmission network being the motorways and the distribution network being smaller roads into towns and cities. These plants are ‘embedded’ into the distribution network, generating and feeding electricity into it at a local level. The ESO does not have real time visibility of most of this embedded generation, which accounts for 29% of generation capacity on the electricity network.

There are several different types of embedded generation, including combined heat and power (CHP) plants, onshore wind, solar farms, and storage devices such as lithium ion batteries. Small plants that come on to meet peak demand, for example diesel generators and gas reciprocating engines, are also one type of embedded generation.

What is Rate of Change Frequency (RoCoF) and Vector Shift?

Distributed generation is not capable of operating safely and supplying consumers unless it is connected to the transmission system through the distribution network. So, they have additional protection devices to detect the absence of this indirect connection and disconnect the site if necessary. For example, this protection would operate to ensure that a small embedded windfarm does not continue to power homes if both have been isolated from the system but are still connected together. These protection devices are:

  1. Rate of Change of Frequency (RoCoF) – which uses the rate at which frequency changes to detect the absence of the connection to the system and disconnects the site if that rate of change of frequency exceeds a specific value
  2. Vector Shift – which uses the change in voltage angles to detect the absence of the connection to the system and disconnects the site if that change in angle exceeds a specific value.