Posted: 27 March 2015
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The delicate balance

If electrical supply and demand lose their balance it can have serious consequences for National Grid. As the SMART Frequency Control project (EFCC NIC) gets under way, Charlotte Grant, Technical Project Manager, explains why performing a careful balancing act is so important.

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The delicate balance

The delicate balance

Langage Power Station, a combined-cycle power plant near the city of Plymouth.

"(Frequency) is a measure of the balance between energy generated and consumed - a constantly shifting number that has to be managed and controlled."

Charlotte Grant, Technical Project Manager.


National Grid is duty-bound to keep frequency within plus or minus 1pc of 50Hz.

Source: National Grid.

Imagine an ever-flickering needle on a screen that needs to remain as close to a certain point as possible. If you’re not providing enough power to meet demand, it slides one way. If you’re providing too much, it moves the other. Any significant change could cause dramatic consequences.

That’s what it feels like to monitor system frequency at National Grid.

Charlotte Grant, Technical Project Manager.

Charlotte Grant, Technical Project Manager.

If you’re not sure what frequency is, let me explain. It’s a measure of the balance between energy generated and consumed – a constantly shifting number that has to be managed and controlled. National Grid is duty-bound to keep it within plus or minus 1pc of 50Hz, with Hz (or hertz) being the unit frequency is measured in.

Boiling point

All sorts of things can affect demand – and, in turn frequency. It can be something as seemingly trivial as an episode of EastEnders finishing and triggering a huge power surge as half a million kettles are flicked on.

The reason it’s such an important consideration is because deviations in frequency can affect the whole power system, causing severe faults. In the worst case, they can result in cascading events that lead to partial or total system blackout.

So how do we go about controlling this?

First of all, we monitor the system and try to keep it in balance. Specialist balancing engineers monitor the supply and demand of electricity in real time, minute by minute. They’re servants of the flickering frequency indicator.

We have experienced forecast teams that predict these peaks in power, but incidents still happen, such as when part of a power station trips out and we lose a large amount of supply.

This uncertainty means we have to have lots of balancing services in place – everything from frequency response to reserve services and beyond.

Frequency response

We use balancing services called frequency response to ensure that sufficient generation and demand is always available. It kicks into action automatically – and in a matter of seconds – when response is rapidly required.

Following a loss in supply, such as a power station trip, the pressure is on. Primary response providers must reach full response after 10 seconds and sustain it for 30 seconds, while secondary units reach full response after 30 seconds and sustain it for 30 minutes. Frequency recovers as this power is generated.

Balancing services

National Grid uses the following three balancing services to control system frequency:

  • Mandatory Frequency Response (MFR) – an automatic change in output from generators in response to a frequency change. All large generators that are connected to the transmission network in the UK have to be able to provide MFR to National Grid.
  • Firm Frequency Response (FFR) – suppliers can tender to supply power when frequency events happen. This helps to widen the number of response providers available to us.
  • Frequency Control by Demand Management (FCDM) – another commercial frequency response service where customers are instructed to stop generating power in order to reduce demand. They’re typically prepared to have their demands interrupted for up to 30 minutes, ten to 30 times a year.

Reserve services are also used in the balancing act, where power stations are put on standby to provide rapid response to peaks or drops in frequency. When this happens, our balancing engineers act in seconds, sending out instructions that fire turbines into life and get frequency moving back into the safe zone.

This delicate balance of demand and supply takes incredible planning and expertise. Who’d have thought that the UK’s love of tea could be a factor?

Swift action

Moving forward, we face fresh challenges in controlling system frequency. Currently, thermal power stations, such as gas and coal plants, are called upon when swift action is required to maintain the balance of power.

These large plants also provide a natural aid to achieving frequency stability because they provide inertia. This is the resistance of an object to any change in its motion – like the feeling you get if you stop a bowling ball rolling towards you. Large rotating generators create lots of this and it is available to the system to help stabilise it.

A low-carbon future, however, means more renewables will come into the energy mix. Newer generation technologies, such as solar PV and wind, create less inertia, which causes greater instability in the system.

Our SMART Frequency Control project (EFCC NIC) aims to solve this conundrum by developing a monitoring and control system that will test the ability of these newer technologies to help control frequency.

You can read all about the innovative project here and we’d love to hear your comments on our approach to keeping the system stable in a low-carbon future, so click here if you’d like to get in touch.

  • Roger Lane

    When will the large tidal schemes of Cardiff and Swansea Bay come on stream and how significant will they be in terms of national grid supply?

  • Charlotte Grant

    Hi Roger, thanks for your question. The Swansea Bay project currently has a connection agreement with National Grid with a completion date of October 2018. No formal application has yet been received for Cardiff Lagoon.

    National Grid’s recently published System Operability Framework (SOF) highlighted a number of system challenges arising from current Future Energy Scenarios (FES) and recommends greater plant flexibility in order to meet these challenges. Tidal lagoons could offer the System Operator operational flexibility that would be beneficial to the system. Its ability to quickly switch between pumping (demand) and generation modes will assist with balancing supply and hence managing system frequency. In this respect tidal energy could provide a complementary response service that can be closely coordinated with existing frequency management tools.

    Charlotte Grant, National Grid

  • Phil Harley

    I have an ancient (c 1980) electronic bedside clock. This relies on counting the 50Hz mains cycles for operation. My understanding is that the Grid used to always ensure the correct number of 50Hz cycles over a 24 hour period were maintained for these clocks to retain their accuracy. Q1 is this still the case or has it changed in some way recently?
    Lately this clock has started indicating a supply drop out at various times in the early hours of the morning, although the time still seems to be maintained OK. There are no other indications of a brown out on other equipment in the house. So, Q2, are these observations linked in some way?

    Phil Harley 22-01-16

  • Nick Martin, National Grid

    Hi Phil, thank you for your questions. This is still the case. National Grid will ensure that over a 24 hour cycle, 50Hz is averaged within an acceptable deviation. In addition, your observations are unlikely to be linked. Whilst frequency adjustments to ensure averaging do tend to happen overnight, these typically are increases rather than reductions in frequency at that time and remain within tight operational limits so are unlikely to be noticeable to the end consumer. Interruptions in supply are more likely to be a result of overnight switching at distribution level supporting maintenance and flexible network operation.

  • Sam Bailey

    Maintaining a constant frequency on the grid using FFR seems very expensive. Would it be easier to widen the tolerances? Presumably the main requirements are based on synchronising devices which are now largely obsolete (CRTs, mains clocks), and synchronous motors in industrial processes. However many of these motors (pump drives etc) would tolerate significant frequency variations before having a detrimental effect on their operations. Would it be easier and cheaper to move the critical devices onto variable frequency drives?

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