Tag: electricity system balancing and ancillary services

The great balancing act: what it takes to keep the power grid stable

What does it mean to say Great Britain’s electricity network needs to be balanced? It doesn’t refer to the structural stability of pylons. Rather, balancing the power system is about ensuring electricity supply meets demand second by second.

From the side of a consumer, the power system serves one purpose: to deliver electricity to homes and businesses so that it powers our lives. But from a generator and a system operator perspective, there is much more at play.

Electricity must be transported the length of the country, levels of generation must be managed so they are exactly equal to levels being used, and properties like voltage and frequency must be minutely regulated across the whole network to ensure power generated at scale in industrial power stations can be used by domestic appliances plugged into wall sockets.

Ensuring all this happens smoothly relies on the system operator – National Grid – working with power generators to provide ‘ancillary services’ – a set of processes that keep the power system in operation, stable and balanced.

Here we look at some of the most important ancillary services at play in Great Britain.

Frequency response

One of the foundations of Great Britain’s power system stability is frequency. The entire power network operates at a frequency of 50 Hz, which is determined by the number of directional changes alternating current (AC) electricity makes every second. However, just a 1% deviation from this begins to damage equipment and infrastructure, so it is imperative it remains consistent.

This is done by National Grid instructing flexible generators (such as thermal, steam-powered turbines like those at Drax Power Station or our planned battery facility) to either increase or decrease generation so electricity supply is matched exactly to demand. If this is unbalanced it affects the network’s frequency and lead to instability and equipment damage. Generators are set up to respond automatically to these request, correcting frequency deviations in seconds.

UK power gridReactive power and voltage management

The electricity that turns on light bulbs and charges phones is what’s known as ‘active power’. However, getting that active power around the transmission system efficiently, economically and safely requires something called ‘reactive power’.

Reactive power is generated the same way as active power and assists with “pushing” the real power around the system but unlike active power it’s does not travel very far. The influence of Reactive power is local and the balance in any particular area is very important to maintain power flows and a stable system.

This means National Grid must work with generators to either generate more reactive power when there is not enough, or absorb it when there is an excess, which can happen when lines are ‘lightly loaded’ (meaning they have a low level of power running through them).

Drax’s ability to absorb reactive power is also vital in controlling the grid’s voltage. Great Britain’s system runs at a voltage of 400 kilovolts (kV) and 275 kV (Scotland also uses 132kV), before it is stepped down by transformers to 230 volts for homes or 11 kV for heavy industrial users. Voltage must stay within 5% of 400 kV before it begins to damage equipment.

By producing reactive power a generator increases the voltage on a system, but by switching to absorbing reactive power it can help lower the voltage, keeping the grid’s electricity safe and efficient.  

System inertia

As well as being able to automatically adjust to keep the country on the right frequency, Drax’s massive turbines, spinning at 3,000 rpm, also have the advantage of adding inertia to the grid.

Inertia is an object’s natural tendency to keep doing what it is currently doing.

This system inertia of the spinning plant is effectively ‘stored’ energy. This can be used to act as a damper on the whole system to slow down and smooth out sudden changes in system frequency across the network – much like a car’s suspension it helps maintain stability.

Reserve power 

Humans are creatures of habit. This means the whole country tends to load dishwashers, turn on TVs and boil kettles at roughly the same time each day, making the rise and fall in electricity demand easy enough for National Grid to predict.

However, if something unexpected happens – a sudden cold snap or a power station breaking down – the grid must be ready. For this, National Grid keeps reserve power on the system to jump into action and fill any sudden gaps in demand and fluctuations in voltage and frequency it could cause.

Ancillary services in an evolving system

As with how electricity is generated across the country, balancing services are undergoing major change. As more intermittent renewables, such as wind and solar, come onto the system to provide low carbon power necessary for Great Britain to decarbonise, that same system becomes more volatile and more difficult to balance.

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More than that, the ancillary services needed to stabilise a more volatile grid can’t be generated by every generation source. Many depend on a turbine rotating at 3,000 rpm, generating electricity at a steady frequency of 50Hz, as is found in thermal generators such as Drax. Intermittent, also known as variable sources of power, are weather dependent. They are often unable to provide the same services as biomass and gas power stations.

While attempts to supply some of these ancillary services by co-locating wind or solar facilities with giant batteries are underway, thermal power stations that can quickly and reliably  balance the system at scale still play an essential role in making the transmission network safe, efficient, economic and stable.

This story is part of a series on the lesser-known electricity markets within the areas of balancing services, system support services and ancillary services. Read more about black startsystem inertiafrequency responsereactive power and reserve power.  Find out what lies ahead by reading Balancing for the renewable future and Maintaining electricity grid stability during rapid decarbonisation.

Keeping the electricity system’s voltage stable

Electricity high voltage sign

In day-to-day life, the electricity system normally plays a consistent, unfluctuating role, powering the same things, in the same way. However, behind the scenes electricity generation is a constant balancing act to keep the grid stable.

Power stations themselves are like living animals, in need of continuous adjustment. Transmission networks need continual maintenance and keeping the whole grid at a frequency of 50 Hz takes careful monitoring and fine-tuning.

One of the other constant challenges for Great Britain’s electricity system is keeping voltage under control.

Keeping the volts in check

Voltage is a way of expressing the potential difference in charge between two points in an electrical field. In more simplistic terms, it acts as the pressure that pushes charged electrons (known as the current) through an electric circuit. 

Great Britain’s National Grid system runs at a voltage of 400 kilovolts (kV) and 275kV (Scotland also uses 132kV). It is then reduced in a series of steps by transformers to levels suitable for supply to customers, for example 11kV for heavy industrial or 230 volts (V) when delivered to homes by regional distribution networks.

UK electricity voltage system

Keeping the voltage steady requires careful management. A deviation as small as 5% above or below can lead to increased wear and tear of equipment – and additional maintenance costs. Or even large-scale blackouts. Power stations such as Drax can control the voltage level through what’s known as reactive power.

“If voltage is high, absorbing reactive power back into the generator reduces it,” says Drax Lead Engineer Gary Preece. “By contrast, generating reactive power increases the voltage.”

Reactive power is made in an electricity generator alongside ‘active power’ (the electricity that powers our lights and devices) and National Grid can request generators such as Drax to either absorb or produce more of it as it’s needed to control voltage.

So how is a generator spinning at 3,000 rpm switched from producing to absorbing reactive power? All it takes is the turn of a tap.

Absorbing reactive power

Taps along a transformer allow a certain portion of the winding – which make up a transformer’s active part with the core – to be selected or unselected. This allows the transformer to alter what’s known as the ‘phase angle’, which refers to the relationship between apparent power (made up of reactive power and active power) and active power. This change in the phase angle regulates the ‘power factor’.

Power factor is measured between 0 and 1. Between 1 and 0 lagging means a generator is producing reactive power and increasing overall voltage, whereas between 1 and 0 leading means it is absorbing reactive power and reducing voltage.

That absorbed reactive power doesn’t just disappear, rather it transfers to heat at the back end of the power station’s generator. “Temperatures can be in excess of 60 degree Celsius,” says Preece. “There’s also a lot of vibration caused by the changes in flux at the end of the generator, this can cause long term damage to the winding.”

As the generators continue to produce active power while absorbing reactive power the conditions begin to reduce efficiency and, if prolonged, begin to damage the machines. Drax’s advantage here is that it operates six turbines, all of which are capable of switching between delivering or absorbing reactive power, or vice versa, in under two minutes.

UK electricity grid

Voltage management in a changing grid

The changing nature of Great Britain’s energy supply means voltage management is trickier than ever. Voltage creeps up when power lines are lightly loaded. The increase of decentralised generation – such as solar panels and small-scale onshore wind farms operating to directly supply specific localities or a number of customers embedded on regional electricity networks –  means this is becoming more common around the grid. This creates a greater demand for the kind of reactive power absorption and voltage management that Drax Power Station carries out.

Grid-scale batteries are being increasingly developed as a means of storing power from weather dependent renewable sources. This power can then be pumped onto the grid when demand is high. In a similar manner, these storage systems can also absorb reactive power when there’s too much on the system and discharge it when it’s needed – bringing the voltage down and up respectively.

Electricity storage

“The trouble is, grid-scale battery storage systems need to be absolutely huge, and a 100 MW facility would be close to the size of football field and double stacked,” explains Preece. “They are also not synchronised to the grid as a thermal turbine generator would be.” Subsequently there is no contribution to inertia.

As Great Britain’s power system continues to evolve, maintaining its stability also needs to adapt. Where once the challenge lay in keeping voltage high and enough reactive power on the grid, today it’s absorbing reactive power and keeping voltage down. It highlights the need for thermal generators that are designed to quickly switch between generating and absorbing to support the wider network.

This story is part of a series on the lesser-known electricity markets within the areas of balancing services, system support services and ancillary services. Read more about reserve powersystem inertiablack start, reactive power and frequency response. View a summary at The great balancing act: what it takes to keep the power grid stable and find out what lies ahead by reading Balancing for the renewable future and Maintaining electricity grid stability during rapid decarbonisation.

The power system’s super subs

Every day we flick on lights, load dishwashers and boil kettles but few of us pause and think of the stress this can cause for the electricity system. We certainly don’t call National Grid in advance to let them know when we plan to do laundry.

But when any number of the 25.8 million households in Great Britain turn on a washing machine, the grid needs to be ready for it. Fortunately, these spikes in demand are often predictable.

“We are creatures of habit,” says Ian Foy, Drax Head of Ancillary Services.

“We all tend to come home from work at the same time and turn on similar appliances, and this keeps the shape of daily electricity demand much the same.”

National Grid, the operator of Great Britain’s high voltage power transmission system, uses this consistent demand to plan when and how much electricity will be needed for the coming days, and agrees contracts with generators to meet it.

However, there are times when the unexpected can happen – an unseasonable cold spell or a power station breakdown – causing sudden imbalances in supply and demand. To plan for this, the grid carries ‘reserve power’, which is used to fill these short-term gaps.

Delivering this doesn’t just mean keeping additional power stations running to deliver last minute electricity. Instead, it involves a range of services coming from different types of providers, technologies and timescales.

In Great Britain, there are four primary ways this is delivered.

  1. Frequency response

The fastest form of reserve is frequency response. It is an automatic change in either electricity generation or demand in response to the system frequency deviating from the target 50Hz.

Generation and demand must be kept balanced within tight limits, second by second. Failure to do so could lead to the whole system becoming unstable, leading to the risk of blackouts. This is why, every second of the day, National Grid has power generators operating in Frequency Response mode. These power stations effectively connect their steam governor or fuel valves to a frequency signal. If frequency falls, generation increases. If frequency rises, generation is reduced.

An issue which has arisen over the past few years is a reduction in system inertia. The inertial forces in a spinning generator help slow the rate of frequency change, acting like dampers on car suspension. Some small power generation technologies, along with some demand, are sensitive to the rate of change of frequency – too high a rate can cause it to disconnect from the system and this unplanned activity can lead to system instability.

Some forms of generation such as wind and solar along with high voltage, direct current (HVDC) interconnectors between Great Britain and the rest of Europe do not provide inertia. As these electricity sources grow on the system, the system operator must find ways to accommodate them. Reducing the size of the largest credible loss, e.g. reducing interconnector load, buying faster frequency response or running conventional generation out of merit are the usual approaches.

  1. Spinning reserve

The next quickest and most common reserve source, spinning reserve, can jump into action and start delivering electricity in just two minutes. This reserve, both positive and negative, is carried on multiple generating units often running at a part load position (for example, if Drax’s biomass Unit 3 is running at 350 MW of a possible 645 MW). Typical delivery rates are 10 to 20 MW per minute.

This type of reserve responds to unexpected short-term changes in power demand or changes in power generation either in size or timing. For example, during a major TV event or if a generator changes output unexpectedly.

During television ad breaks, the millions of people watching may switch on kettles or flick on lights, sending demand soaring. If a programme or sporting event does not end at the scheduled time, National Grid has to quickly change the generation schedule by sending an electronic dispatch to a generator who responds by quickly ramping up or down.

Thermal power stations such as Drax along with hydro and pumped storage, which can increase or decrease output on demand, are the most common providers of this form of reserve, but  newer technologies such as batteries will also be able to offer the speed required.

  1. Short term operating reserve (STOR)

A slightly slower cousin to spinning reserve, STOR is contracted months in advance by National Grid. It is designed to be available within 20 minutes’ notice.

Around 2 gigawatts (GW) of capacity can provide reserves against exceptionally large losses or demand spikes. STOR capacity tends to be plant which cannot make a living in the energy market because it has a high marginal cost. Traditionally STOR has been supplied by low efficiency aeroderivative turbines or diesel engines but a wider variety of technologies from batteries to demand reduction are increasingly being contracted.

  1. Demand turn up

Managing reserve power isn’t just about finding extra generation to meet demand. Sometimes it’s about quickly addressing too much generation.

High levels of generation coming onto the grid without an equally high demand can overload power lines and networks, causing instability and frequency imbalances, which can lead to blackouts. This might happen in the summer months, when the weather allows for lots of intermittent renewable generation (such as wind and solar), but low levels of demand due to factors like warmer weather and longer daylight hours.

To restore balance, the grid is beginning to ask intensive commercial or industrial electricity users to increase consumption or turn off any of their own generation in favour of grid-provided power. In the case of generating units at Drax Power Station, the response starts less than a second from the initial frequency deviation to help slow the rate of frequency change and minimises large frequency swings.

Reserve in a changing energy system

Planning for the unexpected has long been a staple of the electricity system, but as the shape of that system changes, reserve and response delivery is having to change too.

“Carrying reserve is easier on conventional plants because you know what the plant is capable of generating,” says Foy.

“Wind and solar are subject to the weather, which changes over time. The certainty you get with conventional generation you don’t necessarily get with the intermittency of weather-dependant renewables.”

Add to that the way we use electricity, everything from high efficiency lighting, electric vehicles through to smart appliances and we can see the challenges will grow.

As more variable renewables hits the system, storage technologies will become more important in providing a fast-acting source of short term reserve and response. Smart appliance technology too will play a bigger role in spreading demand across the day and reducing the size of demand peaks and troughs which require rapid changes in generation.

Industry has a role to play too. Some of the biggest users of electricity are expected to play an increasingly important role in support of the system operator. National Grid told members of Parliament in 2016, that ‘it is our ambition to have 30-50% of our balancing capacity supplied by demand side measures by 2020.’

Artist’s impression of a Drax rapid-response gas power station (OCGT) with planning permission

Until these technologies and market mechanisms are widespread and implemented at scale across the grid, however, it will fall to thermal power stations such as biomass, gas turbines such as the planned Progress Power in Suffolk and, on occasion, coal to ensure there is the required reserve power available.

This short story is adapted from a series on the lesser-known electricity markets within the areas of balancing services, system support services and ancillary services. Read more about black start, system inertia, frequency response, and reactive power. View a summary at The great balancing act: what it takes to keep the power grid stable and find out what lies ahead by reading Balancing for the renewable future and Maintaining electricity grid stability during rapid decarbonisation.

How electricity is made

Every morning we take electricity as a given. We switch on lights, charge phones and boil kettles without thinking about where this power comes from.

The electronic devices and appliances that make up our daily routines are not particularly energy intensive. Boiling a kettle only uses 93 watts, toasting for three minutes only requires 60 watts, while cooking in a microwave for five minutes takes 100 watts.

However, when people are waking up and making breakfast in almost 30 million households around the UK, those small amounts soon create a significant demand for electricity. On a typical winter’s morning, this combined demand spikes to more than 45 gigawatts (GW).

So this is what it takes to power your breakfast – from the everyday toaster in your kitchen backwards through thousands of miles of cables to the hundreds of thousands of tonnes of machinery in wind farms, hydro-electric dams and at power stations such as Drax where electricity generation begins.

The grid 

The journey starts in the home where all our electricity usage is tracked by meters. These are becoming increasingly ‘smart’, displaying near real-time information on energy consumption in financial terms and allowing more accurate billing. There are already 7.7 million smart meters installed around the UK, but that number is set to triple this year, paving the way for a smarter grid overall.

What brings electricity into homes is perhaps the most visible part of the energy system on the UK’s landscape. The transmission system is made up of almost 4,500 miles of overhead electricity lines, nearly 90,000 pylons and 342 substations, all bringing electricity from power stations into our homes.

Making sure all this happens safely and as efficiently as possible falls to the UK’s nine regional electricity networks and National Grid. Regional networks ensure all the equipment is in place and properly maintained to bring electricity safely across the country, while National Grid is tasked with making sure demand for electricity is met and that the entire grid remains balanced.

The station cools down

One of the most distinctive symbols of power generation, cooling towers carry out an important task on a massive scale.

Water plays a crucial role in electricity generation, but before it can be safely returned to the environment it must be cooled. Water enters cooling towers at around 40 degrees Celsius, and is cooled by air naturally pulled into the structure by its unique shape.

This means those plumes exiting from the top of the towers are, rather than any form of pollution, only water vapour. And this accounts for just 2% of the water pumped into the towers.

Drax counts 12 cooling towers, each 114 metres tall – enough to hold the Statue of Liberty with room to spare. Once the water is cooled it is safe to re-enter the nearby River Ouse.

The station’s bird’s-eye view

The control room is the nerve centre of Drax Power Station. From here technicians have a view into every stage of the power generation process.  The entire system controls roughly 100,000 signals from across the power station’s six generating units, water cooling, air compressors and more.

While once this area was made up of analogue dials and controls, it has recently been updated and modernised to include digital interfaces, display screens and workstations specially designed by Drax to enable operators to monitor and adjust activity around the plant.

The heart of power generation 

At the epicentre of electricity generation is Drax’s six turbines. These heavy-duty pieces of equipment do the major work involved in generating electricity.

High-pressure steam drive the blades which rotates the turbine at 3,000 revolutions per minute (rpm). This in turn spins the generator where energy is converted into the electricity that will eventually make it into our homes.

These are rugged pieces of kit operating in extreme conditions of 165 bar of pressure and temperatures of 565 degrees Celsius. Each of the six turbine shaft lines weighs 300 tonnes and is capable of exporting over 600 megawatts (MW) into the grid.

One of the most important parts of the entire process, turbines are carefully maintained to ensure maximum efficiency. Even a slight percentage increase in performance can translate into millions of pounds in savings.

Turning fuel to fire

To create the steam needed to spin turbines at 3,000 rpm, Drax needs to heat up vast amounts of water quickly and this takes a lot of heat.

The power station’s furnaces swirl with clouds of the burning fuel to heat the boiler. Biomass is injected into the furnace in the form of a finely ground powder. This gives the solid fuel the properties of a gas, enabling it to ignite quickly. Additional air is pumped into the boiler to drive further combustion and optimise the fuel’s performance.


How do you turn hundreds of tonnes of biomass pellets into a powder every day? That’s the task the pulveriser take on. In each of the power plant’s 60 mills, 10 steel and nickel balls, each weighing 1.2 tonnes, operate in extreme conditions to crush, crunch and pulverise fuel.

These metal balls rotate 37 times a minute at roughly 3 mph, exerting 80 tonnes of pressure, crushing all fuel in their path. Air is then blasted in at 190 degrees Celsius to dry the crushed fuel and blow it into the boiler at a rate of 40 tonnes per hour.

The journey begins: biomass arrives

Biomass arrives at Drax by the train-load. Roughly 14 arrive every day at the power station, delivering up to 20,000 tonnes ready to be used as fuel.

These trains arrive from ports in Liverpool, Tyne, Immingham and Hull and are specially designed to maximise the efficiency of the entire delivery process, allowing a full train to unload in 40 minutes without stopping.

The biomass is then taken to be stored inside Drax’s four huge storage domes. Each capable of fitting the Albert Hall inside, the domes can hold 300,000 tonnes of compressed wood pellets between them.

Here the biomass waits until it’s needed, at which point it makes its way along a conveyor belt to the pulveriser and the process of generating the electricity that powers your breakfast begins.