Control of the National Grid (Great Britain)

Control of the United Kingdom's high-voltage power transmission network (the "National Grid"), is exercised from the National Grid Control Centre. Controlling the grid is required in order to match power station output to load, regulate voltage, regulate frequency and avoid disruptions.

Control centre location

The National Grid control centre is based at St Catherine's Lodge, Sindlesham, Wokingham in Berkshire in south east England, sometimes described as being a 'secret' location. As of 2015 the system is under consistent hacker attack via computer systems.

Power generation and transmission statistics

External image
Current grid status Similar data

• Peak winter demand is 57 GW. This peak would be much higher if it were not suppressed by various mechanisms such as maximum demand tariffs, and the system of triad warnings and charges.
• There is generally about 1.5 GW of spinning reserve, typically when a large power station is paid to produce at less than full output.
• National Grid pays to have up to 8.5 GW of additional capacity available to start immediately but not running, referred to as warming or hot standby.
• At any one time some power stations are unavailable due to regular maintenance, being off-line due to a fault becoming apparent, or because of sudden breakdown. Other stations are mothballed or deep-mothballed which means they cannot be readily called upon; even in an emergency it may take several months to de-mothball. In Summer 2006, Fawley Power Station near Southampton was de-mothballed to cope with anticipated power capacity shortages for winter 2006/07.
• 500–550 MW of instantaneous load reduction
• Effectively capacity can be supplied by the #Frequency Service participants.
• Approximately 750 MW of Standing Reserve diesel generators (small generators used by factories, water companies, distribution centres, etc.) and small gas turbines (e.g., 22 MW operated by First Energy). These are used to augment the Frequency Service arrangements.
• 2 GW fast response plant such as large open cycle gas turbines (OCGTs). OCGTs are gas turbines that are in the range 25–100 MW, and which can start in a few minutes (slower to start than diesel engines and marginally less reliable upon start-up). Normally, these are not used for power generation since their low operating efficiency means that the cost per kWh supplied is prohibitively expensive. However, they have low capital cost, as demonstrated by the 100 MW unit at White City, London.
• The pumped storage schemes at Dinorwig and Ffestiniog can offer up to 2 GW of power within 15 seconds. Dinorwig was built to cope with the inflexibility of nuclear power and the inherent unreliability/indeterminacy of large power stations. A further similar station was planned on Exmoor but was never built.
• Britain is connected to neighbouring countries by submarine power cable. The HVDC Cross-Channel can bring in up to 2 GW of power from France at high reliability, while the 1 GW BritNed from Netherlands and the 500MW East–West Interconnector to Ireland exchange power between these areas.

Load Forecasting

To keep a stable voltage and frequency, the power generated at any instant has to be kept close to the load imposed. To do this, National Grid continuously forecasts load ahead on various timescales: 7 years, annually by season (winter outlook and summer outlook), daily, hourly, and shorter time frames.

For the rolling 24 hours ahead, National Grid estimates what the load is then likely to be based mainly what the load is at the moment of forecast, but adjusted for likely weather changes, day of the week and other special events. The required power stations are scheduled, plus a reserve. As time progresses, the forecast is constantly updated until real time is approached, when the balancing is largely automatic using spinning reserve.

Short term and instantaneous load and generation response mechanisms

Spinning reserve

National Grid pays to keep a number of large power station generators partly loaded. These are connected to a frequency controlled governor, and will automatically regulate the steam fed to the turbines to keep the frequency at around 50 Hz. This is generally satisfactory for most large load changes or the loss of a turbine generator set.

Pumped storage

Main article: Hydroelectricity in the United Kingdom

Pumped storage as in Dinorwig Power Station is also used in addition to spinning reserve to keep the system in balance.

Frequency Service

For large perturbations, which can exceed the capability of spinning reserve, NG (National Grid plc) who operate the national grid and control the operations of power stations (but does not own them), has a number of partners who are known as NG Frequency Service, National Grid Reserve Service or reserve service participants.

These are large power users such as steel works, cold stores, etc. who are happy to enter into a contract to be paid to be automatically disconnected from power supplies whenever grid frequency starts to fall. An example of such a participant would be a large steel melting furnace, which may take a day to heat up using an electric arc or induction heater, and is not adversely affected if the process is delayed by twenty minutes. The same applies to a large cold store where interruption in cooling for twenty minutes can readily be accepted in the same way as the normal domestic freezer can happily be shut off for twelve hours without gaining significant temperature rise. These disconnections can obviously assist enormously if a sudden power demand is made on the grid or if there is the sudden loss of generating capacity.

This instant switch off is achieved using a relay provided by NG mounted on the power supply to the major plant switch gear. It is set to detect the falling frequency which can occur when a large power station fails suddenly or there is a sudden rise in demand, and opens the circuit breaker to the furnace or cold store. Ancillary circuits in the factory remain unaffected, such as lights and power sockets. These Frequency Service participants are contracted to stay off for up to twenty minutes.

NG can remotely monitor and control the exact settings on the relay, such as the exact frequency at which the relay disconnects the load, whether the relay is armed or not, whether the customer has temporarily exercised his option to disable the relay, etc.

These Frequency Service participants receive a fee which is of the order of more than several thousand pounds per MW each year—it is a capacity fee per MW not per MWh.

Diesel generators in the National Grid Reserve Service

UK National Grid uses load reduction and diesel generators in private hands, primarily used for emergency power during power failure, but with a secondary role to assist the National Grid.

Until 2007, small diesel generators (150 kW–2 MW) were all started and feeding power into the grid within four minutes from receiving the start signal from National Grid. This delay was mainly due to the time taken to contact each set. There were four auto-diallers and modems each of which having to make the calls in series. The company has now switched to broadband where the calls are instantaneous and the start up time is now less than thirty seconds to full load, which is far less than any conventional power station.

With specially modified diesel engines (all standard factory made modifications such as air pressure vessels to start the turbochargers, air start rather than battery start, jacket warming, continuous pre-lubrication, continuous slow rotation, etc.), start up and full loading can be achieved within one second.

Voltage control

On the 400, 275 and 132 kV systems, voltage control can be achieved using:

• Over/under excitation of generators
• Switching in/out of shunt reactors
• Switching out of overhead line and underground cable circuits
• Tap staggering of both supergrid and grid transformers
• Under/over excitation of synchronous compensators
• Declutching open cycle gas-turbines into the synchronous compensation mode
• Static variable compensators
• Manually switched capacitor banks
• Synchronous compensation of generators
• OLTC of generator step up transformer

Sources of intermittency on the UK National Grid

An industry-wide rate of unplanned scrams (shutdowns) of 0.6 per 7000 hours critical means that such a shut-down without warning is expected to happen about once every year and a half.

In 2008 Sizewell and Longannet power stations both stopped unexpectedly within minutes of each other.

Diesel generators

A large number of small diesel generators are used within the national grid, in order to assist with sudden shortages of capacity or intermittency. This offers the owners and operators of such plant significant benefits. Since the plants are already paid for other reasons, this capacity comes extremely cheaply.

Connecting diesel generators to the National Grid

All modern diesel generators come with an electronic governor and this enables them to be safely synchronised and paralleled with each other and the mains. To do this the generator set must be operating at exactly the same frequency as the mains, and exactly in phase when the circuit breaker is closed. This is all handled automatically by the electronic speed governor and synchronising system.

To meet the technical and safety standards of the electricity distribution companies, special monitors are required so that the set stays paralleled safely and is automatically disconnected if there is a fault. This G59 equipment (as it is colloquially referred to) is relatively inexpensive nowadays.

Diesel generators participating in Reserve Service

There are many private owners and operators of small diesel generators contributing to Reserve Service: Wessex Water is typical and has a total of about 550 diesel powered generators ranging in size from 50 kW to 1.2 MW, with a total capacity of over 110 MW. 32 sets spread across 24 sites totalling 18 MW have already been converted to Load Management, and some of the others are now being converted as well. These are at clean water sewage treatment works.

Many other water companies, hospitals, factories have similar arrangements.

Revenue earning opportunities from third parties

Diesel generator owners also may have contracts with national generating and energy supply companies who pay to operate the diesels remotely from time to time. This is for their own balancing purposes and when they are short of capacity. Notably there was extensive third party running during the gas shortages of winter 2005-2006 when most of the CCGTs that could also switched from gas to run on liquid fuels.

Triads

Triads are both a revenue-earning opportunity and a way of assessing National Grid high voltage transmission costs.

Triad demand is measured as the average demand on the system over three half hours between November and February (inclusive) in a financial year. These three half hours comprise the half hour of system demand peak and the two other half hours of highest system demand which are separated from system demand peak and each other by at least ten days. These half hours of peak demand are usually referred to as triads.

Triad avoidance is a further revenue-earning opportunity for diesel generator set owners separate from Reserve Service, where the diesels can earn substantial amounts by reducing a site's peak demand at National Grid peak periods.

This is because the way the National Grid is paid for is by means of a capacity, not energy, charge (i.e., a charge on kW not kWh). This is levied on the distributors (DNO): Centrica, BGB, Powergen, etc., who then pass it on to their customers in a more or less transparent way.

The charge is calculated in retrospect, by NG looking back over each of the 17,520 half hours and locating the half hours, separated by at least 10 days, of total NG system maximum demand – which at peak might approach 60 GW. Having identified these triad period half hours, it then charges each of the energy supply companies according to their average peak loads in GW on the national grid system during those three half hour periods.

For example, at the extremity of the system, the Western Power Distribution area in the South West, the total annual transmission cost is about £21,000 per MW per year, again charged to supply companies at the average of their loads at the three triad half hours.

So, an operator that can cut its load by 1 MW or start a 1 MW diesel during those periods can save £21,000 for a fuel cost of only £150.

However, it is not easy to predict exactly when the triads are going to occur, so to ensure triad capture, Wessex Water starts its generators about 30 times per year for about 1 hour, expending about £3,000 on fuel. Since the triads always occur at times of high power prices, further savings are obtained from avoiding the purchase of power during the same time, which might be about £3,000 which more or less offsets the cost of diesel.

Wessex Water pays for a triad forecasting service which is typically received at 11:00 for a triad expected at around 17:30 later that day.

Triads usually occur at either 17:00, 17:30 or 18:00 on winter weekdays except Friday, between 1 November and 31 March.

See also

• Brittle Power
• Demand response
• Relative cost of electricity generated by different sources
• Economics of new nuclear power plants (for cost comparisons)
• Energy security and renewable technology
• High-voltage direct current
• Intermittent energy source
• Low-cost solar cell
• Northeast Blackout of 2003
• List of power outages
• Potential energy
• Calculating the cost of the UK Transmission network: cost per kWh of transmission
• Calculating the cost of back up: See spark spread
• Load management
• National Grid Reserve Service
• Energy use and conservation in the United Kingdom
• Diesel-electric transmission
• Motor-generator
• Three-phase electric power
• Load bank