- Control of the National Grid
The National Grid is the high-voltage electric power transmission network in Great Britain, connecting power stations and major substations, and has a synchronized organization such that electricity generated anywhere in Great Britain can be used to satisfy demand elsewhere. There are also undersea connections to northern France (HVDC Cross-Channel), Northern Ireland (HVDC Moyle), the Isle of Man (Isle of Man to England Interconnector), and the Netherlands (BritNed). The National Grid is controlled from the National Grid Control Centre which is located in St Catherine's Lodge, Sindlesham, Wokingham in Berkshire in South East England.
The national grid of the United Kingdom is required to distribute electric power generated in bulk to various grid supply points located across the UK and, in so doing, has to be able to match the supply of generated power to the demand for power which is continuously varying, sometimes gradually and predictably, and sometimes quite sharply. It has to do this and to maintain stability within specified standards of frequency and voltage dealing with both sudden changes in load and sudden changes in the available power output, the latter again being generally quite predictable but suffering random sudden changes as power stations fail from time to time.
Although all such large power grids have their own special historical and geographical peculiarities, they all tend to use methods of control and stabilisation similar to those used by the UK National Grid, to a greater or lesser extent.
Power generation and transmission statistics
- Total generating capacity is about 70 GW, supplied roughly equally by nuclear, coal fired and gas fired power stations.
- In the UK, the peak winter demand is 57 GW. N.B: 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.
- Annual energy used in the UK is around 360 TWh (1.3 EJ). N.B: The average load factor is then 3.6×1011/(8,760 × 57×106) = 72%
- There is generally about 1.5 GW of so called spinning reserve—this is typically a large power station paid to produce at less than full output.
- NG 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 a large number of 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 Response participants mentioned below: steel works, cold stores, etc.
- Approx. 750 MW of Standing Reserve diesel generators (as offered by many other uses of small diesels such as factories, water companies, distribution centres and so on), small gas turbines (e.g., 22 MW operated by First Energy) which as we shall see 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 (incidentally, at the time Dinorwig was built, which was solely 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, so presumably it could still be built if the need arose).
- A cross channel HVDC power line can bring in up to 2 GW of power from France, though this tends to be unreliable.
Matching of power station output to load
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, it 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. it then schedules the required power stations, plus the appropriate reserve. As time progresses, the forecast is constantly updated until real time is approached, when the balancing is largely automatic using spinning reserve.
Dynamics of the UK National Grid
The system frequency target is normally set at 50.00 Hz. Frequency changes as the balance between demand and generation alters. The Grid controller attempts to balance the two by instructing the generators, but there is a range of frequencies between which no instructions will be issued, say 49.95 to 50.05 Hz.
All generators and synchronous machines that are connected to the Grid are locked in to the system frequency. If the demand increases, the frequency will start to fall. There is only a relatively small amount of kinetic energy in the rotating parts of the generators so the governor system of the turbines driving the generators respond by very rapidly increasing the energy input to restore the balance. The characteristic of the governor can be varied to allow a small deviation from the target frequency rather than attempting to return to exactly 50.00 Hz. This results in a more stable system.
The fuel input of a gas turbine can be adjusted very rapidly. With a steam turbine, the energy within the boiler provides a relatively large reserve, and the fuel input to the boiler (or nuclear reactor heat output) can be increased after a minute or two to restore the equilibrium. The combination of some rotational kinetic energy and some potential energy stored in the boiler pressure parts permits a beneficial flywheel effect overall. National Grid Co. needs to quantify the amount of kinetic energy in the system in order to make stability calculations reflecting different scenarios.
The power required to drive a machine is proportional to its speed (depending on the machine characteristics). When the system frequency drops the power of most rotational machinery being driven will drop, which also helps to stabilise the situation. The opposite effects take place when demand falls.
In an AC system it is not desirable to allow voltages to vary more than a small amount with demand. Voltage variations are not a normal power control mechanism (they are a reactive power control mechanism) The generator and Grid system voltages are maintained within quite close limits, otherwise the system can become unstable and parts of it fall out of synchronism, with adverse consequences.
Voltages can be reduced deliberately at the final distribution stages of the system as the first stages of deliberate load reduction, and the system can also be deliberately run at a low frequency (down to 49.00 say) to reduce demand before having to cut customers off. We have not seen such tactics deployed very much in recent decades, but it was a regular feature before the 1970s and we may well see it again in a decade or two.
The financial consequences for energy users will depend entirely on their individual circumstances. They are only paying for energy used but they may suffer from reduced product outputs or quality control problems. Only individuals can answer that.
The fundamentals of AC grid systems everywhere are the same so all will have very similar characteristics to ours provided that there is not a shortage of generating capacity to meet demand.
Short term and instantaneous load and generation response mechanisms
The national grid is organized, and power stations distributed, in such a way as to cope with sudden, unforeseen and dramatic changes in either load or generation. It is designed to cope with the simultaneous or nearly simultaneous failure of 2 × 660 MW sets, which it evidently does with ease as these events happen on average at least once a month.
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 as in Dinorwig Power Station is also used in addition to spinning reserve to keep the system in balance.
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 20 minutes. The same applies to a large cold store where interruption in cooling for 20 minutes can readily be accepted in the same way as the normal domestic freezer can happily be shut off for 12 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 are unaffected, such as lights and power sockets. These Frequency Service participants are contracted to stay off for up to 20 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.
Operating closely with NG Frequency Response is the National Grid Reserve Service now called STOR or Short Term Operating Reserve. NG Standing Reserve participants are small diesel engine owners, and Open Cycle gas turbine generator owners, who are paid to start up and connect to the grid within 20 minutes from the time Frequency Response customers are called to disconnect. These participants must be reliable and able to stay on and run for an hour or so, with a repetition rate of 20 hours.
National Grid has about 500 MW of diesel generators on contract, and 150 MW of gas turbines with about 2,000 MW of disconnect-able load.
Reserve Service partners include utility companies, hospitals, and police headquarters; all of which require small-scale generating facilities as backup for their primary functions.
A typical Reserve Service partner is Wessex Water one of ten water and sewerage companies in England and Wales, covering Somerset, Dorset, Wiltshire and parts of Avon. Wessex Water has about 550 emergency standby diesel engines, totalling 110 MW of capacity whose primary function is to power essential services such as sewage works and water supply works during power failures which happens on average a few hours each year. Of this number, about 33 units totalling 18 MW are also used in a number of non-emergency ways commercially which are called collectively Load Management, and which includes routinely feeding power into the Local Distribution System and ultimately the National Grid. These generators currently have a 4 minute start up and paralleling capability automatically from the control room, and are currently being modified to enable start ups in less than one minute. These units are quite small, 0.24 MW to 1 MW range and are used by the National Grid on a regular call-off basis to supplement its arrangements with power station owners. Essentially this system drastically cuts the amount of expensive Spinning Reserve that would otherwise be needed.
Substantial fees can be earned simply for making these engines available, again backed up by complex contracts with specified levels of reliability, response times, frequency of use, and so on. There are many other such participants including Thames Water and Anglian Water, but these companies have not made detailed information available.
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 30 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.
Use of the Reserve Service and Frequency Service in practice
An example illustrating how the Reserve Service and Frequency Service are used in order to cope with intermittency and variability is given below:
- Consider, if a 660 MW turbine generator set (the standard size of large steam turbine) trips (large power stations usually consist of 2 or 4 sets each of 660 MW). This can happen for all sorts of reasons: a coal crusher might break down, boiler tubes might fail, an alternator might start to overheat, insulation might fail on the alternator. In the event of certain failures, the generating set automatically trips out and the grid suddenly loses 660 MW. On a typical day, this might be 1.3% of the total national grid output. Due to the immediate increased load on the remaining generating sets, grid frequency immediately starts to drop from the standard 50 Hz. (An alternative scenario, leading to the same frequency drop, could be a sudden and unexpectedly large surge in power demand as happens at the end of certain TV programmes, when people all rush to switch on electric kettles etc.)
- As soon as this happens, the under-frequency relays on Frequency Response customers begin to trip off their load as the frequency falls, ultimately to shed total load equal to 660 MW. These relays are set at a range of frequencies between 48.5 and 49.5, so the 660 MW of generation that has been lost is not instantly matched by these relays shedding 660 MW of load simultaneously but progressively as the frequency drops, until enough is shed to exactly match the remaining power station capacity. This will then stabilise the frequency at its now lower level—perhaps 49.3 Hz. This all happens in less than a second. These Frequency Response participants are only contracted to have their loads shed for up to 20 minutes.
- At the same time, the NG Control Room issues start-up signals to sufficient of its Standing Reserve Service participants for the shortfall (in this case, 660 MW) which by contract have to become available within 20 minutes. The NG control room is monitoring the situation and if sufficient Standing Reserve capacity does not come on, then it can order more until it has exactly matched what the Frequency Response relays have shed. (These relays are monitored in real time by NGT's telemetry systems.)
- Due to differing circumstances on the ground, different Standing Reserve participants will have different start-up times and reliabilities which again NG monitors using telemetry. However, when sufficient Standing Reserve has become available, which would be in less than 20 minutes, the Frequency Response loads (steel furnaces, cold stores etc.) are gradually and automatically re-connected by the relays. The original NG Frequency Response relays are then re-armed by NG.
- Up to an hour or so later, the output of the Standing Reserve diesels and gas turbines (which are nearly all in the hands of those not involved in commercial power generation) will have been augmented, and then replaced with new levels of large gas or coal-fired power stations which together will have driven the frequency back to its correct level close to 50 Hz. The diesels can then be stood down, ready for the next emergency.
- The replacement generation sources would have come from increased outputs from other power stations on spinning reserve, resulting in increased output. At the same time new levels of spinning reserve will have been created, which might have been stations on hot standby/warming now switched to running. Increased levels of stations on hot standby will also be called for.
The foregoing is a brief description of how the National Grid organises itself and the dispatch of power stations to cope with sudden, unforeseen and dramatic changes in load or generation.
The point to note is that complicated as this may sound, this has been going on for many years as the loads imposed on the grid, and supply of power from power stations is by itself extremely intermittent already, simply due to the sudden and unpredictable failure of these large 660 MW generating sets, or sometimes entire power stations; and the sudden changes in load which can happen at the end of a major TV programme, or events such as the last eclipse of the sun. These latter can cause surges of several GW which whilst larger in magnitude than the sudden loss of 2 × 660 MW sets, are not instantaneous and so are not as severe a shock to the national grid system.
However reliable a power station is, grid operators have to assume that it will fail, so its replacement must always be running and available.
The value of providing this service is considerable.
There are a number of ways that voltage control is undertaken on the National Grid 400, 275, 132 kV system. This can be done by:
- 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
Sources of intermittency on the UK National Grid
The largest source of intermittency on the UK National Grid is the power stations; in fact, the single largest source is Sizewell B nuclear power station. Whenever Sizewell B is operating the entire 1.3 GW output is liable to stop at any time without warning. Its capacity is 2.16% of the national grid maximum demand, making it the single largest power source and therefore the largest source of intermittency. Despite this issue, NG readily copes with it using the methods outlined above including the use of diesel engines. 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. However, no matter how low the rate of unplanned scrams, this is largely irrelevant - what matters is the fact that it can and does happen, and measures have to be in place to deal with it.
In 2008 both Sizewell and Longannet power stations both stopped unexpectedly within minutes of each other, in fact causing widespread power failures, as substations were tripped off using prearranged under-frequency relays.
Paradoxically, although wind power is inherently intermittent and variable it is in fact much more reliable than conventional plant. Consider a 660 MW plant, which could be replaced by perhaps 900 × 3 MW wind turbines to give the same annual output of energy. On a day when wind strength is enough to give a total output of 600 MW, then these 900 cannot all fail mechanically simultaneously; nor could a drop in wind instantaneously cause all stations to suddenly produce zero output. Whereas 660 MW routinely suffer total and instantaneous shutdowns.
Furthermore, the most reliable form of wind forecasting is to simply look at the total output of the wind turbines themselves – in all probability, what they are producing at one point in time is likely to be produced one hour later, or only a small change from that. If this prediction window is decreased – 20 minutes, 10 minutes 5 minutes, the difference in total national wind power output becomes less and less, and even at 5 minutes, that is ample time to raise or lower spinning reserve accordingly. There is thus ample time to cope with these changes by calling up or standing down more or less plant. If the 5 minute estimates are wrong then the Frequency Service and Reserve Service diesels have the resilience to cope with it.
Perhaps surprisingly, a large number of small diesel generators are currently 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—typically £5/kW each year.
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. These figures are quotable because Wessex Water has gone on the record in a number of conferences and seminars on the subject.
The number of diesel generators in the UK
According to EA Technology nationally there are up to 20 GW of emergency diesels. With the right financial incentives and explanations of the benefits large numbers of these could be brought into the Reserve Service type of scheme. Over 20 years this practice and associated technology will probably become standard.
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.
Transmission cost, distribution costs
The relative costs of both transmission and distribution are reviewed here.
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.
Wessex Water clearly can't be running for triads and third parties when it has taken a capacity payment from NG to keep its generators available. However Wessex Water's contracts enable it to declare its generators unavailable during the anticipated triad periods, so they will call someone else. Wessex Water also declares the status of it generators automatically by the control system in real time to the third party so they also know when they will be available.
- 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
- Three-phase electric power
- Load bank
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- ^ How small emergency generators are used by the National Grid This presentation by also gives statistics for National Grid. Power Convention 2007 10–11 September Imperial College, London 2007
- ^ http://www.claverton-energy.com/download/147/ Claverton Energy group conference
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