The notes below relate to the supply and electrical power transmission system in the UK.     The notes are outline in nature and are intended to provide a general background sufficient for a mechanical engineer to have an awareness of the electrical power supply system as used in the UK

The high-voltage electric power transmission is called the National Grid and transfers the power which is generated in the power stations: [coal , gas, nuclear, hydroelectric, etc ] to local substations which supply power at lower voltages to the various users :[commercial, industrial, domestic ] in the United Kingdom. There are also High- Voltage-Direct-current undersea supply lines form/to northern France (HVDC Cross-Channel)and Northern Ireland (HVDC Moyle).

The power transferred in an electrical transmission line directly related to the (voltage² / resistance ) = E²/R     The power loss in a conductor is the Current² x Resistance = I².R.    Therefore for maximum efficiency of power transmission the electrical power should be transferred at the highest voltage and the lowest current.

In the United Kingdom a range of specific voltages have been adopted to cover adequately various levels of power transmission.

Power stations generators produce electricity at 25kV and 33KV.    The generator output voltages are stepped up, using transformers, for efficient transmission through the National Grid cables, to voltages of 400kV, 275kV and 132kV.

The parts of the system carrying electrical supplies at voltages in excess of 200kV are called the Super grid.

There is also a High-Voltage-Direct-current undersea supply lines form/to northern France (HVDC Cross-Channel).    The undersea section consists of eight 29 mile long 270 kV submarine cables, between Folkstone (UK) and Sangatte (France).    The UK power transmission systems also includes other lower duty HVDC links.

The local transmission voltages to industrial commercial and domestic users include 11 kV, 33 kV, 66 kV.

At the present time the available UK power generation capacity is about 78GW not including a large number of minor generation facilities e.g. small hydroelectric generators, solar panels , factory based diesal generators etc. (Tables showing the capacity are provided at the foot of this webpage).     This capacity is used to meet the ongoing national demand.    This demand, normally, varies on a continuous basis over a 24 hour cycle between a maximum of about 42 GW and a minimum of about 22 GW. (See chart below).

However there significant variations on this pattern with peaks of up the 54GW at times when a large percentage of the population carry out a loading event at the same time and in times of adverse weather.    The generation capacity is also not constant e.g. power stations shut down due to plant failure or maintenance : windpower farms lose their generation capacity when there is no wind.    There are also significant power losses due in the transmission of the power from the generators to the suppliers.     The control of the supply network is therefore critical and it is often necessary to import power from France via the HVDC link

The selection of the electic supplies to the national grid is organised on a basis of availability and cost.    The cost of power generation is extremely difficult to estimate as there are a large number of variables including , fuel cost, capital cost, operating cost, decommssioning costs, costs of generation back-up etc etc. An estimate of the current prices are required to allow the grid controllers to select which suppliers provide the current loading.     It is also necessary to have an estimate of the lifetime costs of future generation to allow investment in new power stations.   Below are tables showing estimated costs for existing facilities and for new facilities.

Important Note: these are indicative and are only useful for comparing costs in the same table.    the two tables are based on information from different sources produced at different dates. The souces also use very different accountancy methods.    Please refer to the linked reports for quality information.   

Indicative Base line operation with no CO₂ removal costs included (2004 values)

Important Note: The report from which these costs has been extracted was issued in 2004.    The fuel costs have risen dramatically since that time i.e coal costs have increased by about 300% and gas costs have increased by at least 200%.    The table should therefore only be used as a guide to indicative relative costs , as noted above.

Note: If gas OCGT is operated on a peaking cycle the costs rise to 6,2 pence/kVh

The Cost of Generating Electricity Royal Academy of engineering

PF=pulverised coal.
CFBC=Circulating fluidised bed combustion
CCS=Carbon capture and storage;
FGD=flue gas desulphurisation;
IGCC=integrated gasification combined cycle;
OCGT=open cycle gas turbine;
CCGT=combined cycle gas turbine;

Indicative - Generation costs for New plant (2010).

UK Electricity Generation Costs Update Mott Macdonald Report

There is little information available for Hydro-Electric generation costs.    I believe that the costs/KWh is comparable to conventional to coal/gas. However there is very little scope for developing new facilities in the UK and the existing larger faciliities are primarily pumped storage systems which are used to allow, low cost, continuous base load operations on other power generation types.

Most of the electrical power supplied to the National Grid is from turbine driven generators located at Coal, Nuclear , Gas, or Oil powered stations.

There is also considerable renewable energy capacity in place in the UK including bio-mass power generation, hydro-electric generation and a considerable number of wind power farms.

The larger power stations have a number of turbines generating voltages about 33kV .     Each generator supplies to the grid via switches and step up transformers.    The switches would typically consist of three oil filled cylindrical tanks, one for each phase, each with two insulators on the top to carry the input and output lines.    At one end would be a box containing the gear to operate the mechanical switches inside the oil-filled tanks.    The Transformers are generally oil cooled and step up the voltage to 275kV or 400kV.    The oil cooling system may include a recirculating system with the oil transferred through large radiator units.
The generators output could also be used at lower voltages to power electrical equipment at the power stations.

Different types of power stations have different performance criteria.    Nuclear power stations need to be operated continuously providing a base load.    Coal powered stations also are most efficient when operated fairly continuously. Gas powered power stations can be reasonably quickly started and shut down.    Diesel Generators have limited capacity but are able to be started and stopped very quickly.    Diesel generators are the primary sources of power for small companies either as back up to the grid or in many cases as the primary source of power to drive large machines.   The wind power generators supply energy continuously when the wind force and direction are favorable.    Hydro electric storage generators are very useful for balancing the supply with the demand.

There are a number of Hydro-electric power stations, the majority of which, are located on the scottish lochs.     The larger ones (over 1 MW) are listed in the table at the foot of this webpage

The largest hydro-electric power station( Dinorwig) is located in Wales, has a generation capacity of 1.7GW.    This is a pumped water storage power station operated as a short-term-operating-reserve, or fast response plant which is brought on line in response to short term rapid changes in power demand or sudden loss of power stations.    This station pumps water into a high level reservoir.    The power to do this is derived from the grid during periods of low national demand. When there is a sudden increased demand the water is allowed to flow back to a lower level reservoir generating power for the grid.

The UK national Electricity supply industry also includes a large number of wind powered generators grouped in farms in windy areas on land and offshore in the seas around the UK coast.

Individual wind turbines are interconnected with a medium voltage (usually 34.5 kV) power collection system and communications network.    At a substation, this medium-voltage electrical current is increased in voltage with a transformer for connection to the local high voltage transmission system.    Construction of a wind farm requires installation of the collector system and substation.    Wind farms are generally located near rural areas of comparative low local power loading.    Voltage / power fluctuations resulting from wind speed/direction variation and start-up and shut downs can be significant problems to the local power supplies in these regions.

As the present time (May 2011), there are about 292 operational wind farms in the UK, with 3 184 turbines and 5,3GW of installed capacity.     A further 3,9GW worth of schemes are currently under construction, and about 5.4 GW have planning consent and some 8.7GW are in the process of planning awaiting approval.    There is a growing utilisation of renewable resources in the UK but these are not do not provide continuous reliable availability and there will be a continuing requirement for a combination of coal, gas and nuclear capacity.

There is an on-going argument as to the level of power back-up that is required for wind power generated supplies.    The pessimistic view is that 80%- back-up is required.

Most of the electrical power is transferred from the generators to the users via cables suspended from large engineered pylons.    These are aesthetically not pleasing but they are far cheaper than the alternative of underground cables which have to be provided with highly engineered insulation to cater for the high voltages and the arduous conditions underground.    The suspended high voltage cables are generally based on steel cores surrounded with a layer of conducting aluminium .    The cables are generally supported from the arms of the pylons with engineered insulators.    The diagram below shows the main principles of the UK National grid system.

National Grid System.. Detailed download map showing UK Electrical Distribution System as of 2004

Virtually all public electricity supplies are AC today.

Users of large amounts of DC power including electric railways, telephone exchanges and industrial processes such as aluminium smelting usually either have adjacent dedicated generating equipment, or use rectifiers to derive DC from the public AC supply.

The electrical supplies to industrial users are invariably three phase.    Three-phase supplies are essential for operating heavy industrial equipment.     A ground is normally provided, connected to conductive enclosure and other safety equipment, to keep current away from equipment and people.    The delivered voltage is supplied via step- down transformers

The standards 3 phase supply in the UK is 400 V -6% to +10% (376V and 440 V) .    For many years the 3 Phase supply has been a nominal 415V supply.    In many areas, "delta" three phase service is common.    Delta service has no distributed neutral wire and is therefore less expensive.     Ground is provided as a low resistance earth ground, sometimes attached to a synthetic ground made by a transformer in a substation.

The standard nominal supply voltage in domestic single-phase 50 Hz installations in the UK is still 240V AC (RMS).     However, to conform with European codes, the official voltage is 230 V+10%-6% (216.2�253 V).    There is a proposal to modify this to 230 V �10% (207-253 V).    .

The electrical supply to a typical UK house is via an electricity board on which is mounted the meter which records the quantity of electricity used and a consumer unit which includes a power main switch and fuses or circuit breakers for the various circuits feeding the sockets and devices located around the home.

There are five different circuit types used in the typical domestic system

Important: The above notes on a domestic system are only general in nature . Detailed requirements are found in the relevant regulations. (BS 7671: 2008, 17th Edition).;

Total of UK generation Capacity in UK = 78GW (2009), maximum simultansous UK load 60 GW (2009)..Governemnt statistics (2010) ref Department of Energy and Climate Change- Statistical Press Release 29/Jul/2010

These lists and totals do not include tidal power generators, and power stations /generators with capacities less than 1 MW.
The lists are based on information available on the internet on 25/05/2011.   Stations are closing down and new power stations / wind farms are being constructed over time and so these lists represent a rough snapshot of the situation at a set time.

Coal..... Nuclear..... Gas..... Oil..... Hydroelectric..... Windpower - Offshore..... Windpower - Onshore..... Biomass.....

Note: Kilroot is a coal ,Oil and Bio-mass combined station.