| Ancillary services | Services procured by a system operator to balance demand and supply and to ensure the security and quality of electricity supply across the transmission system. These services include reserve, frequency control and voltage control. In GB these are known as balancing services and each service has different parameters that a provider must meet. |
ACS | Average cold spell | Average cold spell is defined as a particular combination of weather elements which gives rise to a level of winter peak demand which has a 50% chance of being exceeded as a result of weather variation alone. There are different definitions of ACS peak demand for different purposes. |
| Boundary allowance | An allowance in MW to be added in whole or in part to transfers arising out of the NETS SQSS economy planned transfer condition to take some account of year-round variations in levels of generation and demand. This allowance is calculated by an empirical method described in Appendix F of the Security And Quality of Supply Standards (SQSS). |
| Boundary transfer capacity | The maximum pre-fault power that the transmission system can carry from the region on one side of a boundary to the region on the other side of the boundary while ensuring acceptable transmission system operating conditions will exist following one of a range of different faults. |
CBA | Cost-benefit analysis | A method of assessing the benefits of a given project in comparison to the costs. This tool can help to provide a comparative base for all projects to be considered. |
CCS | Carbon capture and storage | Carbon capture and storage is a process by which the CO2 produced in the combustion of fossil fuels is captured, transported to a storage location and isolated from the atmosphere. Carbon capture and storage can be applied to large emission sources like power plants used for electricity generation and industrial processes. The CO2 is then compressed and transported for long-term storage in geological formations or for use in industrial processes. |
| Climate change targets | Targets for share of energy use sourced from renewable sources. The 2020 UK targets are defined in the Directive 2009/28/EC of the European Parliament and of the Council of the European Union, see http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/ ?uri=CELEX:32009L0028&from=EN#ntc1-L_2009140EN.01004601-E0001 |
CCGT | Combined cycle gas turbine | Gas turbine that uses the combustion of natural gas or diesel to drive a gas turbine generator to generate electricity. The residual heat from this process is used to produce steam in a heat recovery boiler which, in turn, drives a steam turbine generator to generate more electricity. |
CHP | Combined heat and power | A system whereby both heat and electricity are generated simultaneously as part of one process. Covers a range of technologies that achieve this. |
CT | Consumer Transformation | This scenario achieves the 2050 decarbonisation target in a decentralised energy landscape. |
| Contracted generation | A term used to reference any generator who has entered into a contract to connect with the National Electricity Transmission System (NETS) on a given date while having a transmission entry capacity (TEC) figure as a requirement of said contract. |
| Deterministic | A deterministic system is a system in which no randomness is involved in the development of future states of the system. |
| Double-circuit overhead line | In the case of the onshore transmission system, this is a transmission line which consists of two circuits sharing the same towers for at least one span in SHE Transmission's system or NGET’s transmission system or for at least two miles in SP Transmission’s system. In the case of an offshore transmission system, this is a transmission line which consists of two circuits sharing the same towers for at least one span. |
DC | Direct current | An electric current flowing in one direction only. |
DSR | Demand side response | A deliberate change to an industrial and commercial user’s natural pattern of metered electricity or gas consumption, brought about by a signal from another party. |
DNO | Distribution Network Operator | Distribution Network Operators own and operate electricity distribution networks. |
| Embedded generation | Power generating stations/units that don’t have a contractual agreement with the Electricity System Operator (ESO). They reduce electricity demand on the National Electricity Transmission System. |
ENTSO-E | European Network of Transmission System Operators | Electricity ENTSO-E is an association of European electricity TSOs. ENTSO-E was established and given legal mandates by the EU’s Third Legislative Package for the Internal Energy Market in 2009, which aims at further liberalising electricity markets in the EU. |
ESO | Electricity System Operator | An entity entrusted with transporting electric energy on a regional or national level, using fixed infrastructure. Unlike a TO, the ESO may not necessarily own the assets concerned. For example, National Grid ESO operates the electricity transmission system in Scotland, which is owned by Scottish Hydro Electricity Transmission and Scottish Power Transmission. |
EU | European Union | A political and economic union of 28 member states that are located primarily in Europe. |
FACTS | Flexible alternating current transmission system | FACTS devices are static power-electronic devices that utilise series and/or shunt compensation. They are installed in AC transmission networks to increase power transfer capability, stability, and controllability of the networks. |
FES | Future energy scenarios | The FES is a range of credible futures which has been developed in conjunction with the energy industry. They are a set of scenarios covering the period from now to 2050, and are used to frame discussions and perform stress tests. They form the starting point for all transmission network and investment planning, and are used to identify future operability challenges and potential solutions. |
GEP | Grid entry point | A point at which a generating unit directly connects to the National Electricity Transmission System. The default point of connection is taken to be the busbar clamp in the case of an air insulated substation, gas zone separator in the case of a gas insulated substation, or equivalent point as may be determined by the relevant transmission licensees for new types of substation. When offshore, the GEP is defined as the low voltage busbar on the platform substation. |
GSP | Grid supply point | A point of supply from the GB transmission system to a distribution network or transmission-connected load. Typically only large industrial loads are directly connected to the transmission system. |
GTYS | Gas Ten Year Statement | The GTYS illustrates the potential future development of the (gas) National Transmission System (NTS) over a ten year period and is published on an annual basis. |
GW | Gigawatt | 1,000,000,000 Watts, a measure of power. |
GWh | Gigawatt hour | 1,000,000,000 Watt hours, a unit of energy. |
GB | Great Britain | A geographical, social and economic grouping of countries that contains England, Scotland and Wales. |
HVAC | High voltage alternating current | Electric power transmission in which the voltage varies in a sinusoidal fashion, resulting in a current flow that periodically reverses direction. HVAC is presently the most common form of electricity transmission and distribution, since it allows the voltage level to be raised or lowered using a transformer. |
HVDC | High voltage direct current | The transmission of power using continuous voltage and current as opposed to alternating current. HVDC is commonly used for point to point long-distance and/or subsea connections. HVDC offers various advantages over HVAC transmission, but requires the use of costly power electronic converters at each end to change the voltage level and convert it to/from AC. |
| Interconnector | Electricity interconnectors are transmission assets that connect the GB market to Europe and allow suppliers to trade electricity between markets. |
LCPD | Large Combustion Plant Directive | The Large Combustion Plant Directive is a European Union directive which introduced measures to control the emissions of sulphur dioxide, oxides of nitrogen and dust from large combustion plant. |
LW | Leading the Way | A scenario from the Future Energy Scenarios (FES) where net zero is achieved at the fast pace with a high level of societal change and a rapid speed of decarbonation |
| Load factor | The average power output divided by the peak power output over a period of time. |
| Marine technologies | Tidal streams, tidal lagoons and energy from wave technologies (see http://www.emec.org.uk/). |
MW | Megawatt | 1,000,000 Watts, a measure of power. |
MWh | Megawatt hour | 1,000,000 Watt hours, a measure of power usage or consumption in 1 hour. |
| Merit order | An ordered list of generators, sorted by the marginal cost of generation. |
MITS | Main Interconnected Transmission System | This comprises all the 400kV and 275kV elements of the onshore transmission system and, in Scotland, the 132kV elements of the onshore transmission system operated in parallel with the supergrid, and any elements of an offshore transmission system operated in parallel with the supergrid, but excludes generation circuits, transformer connections to lower voltage systems, external interconnections between the onshore transmission system and external systems, and any offshore transmission systems radially connected to the onshore transmission system via single interface points. |
NETS | National Electricity Transmission System | The National Electricity Transmission System comprises the onshore and offshore transmission systems of England, Wales and Scotland. It transmits high-voltage electricity from where it is produced to where it is needed throughout the country. The system is made up of high voltage electricity wires that extend across Britain and nearby offshore waters. It is owned and maintained by regional transmission companies, while the system as a whole is operated by a single Electricity System Operator (ESO). |
NETS SQSS | National Electricity Transmission System Security and Quality of Supply Standards | A set of standards used in the planning and operation of the National Electricity Transmission System of Great Britain. For the avoidance of doubt, the National Electricity Transmission System is made up of both the onshore transmission system and the offshore transmission systems. |
NGET | National Grid Electricity Transmission plc | National Grid Electricity Transmission plc (No. 2366977) whose registered office is 1-3 Strand, London, WC2N 5EH. |
| Network access | Maintenance and system access is typically undertaken during the spring, summer and autumn seasons when the system is less heavily loaded and access is favourable. With circuits and equipment unavailable, the integrity of the system is reduced. The planning of system access is carefully controlled to ensure system security is maintained. |
NOA | Network Options Assessment | The NOA is the process for assessing options for reinforcing the National Electricity Transmission System (NETS) to meet the requirements that the Electricity System Operator (ESO) finds from its analysis of the Future Energy Scenarios (FES). |
OFGEM | Office of Gas and Electricity Markets | The UK’s independent National Regulatory Authority, a non-ministerial government department. Their principal objective is to protect the interests of existing and future electricity and gas consumers. |
| Offshore | This term means wholly or partly in offshore waters. |
| Offshore transmission circuit | Part of an offshore transmission system between two or more circuit breakers which includes, for example, transformers, reactors, cables, overhead lines and DC converters but excludes busbars and onshore transmission circuits. |
| Onshore | This term refers to assets that are wholly on land. |
| Onshore transmission circuit | Part of the onshore transmission system between two or more circuit breakers which includes, for example, transformers, reactors, cables and overhead lines but excludes busbars, generation circuits and offshore transmission circuits. |
OCGT | Open cycle gas turbine | Gas turbines in which air is first compressed in the compressor element before fuel is injected and burned in the combustor. |
| Peak demand | The maximum power demand in any one fiscal year: Peak demand typically occurs at around 5:30pm on a week-day between December and February. Different definitions of peak demand are used for different purposes. |
PA | Per annum | per year. |
PV | Photovoltaic | A method of converting solar energy into direct current electricity using semi-conducting materials. |
| Planned transfer | A term to describe a point at which demand is set to the National Peak when analysing boundary capability. |
| Power supply background (aka generation background) | The sources of generation across Great Britain to meet the power demand. |
| Probabilistic | Model or approach where there are multiple possible outcomes, each having varying degrees of certainty or uncertainty of occurrence. This is based on the idea that you cannot be certain about results or future events but you can judge whether or not they are likely, and act on the basis of this judgment. |
QB | Quadrature booster | A quadrature booster is a type of transformer also known as a phase shifting transformer and it is used to control the amount of real power flow between two parallel lines. |
| Ranking order | A list of generators sorted in order of likelihood of operation at time of winter peak and used by the NETS SQSS. |
| Reactive power | Reactive power is a concept used by engineers to describe the background energy movement in an alternating current (AC) system arising from the production of electric and magnetic fields. These fields store energy which changes through each AC cycle. Devices which store energy by virtue of a magnetic field produced by a flow of current are said to absorb reactive power; those which store energy by virtue of electric fields are said to generate reactive power. |
| Real power | This term (sometimes referred to as “Active Power”) provides the useful energy to a load. In an AC system, real power is accompanied by reactive power for any power factor other than 1. |
| Seasonal circuit ratings | The current carrying capability of circuits. Typically, this reduces during the warmer seasons as the circuits’ capability to dissipate heat is reduced. The rating of a typical 400kV overhead line may be 20% less in the summer than in winter. |
| SHE Transmission | Scottish Hydro-Electric Transmission (No.SC213461) whose registered office is situated at Inveralmond HS, 200 Dunkeld Road, Perth, Perthshire PH1 3AQ. |
SP | Steady Progression | This scenario makes progress towards decarbonisation through a centralised pathway, but does not achieve the 2050 target. |
| SP Transmission | Scottish Power Transmission Limited (No. SC189126) whose registered office is situated at Ochil House, 10 Technology Avenue, Blantyre G72 0HT. |
| Summer minimum | The minimum power demand of the transmission network in any one fiscal year. Minimum demand typically occurs at around 06:00am on a Sunday between May and September. |
| Supergrid | That part of the National Electricity Transmission System operated at a nominal voltage of 275kV and above. |
SGT | Supergrid transformer | A term used to describe transformers on the NETS that operate in the 275–400kV range. |
| Switchgear | The term used to describe components of a substation that can be used to carry out switching activities. This can include, but is not limited to, isolators/disconnectors and circuit breakers. |
| System inertia | The property of the system that resists changes. This is provided largely by the rotating synchronous generator inertia that is a function of the rotor mass, diameter and speed of rotation. Low system inertia increases the risk of rapid system changes. |
| System operability | The ability to maintain system stability and all of the asset ratings and operational parameters within pre-defined limits safely, economically and sustainably. |
SOF | System Operability Framework | The SOF identifies the challenges and opportunities which exist in the operation of future electricity networks and identifies measures to ensure the future operability |
| System stability | With reduced power demand and a tendency for higher system voltages during the summer months, fewer generators will operate and those that do run could be at reduced power factor output. This condition has a tendency to reduce the dynamic stability of the NETS. Therefore network stability analysis is usually performed for summer minimum demand conditions as this represents the limiting period. |
ST | System Transformation | Scenario from the Future Energy Scenarios (FES) where the target of reaching net zero is achieved by a moderate level of societal change and a low-moderate level of decarbonisation |
| Transmission circuit | This is either an onshore transmission circuit or an offshore transmission circuit. |
TEC | Transmission entry capacity | The maximum amount of real power deliverable by a power station at its grid entry point (which can be either onshore or offshore). This will be the maximum power deliverable by all of the generating units within the power station, minus any auxiliary loads. |
| Transmission losses | Power losses that are caused by the electrical resistance of the transmission system. |
| TO Transmission Owners | A collective term used to describe the three transmission asset owners within Great Britain, namely National Grid Electricity Transmission, Scottish Hydro–Electric Transmission Limited and SP Transmission Limited. |
TSO | Transmission System Operators | An entity entrusted with transporting energy in the form of natural gas or power on a regional or national level, using fixed infrastructure. |
