ETYS documents and appendices

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.

CSNP

Centralised Strategic Network Plan 

The ESO's new electricity transmission network planning output which addresses all load related planning. 

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 SSEN 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.

HND 

Holistic Network Design

The Holistic Network Design (HND) is a first of its kind, integrated approach for connecting 23GW of offshore wind to Great Britain. By considering future offshore generation out to 2030, infrastructure can be planned to bring power to the grid cohesively, ensuring maximum benefit for consumers, local communities and the environment.

HNDFUE

Holistic Network Design Follow up Excercise 

Following the publication of the HND and the NOA Refresh, the ESO is undertaking an HND follow-up exercise known as “HNDFUE”, to consider additional offshore wind connections from the Celtic Sea and ScotWind offshore leasing rounds. This is expected to be published in March 2023.

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.

SSEN 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

Thermal Constraint 

The maximum power transfer achievable without exceeding the heat dissipation limitations of the circuits.

Transmission circuit

This is either an onshore transmission circuit or an offshore transmission circuit.

tCSNP

Transitional Centralised Strategic Network Plan 

The Transitional Centralised Strategic Network Plan was put in place to ensure that the ESO can identify key investments for the onshore network to facilitate offshore wind generation targets by 2030 .

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 electricity on a regional or national level, using fixed infrastructure.

Voltage Constraint

The transmission operation limitation to maintain transmission assets within statutory limits. The limits are to keep assets within safe and secure voltage ranges.