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For information on frequently asked questions about this document, please read on.

Where does electricity go after it is generated?

Electricity cannot be stored in large quantities, so electricity entering the system is moved around the country for immediate use. In Great Britain, this national electricity supply is maintained by National Grid in its role as the National Electricity Transmission System Operator (NETSO).  A key function of the NETSO is to constantly ‘balance’ the supply and demand across the system. This is a process of always ensuring that the grid has sufficient electricity at any moment it is needed, which can vary greatly due to changing domestic and industrial demand and variable supply due to power station faults or changing wind conditions for wind energy generation.

Most consumers are not connected to the transmission system. As such, electrical energy is transferred from the transmission system to local distribution networks. These local networks run at lower voltages (up to and including 132kV in England & Wales and below 132kV in Scotland). Electricity from distribution networks is in turn transformed at a neighbourhood level to 400V (i.e. 240V in single phase) for domestic consumption.

How will variable wind energy adequately supply our continuous demand?

All electricity systems must accommodate a constantly varying level of demand for power. The electricity grid is designed to accommodate this variability and the mechanisms that enable changes  in  demand  to  be  accommodated  can  also  accommodate  changes  in  output  from variable generation sources like wind power. 

In Great Britain, National Grid studies show how the GB electricity system could be operated satisfactorily  to  meet  targets  for  2020  with  27  GW  of  wind  generation,  though  with  some additional costs, for example for additional reserve capacity (National Grid. Electricity Ten Year Statement 2012. Accessible at: http://www.nationalgrid.com/uk/Electricity/ten-yearstatement/ and National Grid. Operating the electricity transmission networks in 2020 – Update June 2011. Accessible at: http://www.nationalgrid.com/uk/Electricity/Operating+in+2020/). This assertion is supported by comparison with other weakly-connected networks with large levels of wind energy penetration internationally, such as Iberia (Spain and Portugal) and Ireland (Republic of Ireland and Northern Ireland). In the longer term there are numerous technological options to facilitate much greater amounts of wind power – such as improved interconnection with other countries, intelligent management  of  supply  and  demand  through  a  ‘smart  grid’  and  adequate  energy  storage capacity.

How can offshore renewable energy projects help meet renewable energy targets?

The Committee on Climate Change advised the government that 30% of electricity generation in the  UK should come from renewable sources by 2020. This is in line with the government’s targets  under the Renewable Energy Directive to achieve 15% of energy use from renewable sources by 2020.

How do you assess possible environmental impacts of offshore renewable energy projects and their associated electrical transmission infrastructure?

Large electrical transmission developments are required, under EU and UK law, to undertake a statutory  Environmental  Impact  Assessment  (EIA)  into  the  possible  effects  of  construction, operation and decommissioning upon sensitive environmental receptors. This legal requirement is set down in the EIA directive (85/337/EEC) of the European Council, as transposed into UK law in the context of electrical transmission by the Infrastructure Planning (Environmental Impact Assessment) Regulations 2009.

The Environmental Impact Assessment process consists of the following:

  • Screening – assessing a site for whether an EIA is to be required.
    • Scoping – ruling out environmental receptors to which no impact is likely, and focussing on  receptors where there is a risk of impact. This process should involve consultation with  external stakeholders to provide outside opinion of  where impacts may require further investigation.
  • Impact Assessment process – standardised process of assessing impacts of development upon environmental receptors, carried out by technical specialists.
  • Consultation – community and stakeholder consultation to determine acceptability
    • Publication – submission and publication of an Environmental Statement (ES) based on the outcomes of the impact assessment process. The ES will include, as a minimum:

a) A description of the development comprising information on the site, design and size of the development;

b) An outline of the main alternatives studied by the applicant and an indication of the main reasons for the applicant’s choice, taking into account the environmental effects;

c) The data required to identify and assess the main effects, which the development is likely to have on the environment;

d) A  description  of  the  measures  envisaged  in  order  to  avoid,  reduce  and,  if possible, remedy significant adverse effects; and

e) A non-technical summary (NTS) of the information provided.

What are the wider community benefits associated with transmission infrastructure construction?

Electrical  transmission  is  heavily  reliant  on  the  local  economy  during  construction.  Local contractors and materials can be required for fencing, cable ducting, substation construction and machinery  hire.  Local  contractors  will  in  addition  be  required  for  substation  maintenance. Substations do not fall under the requirements of the community infrastructure levy, however to achieve community benefit through transmission development developers may have the option to allocate a community fund for community projects after construction; however there is no legal requirement.

What are electromagnetic fields, and what risk do they pose?

Electromagnetic  fields  (EMFs)  are  produced  where  any  electrical  current  flows,  hence electromagnetic fields are emitted by all electrical devices or natural electrical phenomenon such as lightning. Higher voltage and greater current produce stronger fields.

Electromagnetic fields associated with electricity produce non-ionising radiation, i.e. radiation which is insufficiently powerful to break molecular bonds. Chief health concerns regarding this form  of  radiation  are  linked  to  its  capacity  to  heat  up  human  tissue  where  the  fields  are sufficiently powerful. Despite this concern, the WHO (World Health Organisation) guidelines51 currently state that no conclusive evidence has been found to indicate that there are any health consequences resulting from exposure to low level electromagnetic fields.

The WHO guidelines indicate that safe levels of magnetic fields are 100 µT for the general public, or 500 µT for workers. The magnetic fields from buried cables exceed these limits, with the strength of the fields mitigated by passage through the earth to reach the surface. Electric fields underneath overhead lines can be as high as 10 kV/m, which is higher than the WHO safe limit of  5kV/m. However, the strength of fields drop off rapidly with distance from source, with values returning to background levels within 50-100m from overhead lines. Substations produce almost no  electrical fields, as the grounded metal security fence surrounding the compound dissipates the majority of the fields and prevent them from emanating from the site.

In the UK, the Government policy on EMFs is that power lines should comply with the 1998

International Commission on Non-Ionizing Radiation Protection (ICNIRP) Guidelines on exposure to EMFs in terms of the 1999 EU Recommendation.  In 2009 two voluntary Codes of Practice were   developed  and  agreed  between  the  Energy  Networks  Association  (ENA)  and  the Government. Both Codes of Practice have been applied in England, Scotland and Wales.

What is the difference between MW and MWh?

MW (Megawatts) refers to power, i.e. the amount of energy being produced or transmitted per second.

MWh (Megawatt-hours) refers to energy.  If a wind turbine produces 1 MW for a duration of one hour, it has produced 1 MWh of electrical energy.  Similarly, if a domestic electric kettle (typically 3 kW rating) is switched on for 2 minutes, it has used 6 kW-minutes of electrical energy, or 0.1 kWh, or 0.0001 MWh.

What is the difference between MW and MVA?

For the purposes of this document, MW and MVA can be assumed to be the same.  Both describe the amount of electrical energy per second (i.e. power, see above) which is being produced  or  transmitted.  Alternatively,  if  used  to  describe  electrical  equipment,  the  terms describe the maximum amount of electrical energy per second which the equipment is capable of producing or transmitting.

The difference between MW and MVA is that the former term strictly covers only electrical power, i.e. capable of doing useful work, whereas the latter term also includes the ‘reactive power’ which flows without producing useful work (see below).

What is the difference between power factor and capacity or load factor?

Power factor is a term used in electrical engineering to quantify the reactive power (see below) flowing in a circuit, normalised by the ‘active’ or useful power.  The exact relationship is slightly more complex than a simple ratio of these quantities, but for the purposes of this document it is sufficient to note that electrical engineers will seek to design and operate the system to keep the power factor  close to 1.0 or ‘unity power factor’.     Power factor can never exceed 1.0, and normally is in the range 0.85 to 1.0.

The terms ‘capacity factor’ and ‘load factor’, which can be used interchangeably, mean the ratio, over a long period such as a year or longer, of the energy actually produced by the generator, to the energy it could have produced if it had operated at full output for the entire period. 

Wind generation typically achieves capacity factors in the range 20% to 35%, depending on the location and the design.  In extremely windy locations such as Shetland, capacity factors of over 50% may be achieved.    Conventional fossil-fired electricity generation is not constrained by availability of the ‘fuel’, unlike wind, but is constrained by reliability, and by its ability to compete on  price against other generators for a finite electricity demand.   The cheapest may achieve capacity factors of 80% or possibly 90%, whereas the most expensive generators are used only to meet very rare peaks in electricity demand, and may only operate for a few hundred hours a year, with capacity factors of a few percent.

What is the reactive power and why does it need to be compensated?

‘Reactive power’ is a concept used by electrical engineers to describe the flows of electrical energy between elements of an AC electrical system, without producing useful work in the ‘loads’.

 Some elements of the electrical system act as capacitors, i.e. they store energy in electric fields within their structure.   Cables are a good example: the conductors are charged up to some voltage, whereas on the other side of the insulating material there is a metallic screen or armour which is effectively at earth potential.  There is therefore an electric field between the conductor and the earthed elements, and energy is stored in this field.  For an AC system, this energy is constantly charged and discharged as the instantaneous voltage alternates.

Similarly, other elements of the electrical system act as inductors, i.e. they store energy in magnetic fields within their structure.  The most common source of ‘inductance’ is a transformer, which depends on creating magnetic fields in its core in order to function.  Electrical energy is stored in this magnetic field, and is constantly charged and discharged as the instantaneous voltage alternates.

The net effect is that instantaneous electrical power oscillates between the inductances and the capacitances, changing direction with every cycle of the AC voltage.  This power is real, but does no useful or active work. It is hence called ‘Reactive Power’. In fact, Reactive Power’ contributes to resistive losses.  Electrical engineers seek to minimise reactive power flows to the economic minimum. Reactive power compensation is one way to do so.

There is an additional complication, which is that in some cases reactive power flows can be useful in controlling the voltage at points on the electricity system.

What is monopole HVDC and what is bipole HVDC and sea return?

For an HVDC connection, pairs of cables (a ‘bipole’ arrangement) may be used, one with a positive polarity conductor, and another a negative polarity conductor. The cables in this case are a single core construction.

It is possible for HVDC transmission to be achieved with a single cable (called ‘monopole’). In this case, the return path is provided by current flowing through the earth or through the sea (‘sea return’).   This may cause greater electromagnetic fields.   This may be used as a short-term measure when there is a failure of one cable or its associated equipment.

Where can I find more information?

Further information on energy legislation, targets and renewable energy policy can be found on the DECC website: http://www.decc.gov.uk/en/content/cms/meeting_energy/renewable_ener/renewable_ener.aspx

Further information about wind and marine renewable energy, including state of the industry reports and industry information, can be found on the RenewableUK website: http://www.renewableuk.com/index.html

Further information on the electricity transmission system within the UK, including technical details of electricity transmission, can be found on the National Grid website: http://www.nationalgrid.com/uk/Electricity/

Further information on marine (wave and tidal) energy production can be found on the EMEC website: http://www.emec.org.uk/marine-energy/

Further information on electricity regulation in Great Britain can be found on the Ofgem website:http://www.ofgem.gov.uk/Networks/Pages/Ntwrks.aspx

Further information on electricity regulation in Northern Ireland can be found on the NIAUR website: http://www.uregni.gov.uk

What is the purpose of the document?

The aim of the document is to provide reference information on the technical and environmental aspects of the electrical transmission infrastructure associated with connecting offshore renewable energy generation projects to the national electricity transmission system.  The document includes material on the potential nature, type and functions of infrastructure, provides a range of images of equipment, and outlines the key technical and environmental considerations associated with installation and operation.  It also includes high level descriptions of the relevant planning and regulatory frameworks applicable to this sector, and also a comprehensive library of further information on relevant environmental and socio-economic considerations.

This document is not intended to be a guide to developing grid connections or to be used as a form of checklist by readers as to what should/will be incorporated into a particular project. Each offshore generation project will have its own unique characteristics which will drive what the specific connection requirements are.

Who is the target audience for this material?

This document is primarily targeted at the planning and consenting community and is focussed accordingly.  However, we expect that it will be useful as reference material across a range of stakeholders.