Active power is a real component of the apparent power, usually expressed in kilowatts (kW) or megawatts (MW), in contrast to reactive power.
Adequacy: a measure of the ability of the power system to supply the aggregate electric power and energy requirements of the customers within component ratings and voltage limits, taking into account planned and unplanned outages of system components. Adequacy measures the capability of the power system to supply the load in all the steady states in which the power system may exist, considering standard conditions.
Ancillary services are services such as provision of reactive power, or frequency response, necessary for control or operation of the power system, and which it may be beneficial to provide from generators or other system users.
Annualised net metering is the same as net metering, but in this case the regulator averages a user’s net electricity consumption or production over the span of one full year, rather than a shorter period.
ASACS: UK Air Surveillance and Control Systems.
Auxiliary costs are other than those of the turbine itself, in other words foundation, grid connection, electrical installation, road construction, financial charges and so on.
Availability describes the amount of the time that the wind turbine is actually functional, not out of order or being serviced.


Balance of Plant (BOP): the infrastructure of a wind farm project, in other words all elements of the wind farm, excluding the turbines. Includes civil works, SCADA and internal electrical system. It may also include elements of the grid connection.
Black start capability: some power stations have the ability to start up independently of a power grid. This is an essential prerequisite for system security, as these plants can be called on during a blackout to re-power the grid.
Boundary layer profile: see wind shear profile.


Capacity is the rated continuous load-carrying ability of generation, transmission or other electrical equipment, expressed in megawatts (MW) for active power or megavolt-amperes (MVA) for apparent power.
Capacity credit: a wind turbine can only produce when the wind blows and therefore is not directly comparable to a conventional power plant. The capacity credit is the percentage of conventional capacity that a given turbine can replace. A typical value of the capacity credit is 25 per cent (see capacity factor).
Capacity factor (load factor) is the ratio between the average generated power in a given period and the installed (rated) power.
Capital costs are the total investment costs of the project, including auxiliary costs.
Carbon dioxide (CO2) is a naturally occurring gas, and also a by-product of burning fossil fuels and biomass, as well as land-use changes and other industrial processes. It is the principal anthropogenic greenhouse gas that affects the Earth’s radiative balance.
Citizen engagement can be defined as being responsive to lay views and actively seeking the involvement of the lay public in policymaking and decision-making. Considered a central motif in public policy discourse within many democratic countries, it is acknowledged as an important component of good governance.
Climate change is a change of climate attributed directly or indirectly to human activity which alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.
Cogging: variation in speed of a generator due to variations in magnetic flux as rotor poles pass stator poles. Cogging in permanent magnet generators can hinder the start-up of small wind turbines at low wind speeds.
Community acceptance refers to the acceptance of specific projects at the local level, including affected populations, key local stakeholders and local authorities.
Contingency is the unexpected failure or outage of a system component, such as a generator, transmission line, circuit breaker, switch or other electrical element. A contingency may also include multiple components which are related by situations leading to simultaneous component outages.
Control area is a coherent part of the UCTE Interconnected System (usually concurrent with the territory of a company, a country or a geographical area, and physically demarcated by the position of points for measurement of the interchanged power and energy to the remaining interconnected network), operated by a single transmission system operator (TSO), with physical loads and controllable generation units connected within the control area. A control area may be a coherent part of a control block that has its own subordinate control in the hierarchy of secondary control.
Control block comprises one or more control areas, working together in the secondary control function, with respect to the other control blocks of the synchronous area to which it belongs.
Costs of generated wind power: see levelised costs.
Curtailment means a reduction in the scheduled capacity or energy delivery.


D is the wind turbine rotor diameter (measured in metres).
Darrieus rotor is a sleek vertical axis wind turbine developed by French inventor G. J. M. Darrieus in 1929 based on aerodynamic profiles.
dB(A): The human ear is more sensitive to sound in the frequency range 1 kHz to 4 kHz than to sound at very low or high frequencies. Therefore, sound meters are normally fitted with filters adapting the measured sound response to the human ear.
Decibel (dB) is a unit of measurement that is used to indicate the relative amplitude of a sound or the ratio of the signal level such as sound pressure. Sound levels in decibels are calculated on a logarithmic scale.
Diffuser is a downwind device that diffuses the wind stream through a rotor.
Direct drive is a drive-train concept for wind turbines in which there is no gearbox and the wind turbine rotor is connected directly to a low-speed electrical generator.
Direct employment is the total number of people employed in companies belonging to a specific sector.
Discount rate is the interest rate (in ) used to calculate the equivalent present-day costs  or value of  future expenditure or income.
Distributed generation means single or small clusters of wind turbines spread across the landscape, in contrast to the concentration of wind turbines in large arrays or wind power plants.

Doppler shift principle: when a source generating waves moves relative to an observer, or when an observer moves relative to a source, there is an apparent shift in frequency. If the distance between the observer and the source is increasing, the frequency apparently decreases, while the frequency apparently increases if the distance between the observer and the source is decreasing. This relationship is called the Doppler effect (or Doppler shift) after Austrian physicist Christian Johann Doppler (1803–1853).
Doubly fed induction generator (DFIG) is an electrical machine concept in which variable-speed operation is provided by using a relatively small power electronic converter to control currents in the rotor, such that the rotor does not necessarily rotate at the synchronous speed of the magnetic field set up in the stator.
DTI: former Department of Trade and Industry of the UK Government, now Department of Business, Enterprise and Regulatory Reform (BERR).


Efficiency for a turbine describes the amount of active electrical power generated as a percentage of the wind power incident on the rotor area.

Electricity demand is the total electricity consumption in GWh consumed by a nation annually.
Emissions are the discharges of pollutants into the atmosphere from stationary sources such as smokestacks, other vents, surface areas of commercial or industrial facilities, and mobile sources such as motor vehicles, locomotives and aircraft. With respect to climate change, emissions refer to the release of greenhouse gases into the atmosphere over a specified area and period of time.
Energy payback is the time period it takes for a wind turbine to generate as much energy as is required to produce the turbine in the first place, install it, maintain it throughout its lifetime and, finally, scrap it. Typically, this takes 2–3 months at a site with reasonable exposure.
Equivalent sound level (dBLeq) quantifies the environmental noise as a single value of sound level for any desired duration. The environmental sounds are usually described in terms of an average level that has the same acoustical energy as the summation of all the time-varying events.
ETSU: Energy Technology Support Unit of the UK Government.
EWEA: European Wind Energy Association.
Experience curve relates the cumulative quantitative development of a product with the development of the specific costs. The more this product is produced, the more efficient the production process and the cheaper it becomes.
External costs are those costs incurred in activities which may cause damage to a wide range of receptors, including human health, natural ecosystems and the built environment, and yet are not reflected in the price paid by consumers.


Fault ride-through (FRT) is a requirement of many network operators, such that the wind turbine remains connected during severe disturbances on the electricity system, and returns to normal operation very quickly after the disturbance ends.
FINO 1 is an offshore research platform in the North Sea, off Germany.
Fuel cycle: the impacts of power production are not exclusively generated during the operation of the power plant, but also in the entire chain of activities needed for the electricity production and distribution, such as fuel extraction, processing and transformation, construction and installation of the equipment, and the disposal of waste. These stages, which constitute the chain of electricity production and distribution, are known as the fuel cycle.
Full load hours is the turbine’s average annual production divided by its rated power. The higher the number of full load hours, the higher the tubine’s production at the chosen site.
Furling is a passive overspeed control mechanism which functions by reducing the projected swept area, by turning the rotor out of the incident wind direction.


Gate closure is the point in time when generation and demand schedules are notified to the system operator.
Generation mix is the percentage distribution by technology (nuclear, thermal, large hydro, renewables) of the capacity of operational generation plants.
Geographical information system (GIS) is a software system which stores and processes data on a geographical or spatial basis.
Giromill (or cycloturbine) is a vertical axis H configuration wind turbine with articulating straight blades.
Greenhouse gases are those gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of infrared radiation emitted by the Earth’s surface, the atmosphere and clouds. Human-made greenhouse gases in the atmosphere such as halocarbons and other chlorine- and bromine containing substances are dealt with under the Montreal Protocol. Beside carbon dioxide, nitrous oxide and methane, the Kyoto Protocol deals with the greenhouse gases sulphur hexafluoride, hydrofluorocarbons and perfluorocarbons.
Grid-connected: a wind turbine is grid-connected when its output is channelled directly into a national grid (see also stand-alone system).
Grid reinforcement: a weak grid can be reinforced by up-rating its connection to the rest of the grid. The cost of doing this may fall to the wind farm developer.


High voltage (HV): typically 100 to 150 kV.
Horizontal axis wind turbine (HAWT): a wind turbine whose rotor axis is substantially parallel to the wind flow.
Hub: the rotating component of the wind turbine to which the rotor blades are fixed.
Hub height is the height of the rotor axis above the ground.
Hybrid power systems (HPS) are combinations of renewable technologies (such as wind turbines or solar photovoltaics) and conventional technologies (such as diesel generators) that are used to provide power to remote areas.


IEC: International Electrotechnical Commission.
Impact pathway approach is developed by ExternE to establish the effects and spatial distribution of the burdens from the fuel cycle to find out their final impact on health and the environment. Subsequently, the economic valuation assigns the respective costs of the damages induced by a given activity.
Independent power producer (IPP): a privately owned and operated electricity production company not associated with national utility firms.
Indirect employment refers to those employed in sectors and activities supplying intermediate products/components to, for example, wind turbine manufacturers. Indirect employment includes employment throughout the production chain.
Input-output: the national accounts of a country’s or region’s economic transactions keep track of all the inputs and outputs between economic sectors.
Installed capacity is the total MW of operational generation plant of a given technology.
Institutional capacity building refers to the process of creating more effective institutions through the increase of shared knowledge resources, relational resources and the capacity for mobilisation. It is usually related to the capacity to facilitate open policy- and decision-making processes (at national and local levels) that provide access to relevant stakeholders and room for various types of knowledge resources.
Institutional framework is a concept used to refer to the policy and regulatory elements affecting energy developments. In the wind energy context, this would include issues such as political commitment, financial incentives, planning systems, presence and roles of landscape protection organisations, and patterns of local ownership.
Interconnected system: two or more individual electric systems that normally operate synchronously and are physically connected via tie-lines (see also: synchronous area).
Interconnection is a transmission link (such as a tieline or transformer), which connects two control areas.
Intermedial load refers to those electricity-generation technologies contributing to satisfy the demand in a range between the base load and peak load of the electricity system. A generating unit that normally operates at a constant output (amount of electricity delivered) takes all or part of the base load of a system. In contrast, a peak load unit is only used to reach specific peak periods of a few hours when the demand is high.
Investment costs are the costs of the turbine itself, including transport from the factory to the place where the turbine is erected.
ISO: International Organization for Standardization.


K-factor is a weighting of the harmonic load currents according to their effects on transformer heating. A K-factor of 1.0 indicates a linear load (no harmonics). The higher the K-factor, the greater the harmonic heating effects. The K-Factor is used by transformer manufacturers and their customers to adjust the load rating as a function of the harmonic currents caused by the load(s). Generally, only substation transformer manufacturers specify K-factor load de-rating for their products. So, for K-factors higher than 1, the maximum transformer load is de-rated.
Kilohertz (kHz) is a unit of measurement of frequency. It is a unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one thousand hertz (1000 Hz).


Learning rate is a learning curve parameter. It is estimated on available data for wind turbines and tells you the achieved reduction in specific production costs.
Levelised costs: the present-day average cost per kWh produced by the turbine over its entire lifetime, including all costs – (re-)investments, operation and maintenance. Levelised costs are calculated using the discount rate and the turbine lifetime.
Load means an end-use device or customer that receives power from the electricity system. Load should not be confused with demand, which is the measure of power that a load receives or requires.
Load-frequency control (LFC): see secondary control.
Local ownership is a way of community involvement based on the fact that local residents can own shares in and obtain personal benefits from local developments. There is a significant relationship between share ownership and positive attitudes towards wind farms, and local ownership and levels of wind implementation.
Low voltage (LV): below 1000 V.
Low-voltage ride-through (LVRT): see fault ride-through.


Market acceptance refers to the process by which market parties adopt and support (or otherwise) the energy innovation. Market acceptance is proposed in a wider sense, including not only consumers but also investors and, very significantly, intra-firm acceptance.
Medium voltage (MV): typically 10 to 35 kV.
Met mast: a mast or tower which carries meteorological instrumentation (typically wind speed transducers at several heights and wind direction, air temperature and pressure transducers).
Microvolts/cm (μVcm−1) is a unit of measurement of electrical fields.
Millitesla (mT) is a unit of measurement of static magnetic fields.
Minigrid is a distribution network usually operating only at low voltage and providing electricity supply to a community. It is supplied by one or more diesel generators, wind turbines, mini-hydro generators or solar photovoltaics.
Minute reserve (15-minute reserve): see tertiary control.
Multiplier/multiplicator: for employment, this measures the direct and indirect employment effect of producing €1 million worth of output from the wind turbine manufacturing sector. Basically, it assumes that it is valid to multiply total wind turbine manufacturing in euros with a factor giving the necessary employment to produce this output. Series of multipliers for historical national account statistics exist.


N-1 criterion is a rule that requires elements remaining in operation after the failure of a single network element (such as a transmission line/transformer or generating unit, or in certain instances a busbar) to be capable of accommodating the change of flows in the network caused by that single failure.
(N-1)-safety means that any single element in the power system may fail without causing a succession of other failures, leading to a total system collapse. Together with avoiding constant overloading of grid elements, (N-1)-safety is a main concern for the grid operator.
Net metering is a policy implemented by some states and utilities to ensure that any extra electricity produced by an on-site generator, such a small wind system, can be sent back into the utility system, and where the owner is charged for energy on the basis of the net import.

Net transfer capacity is the maximum value of generation that can be wheeled through the interface between the two systems without leading to network constraints in either system, taking into account technical uncertainties about future network conditions.
Network power frequency characteristic defines the sensitivity, given in megawatts per hertz (MW/Hz), usually associated with a (single) control area/block or the entire synchronous area, which relates the difference between scheduled and actual system frequency to the amount of generation required to correct the power imbalance for that control area/block (or, vice versa, the stationary change of the system frequency in the case of a disturbance of the generation/load equilibrium in the control area without being connected to other control areas). It should not be confused with the K-factor (K). The network power frequency characteristic includes all active primary control and self-regulation of load and changes, due to modifications in the generation pattern and demand.
NIMBY is the acronym for ‘not in my back yard’ and refers to an explanation of the local rejection to technological projects. Recent research is questioning the traditional explanation of local rejection to technological projects based on the NIMBY concept, as this may be giving an incorrect or partial explanation of all the variables at stake.
Nitrogen oxide (NOx) is a product of combustion from transportation and stationary sources. It is a major contributor to acid depositions and the formation of ground level ozone in the troposphere. It is formed by combustion under high pressure and high temperature in an internal combustion engine. It changes into nitrogen dioxide in the ambient air and contributes to photochemical smog.
Numerical weather prediction (NWP) means weather forecasting by computational simulation of the atmosphere.



Offshore: wind generation plant installed in a marine environment.
Offshore wind developments are wind projects installed in shallow waters off the coast.
Onshore wind developments are wind farms installed on land.


Participatory planning is a planning process open to higher levels of public engagement. Successful wind farm developments are linked to the nature of the planning and development process, and public support tends to increase when the process is open and participatory. Thus, collaborative approaches to decision-making in wind power implementation are suggested to be more effective than top-down imposed decision-making.
Permanent magnet generator (PMG) is a synchronous electrical generator design based on the use of permanent magnets on the rotor.
Photovoltaic generation (PV) is the generation of electricity from sunlight or ambient light, using the photovoltaic effect.
Point of common coupling (PCC) is the point on the public electricity network at which other customers are, or could be, connected. Not necessarily the same location as point of connection.
Point of connection (POC) is the point at which the wind farm electrical system is connected to the public electricity system.
Pollutant: a substance that is present in concentrations that may harm organisms (humans, plants and animals) or exceed an environmental quality standard. The term is frequently used synonymously with contaminant.
Power curve depicts the relationship between net electric output of a wind turbine and the wind speed measured at hub height on a 10-minute average basis.
Primary control (frequency control, primary frequency
) maintains the balance between generation and demand in the network, using turbine speed governors. Primary control is an automatic decentralised function of the turbine governor to adjust the generator output of a unit as a consequence of a frequency deviation/offset in the synchronous area. Primary control should be distributed as evenly as possible over units in operation in the synchronous area. The global primary control behaviour of an interconnection partner (control area/block) may be assessed by the calculation of the equivalent fallout of the area (basically resulting from the fallout of all generators and the self-regulation of the total demand). By the joint action of all interconnected undertakings, primary control ensures the operational reliability for the power system of the synchronous area.
Primary controller: decentralised/locally installed control equipment for a generation set to control the valves of the turbine, based on the speed of the generator (see primary control). The insensitivity of the primary controller is defined by the limit frequencies between which the controller does not respond. This concept applies to the complete primary controller-generator unit. A distinction is drawn between unintentional insensitivity, associated with structural inaccuracies in the unit, and a dead band set intentionally on the controller of a generator.
Primary control power is the power output of a generation set due to primary control.
Primary control range is the range of adjustment of primary control power, within which primary controllers can provide automatic control, in both directions, in response to a frequency deviation. The concept of the primary control range applies to each generator, each control area/block and the entire synchronous area.
Primary control reserve: the (positive/negative) part of the primary control range measured from the working point prior to the disturbance up to the maximum primary control power (taking account of a limiter). The concept of the primary control reserve applies to each generator, each control area/block and the entire synchronous area.
Productivity is used here as employees per output unit in fixed prices. The 2 per cent increase in productivity used as a basic assumption implies that 2 per cent less people are needed to produce the same output every year. If additional cost reductions of turbines are assumed, this must partly be attributed to additional productivity increases further reducing the need for employees.
Progress ratio is related to the learning rate (see learning rate) – if the learning rate is 15 per cent, then the progress ratio is 85 per cent.
PX is a power exchange scheduling coordinator and is independent of system operators and all other market participants.


Rated wind speed is the lowest steady wind speed at which a wind turbine can produce its rated output power.
Reactive power is an imaginary component of the apparent power. It is usually expressed in kilo-vars (kVAr) or mega-vars (MVAr). Reactive power is the portion of electricity that establishes and sustains the electric and magnetic fields of alternating-current equipment.
Reactive power must be supplied to most types of magnetic equipment, such as motors and transformers, and causes reactive losses on transmission facilities. Reactive power is provided by generators, synchronous condensers or electrostatic equipment such as capacitors and directly influences the electric system voltage. The reactive power is the imaginary part of the complex product of voltage and current.
Reinvestments are the costs of replacing a larger and more costly part of a turbine.
Reliability describes the degree of performance of the elements of the bulk electric system that results in electricity being delivered to customers within accepted standards and in the amount desired. Reliability at the transmission level may be measured by the frequency, duration and magnitude (or the probability) of adverse effects on the electric supply/transport/generation. Electric system reliability can be addressed by considering two basic and functional aspects of the electric system:

  1. adequacy: the ability of the electric system to supply the aggregate electrical demand and energy requirements of customers at all times, taking into account scheduled and reasonably expected unscheduled outages of system elements; and
  2. security: the ability of the electric system to withstand sudden disturbances such as electric short circuits or unanticipated loss of system elements.

Reynolds number: a dimensionless number describing the aerodynamic state of an operating aerofoil. The number is used along with the angle of attack to describe the limits of a particular aerofoil’s lift-to-drag ratio and the conditions at which stall occurs. Small wind turbine aerofoils typically operate in a low Reynolds number range, from 0.150 to 0.5 million.
Rural electrification provides a regular supply of electricity to rural residents. It implies the extension of power lines to rural areas, or the use of stand-alone or isolated power systems.


Savonius rotor (S-rotor): a simple drag device producing high starting torque developed by the Finnish inventor Sigurd J. Savonius.
SCADA: see supervisory control and data acquisition system.
Secondary control is a centralised automatic function to regulate the generation in a control area, based on secondary control reserves in order to maintain its interchange power flow at the control programme with all other control areas (and to correct the loss of capacity in a control area affected by a loss of production) and, at the same time, in the case of a major frequency deviation originating from the control area, particularly after the loss of a large generation unit, to restore the frequency to its set value in order to free the capacity engaged by the primary control (and to restore the primary control reserves). In order to fulfil these functions, secondary control operates by the Network Characteristic Method. Secondary control applies to selected generator sets in the power plants comprising this control loop. Secondary control operates for periods of several minutes, and is therefore dissociated from primary control. This behaviour over time is associated with the PI (proportional-integral) characteristic of the secondary controller.
Secondary control range: the range of adjustment of the secondary control power, within which the secondary controller can operate automatically, in both directions at the time concerned, from the working point of the secondary control power. The positive/negative secondary control reserve is the part of the secondary control range between the working point and the maximum/minimum value. The portion of the secondary control range already activated at the working point is the secondary control power.

Security limits define the acceptable operating boundaries (thermal, voltage and stability limits). The TSO must have defined security limits for his own network and must ensure adherence to these security limits. Violation of these limits for a prolonged period of time could cause damage and/or an outage of another element that could cause further deterioration of system operating conditions.

Small-signal stability is the ability of the electric system to withstand small changes or disturbances without the loss of synchronism among the synchronous machines in the system, while having an adequate damping of system oscillations (sufficient margin to the border of stability).
Small wind turbine (SWT):
a system with 300m2 rotor swept area or less that converts kinetic energy in the wind into electrical energy.
Social acceptance:
in the energy and technology policy context, this concept refers to the responses of the lay public (including the hosting communities), and of stakeholders, such as industry and non-governmental, governmental and research organisations, to a specific energy innovation. The most recent and comprehensive approach to the social acceptance of renewable energies proposes the ‘triangle model’, integrating three key dimensions: socio-political acceptance, community acceptance and market acceptance.
Social trust:
in technological and risk contexts, this refers to the level of trust individuals have towards organisations and authorities managing technological projects. It is increasingly regarded as a significant element in social reactions to technological developments. Trust can be created in careful, sophisticated decision-making processes that take time, but it can be destroyed in an instant.
Socio-political acceptance
refers to the acceptance of both technologies and policies at the most general level. This general level of socio-political acceptance is not limited to the ‘high and stable’ levels of acceptance by the general public, but includes acceptance by key stakeholders and policymakers.
is the ability of an electric system to maintain a state of equilibrium during normal and abnormal system conditions or disturbances. Stand-alone systems are electric power systems independent of the network or grid, often used in remote locations where the cost of providing lines from large central power plants is prohibitive.
Static load flow calculations
investigate the risk of system overload, voltage instability and (N-1)-safety problems. System overload occurs when the transmitted power through certain lines or transformers is above the capacity of these lines or transformers.
System static voltage instability
may be caused by a high reactive power demand from wind turbines. Generally speaking, a high reactive power demand causes the system voltage to drop.
Sulphur dioxide (SO2)
is a heavy, pungent, colourless gas formed primarily by the combustion of fossil fuels. It is harmful to human beings and vegetation, and contributes to the acidity in precipitation.
Supervisory control and data acquisition system (SCADA)
is the wind farm monitoring system which allows the owner and the turbine manufacturer to be notified of faults or alarms, remotely start and stop turbines, and review operating statistics.
Surface roughness (Z
o) is a parameter used to describe the roughness of the surface of the ground.
Synchronous area:
an area covered by interconnected systems. These systems’ control areas are synchronously interconnected with the control areas of members of the association. Within a synchronous area the system frequency is commonly steady. A certain number of synchronous areas may exist in parallel on a temporary or permanent basis. A synchronous area is a set of synchronously interconnected systems that has no synchronous interconnections to any other interconnected systems.
System frequency
is the electric frequency of the system that can be measured in all network areas of the synchronous area under the assumption of a coherent value for the system in a timeframe of seconds (with minor differences between different measurement locations only).


Tertiary control is any automatic or manual change in the working points of generators (mainly by rescheduling) in order to restore an adequate secondary control reserve at the right time. The power that can be connected automatically or manually under tertiary control in order to provide an adequate secondary control reserve is known as the tertiary control reserve. This reserve must be used in such a way that it contributes to the restoration of the secondary control range when required.
Thrust curve:
a graph which shows the force applied by the wind at the top of the tower as a function of wind speed.
Tip speed:
speed (in m/s) of the blade tip through the air.

Transformer: a piece of electrical equipment used to step up or down the voltage of an electrical signal. Most turbines have a dedicated transformer to step up their voltage output to the grid voltage.
Transient stability
is the ability of an electric system to maintain synchronism between its parts when subjected to a disturbance of specified severity and to regain a state of equilibrium following that disturbance.
Transmission system operator (TSO):
a company that is responsible for operating, maintaining and developing the transmission system for a control area and its interconnections.
Turbine lifetime
is the expected total lifetime of the turbine (normally 20 years).
Turbulence intensity
measures the ‘roughness’ of the wind, calculated for a time series of wind speed data, as the standard deviation divided by the mean wind speed.


UNEP-GEF: United Nations Environment Programme, Division of Global Environment Facility Coordination.

Unity (or harmony) with the landscape is the degree to which individuals perceive wind turbines to be integrated with the landscape. The perceived impact on landscape seems to be the crucial factor in public attitudes towards wind farms, and opposition to the visual despoliation of valued landscapes has been analysed as the key motivation for opposition to wind farms.
U-shape curve
is a model stating that public attitudes towards wind farms change from being very positive, before the announcement of the project, to negative, when the project is announced, to positive again, after the construction. It is related to the familiarity factor, considered a key element in individuals’ perception of technological developments.
is the incumbent electricity supplier to end users (usually state-owned at some period), which may own and operate other electricity supply assets, including transmission networks and usually generation plant.


Value chain is the set of interconnected activities, consisting of discrete value-adding market segments, that comprise an industry. In the case of the wind energy industry, this may include (but is not restricted to) wind turbine manufacturing, project development, financing, asset ownership, operations and maintenance, and electricity distribution.
Value of statistical life (VSL)
is an approach measuring a society’s willingness to pay to avoid additional cases of death. This can be seen in spending for improved safety in the aircraft or car industry. In the EU and the US, values of between 1 and 10 million US$ or € per life saved have been found in different studies. Earlier versions of the ExternE project adopted a figure of US$3 million per life saved for VSL calculations. In these calculations the age of a person saved does not matter.
Vertical axis wind turbine (VAWT):
a wind turbine with a vertical rotor axis.


Wind Atlas Analysis and Application Program (WAsP): a program for predicting wind climate and energy production from wind farms.
Wind farm design tool (WFDT):
software to aid in the design and optimisation of a wind farm.
Wind home system (WHS):
a wind-based system to provide basic lighting and entertainment needs to an individual home, with a capacity typically in the range of hundreds of watts.
Wind rose:
a circular diagram giving a visual summary of the relative amounts of wind available in each of a number of direction sectors (often 12) at a given location, and the speed content of that wind.
Wind shear profile (α):
the increase in wind speed with height above ground or sea level.
Wound rotor:
a type of synchronous electrical machine in which the magnetic field on the rotor is established by passing a current through coils on the rotor. The alternative is to establish the magnetic field using permanent magnets (see PMG).
Wound rotor induction generator (WRIG):
see doubly fed induction generator.


Years of life lost (YOLL): the YOLL approach takes into account that due to different causes people in very different age groups may be at risk. In the case of a chronic disease leading to the death of very old people, only the years of life lost due to the disease, as compared to the average life expectancy, are taken into account. For each year of life lost, approximately 1/20th of the value of statistical life is used.


Zone of visual influence (ZVI): the land area around a wind farm from which a specified number of wind turbines can be seen. Often presented as a map, with areas coloured depending on the number of turbines which can be seen.

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