# Relevant parameters in wind farm production

After the connection of a Wind Farm to the grid several parameters are used to analyze the smooth operation of the installation.

The more relevant are:

Capacity Factor

$CF=\left( \frac{E(kWh)}{P(kW) T(h)}\right )$

Is a parameter used frequently in power producing  plant. A high capacity factor means that the plant is working almost continuously (for instance a nuclear plant), while a low capacity factor may characterize a power plant working only in peak hours (like some hydro plants).

In the case of wind farms, capacity factor depends more on the wind that on the needs of the grid.

To be economically reasonable, a wind farm needs to have a capacity factor of more than 25%. Translated in hours, it would be around 2190 equivalent hours.

This parameter is probably too “global”, as it doesn’t add information about why the wind farm was not producing: was it for a low wind, for a technical problem of the WTG, for a disconnection from the grid?

Or maybe it is due to a scheduled maintenance or to the wind sector management (the automatic planned disconnection of some WTGs in particular wind conditions)?

Technical Availability

$TA=\left( \frac{T(Available)}{T(Total)}\right )$

This is easily defined: basically is the ratio between the hours the WTG was available for production an the total number of hours in the considered period. If there is a fault in the grid, or if wind condition is above or below the maximum, it doesn’t count as “unavailability”.

Production Availability

$PA=\left( \frac{T(Producing)}{T(Total)}\right )$

This is parameters start to be interesting from an economical point of view: is the ratio between the total number of hours producing and the hours in the considered period. It will be less than 1 due to grid disconnections, WTG problems and wind outside the operational limits.

Effective Availability

$EA=\left( \frac{T(producing)}{T(wind)-T(disconnected)-T(stop)}\right )$

This parameter give a very solid information about the quality of the turbine, and the “real” availability: is the ratio between the hours of production and the hours of wind speed between the operational limits, minus the hours disconnected by the grid (for a grid problem or order) minus the justified stops (for visits, preventive maintenance, etc.)

# Wind power plant concepts and performance indicators

Wind turbines are usually placed in clusters called wind farms, with sizes ranging from a few MW (sometimes even a single wind turbine is sold, for instance to a private investor or to give power to an energy intensive factory like a cement plant) up to several hundred MW.

These clusters are connected to the grid as single generation units, therefore the term wind plants is the best suited. Whereas initially the emphasis on wind farm design was mainly on efficient and economic energy production that respected the rules of the grid operators, nowadays, with increasing wind power penetration, the demands of the grid operators have changed.

In response to these demands, modern wind turbines and wind farms have developed the concept of the so called “wind energy power plant”. The concept is essentially a wind farm with properties similar to a conventional power plant, with the exception that the fuel injection is variable. The operation of a wind energy power plant is designed in such a way that it can deliver a range of ancillary services to the power system.

Its control system is designed such that the power can be actively controlled, including ramping up and down similar to conventional generation plants. Wind power plants can and do positively contribute to system stability, fault recovery and voltage support in the system.

The properties described above greatly enhance the grid integration capability of wind power. In order to achieve high penetration levels, active control properties are essential to optimally share the power supply tasks together with other plants and to enhance network security.

An essential difference between wind plants and conventional power plants is that the output of wind plants very strongly depends on the characteristics (mainly the local wind climate) of the site where they are installed. The rated power, also known as the nameplate power, is the maximum power, which is reached only 1% to 10% of time. Most of the time wind turbines operate at partial load, depending on the wind speed. From the point of view of the power system, wind turbines can be regarded as production assets with an average power corresponding to 20 to 40% of the rated power, with peaks that are three to five times higher.

Wind power performance indicators are related to the principal wind turbine specifications, that is rated power, and rotor diameter. The specific rated power is in the range of 300 - 500 W/m2, where the area is the "swept area" of the rotor. Wind turbine electric power output will vary with the wind: it is measured according to IEC and is graphically represented in a power curve (a graphical representation of the power output at several wind speeds).

This energy output can be standardized to a long-term (20 years) average energy output and to derive the power output in short-term forecasting from 10 minute average wind speed values produced by forecast models.

Here you have an example of range and typical values for several relevant technical characteristics both for wind turbines and wind farms:

 Wind Turbine characteristic Range Typical value Rated power (MW) 0.850-6.0 3 Rotor diameter (m) 58-130 90 Specific rated power (W/m2) 300-500 470 Capacity factor onshore 18-40 Varies Capacity factor offshore 30-45 Varies Full load equivalent onshore 1600-3500 Varies Full load equivalent offshore 2600-4000 Varies Specific annual energy output (kWh/m2 year) 600-1500 Varies Technical availability onshore 95-99 97.5

 Wind Farm characteristic Range Rated wind farm size (MW) 1.5-500 Number of turbines 1-hundreds Specific rated power offshore (MW/Km2) 6-10 Specific rated power onshore (MW/Km2) 10-15 Capacity factor (load factor) onshore 18-40 Capacity factor (load factor) offshore 30-45 Full load equivalent onshore 1600-3500 Full load equivalent offshore 2600-4000 Technical availability onshore 95-99

Sources for this post:

Powering Europe. Wind energy and the electric grid (EWEA, November 2012)

IEC, 2005 Power performance measurements of electricity producing wind turbines

TradeWind 2009. Integrating wind – developing Europe’s power market for the large-scale integration of wind power.

# Power curve: what is it and how to measure it

A power curve is a relationship between free wind speed at the WT location and the associated expected power being produced. Power curve warranties are often included in contracts, to assure the wind turbine performs according to specification.

Here you have an example of how it looks like:

It is measured following international standards to demonstrate compliance with warranted values.

Following a standard not only reduces variance due to data analysis, but it also provides solutions to different problems, for example:

• How to judge if an anemometer is good enough.
• How to deal with complex terrain and obstacles.
• How to correct for low air pressure.
• How to measure the production and with which devices.

Basically, it provides an accepted way of estimating the result’s uncertainty. Any deviations from the standard (in procedure or interpretation) will cause severe deviations and uncertainties that will make conclusions very hard to take.

The standards in use to measure the power curves are made by the International Electromechanical Commission (IEC):

IEC 61400-12, in use since 1998, has been replaced by IEC 61400-12-1 (December 2005), which includes MEASNET requirements and has additional requirements for anemometer.

The following version is IEC 61400-12-2, addressing nacelle anemometry.

IEC 61400-12-1 without site calibration is the method used on flat terrain.

Basically, a meteorological tower (met mast) is positioned near a turbine to measure the wind, and a uniform flow at mast and turbine is assumed. The normal uncertainty is 5-8 %, depending on annual wind.

The best position for the mast is in the middle of sector, and it has to be between 2 and 4 diameters in front of the turbine (for instance for a V90 it would be between 180 and 360 meters). It is often erected at a 2.5 diameters distance.

The met mast can be erected before or after turbines are on site.

IEC 61400-12-1 with site calibration is the method used on complex terrain.

In this case, a temporary met mast is placed in the future position of the selected turbine, and at the same time the wind farm definitive met mast is erected.

The 2 masts works together for a certain amount of time, until a relationship between the wind measured by the definitive met mast and the temporary met mast can be found.

After, the temporary met mast is disassembled, the WTG is erected in his position and it can be defined if the energy produced is in line with the expected generation with the wind speed (extrapolated using the data of the wind farm met mast and the previously defined correlation).

# Wind turbines blades materials

A fundamental aspect in the wind blades design is the choice of the correct materials, because several parameters (weight, load and fatigue behaviour, physical properties, etc.) are influenced by this basic decision.

Several years ago materials as varied as wood, steel and aluminium where used to produce wind blades.

The first wind blades where made using wood, a low cost, low density material with a good resistance to fatigue. However, wood has a strong anisotropy and a huge potential variation of his properties, together with a notable tendency to water absorption that can lead to a reduction of resistance.

NASA MOD-0 prototype (a 38 metre diameter downwind two-bladed rotor developed in 1975) was initially made with aluminium blades, but soon due to structural problems wood blades were installed.

Wood was used in the 80’s in small (< 10 m.) blades, and can be still seen today as part of composite materials together with epoxy resins.

Steel was used in the first years of the 80’s in turbines as Growian ("Große Windkraftanlage" – big wind turbine), a Goliath for his times with steel spars and fibreglass skin. The structural properties of this material are exceptional, while the main drawback is its elevated density, leading to an increase of inertial and gravitational loads.

Aluminium was the initial choice for the MOD-0 prototype. Blades were developed by Lockheed, but were removed do to problems at the root caused by the rapidly changing loads (the WTG was downwind). It has a low density and a good resistance to corrosion, but a low fatigue resistance. It has been traditionally used in vertical axis wind turbines.

Today, the materials commonly used are fibreglass polyester and fibreglass epoxy (called GRP, glass reinforced plastic).

GRP was previously used in the naval sector: among his advantages, good structural and fatigue resistance, fabrication versatility. Other interesting physical properties of this material is that it has a low electrical conductivity (useful if the blade is struck by a lightning).

The evolution of the sector is leading to the introductions of new materials, as Kevlar and carbon fibre (CFRP, carbon fiber reinforced plastic) which are lighter but still too expensive for mass production.

It must be observed that several variables influence the choice of the materials: cost, weight, resistance, number of blades of the rotor (3 blades WTGs with fixed hubs are more robust than 2 blades WTGs with tilting hubs). An important parameter is the specific weight, the relation between the weight of the rotor and the swept area (in kg/m²).

Big manufacturers such as Enercon, Vestas, Gamesa, etc. produce blades by themselves, while others buy blades produced by specialized manufacturers on the market.

# Wind turbine tower

Towers are an important element in the structure of a WTG, not only for structural reasons (they transmit the loads from the nacelle to the foundation) but also for economical reasons: they highest the tower, the biggest the amount of energy produced.

For instance, with an increase of 20 meters from the standard 80 meters height an additional energy production around 5% can be obtained. In the future higher towers are expected, also because in mature market the most productive locations have already been used.

The average height of tower installed in Europe is around 80 meters. Modern towers have a lift inside, a ladder and several intermediate platforms. Another item that can be hosted in the tower, normally at the bottom, is the MW transformer.

Towers, above all standard steel towers, aren’t a high technology product and there are several companies with the expertise and capabilities to do them. For this reason they are often outsourced.

Towers can be made of several material following different design concepts:

• Lattice
• Steel (tubular or segmented)
• Concrete
• Hybrid

Lattice tower were common in the past when turbine where smaller (lass than a MW), but are seldom used today. Their biggest problems are a notable visual impact, and higher construction and maintenance costs. They have several advantages: they use less materials (about 50% of a standard steel tower with the same stiffness) and they produce less shadow.

Tubular steel towers are the most widely used solution. They normally have a conical shape and a diameter varying from approximately 4.5 meters at the base to 2 meters at the top, divided in 3 or 4 sections assembled at the wind farm (they are bolted together). The length of a section can vary from 20 to 30 meters. Basically they are manufactured with steel sheets cut, rolled and welded.

The new steel towers of more than 100 meters have a base section diameter over 5 meters: this can be a problem, because in many countries the maximum transportable size by road is less than 4.9 meters.

Siemens is currently working with Andresen Towers to a longitudinally, on site bolted steel-shell tower.

Concrete towers are a solution in countries were steel price is unusually high (for instance in Brazil, where steel production is almost a monopoly). They are made of several smaller precast pieces assembled on site. This solution allows an easier transportation due to the smaller dimension of the components and a good control of the quality of the materials. They biggest problem is the weight (unless they are designed in a biggest number of pieces, they can weight more than the nacelle)

Hybrid towers are another solution used by several manufacturer to reduce the exposition to the steel price volatility, the main drawbacks is that they are quite complicated to assembly, so they have higher installation costs.

Other solutions are available (for instance guy-wired pole tower), but they are used only in very small turbines.

# Windfarm SCADA system characteristics

SCADA is an acronym that stands for "Supervisory Control And Data Acquisition".
It is a fundamental tool to monitor and control several parameters of the WTGs, and it’s usual to sell it together with the turbines.
It allows combining, in a single point of control, all the info about WTGs, meteorological mast and substation, serving as the primary interface between the wind plant operator and the wind farm equipment, sending control signals from the wind plant operator to the wind park controller, retrieving and storing operational data from the WTGs and interrogating alarm signals.

The fundamentals characteristics required to a wind farm SCADA system are:

• To integrate in a single system WTGs, substation and meteorological tower info.
• To allow an interrupted access to the data from the local PC (normally located in the substation building) and from remote computers.
• To allow to modify several parameters of the control system of the turbines.
• Each communication protocol used by a part of the system must be compatible with the others.
• A clear hierarchy of all the users must be defined (basically to guarantee the safety of people working inside the machine)
• Several parameters for each component must be shown. Normally, at least the following are required:
1.  WTGs: status (working, ready, paused, stopped), power (kW), cos j, speed (rotor, generator, wind), temperatures, voltage and current of each phase, active alarms.
2.  Met Mast: Wind speed and direction, temperature, pressure, battery status.
3.  Substation: Line voltage and current, active and reactive power delivered, status of alarms and protections.
• The user must be able to change at any time various parameters:
1. WTGs: start and stop of the WTG, use of the orientation system, transfer of production data.
2. Substation: opening and closing the main switch.

Several reports can be produced with the data provided by the SCADA system, such as determination of the power curve, generated power, availability of the turbine, failures statistics, wind data (speed and turbulence), active and reactive power and cos φ at the substation.

SCADA systems retrieve, store and exports huge amount of data to a variety of stakeholders, everyone with different needs:

Remote operation center: they must be able to use the alarm condition in a quick and efficient manner, discerning the root cause of a fault without being submerged by cascading alarms.

Remote monitoring and diagnostic: after the first intervention from the remote operating center they must be able to interpret data quickly to solve the problem. Additionally historical SCADA data can be used to validate computational models or develop new models.

Asset owners: they will use SCADA output for power revenue calculation, calculation of lost energy, etc.

# WTG earthing system

A wind farm earthing system is designed for two different functions: to protect human beings and the components in case of power systems fault, and to protect them in case of a lightning hitting the structure.

Wind turbines have a considerably high possibility of being struck by a lightning during their lifetime. For this reason it is essential to give to the electrical current a low impedance path.

The main characteristics of an earthing system are:

• Ensure that living beings in the vicinity of the earthing system are not exposed to dangerous potentials
• Retain system voltages within reasonable limits in case of faults
• Provide a sufficiently low impedance

The main factors to be considered designing a normal earthing system are soil resistivity, level of lightning protection required and power system earth fault level. In the case of wind turbines the earthing system is designed to be independent from the soil condition.

In modern wind farms all the WTGs are interconnected with a copper cable buried directly in the ground. It is called a multi-earthed system, and it reduces step voltage to a tolerable level in order to protect living beings from injuries.

Equipotentialisation of turbines and surrounding soil by means of a meshed earthing system reduces step voltage to a tolerable level in order to protect living beings from injury. This is obtained by connecting the entire interconnected foundation earthing (all the steel reinforcement bars in the foundation) to the earth interconnection wires between the wind turbines.

Basically the copper cable coming from the nearest turbine through the trench is tied to the reinforcement bars of the foundation. It goes all around the lower and upper reinforcement, and than is bond to the same earthing bar of all the electrical components of the WTG, such as the MV transformer and the switchgears.

Using this solution the soil conditions around the wind turbines and the resistance to remote earth of the earthing system are not a concern because all fault currents are handled by direct connections of known dimensions and lengths.

# WTG Components: an introduction

Blades: Most wind turbines have three blades, though there are some with two blades and even with a single blade. Blades are generally 30 to 50 meters (100 to 165 feet) long, with the most common sizes around 40 meters (130 feet). Longer blades are being designed and tested, but the main problem with bigger blade is transportation. Blade weights vary, depending on the design and materials—a glass fibre blade of about 45 meters weighs more than 6,000 kg

Controller: There is a controller in the nacelle and one at the base of the turbine. The controller monitors the condition of the turbine and controls the turbine movement.

Gearbox: Many wind turbines have a gearbox that increases the rotational speed of the shaft. A low-speed shaft feeds into the gearbox and a high-speed shaft feeds from the gearbox into the generator. Some turbines use direct drive generators that are capable of producing electricity at a lower rotational speed. These turbines do not require a gearbox.

Generators: Wind turbines typically have a single AC generator that converts the mechanical energy from the wind turbine’s rotation into electrical energy.

Nacelles: The nacelle houses the main components of the wind turbine, such as the controller, gearbox, generator, and shafts. It is made of fibreglass and it is equipped with several sensors. It is normally produced in house, while sometimes tower or even blades are outsourced.

Rotor: The rotor includes both the blades and the hub (the component to which the blades are attached). Standard diameters used in this years are from 80 to 115 meters.

Towers: Towers are usually tubular steel towers 60 to 80 meters (about 195 to 260 feet) high that consist of three or more sections of varying heights. Several companies use concrete or mixed (with a concrete section and a steel section) towers, above all where the price of steel is very high (right now in Brazil).