DANISH MINISTRY OF FOREIGN AFFAIRS
DANIDA
DANIDA’S MIXED CREDIT PROGRAMME
GUIDELINES FOR
PREPARATION AND EVALUATION
OF
INVESTMENTS IN WIND FARMS
VOLUME 2:
ANALYSIS OF TECHNICAL WIND FARM ISSUES
August 2001
Prepared for:
DANIDA, Ministry of Foreign Affairs
2, Asiatisk Plads
DK-1448 Copenhagen K
Phone: +45 3392 0000
Fax:
+45 3154 0533
Danida file no.: 104.O.30.Kina
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 1
TABLE OF CONTENTS
0
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Requirements and Recommendations for the Collection of Wind Data . . . . . . . . . .
1.1
Measuring Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2
Number of Masts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3
Location and Height of Masts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4
Number of Sensors and Mounting Requirements . . . . . . . . . . . . . . . . . . . . .
1.5
Calibration and Deviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.6
Maintenance / Surveillance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7
Indication of Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
4
4
5
5
5
6
6
6
2
2.1
2.2
2.3
2.4
Conditions at the Project Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Climatic Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Wind Farm Area and Surroundings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Practical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Grid conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3
3.1
3.2
3.3
3.4
3.5
Technical Description of the Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Wind Farm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Wind Turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Central Monitoring System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Civil Infrastructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Grid Connection (Electrical Infrastructure) . . . . . . . . . . . . . . . . . . . . . . . .
14
14
14
17
17
18
4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
Annual Energy Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Objectives and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Analysis of the Wind Regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Micro Siting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Wind Turbine Power Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AEPgross Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Correction and Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AEPnet Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Uncertainty Estimate / Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . .
24
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27
28
28
28
30
31
5
5.1
Tendering and Bid Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
General Rules and Guidelines for Procurement under Danish Mixed Credits to
China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.2
Quotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.3
Bid Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.4
Technical Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.5
Organizational Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Appendix 1 - 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 2
ABBREVIATIONS, ACRONYMS AND DEFINITION OF TERMS
AEP
Annual Energy Production
AEPGross
Annual Energy Production before applying corrections and loss
AEPNet
Annual Energy Production after applying corrections and loss
Anemometer
Instrument measuring the wind speed
CMS
Central monitoring system
Complex terrain
Hilly or mountainous terrain.
Connection point
The point where the wind farm is connected to the grid.
EMI
Electromagnetic interference
HT
High Tension
Hub height
The distance from the ground level to the centre of the wind turbine rotor
kWh
Kilo Watt hours
MWH
Mega Watt hours = 1.000 kWh
GWH
Giga Watt hours = 1.000 MWh
Micro siting
The process in which the individual wind turbine sites are determined and the
production of the wind farm is optimized.
On site
“On site” mean on the project site. Ex. on site measurements are wind
measurements recorded on the project site opposite to wind data collected off
the site, for instance in an airport.
PC
Power Curve
Power curve
The power output of a wind turbine as function of the wind speed
RFQ
Request for quotation
RMB
Renminbi, equivalent to Yuan
RPM
Rotations Per Minute
Turbulence intensity Standard deviation of the wind speed divided by the mean value.
USD
US Dollars
UTM coordinates
Universal Transverse Mercator coordinates
s
WA P
Wind Atlas Analysis and Application Programme
WD
Wind distribution
WDD
Wind direction distribution
Wind regime
“Wind regime” is the general expression of the wind conditions i.e. wind and
direction distribution, maximum wind speed, turbulence intensity etc.
Wind shear
The wind speed as a function of the height above ground level.
Wind vane
Instrument measuring the wind direction
WT
Wind Turbine
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Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 3
0 Introduction
This manual provides guidance for the preparation of feasibility studies of wind farm projects in
China. It is directed at new investors in wind farms that wish to apply for soft-financing under
Danida's Mixed Credit facility, and their consultants. In addition, it can be used by officials from
financing institutions that cooperate with Danida's Mixed Credit Program, and who deal with
requests for the financing of wind farm projects. Even though the manual is prepared for Mixed
Credit projects in China there are many universally applicable elements in the manual, making it a
valuable tool for anybody who is contemplating making use of Danida Mixed Credits for a wind farm
project.
The manual is divided into four volumes:
•
Volume 1: “Checklist for Feasibility Studies” introduces the typical outline of a Danida feasibility
report for a wind farm project. It provides a list of the content to be covered. Whether a
feasibility study follows the proposed structure is not important (form). But it should provide the
same amount of information (substance).
•
Volume 2: “Analysis of Technical Wind Farm Issues” gives recommendations for the data
collection phase and detailed recommendations on procedures and methodologies to be
followed in the technical analysis of a wind farm.
•
Volume 3: “Environmental Assessment of Wind Farms” informs about the environmental
consequences of investments in and the operation of wind farms.
•
Volume 4: “Methodology for Economic and Financial Analysis of Wind Farm Projects” gives
details of the economic and financial analyses that are required in a feasibility study. It is subdivided into two sections. Volume 4A explains the methodology using as a case study the
appraisal of the Shanwei Wind Farm project in China by Danida. Volume 4B explains the use of
a spreadsheet model for the financial and economic analysis of wind farms developed by Danida.
Volume 3 provides information about the recommended depth of information for sound project
preparation within the area of technical project analysis. The chapters in this volume are linked to
specific sub-sections of the checklists in volume 1.
The text boxes highlight issues and illustrate points made in the text with practical examples of
“worst and best cases”.
The approach is pragmatic. The ambition has been to prepare a practical practitioner’s guide to
project preparation, not to prepare an ideal best practice manual.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 4
ANALYSIS OF TECHNICAL WIND FARM ISSUES
1 Requirements and Recommendations for the Collection of Wind Data
The objective of a wind measurement programme is to form the basis of a reliable estimate of the annual
energy production of a wind farm. The term reliable is relative in the sense that it represents a degree
of uncertainty, which may be expressed as a percentage of the real value, e.g. +/- 15 %, but the nature
of the basic data and the calculation methods makes it often hard to determine the exact uncertainty.
The overall concept in a wind study is that the past is equal to the future or in other words that an
analysis of the past is equal to a prediction of the future.
The uncertainty of the estimate of the wind conditions on the site is closely connected to five separate
circumstances:
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the duration of the measurement
the number of measuring units
the location of the measuring units and the height of the sensors above ground level
the quality of the equipment and the mounting of the sensors
the surveillance or maintenance of the measuring equipment
The ideal situation is a 20 year measuring period, a mast on each wind turbine site, anemometers
mounted at hub height, correct mounting of calibrated sensors and a maintenance programme securing
against loss of data.
As the ideal situation is not operational, the question is, what is acceptable ? There is no simple answer
to this question as it depends on the size of the project, the topography, the wind direction distribution,
the size and shape of the site, the availability of long term data, which can be correlated to the site
measurements, etc. The bottom line is that a wind measurement programme should be specially
designed at an early stage of the development of every wind power project.
It should be kept in mind and understood by investors and developers of wind power projects, that wind
energy specialists do not have a crystal globe permitting them to look into the future, they only have
their experience in handling wind data on which to form a conclusion based on the available information.
1.1
Measuring Time
If long term measurements are available (10 - 20 years) and a correlation between the site and the long
term station exists, the measuring time for the masts on the project site shall be at least 1 year and
preferably 2 - 3 years.
If long term measurements are not available or a correlation between the site and the long term station
does not exist, the measuring time for the masts on the project site shall be at least 2 - 3 years.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
1.2
Page 5
Number of Masts
The number of masts depends primarily on the size of the project and the orography. A small project
located in flat terrain may need only one mast, whereas a large project (50 MW) located in complex
terrain may require up to 8 masts.
The actual number of masts may be reduced, if some (e.g. 2) are stationary for the entire measuring
period and some, (e.g. 3), are moved about on the site only measuring for a shorter period in each
position. In order to use this strategy, correlations between the primary and the secondary masts shall
be established. In this connection it shall be kept in mind, that the correlation between two masts can
change when the wind direction change, depending on the roughness and orography of the surrounding
terrain.
If a wind flow modelling program is used for the micro-siting and the individual production estimates
at the wind turbine sites, it is recommended to install at least two mast in order to be able to check the
software’s capability of handling the actual site.
1.3
Location and Height of Masts
The primary mast(s) should be located at (a) central position(s) on the site where the wind flow is
deemed to represent the flow at an average wind turbine site. If supporting measuring masts are installed,
these should be located in areas, which have a special flow due to obstacles, changes in the roughness
of the terrain in the main wind direction or at small ridges with steep slopes.
The height of the masts should be at least 30 m and preferably be equal to the height of the expected
turbines, i.e. about 40 to 50 m for 600 kW wind turbines.
1.4
Number of Sensors and Mounting Requirements
Each mast should be equipped with at least two anemometers and one wind direction sensor.
Furthermore, the primary mast should be equipped with a temperature sensor and preferably a pressure
sensor. One anemometer should be mounted on top of the mast and the wind direction sensor
approximately 1 m below. The second anemometer should be mounted at a height of 20 or 30 m and the
temperature and pressure sensors as high as possible, without disturbing the wind flow to the other
sensors. When an anemometer is mounted on a boom, the distance from the mast should be 7 times the
“diameter” of the mast and the cups should be at least 7 times the boom-diameter above the horizontal
part of the boom. The booms shall be mounted in a direction perpendicular to the main wind direction
in order to avoid major influence from the mast.
The importance of the accuracy of wind direction measurements is often not recognized and one typical
error is that the “north-mark” on the wind direction sensor is not pointing towards north. It is strongly
recommended that the “north-mark” is made visible from the ground in order to determine the deviation
from north and correct the error in the data logger or in the data handling process.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
1.5
Page 6
Calibration and Deviation
All anemometers should be calibrated in a wind-tunnel in order to secure the accuracy of the wind
measurements. The calibration equation should be entered into the data logger if possible and this
connection it should be noted, that a common error in wind studies is, that the calibration expression
actually found is not incorporated in the analysis. If an anemometer has been measuring on a mast for
several years it is recommended that a calibration is repeated after the anemometer has been dismounted
in order to determine whether the calibration expression has changed.
Regarding wind direction there is a deviation between magnetic North and geographic North and this
deviation should be identified and incorporated in the measurement or analysis.
1.6
Maintenance / Surveillance
It is recommended to have a surveillance programme as part of the measuring programme in order to
diminish possible data loss due to sensor or data logger problems. Furthermore, it is recommended to
have a logbook for each data logger in which all incidents are noted as this will ease the data handling
process and diminish the risk of data loss.
If, for example, the batteries in the data loggers are changed every third month and in addition to this
an inspection of the data loggers and sensors is carried out every 2 weeks, the loss of an anemometer
cup will only mean the loss of up to two weeks of data (plus the time of actually changing the sensor),
whereas if no surveillance programme had been initiated the loss would be up to 3 months (plus).
1.7
Indication of Costs
The data-logger, which is the main part of the wind data collection system, should work on a real time
basis, have a chip or card recorded memory and an electric battery supply of at least one month.
There are several modern data logger systems, which can be used in a wind measurement programme.
The price differs, but the following should give an impression of the size of the hardware investment in
a measurement programme. The approximate prices are in US dollars and without freight, taxes and
duty.
1 data logger
2.000 USD
2 calibrated anemometers, 1 wind vane, 1 temperature sensor, 1 pressure sensor
1.600 USD
Accessories
400 USD
One 40 m mast
6.000 USD
Total for a 40 m measuring station
10.000 USD
In addition to the above, there will be the cost of a chip reader and reading software of approximately
1.000 USD, but these items will cover a whole measuring programme.
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Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 7
2 Conditions at the Project Site
The objective of the data collection is to identify all relevant issues, collect the relevant data and
incorporate their impact in the project planning. Some issues will
i) influence the energy output of the project (AEP)
ii) affect the required hardware design (wind turbine, foundation, infrastructure, etc.)
iii) influence the wind farm design (lay-out/micro-siting and infrastructure).
All of them will to some extent have an impact on the project economy. In all projects, some information
is collected on the project site and some are collected from outside the project area. Many of these data
have to be transferred and/or transformed before they are relevant in the context of the project.
The conditions relevant to the project site can be divided into
four almost separate parts:
1) conditions relating to the climate
2) conditions or restriction relating to the surroundings
3) conditions relating to the wind farm design and practical
implementation of the project
4) conditions relating to the grid
2.1
If the wind is measured at a height
of 20 m and a wind distribution
can be obtained, this is not
directly relevant to the project
until it has been transferred to the
hub height of the wind turbines
(40 - 60 m).
Climatic Conditions
As regards climatic conditions, both
i) general and
ii) extreme conditions relevant to the project site shall be established.
The feasibility study shall include a description of the source of the available information and, most
important, it shall include an assessment of the validity of the information.
The objective is to estimate the mean and/or extreme values of all relevant parameters, the distributions,
the variations and the uncertainty involved in the estimate. The impact on the project, i.e. the wind
turbine design, the project design, the project implementation and the project economy, shall always be
kept in mind, whenever a parameter is addressed.
It should be noted that it is impossible to lay down general rules for the appropriate analysis of available
information. This is due to the fact that the information available differs widely from project to project
and the approach in the individual evaluation has
to be based on the format and the content of the Most standard wind turbines operate in the
information. Furthermore, the absolute value
temperature interval of - 20/ C to + 35/ C and it
(probable interval) of the parameter has to be is not important to determine whether the
taken into consideration, whenever the need for
extreme high temperature is 25/ C or 30/ C. On
accuracy is to be determined.
the other hand it is important to determine
whether the extreme low temperature is - 20/ C
or - 30/ C and for how long periods, as the
consequence could be requirements to installation
of heaters or procurement of special designed
“cold climate” wind turbines.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 8
The climatic parameters are:
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wind regime
air temperature
air pressure
precipitation
lightning
icing
saline air
insolation
particles or insects in the air
All information shall have a reference to the actual measurement, which shall include all relevant
information, i.e. organization, location, instruments, calibration, measuring period, etc.
Wind Regime
The objective of the analysis of the wind regime is to collect and present all available information on the
wind relevant to the project site. The information of the wind regime shall form the base of:
1)
2)
3)
the calculation of the energy production of the wind turbines
the micro-siting of the wind turbines, i.e. the optimization of the lay-out and
the assessment of whether the wind turbine is suitable for the wind regime.
The analysis of the wind regime shall be based on wind measurements on the site or close to the site.
These measurements are normally carried out as short term measurements, i.e. in a period of 1 to 3
years. Furthermore, long term wind measurements, i.e. more than 10 years, which are representative of
the variations on the site shall be analysed and assessed, if possible. The long term wind data are usually
provided by local meteorological stations.
The requirements to the equipment, the amount of wind data and the location of the measuring stations
are determined by the project size and the complexity of the site and, hence, not fixed. A “general” wind
measuring program would be the following:
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Two or more measuring stations on the site.
Each mast equipped with anemometers at two or more levels. The top anemometer mounted at the
hub height of the wind turbine. Measurements carried out with two or more anemometers at the
same mast make it possible to determine the wind shear, i.e. the increase of the wind speed by the
height above the ground.
Each mast equipped with one wind direction sensor (a wind vane).
Measuring period on the site: 1 - 3 years.
Long term measurements outside the project site, which can be correlated to the site
measurements. Measuring period: 10 - 20 years. It should be kept in mind that it is not the distance
between the site and the long term measurement that is important, but whether there actually is a
correlation between the two measurements.
Modern data loggers, calibrated sensors and a high level of surveillance in order to diminish data
loss due to sensor or logger malfunction.
Correct mounting of all sensors.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 9
The number of measuring masts, anemometers and years of measuring have a strong influence on the
uncertainty of the determination of the long term wind regime, i.e. a large number of anemometers and
long measuring periods will result in a low degree of uncertainty, whereas a low number of anemometers
and short periods will have the opposite effect. Minimum requirements cannot be fixed, as they depend
on the economy of the project. A marginal project economy demands a low degree of uncertainty and
a strong project economy can accept a higher degree of uncertainty.
The analysis and the assessment of the wind data shall include, but are not limited to, the following:
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Distribution of the mean annual wind speed.
Distribution of the mean annual wind direction.
Diurnal, monthly, seasonal and annual variations.
Wind shear. Analysis and considerations.
Turbulence intensity. Analysis and considerations.
Maximum wind speeds, including a specification of averaging time: 10 minutes, 1 minute, 10
seconds or 2 seconds. The assessment shall include locally applicable codes or guidelines for wind
loads on structures.
Mean annual wind speed variations. Long term data.
Air Temperature
The air temperature is one of the parameters, which determine the air density. The minimum and
maximum values determine the requirements to the specifications of the wind turbine. Temperature
should be measured at the site along with the wind measurements, but may be transferred from
meteorological stations located not far from the site. If exposed to the same climatic conditions, the
transfer is primarily a function of the difference in height above sea level between the site and the
location of the measurement.
The analysis and assessment of the temperature data shall include the following:
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Monthly and annual mean air temperature
Minimum and maximum temperatures
Assessment of the possibility of extreme values outside the min-max interval
Assessment of the duration of temperatures close to the extremes
Air Pressure
The air pressure is the other parameter, which determines the air density. In many cases air pressure is
not measured on the project site and may be transferred from a meteorological station located not far
from the site. The transfer is primarily a function of the difference in height above sea level between the
site and the measurement location.
Precipitation
Information regarding precipitation is most
important in relation to project implementation, as
heavy rain can make site access, infrastructure
work and erection of wind turbines very difficult,
if not impossible. Risk of special incidents caused
by rain (flooding, streams, landslides etc.) shall be
evaluated.
Part of a wind farm in California, USA was
located in a wadi in a desert area. Many years
without water in the wadi excluded the
possibility of a potential major problem.
However, one year it rained heavily in the
mountains and the wadi turned into a violent
stream, which undermined the wind turbine
foundations and damaged some of the electrical
infrastructure. The economic loss was
considerable.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 10
Lightning
The frequency of thunderstorms in the project site area shall be evaluated in order assess the risk of
lightning striking a wind turbine. The most threatened components are the blades and the controllers and
if not protected, the economic consequence of a strike of lightning can amount to 20 - 25 % of the total
price of the wind turbine.
Most Danish wind turbines have lightning protection as standard equipment and some manufacturers
offer extra protection as an option at extra costs.
Icing
If relevant, the risk of icing shall be assessed. If the problem is deemed severe, special precautions shall
be taken and discussed with the wind turbine manufacturer. If there is a risk, but not a severe one,
special anemometers and wind vanes equipped with heating elements may be the only measure necessary.
Saline Air
The possibility of saline air shall be assessed as the corrosive effect on the wind turbines can be severe.
Normally, inspection of existing steel structures in the site area will determine whether salinity is a
potential problem.
Insolation
The hours of insolation can be important in some areas as ultraviolet light in the long run destroys some
materials, such as rubber and electrical isolation material. Especially the cables on wind turbines on
lattice towers will be exposed to insolation.
Particles or Insects in the Air
The possibility of particles and/or insects in the air
shall be assessed as the impact on the power curve
can be significant. If the problem is assessed to
exist, a blade washing programme hall be
incorporated in the operation and maintenance
schedule.
2.2
In some areas insects hatch in enormous
numbers at certain times of the year and the
negative impact on the wind turbine production
can amount to 20 % depending on the wind
speed. In order to cope with the problem,
blade washing programme are being launched
in these periods.
Wind Farm Area and Surroundings
All relevant issues which have an impact on the wind farm or wind turbine design shall be identified and
described in the feasibility study in order that the necessary precautions may be taken.
Terrain Parameters
In complex terrain there will be limitations to the micro-siting of the turbines, as the conditions of the
wind turbine approvals require a minimum distance to slopes above a certain value. (The criterion is
basically a load criterion converted into a siting criterion). The site area and the surroundings shall be
shown in an iso-height contour map in order to make it possible to evaluate the potential problem and
determine restrictions, if any.
Furthermore, the vegetation (surface roughness) on the site and in the surroundings shall be described
in order to permit the assessment of the possible impact on the output of the wind turbine. The
roughness of the terrain at the site and in the surroundings influence the distribution of the wind energy
potential over the site and the wind shear at all turbine locations in all directions.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 11
Earthquake
If the project site is located in a region exposed to earthquakes the magnitude of possible earthquakes
shall be assessed and presented, e.g. as a figure on the Richter scale.
Volcanic Activity
If relevant, it shall be assessed whether the project site is exposed to volcanic activity.
Dwellings - Local Inhabitants
Many countries have regulations determining the minimum distance to dwellings and other buildings,
either measured in meters or in acceptable noise levels. All major Danish wind turbine manufacturers
have computer programs, which calculate the noise level originating from the wind turbines. Buildings
on the project site and in its close surroundings shall be shown on a site map.
High Tension Lines and other Structures
Requirements to distance to existing or planned high tension lines, wind turbines, antennas or other
structures shall be evaluated and incorporated in the project planning.
Telecommunication
Telecommunication systems broadcast at a variety of frequencies
and in a number of ways. Interference with telecommunication
systems is known as electromagnetic disturbance or electromagnetic
interference (EMI). The potential interference shall be evaluated as
well as the need for approvals by authorities.
In Australia, a radio link was
passing through a project site
and the wind turbines were
not allowed within a distance
of 50 m from the path.
Air Traffic
As is the case for all tall structures, air traffic present a potential
problem at some locations. It shall be assessed whether rules and regulations limit the tower height or
require lighting.
Other Considerations
If special local conditions not mentioned above are determined to influence the wind turbine or the
project, the impact shall be evaluated.
2.3
Practical Considerations
The practical considerations include, but are not limited to, the following issues:
C
C
C
C
C
Soil conditions
Concrete aggregates
Access roads
Crane availability
Routing of high tension overhead lines
Soil Conditions
The investigation of the soil conditions on the project site, which is relevant to installing wind turbines
on concrete foundations, shall be assessed. The investigation of the ground water level is normally part
of the soil investigation and it shall be assessed whether the project design complies with the result of
the investigation.
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Volume 2: Analysis of Technical Wind Farm Issues
Page 12
If a solution other than a concrete foundation is considered, e.g. founding direct on solid rock, the
necessary parameters shall be assessed.
Concrete Aggregates
The availability of concrete aggregates as well as water of a sufficient quality shall be investigated and
evaluated. In some areas the lack of concrete aggregates and water of sufficient quality raise the price
of foundations significantly.
It should be noted that cooling of the foundations in the period from pouring to hardening may be
needed, if the surrounding temperature is very high.
Access Roads
The assessment of the accessibility of the project
site includes harbour, train, public roads, and local
roads. Most parts of the wind turbines are packed
in 40 foot containers, but the size of the blades and
the towers makes container transport of these
items impossible. The normal means of
transportation to the site is ship, (train), and truck.
The constraints may be the load capacity of roads
and bridges, as well as sharp curves.
The length of a 40 foot container is
approximately 12 m. The length of a blade for
a 600 kW wind turbine is approximately 21 - 23
m. The size of a tubular tower varies, but the
diameter of the bottom section is approximately
2.2 - 2.8 m. Lattice towers can be transported
in containers.
The curves of a public access road to a site in
Greece were to sharp and it was impossible to
transport the blades to the site. Several schemes
were put into execution and in one instance the
project had to buy a house and tear it down in
order to straighten the road and make truck
transportation possible.
Crane
The availability of mobile cranes shall be
investigated. The investigation shall focus on the
crane capacity necessary for the construction of the
wind farm and the erection of the wind turbines
and the capacity needed for operation and
maintenance. If sufficient crane capacity is not available locally, the possibility of renting and transporting
the necessary mobile crane to the site for construction and erection shall be investigated.
Some wind turbines (up to 600 kW) can be erected by a pull and erect system without a large mobile
crane. A smaller crane for handling and assembling the wind turbine on the ground is a necessity.
Routing of the High Tension Overhead Line
Feeders connecting the wind farm to the grid will often be over head lines. However, regardless of the
connection being based on overhead lines or cables, the trace has to be assessed. It should be ensured
that the trace does not impose difficulties on the project. Difficulties may arise from:
C
C
C
C
ownership or rights of land
general or specific environmental or other restrictions
topographical, infrastructural or other hindrances for the trace itself or for accessing the trace
during construction
other
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
2.4
Page 13
Grid conditions
In the ideal situation, the grid is available at all times and is able to maintain the nominal voltage and
frequency at the connection point. It is, furthermore, at all times able to supply the needed power –
active or reactive, and able to absorb the produced power – active or reactive.
This is, however, not always the case, and for the specific project and the specific conditions it must be
assessed how severe the deviations from the ideal situation are, and to what extent they will influence
technical, economic and practical conditions.
If the wind climate is of a nature that implies sudden and/or fast changes in the power production from
large wind farms, the ability of the grid to attune equally fast shall also be assessed.
In the data collection phase it is therefore important to gather all necessary information relating to the
capability and operational conditions of the grid in general and of potential connection points including
'electrical vicinity' in particular.
C Static grid data
B Grid configuration (diagram) and basic data regarding the grid in terms of short circuit impedance
or power, line impedances and admittances, transformer ratios and impedances etc. should be
collected for the technical calculations.
C Voltage level and voltage fluctuations
B Retrieving information regarding, or calculation of, short circuit level at the grid connection point.
B Gathering information regarding voltage levels, for instance in the form of duration curves or
tables of statistical frequency distributions.
C Average frequency and frequency variations
B Gathering information regarding grid frequency, for instance in the form of duration curves or
tables of statistical frequency distributions.
C Grid availability / Uptime
B Gathering of statistical data regarding grid failures and black-outs at relevant points in the grid.
C Potential grid connection points
B Alternative possible connection points should be presented and the above mentioned information
should be collected for each of these alternatives.
C Conditions for grid connected equipment and plants
B Any regulations/conditions regarding flicker resulting from operation of grid connected equipment
and plants shall be identified.
B Any regulations/conditions regarding harmonics induced by grid connected equipment and plants
shall be identified.
C Large Wind Farms
B Limitations in acceptable MW/min or MVAr/min – both either positive or negative due to
transformer tab changing, power plant regulation or something else shall be established.
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Volume 2: Analysis of Technical Wind Farm Issues
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3 Technical Description of the Project
The technical description of the project in the feasibility study shall include a thorough description on
whether the project is in compliance with the requirements derived from chapter 2, Conditions at the
Project Site. If the parameters in the said chapter are not representative of the project site or are not in
a comparable format, the first step is to transfer or transform these parameters to cover the site
conditions in a format, which is comparable to the specifications of the wind farm or wind turbine. The
appraisal of the project shall secure that all issues are treated in an appropriate manner and are
incorporated in the project.
3.1
Wind Farm
Location
The location of the wind farm shall be shown on maps:
C Large scale - country/state level
C 1 : 25 - 50.000 - showing the wind farm borders, roads, towns/villages, plantation etc. If possible,
this map shall include iso-height contour lines and UTM coordinates.
The description of the site location shall be unambiguous and a site visit by the appraisal team shall verify
the information.
Lay-out and Infrastructure
The feasibility study shall include a drawing of the project including all restrictions determined to have
an impact on the project lay-out. The project drawing (1 : 5.000) shall include the location of the wind
turbines, the wind measuring stations, buildings, transformers,
high tension lines, roads, etc. All restrictions shall be treated to
Local or State regulations may
show how the project complies with each restriction.
include stipulations regarding
The assessment shall include, but is not limited to, all the issues
minimum distance to the borders
treated in chapter 2 - Conditions at the Project Site. The
of the project site, restrictions
assessment shall also include the restrictions determined in the
due to visual impact etc.
environmental study as well as laws and regulations issued by
governmental bodies or state organizations.
3.2
Wind Turbine
The objective of the appraisal of this
If the maximum wind speed at a height of 20 m at the site
part is to ensure that the chosen wind
is estimated at 35 m/s as a 10 min. average value and the
turbine is suitable for the project. The
limit according to the wind turbine specifications is 60 m/s
assessment of the suitability of the wind
as a 3 sec. gust at a height of 50 m, these values are not
turbine to the conditions of the project
directly comparable, but have to be transformed before
is merely a comparison of the wind
comparison.
turbine specifications and the project
conditions. The main problem in the
comparison is that some parameters are not in a directly comparable format and needs to be transformed
before the comparison.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Technical Description
The technical assessment of the wind turbine
shall include:
C Manufacturer
C Wind turbine description & specifications
C Power curve
C Approvals and site conditions
Page 15
The wind turbine specifications normally lay down
rules for the minimum distance between the turbines
on a site, but this presumes a general/uniform wind
direction distribution. If the major part of the wind
energy comes from two opposite directions, it is
possible to decrease the distance between the
turbines in a row to less than stated in the general
approval, but this has to be approved by the
institution issuing the approval of the turbine.
Manufacturer
Identification of the manufacturer of the wind
turbine. If the wind turbines are manufactured
and/or assembled partly abroad and partly locally, the relevant companies shall be identified and deemed
capable of fulfilling the contractual obligations. The assessment shall include experience and technical
capacity. Furthermore, it shall be assessed whether the type of wind turbine has an acceptable track
record.
Wind Turbine Description & Specifications
The feasibility study shall include a specific description of the wind turbine, i.e hub height, upwind or
downwind, number of blades, power regulation (pitch or stall), fixed or variable speed, 1 or 2
generators, lattice or tubular tower etc.
The wind turbine shall have a type approval issued by an internationally recognized institution in
compliance with the Danish rules for wind turbine approvals. Specifications of the wind turbines shall
be supplied by the manufacturer. It shall be assessed whether these specifications are complying with the
turbines intended to be supplied to the project, or if they are just general specifications.
The specifications shall include, but are not limited to, the following:
C Specification of main components, i.e. rotor, main shaft, main bearings, gear box, generator, yaw
system, tower, controller, lightning protection,
C Operational features, i.e. RPM, design frequency and voltage, cut-in and cut-out wind speed, survival
wind speed, turbulence intensity, temperature range, humidity, corrosion class/protection,
C Siting specifications, i.e. distance to slopes, slope angle, minimum distances between turbines,
C Requirements to crane capacity for construction and during operation, and requirements to access
roads and operation area during erection.
The description shall include special features, which correspond to special requirements, i.e. special
heating or cooling systems, special lightning protection, colour, etc.
The hub height of the wind turbines shall comply with all external and internal restrictions and
regulations and be equal to the height incorporated in the calculations of the annual energy production.
Power Curve
It shall be assessed whether the power curve used in the calculations of the annual energy production
is identical with the power curve, which is part of the approval. Furthermore, it shall be checked that this
power curve is equal to the one that the manufacturer guarantees in the contract for the supply of the
wind turbine.
It should be noted that power curves are normally presented in accordance with standard conditions, i.e.
air density = 1.225 kg/m3, temperature = 15/ Celsius , pressure = 1013 hPa, frequency = 50 Hz and a
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 16
turbulence intensity of less than 15 %. The re-calculation to site conditions is part of the corrections in
the calculations of the annual energy production.
Approvals and Site Conditions
If the site conditions are assessed to be within the specifications of the general wind turbine approval this
can be accepted, but if the site conditions are assessed to exceed the limits of the general approval, a site
specific approval shall be issued by an approval institution.
The assessment shall include, but is not limited to the following:
C Extreme wind conditions, wind speed, turbulence intensity
The maximum wind speed, which the wind turbines will experience in the lifetime of the project shall
be assessed and a comparison made with the wind turbine specifications. The assessment of this
parameter may be crucial to the project or the project economy. If underestimated, the risk of damage
to the turbines is increased, and if overestimated, the risk of not choosing the best wind turbine for
the project is increased. Furthermore, the objective is to assess the turbulence level at the project site
and compare it with the wind turbine specifications.
C Temperature interval for operation and survival
It shall be assessed whether the wind turbine specifications comply with the expected temperatures
on the site. If icing is a risk, the problem shall be addressed.
C Lightning protection
If thunderstorms are a possibility at the project site it shall be assessed whether the lightning
protection system is adequate or if additional protection will be required. In some countries back-up
by an insurance policy is a possibility.
C Site conditions, maximum slope of terrain
In complex terrain (hilly or mountainous) it shall be assessed whether the wind turbine siting (the
wind farm lay-out) complies with the specifications stated in the wind turbine approval regarding
distance to slopes. If it does not, the approval institution shall accept the discrepancies or issue a site
specific approval.
C Earthquake
Wind turbines are generally not very sensitive to earthquakes, but in regions exposed to earthquakes,
the risk of physical damage shall be evaluated. Input from the seismological institution of the
country/region shall be compared with information from the wind turbine manufacturer. If the risk
of major physical damage is high, the project shall be insured, if possible.
C Volcanic activity
If the wind farm is located in an area, which is exposed to volcanic activities, the assessment shall
include the risk of physical influence as well as the risk and influence of detrimental gasses.
C Saline air
If there is a possibility of salinity in the air, it shall be assessed whether the corrosion protection of
the wind turbine and electrical infrastructure is adequate.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
3.3
Page 17
Central Monitoring System
All modern wind turbines are designed for unattended, fully automatic operation. A computerized
controller manages all the functions of the turbines including stop and automatic start after grid failures
and other non critical incidents.
The controller normally also includes facilities for gathering data and information on the performance
of the wind turbine. The number of hours in operation, the energy production, the power curve and so
on are examples of data collected by the controller. A log book containing incidents and faults will
normally also be included in the controller.
The controller is often implemented with a main unit placed at the bottom of the tower and possibly a
sub-unit placed in the nacelle for collecting measurements and/or control of auxiliaries. It is also common
to include a possibility for remote data communication with the controller. Such a link makes it possible
to monitor the operation and performance of many turbines from one or more central computers.
If a central monitoring system is part of the project design, it shall be assessed whether it complies with
expectations for the operation and maintenance of the project. It is recommended that the central
monitoring system is part of the wind turbine supply.
3.4
Civil Infrastructure
The civil infrastructure includes roads, leveled space beside the turbines, the foundations of the wind
turbines and transformers and buildings for operation and maintenance.
Roads and Space for Cranes
It shall be assessed whether the roads are adequate for transport of the turbines on trucks and for
transport of the crane for erection. Furthermore, it shall be assessed whether the leveled space beside
the turbines is adequate for unloading and erection of the wind turbines.
Foundations
The design of the foundations shall be part of the
wind turbine supply or approved by the wind
turbine manufacturer. It shall be assessed whether
the wind turbine foundation design complies with
the soil investigation and other requirements.
Furthermore, it shall be assessed if the solution is
reflected in the cost estimate of the civil
infrastructure.
If the soil is determined to have a low load
capacity, the cost of the foundations may be
double or triple the cost of normal foundations.
Rock foundations may be cheaper as well as
more expensive than normal foundations
depending on the solidity of the rock and the
experience of the contractor.
It should be noted, that the quality requirements to wind turbine foundations are often not recognized
by local contractors and it is important to have a common understanding of the necessity for high quality
concrete and workmanship in the construction of the foundations. The possible need for inspection under
construction shall be assessed. Furthermore, the availability of concrete aggregates and water of
adequate quality shall be assessed and the influence on the cost of the foundations incorporated in the
cost estimate.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Buildings
It shall be assessed whether the buildings are adequate
for the intended use. The assessment shall include:
C Facilities for operation, including space for the
central monitoring system
C Maintenance facilities, including repair shop
(overhead crane), storing facilities for spare parts,
garage for trucks, crane, etc.
C Office facilities
C Housing for maintenance crews and security
3.5
Page 18
At a wind turbine location in a project, the
foundation bolts and the concrete reinforcement
were not “overlapping” as designed. In the first
major storm, the foundations bolts were pulled
out of the concrete and the turbine overturned.
Several examples of foundation problems could
be mentioned, relating to insufficient quality of
the materials used, incorrect vibration of the
concrete, or insufficient curing due to lack of
water or high temperature.
Grid Connection (Electrical Infrastructure)
The specific project objectives regarding various subjects related to grid connection and grid quality are
as follows:
Grid Voltage:
The wind turbine must be able to function correctly at the normal voltage
level and the voltage fluctuations actually found at the planned grid
connection point.
Extreme voltage levels must not harm the wind turbines or other wind farm
equipment.
The operation of the wind turbines – or wind farm must not imply an
unacceptable flicker level. Neither must the wind turbines emit an
unacceptable amount of harmonics.
Frequency:
The wind turbine must be able to function correctly at the frequencies
actually found in the grid. “Function correctly” could either be to shut
down during occasional instances of low frequency or to continue
operation at the lower frequency, in case such instances are more frequent.
The estimate of the energy production must include a correction factor
representing modifications of the power curve caused by a deviant grid
frequency.
Grid availability/uptime:
The grid must be available in order for the turbines to be able to feed the
produced energy into it.
A significant number of grid failures must not harm the wind turbine
structure, brakes or other wind farm equipment.
Power factor / phase angle: The wind turbines’ consumption of reactive power from the grid must not
exceed technical limits or limits imposed by utility policies.
Load and load variations:
The wind turbines must be able to feed energy into the grid whenever the
wind is blowing, thereby requiring the grid load to be equal or larger than
the wind generated power plus the minimum power from controllable
power plants.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Grid connection location:
Page 19
The grid connection location should not impose severe technical, economic
or practical problems due to the distance to the wind farm, the connection
voltage level, the proposed routing of cables or over-head-lines or other
causes.
The project must comply with many externally given conditions, but in addition to these conditions, the
chosen internal electrical layout and configuration including low voltage lines, step-up transformers,
capacitor banks, medium and high voltage feeders etc. will be of major importance for assessment of the
various subjects. These choices will often be a balancing of advantages and disadvantages in terms of
investment, operational costs, loss, flexibility, convenience etc. and hence, they will obviously influence
the financial calculations. They are, however, equally important in connection with assessment of many
of the technical subjects.
When appraising the electrical aspects of a wind farm project it should therefore initially be ensured that
the internal layout has been well defined and presented in the feasibility study phase. It would often be
expedient to have the layout presented in the feasibility study report as well and thereby immediately
available to the appraisal team.
The electrical infrastructure comprises electrical installations from the wind turbines to the grid
connection points and includes transformers, cables (underground and overhead) and possibly a
substation or part of a substation.
The analysis carried out as part of the feasibility study regarding grid connection should cover the
mentioned subjects and overall major conclusions should be extracted and presented under a separate
heading. The conclusions may be subdivided under the following sub-headings:
C Exceptional technical details; i.e. whether alterations of standard turbines and/or other equipment will
be needed and whether special equipment or technical solutions, the project taken as a whole, has
been or has to be employed.
C Influence of the electrical conditions on the project including any impact on wind turbine
performance, maintenance requirements or life time and any impact on energy yield.
C Impact of the wind farm on the existing electrical grid.
Even if the feasibility study has not employed this or a similar structure, it may in any case be useful for
the appraisal team to bear the mentioned subjects in mind.
The assessment shall include transformer and cable specifications, need for maintenance and the expected
electrical loss from the turbines to the grid connection point. Furthermore, it shall be assured that the
electrical loss are included in the economic calculations.
In sections following below, more detailed proposals for appraisal of grid connection related issues are
given. As projects and conditions may differ widely the presented proposal must be taken as a proposal,
only. In each case it should be carefully considered, whether further subjects have to be assessed and/or,
on the contrary whether some of the presented subjects are less important or directly irrelevant.
At this point, it may be worth stressing again that a carefully prepared feasibility study can significantly
ease this process and thereby speed up the appraisal.
Detailed Proposals
Having established the basic electrical <map’ of the wind farm, it is suggested that the appraisal ascertains
that the following points have been assessed and the results duly incorporated in the project.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 20
Voltage level and voltage fluctuations
It should be ascertained that:
C The normal operation voltage level does not deviate from nominal values to an extent which will
cause the wind turbine controllers to cut off the turbines (too often). The analysis should cover noload, low load and high load.
C The wind turbines do not cause unacceptable voltage variations and/or fluctuations (flicker).
C The wind turbines do not emit unacceptably large quantities of harmonics into the grid.
An assessment of these topics could include:
C Description of capacitor bank controller algorithms.
C Calculation of the fraction of total time in which the wind turbines will be cut off from the grid due
to high or low voltage levels.
C Load-flow calculations for the internal electrical grid and perhaps the nearest electrical surroundings.
C Collection of sample wind turbine data.
Some of the major pitfalls and peculiarities worth mentioning at this point are:
C Some of the often employed simplified methods for calculation of voltage drops, phase angle changes
etc. cannot be immediately adopted when dealing with distributed power generation facilities such
as wind turbines. The reason is that the simplifications are based on the concept of centralized
electricity generation plants supplying at high voltage levels and energy flowing in the electrical grid
from higher voltage levels to the electricity consumers connected at lower voltage levels. It is
therefore necessary to adopt the full and basic set of equations expressing electrical regularities.
C Too high voltage levels at the wind turbines are, of course, critical as the electrical equipment may
be damaged as a direct result of the voltage level itself. Low voltage levels are however, also critical
as they, given the same wind speed and thereby the same power production, will lead to increased
currents which in their turn can be directly or indirectly
harmful to the electrical equipment.
In conventional electricity distribution
design a frequently applied rule-ofC One of the often applied methods or system for
thumb for dimensioning aluminium
maintaining a constant voltage level in electrical
cables is a 2.0 A/mm2 limit. However,
distribution grids is compound regulation of
application of this rule in connection to
transformer tap-changers. This system is, however,
wind farms may lead to voltage levels
incompatible with distributed power generation facilities
20 % above the nominal values at the
supplying energy at low voltage levels, as it will further
wind turbine as shown in Appendix 1
amplify the voltage increase at the wind turbines at high
energy production levels.
C Voltage variations exceeding the wind turbine controller’s maximum or minimum limits for even very
short periods will often lead to outage time for the turbine of at least 10 minutes as a <dead-time’ is
commonly part of the controller algorithms. In a situation in which the voltage level exceeds the limits
for just one second, five times during a day (just five seconds in total) the wind turbine will be out
of operation for a minimum of five times ten minutes or as much as 3.5 percent of the time.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 21
Average Frequency and Frequency Variations
It should be ascertained that:
C The normal operation grid frequency does not deviate from the nominal value to an extent which will
cause the wind turbine controllers to cut off the turbines (too often).
C The wind turbine power curve used in the AEP calculations – and in turn in the economic calculations
is based on actually occurring frequencies rather than nominal values.
C The wind turbines do not cause unacceptably high levels of harmonics to be fed into the electrical
grid.
An assessment of these topics could include:
C Calculation of the fraction of total time in which the wind turbines will be cut off from the grid due
to too high or too low grid frequency.
C Transformation of wind turbine power curve applicable under standard conditions.
C Collection of sample wind turbine data.
A common situation in electrical grids suffering from low generation capacity reserve is a lowered grid
frequency as this can be used to reduce grid load and thereby stabilizing the whole grid. However, it
must be noted that even a very small change in frequency will significantly change the wind turbine
power curve. Energy production calculations must consequently be based on one or more modified
power curves, when grid frequency deviates from the nominal value for a considerable part of the total
time.
Grid Availability / Uptime
It should be ascertained that:
C Grid availability is satisfactory.
C Wind turbine outages as a result of grid failures/black-outs are considered when the energy
production estimate is calculated.
C Repeated grid failures will not harm wind turbine structure and/or reduce wind turbine life-time.
C Possible increased operation and maintenance costs due to grid failures and frequent turbine shut
downs are considered in the economic calculations.
An assessment of these topics could include
C Collection of sample wind turbine data.
Again to illustrate: Comparable to the situation regarding voltage variations exceeding the wind turbine
controller’s maximum or minimum limits, even very short grid <black-outs’ will often lead to outage time
for the turbine of at least 10 minutes as the mentioned <dead-time’ also applies to grid failures. Similarly,
a situation in which the grid experiences even very short black-outs of only a fraction of a second or less
for instance five times during in a day (just five seconds in total), the wind turbine will be out of
operation for a minimum of five times ten minutes or as much as 3.5 percent of the time.
Power Factor Compensation
It should be ascertained that:
C The grid is capable of supplying the needed reactive power without destabilizing the grid.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 22
C Local reactive power compensation is included in each wind turbine enabling reduced voltage
increases in feeders, reduced loss and lowered demand on feeder dimensions.
C Possible capacitor banks are controlled so that a situation in which the wind farm is cut off from the
grid will not result in potentially damaging high voltage levels
within the wind farm.
Inclusion of capacitor banks in
C The possibility of offering reactive power support to the grid
the wind turbine can reduce
using internal, central capacitor banks has been assessed.
currents by 10 % thereby
An assessment of these topics could include:
reducing loss by 20% and
demands on cables etc. See
C Analysis of controller algorithms.
Appendix 2
C Load-flow analysis.
C Collection of sample wind turbine data.
It is worth bearing in mind that with minor changes of a wind farm design it is possible to provide the
utility with a support of the grid giving stabilized grid node voltages and reduced grid loss. It will require
controlled infusion of reactive power to the grid, but the increased costs can be counterbalanced by
better PPA provisions or by a special agreement regarding supplied reactive power. It is necessary to
assess the diurnal, monthly and yearly variations of the wind speeds as the available free reactive capacity
is a result of the wind farm power production and the resulting internal demand for reactive power.
Energy Need/Demand/Consumption: Load Variations
It should be ascertained that the energy demand in the grid at all times is sufficient taking into
consideration the technical minimum production of existing power plants being on-line.
An assessment of this topic could include:
C Gathering of statistical data on grid load.
C Forecasting of grid load development.
C An analysis of wind generated electricity based on statistical data on the wind variations.
The grid load will often vary periodically on a daily, monthly and annual basis as a result of the electricity
consumers’ pattern of consumption.
In grids where the planned and existing amount of wind power will be comparable within a range to the
minimum grid load, there is a risk that the sum of the technical minimum production on-line power plants
and electricity produced on wind turbines will exceed the immediate grid load. Such situations are
undesirable as they will result in loss of energy and, probably, money. What will actually happen cannot
be foreseen in detail because it will depend on the specific design and adjustment of power plant
controllers and wind turbine controllers.
Wind generated electricity will vary with the periodical and random changes in wind speeds and with any
systematic, or periodic variations in the wind. Examples are morning and evening sea breeze and similar
phenomena in connection with plateaus/tablelands. If the wind farm in question is significant in relation
to the occurring power flows in the grid, it shall be evaluated whether the changes in power output from
the wind farm is exceeding the capabilities of the grid ( typically regulation of transformers, capacitor
banks etc.)
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 23
Grid Connection Location
It should be ascertained that an optimum grid connection point has been elected.
If alternatives exist, the assessment will often include
C an analysis of short circuit levels, voltage variations etc
C an assessment of the various implications of the alternative connection points
C an economic assessment
Electrical Equipment Specifications
Depending on the actual project site, it may be relevant to pay special attention to certain electric
components of the wind turbine. Some of the major subjects in this respect are presented in the
following.
Humidity and Salinity
At sites with high salinity content of the air and/or high humidity, special care must be taken to ensure
satisfactory operation of the components throughout the anticipated lifetime of the wind turbine. This
applies both to components placed outside such as sensors, transformer equipment etc. and to
components placed inside the nacelle, in a possible lattice tower or in a cubicle.
Insolation
At sites with significant insolation, cables hanging from the nacelle, sensor cables and other electrical
equipment exposed to the sun must be designed bearing this in mind.
Generator
The class of the isolation is often F and the temperature rise at nominal power is according to class B.
This allows the wind turbine to produce electricity also under meteorological conditions giving a power
production in excess of the nominal power without damaging the isolation material.
Depending on the climatic conditions, generators must be fitted with heating elements preventing
condensation of water vapour inside the generator and special attention should be paid to heating of the
coil ends.
Circuit Breakers
If frequent tripping of the circuit breakers will occur due to grid conditions or other causes, it should
be ensured that this has been included in the design basis.
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Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
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4 Annual Energy Production
Estimation of the annual energy production (AEP) of a wind turbine or a wind farm is basically a
multiplication of the wind turbine power curve by the wind distribution at the wind turbine site, followed
by appropriate corrections for loss and special conditions.
The gross annual energy production (AEPgross) estimate is a
fictive figure expressing the result of the multiplication of the
wind distribution and the power curve without corrections and
loss. In this document AEPgross is used to denote this figure
taking the wind farm as a whole. In certain special situations,
some of the corrections may have been included in the
calculation of AEPgross.
If the power curve is available for
different air densities and the
power curve used in the AEPgross
calcu la t io n is t he o ne
representative of the air density
on the site, this correction is
incorporated in the AEPgross
figure.
The AEPnet is the modified AEPgross, which takes loss and
corrections into consideration. The approach is described in
following paragraphs. The financial calculations shall be based on the AEPnet figure.
4.1
Objectives and Methods
Objectives
The objective of the appraisal of the AEP calculation is to ensure that a realistic figure for the expected
output of the wind farm measured in MWh/year is used in the financial calculations.
The objective of calculating the net annual energy production (AEPnet) is to estimate the long term mean
annual production (MWh/year) that can be expected to be supplied from the wind farm to the grid at the
metering point.
The objective of the wind data analysis is to determine the long term mean distributions of annual wind
speed and wind direction, representative of the conditions on the project site in the lifetime of the
project.
Method
The prediction of the behaviour of the wind rests on an analysis of the past. There are a number of
methods applicable to the task of assessing the basic data and reaching the result. A detailed description
of just one AEPnet calculation is therefore not desirable.
The first step in the general method is to establish a wind distribution and a wind direction distribution,
which are representative of one point on the site at the height equal to the hub-height of the wind
turbines. This wind distribution is often referred to as the “project wind distribution”, but it should be
kept in mind that it actually refers to one point at the site, which is not necessarily an average point.
The general procedure is as follows:
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
1.
2.
3.
4.
Establishing the mean annual wind distribution:
Determining the wind turbine power curve:
Calculation of the gross annual energy production:
Determining correction and loss factors:
5. Combining correction and loss factor:
6. Calculation of the net annual energy production:
Page 25
WDProject, MA
PCWT
AEPGross = WDProject, MA @ PCWT
fc1, fc2, fc3, ... and
fl1, fl2, fl3, ... respectively
fcombined = fc1 @ fc2 @ fc3 @ fl1 @ fl2 @ fl3, .....
AEPnet = AEPgross @ fcombined
The most important and difficult part is the assessment and determination of the wind energy resource
on the project site for the lifetime of the project represented by WDProject, MA, although the assessment of
the corrections and loss also have a significant impact on the final result.
The calculation of the AEPnet must include all relevant parameters, which means that all issues have to
be taken into consideration and applied to the calculation in an appropriate manner.
Uncertainties
Both the data and the methods apply uncertainties to the AEPnet estimate and whenever possible
uncertainties shall be assessed and/or sensitivity analyses conducted. If a great deal of reliable
information on the past is available, it means less uncertainty. If, on the other hand, the information is
less reliable, it means increased uncertainty. It shall be assessed whether the reliability of the basic data
is acceptable.
4.2
Analysis of the Wind Regime
The aim of the wind data analysis is to determine the long term mean annual wind speed and wind
direction distribution, which is representative of the conditions on the project site based on available
wind measurements. In the preparation of the establishment of larger wind farms (more than 5 to 10
MW) wind data are often collected by more than one measuring station at the site and there are two
methods, which can be used in the calculation of the AEP's. The choice of method depends on the length
of the individual measuring periods and the complexity of the site (the orography).
Method 1 One mast is nominated as reference mast and the others as secondary or supporting masts.
If a flow modeling computer program is used in the analysis, the secondary masts are used
to check the program’s capability of calculating the wind energy over the site area and
possibly determine correction factors for certain areas of the site.
This method is normally used when one measuring mast is placed in a central position on the
site and has a long measuring record whereas the other measuring masts are located near the
borders and have short measuring records.
Method 2 Each mast covers the local area in the vicinity of the mast. This method is normally used in
complex terrain and when all masts have long measuring records.
On-Site Wind Measurements
The analysis of the on-site wind data shall include:
C mean annual wind distribution
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 26
C monthly wind distribution
C periodical variations, energy content.
C wind direction distribution, energy content in all directions.
If the measurement period is not a number of whole years, the data shall be converted to cover annual
periods taking into consideration that a straightforward ratio calculation is not correct.
Vertical Transfer (Wind Shear)
In many projects the wind speed is not measured at a height equal to the hub height of the wind turbines
and a vertical transfer of the data is therefore necessary. The vertical transfer is expressed in the wind
shear, which is the wind speed as a function of the height above ground level, which in flat homogeneous
terrain can be approximated by the formula:
V2/V1 = (H2/H1)" where " is the wind shear exponent
The shape of the wind shear is basically a
function of the terrain roughness, but in hilly
terrain it is also a function of the orography. This
means that the wind shear varies from place to
place over the project site and in all directions
and it shall be kept in mind that the general
formula must only be used with great caution.
In a project in California located in complex
terrain the turbines were installed in accordance
with the “wind wall” concept according to
which the turbines are installed in one very
closely spaced row with different hub heights.
In some areas there was a negative wind shear
(i.e. a decrease in wind speed with increased
height above ground level) which made the
lowest turbines produce the most energy.
The vertical transfer of the wind speed has a great
impact on the calculation of the wind distribution
at hub height and it is recommended to measure
the wind speed as close to hub height as possible in order to reduce the correction factor and thereby
diminish the uncertainties. The assessment of the wind shear and how it is applied in the calculation shall
be carried out in due consideration of the orography and the surface roughness. It is recommended to
conduct a sensitivity analysis of the vertical transfer. See Appendix 3.
Horisontal Transfer
The aim of the horisontal transfer is to determine the wind speed distribution at locations other than the
location of the measurement, in particular at the individual turbine sites.
There are various approaches and the choice of the one to be used in a project depends on the format
and the volume of the wind data and the topography of the project site and its surroundings.
Wind Flow Modeling
Computer programs, such as WAsP developed by RISØ, Denmark, models the wind flow over the site
based on measurements and a thorough description of the topography on the site. This description
includes digitized maps, determination of terrain roughness in a circumference of 20 km from the site,
and determination and description of obstacles. The transfers made by the programs are both vertical
and horisontal, but it should be kept in mind that most programs are not intended for use in complex
terrain.
It is recommended that wind data are obtained from at least two measuring masts in order to check the
program’s capability of modeling the wind flow on the site.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 27
Correlation Calculation
Calculations of correlations are used when having two stations measuring at the same time. The wind
speed data pairs (V1,V2) can be plotted in a diagram to make the correlation curve showing the
correlation between the two sites.
If used with caution the correlation can be used to extrapolate a short measurement period from one
station to cover the longer measurement period of another station. It should be noted that the correlation
changes by direction (orography and surface roughness) and that it may not exist. Furthermore, it may
not be the same in a high and a low wind season. If the correlation line is determined by the least square
root distance to plotted points, the standard deviation is a measure of the uncertainty of the correlation.
The method is used when having measurements from one station (reference station) for a long period
(one to several years) and other stations for shorter periods.
Ratio Calculation
The “ratio” method has been widely used in the USA and is a simplified version of the correlation
method. The ratio between the mean wind speed of two measuring stations for a certain period expresses
the ratio for all periods. The method is not recommended as better options are usually available.
4.3
Micro Siting
The wind turbines of a project can be situated on the project site in accordance with different lay-outs.
The micro siting is the determination of the optimum lay-out, i.e. the siting of the wind turbines, which
optimizes the project economy.
The micro siting process has two purposes:
1)
2)
to determine the best configuration of the wind turbines on the site with respect to
restrictions, wind conditions and project economy
to determine the corresponding AEPnet
How much the micro siting process can improve the AEP depends on the size of the project, the
complexity of the site, the constraints and the wind direction distribution, but in many projects it is not
unrealistic to expect the micro siting process to improve the AEP by 2 to 4 %. As shown in Appendix
4, the cost of the measuring campaign and the micro siting may be paid back by the improved production
in 3 to 5 years.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
4.4
Page 28
Wind Turbine Power Curve
The power curve, which is guaranteed by the wind turbine manufacturer shall be used in the AEP
calculation. All conditions attached to the power curve shall be compared with site conditions and the
appropriate corrections applied in the calculation of the AEPnet.
4.5
AEPgross Calculation
After establishing a “site” wind distribution at a specific point on the site and at a height equal to the hub
height of the wind turbines, the AEPgross can be calculated as previously described in this chapter by
applying a multiplication-like technique.
When “multiplying” the power curve by the wind distribution it is of great importance that the wind
speed intervals at the X-axis of the two curves are equal. It is often seen that e.g. V = 6 m/s is the
interval from 5 m/s to 6 m/s with regard to the wind distribution and that it is the interval from 5.5 m/s
to 6.5 m/s with regard to the power curve. This error could lead to an overestimate of the AEP of 10 15 %.
If the wind distribution is calculated at each individual wind turbine site the AEPgross will be the sum of
the individual estimates. If the wind distribution is calculated for one specific site, the AEPgross will be
the estimate of this site multiplied by the number of turbines and a correction factor. The correction
factor shall be 1.0 if the specific site is calculated as an average site compared with the wind turbine sites.
4.6
Correction and Loss
The corrections and loss are treated as being independent, i.e.
the combined correction and loss factor is a multiplication of
the individual factors. All relevant parameters/issues shall be
discussed and the values assessed.
If one loss is assessed at 14 %
and another loss is assessed at 16
%, the combined loss of these
two parameters will be 0.86 *
0.84 = 0.72, i.e a combined loss
of 28 %.
Air Density
The air density is a function of the air temperature and the air pressure. The mean air density at the site
can be calculated by
where T is the mean temperature and P is the mean pressure at the site. T0 = 15oC and P0 = 1.013 hPa
are standard conditions, which correspond to the standard air density of Do = 1.225 kg/m3.
It shall be kept in mind that the power curve, which is part of the wind turbine approval, is only valid
under standard conditions and that the influence of the actual air density on the site shall be incorporated
in the AEP calculation.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 29
The energy of the wind is proportional to the air density and so is the output from a stall regulated wind
turbine from cut-in wind speed up to the stall level. For a pitch regulated turbine, the power curve does
not change in proportion to the change in the air density and in that case a recalculated power curve
(valid under site conditions) has to be used either in the AEPgross calculation or in the determination of
the correction factor.
Correction factor, air density, stall regulated WT’s
fc = D / Do
If the actual air density has not been incorporated in the AEPgross calculation, a correction has to be
applied in accordance with the conditions on site.
Wake Loss
A turbine situated behind another turbine, as seen from the wind direction, will experience a decrease
in wind speed compared to the situation in which there is no turbine in front of it. This effect is called
the “wake loss” and one of the objectives of the micro siting process is to minimize the loss in due
consideration of the project constraints. The computer program PARK, or a similar program can be used
to calculate the wake loss, but it shall be kept in mind that a prerequisite for using the program is that
the project is located in flat terrain. If the project is located in complex terrain the wake loss calculated
by PARK will be conservative in most situations.
The wake loss shall be assessed and corrections applied in accordance with the lay-out.
Blade Contamination
The power curve used in the calculation of the AEP is valid only if the blades are clean and smooth. In
some areas bugs and/or dirt may build up on the surface of the blades and this will have a negative
impact on the output from the turbines. The effect may be reduced by washing and polishing the blades
as part of the maintenance programme.
The potential problem shall be addressed, although it can be hard to quantify, if data from existing wind
turbines in the area are not available. The influence on the power curve is most severe in the stall area,
i.e. wind speed above 10 m/s, and hence most critical in the high wind season. It shall be kept in mind,
that if the potential problem is not quantified, the impact is automatically determined to be none.
Availability Loss - Wind Turbine
The wind turbines are not always available due to maintenance
or repairs. The availability loss due to scheduled maintenance
and small repairs should not exceed 1- 2 %, but depend, of
course, very much on the service organisation and the spare
part inventory.
The availability loss of modern turbines (designed after 1986 88) has proven to lie in the interval of 1 - 5 % for the first 10
years. There is, of course, no experience of the availability for
the 10 - 20 year period.
The availability loss shall be assessed and the corresponding
correction applied in the calculation.
In a project in China, 3 out of 6
wind turbines experienced cracks
in the brake disk, which needed to
be replaced. The manufacturer
immediately shipped 3 new brake
disks to China and the down time
of the turbines should not have
exceeded 2 weeks. Unfortunately,
the shipment was delayed in
customs and it took 3 months
before it was released.
The impact on the availability of
this incident should have been 2 %
(3/6 A 2/52 A 100), but ended up as
12.5 % (3/6 A 3/12 A 100) for the
whole project in a one year period.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 30
Transformer and Line Loss
The transformer and line loss is an electrical loss calculation, taking the dimensions of the cables and
transformers into consideration. The loss is very much depending on the distance between the turbines
and the point of connection to the grid.
In most projects the transformer and line loss lies in the interval of 2 - 4 % after optimization.
Grid and Controller Related Loss
Grid failures such as black-outs, brown-outs, frequency variations etc. will result in the controller
shutting down the turbine and preventing it from starting again for a certain period (e.g. 10 to 15
minutes.
Similarly, extraordinarily high wind speeds or turbine vibrations caused by wind turbulence will cause
the turbine to shut down for a certain period of time equal to or longer than the 10 to 15 minutes
mentioned above.
If such situations will occur frequently the consequence in terms of reduced energy output from the
turbine has to be included in the AEP calculation as one or more loss factors.
Long Term Correction
The wind regime and the mean annual wind speed change from year to year and these inter annual
variations differ widely from place to place. It is not unusual that the mean annual wind speed of an
individual year is up to 10 % from the average or long term mean annual wind speed. Expressed in
energy production from a wind turbine or the wind farm, the inter annual variations are up to +/- 15-25
% of the long term average.
Due to these variations, it is very important to assess the level of the measuring period used in the
calculations and compare it with the long term average. The long term correction can either be applied
to the site wind distribution before the AEPnet is calculated or after. If the correction is applied after
calculating the AEPnet , the correction shall be the ratio in terms of energy production and not the wind
speed.
Long term wind data are often very hard to obtain. If available, the quality of the data is often poor
and/or they are measured far from the site and/or under topographical conditions, which make the them
difficult to use. A long term period should preferably be more than 10 years - and mathematically more
years means a better estimate of the long term average. However, it should be kept in mind that many
sensors at meteorological stations become worn over time and that local obstacles such as houses and
trees do not remain the same, which can have a significant influence on the measurement, seen over a
period of 10 to 20 years.
The assessment of the long term mean annual wind speed distribution and the assessment of how much
the period used in the AEP calculation differs from the long term mean is often one of the biggest
contributions to the uncertainty of the AEP estimate.
4.7
AEPnet Calculation
The AEPnet is the output from the wind farm which as an average is expected to be supplied to the grid
at the connection point and it is the figure that will form the basis of the calculations of the income.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 31
The AEPnet is calculated by multiplying the AEPgross by the combined loss and correction factor.
AEPnet = fcombined * AEPgross
4.8
Uncertainty Estimate / Sensitivity Analysis
The AEPnet estimate is one of the most important parameters in the economic calculation of the project’s
feasibility and it is recommended to present
a base case scenario
a low case scenario
a high case scenario
in which the latter two reflect the uncertainties involved in the estimate of the base case.
- The base case estimate shall represent the situation, which is assessed to have the highest probability
of occurring
- The low and high case estimates shall cover a 95 % confidence interval, i.e. the interval within which
the mean AEPnet (actual in the lifetime of the project) will lie with a probability of 95 %.
The assessment of the uncertainties is not always a mathematical calculation, but rather a subjective
assessment of all the parameters and their impact on the project made by an experienced wind energy
specialist.
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Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 32
5 Tendering and Bid Evaluation
The objective of the appraisal of the tender process is to assure that the bid evaluation process is
conducted in accordance with applicable rules and regulations and that the comparison is made on
equal/comparable terms.
5.1 General Rules and Guidelines for Procurement under Danish Mixed Credits to China
Procurement procedures must adhere to the ‘Rules and Guidelines for procurement under Danish Mixed
Credits to China, September 2000'. This document is available at Danida’s Secretariat for Mixed Credits.
5.2 Quotation
Before issuing a request for quotation (RFQ), the approximate size of the turbine should be determined
and the market examined for relevant wind turbines and manufacturers. Danida only accept approved
Danish wind turbines and a list of applicable wind turbines will be provided by Danida upon request.
At least 3 wind turbine manufacturers should be invited to bid on the project and the quotation should
include and be subdivided into the following points:
C
C
Supply of wind turbines including transport, insurance, erection and commissioning.
The supply of wind turbines should include a 95 % power curve guarantee and a statement on how
the offer complies with special requirements in the RFQ.
Terms of delivery and payment.
If relevant:
C
C
C
C
C
Consumables and spare parts for 2 - 3 years of operation
Training package in operation and maintenance
Central Monitoring System (CMS)
Tools for erection and maintenance and technical documentation
Foundations (optional)
5.3 Bid Evaluation
When comparing the bids, the main parameter is the price per kWh. However, other technical and
organizational issues should be taken into consideration.
If all other issues apart from the price and the annual energy production (AEP) are assessed to be equal,
the offer having the lowest price/kwh should be considered the best offer.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 33
Project Cost
The objective of estimating the project cost for each individual offer is to determine the cost side of the
comparison on equal terms.
If the RFQ is based on a turn-key project, the prices should be adjusted to cover comparable supplies.
For example, if one quote is including an extra set of spare part blades and the other quotes do not have
this included, the first quote needs to be adjusted.
If the RFQ is based on a wind turbine supply including erection and commissioning, estimates on
foundation prices, civil and electrical infrastructure, CMS etc. for each individual offer should be added
to form an equal base for comparison.
If different terms of payment or delivery can be quantified, the relevant adjustments should be included
in the comparison.
AEP Calculations
The calculations of the expected AEP for each offer have to be based on the same prerequisites and be
as close to the realistic values as possible, without making individual wind studies for each offer.
The calculations of the individual AEPs pertinent for each offer in the comparison, are a multiplication
of the guaranteed power curve (included in each offer) by the project wind distribution corrected by
applicable correction factors.
Project Wind Distribution
From the wind data analysis or the final wind study, a project wind distribution can be determined. This
wind distribution is valid for a specific height and in principle also for one location on the project site,
which should be an average wind turbine site.
If the turbines are offered with different hub heights, the wind distribution has to be transferred to the
relevant hub height by a “wind shear” transformation. It should be noted that an overestimate of the wind
shear will give an advantage to the highest turbines and an underestimate will give an advantage to the
lowest turbines.
AEP Corrections
Corrections of the AEP should follow the lines of the AEP calculation in the wind study, although the
determination is less critical if the same value is used for all offers. The AEP does not necessarily have
to be the AEPNet as long as the estimates are comparable and are based on the same prerequisites.
Only loss and corrections significantly different in the various offers should be included in the
calculations.
C
Air Density and Grid Frequency
It should be noted that wind turbines based on different concepts (stall, pitch or variable speed) do
not loose equally amount of energy due to low air density or low grid frequency.
C
Wake and Transformer and Line Loss
Only if different numbers of turbines and the project site are assessed to make the wake loss or
transformer and line loss significantly different, these loss should be included.
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 34
C
Turbulence and Control Loss
Only if this loss is assessed to be significantly different in the comparison of the different wind
turbines, the loss should be included.
C
Blade Contamination
It should be noted that wind turbines based on different concepts (stall, pitch or variable speed) do
not loose equally amount of energy due to blade contamination.
C
Availability loss, Wind Turbines
This loss could be different due to different forms of co-operations between the Danish manufacturer
and the local part responsible for the O & M. If nothing indicates a difference, this loss is equal for
all offers.
5.4 Technical Assessment
Technical specifications should be included in the RFQ and it should be assessed whether special
requirements will have an economic influence on the project. Furthermore, it has to be assessed whether
the turbines are suitable for the project and project organisation.
The assessment shall include, but not be limited to, the following:
C If a site specific approval is necessary.
C The chosen wind turbines should have an acceptable track record.
C It has to be assessed whether the turbines need special crane capacity or special requirements to the
access road.
C It should be assessed whether the offered turbines have special requirements to operation and
maintenance, such as size of service crane.
5.5 Organizational Assessment
As part of the evaluation of the offers, it shall be assessed whether the individual manufacturers are
strong enough (economically and technically) to live up to their obligations, including delivery time,
training capacity and experience.
The organizational assessment should include a company profile (size, market share - local and global,
local representatives, maintenance organisation etc.).
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Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Appendix 1 - 4
Page 35
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 36
Appendix 1
Voltage level calculation, an example
Wind turbines of the typical Danish concept1 are in general producing active power and consuming
reactive power. This is not the usual situation in connection with conventional design of low voltage
feeder design and calculations to be used for electricity distribution. Therefore, special care should be
taken. The ruling factor is in the case of designing wind farm connections normally not the cross section
of the cables used and it is therefore not sufficient to confirm whether the chosen cables are capable of
carrying the currents in question.
To illustrate this fact, we look at a
sample wind farm electrically connected
as sketched in the figure.
Node A is the connection of the wind
farm to the utility grid. The connection A
to B is the feeder, and B is the switch
board at the wind farm. Connection B to
C is high voltage connections within the
farm and C to WT is one of the low
voltage connections of the turbines.
In the example, we assume that node A
(the grid) has a voltage level of 11.0 kV
with allowable variations of ± 5%. Node
B is connected to node A via two
parallel cables. The length is 20 km and
the cross section of each cable is
Figure 1, Sample grid connection
3×150 mm².
At node B, a number of cables are connected, each feeding the power produced on a group of turbines.
The details are shown for one of the groups only.
The shown group consists of a 900 m long 3×150 mm² cable connected to a 11.0/0.4 kV transformer.
The low voltage side of the transformer is connected to a bus, to which three wind turbines are
connected; only one shown in detail. The length of the cables from the bus to the wind turbine shown is
350 m and the cross section of one cable is 3×150+75 mm². The 'WT' on the drawing denotes wind
turbine.
Based on the active and reactive power, P and Q, we can in general calculate the current in a cable:
(1)
Applying this formula, we calculate the phase-currents Iph in each of the cables WT to C, C to B and B
to A as follows:
1
By the Danish concept is – at least as of year 2000 – normally understood a threebladed wind turbine with the rotor placed up-winds, equipped with a gearbox and
a asynchronous generator.
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Volume 2: Analysis of Technical Wind Farm Issues
Page 37
(2a-c)
We notice that none of the currents exceeds 2.0 A/mm², which is a rule-of-thumb limit for aluminium
cables often applied in connection with design of conventional electricity distribution systems.
If the production in the wind farm is zero and the voltage level at node A is 11.0 kV, the voltage level
at the wind turbine will be 400 V, as there are no currents and hence no voltage drops in the cables. (The
cables' charging currents have been ignored and the nominal voltage level is assumed to be 400 V).
We will now calculate the voltage at the wind turbine, if all the turbines are producing the nominal power
and the voltage level at node A is 11.0 kV plus the allowed variation 5% equal to 11.6 kV. The voltage
level at node B can be calculated as:
(3)
where
(4)
The voltage levels of the nodes B and at the high tension side of the transformer at C is now calculated
as follows: (the phase angles are ignored in the results)
(5a-b)
The voltage level at the low tension side of the transformer is calculated from the nominal ratio:
(6)
Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 38
The voltage level at the wind turbine can now be calculated:
(7)
We see that in the case the wind turbine and its components will be exposed to a voltage level 20 %
higher that the nominal voltage, which is not acceptable.
We also see that the origins are as follows: 5 % from variations in voltage at the point of connection to
the grid, 7 % in the high tension feeder and 6 % from voltage increase in the low voltage connection.
A wind turbine is normally not designed to withstand a voltage increase of more than 10%. Whenever
the voltage level exceeds that limit the controller will stop and disconnect the turbine in order not to
damage the generator, motors and other components. A design as outlined above will hence result in a
great loss of energy as the turbine will often be halted due to 'over voltage'. Following rules of thumb are
therefore normally applied:
- maximum voltage variations at the connection to the grid is ± 5 %,
- maximum voltage drop/raise along the high tension feeder is 1.5 % and
- maximum voltage drop/raise in the low tension connection is 2.5 %
This rules leaves approximately 1 % as margin.
It is hereby demonstrated that calculations of the voltage drops are crucial and often the ruling factor for
selection of cables!
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Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 39
Appendix 2
Current and compensation calculation, an example
In order to give an idea of the size of the currents we calculate the phase current Iph for a 400 V generator
with a nominal power of 300 kW and a cosn of 0.89:
(1)
Using the same example as above we can calculate the nominal size of the reactive power consumption
of the generator, Qnom:
(2)
The reactive power, or part hereof, needed for excitation of the squirrel cage generator is, as mentioned
earlier, generated by capacitor banks placed in the cabinet.
In order to minimize the risk of isolated operation of the wind turbine and the drastically increased
voltage levels prone in such situations, the capacitors installed in the turbine are limited to the size of the
consumption of reactive power at idle operation of the generator.
If the wind turbine is equipped with a 100 kVAr capacitor bank for compensation, the phase current
Iph,comp is then limited to:
(3)
Inclusion of a 100 kVAr capacity bank will, hence, reduce the phase currents by 10%. Line loss are
proportional to the square of the line current and the loss will hence, all other things being equal, be
reduced by some 20%.
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Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 40
Appendix 3
The 1/7 -law
The wind shear formula is often referred to as “the 1/7 power law”, which indicate that the exponent, ",
in the formula is 1/7 or approximately 0.14. This is actually a specific value corresponding to “levelled
grass plains” or a surface roughness of 0.01 m and must only be used under these conditions.
In order to quantify the influence on the AEP calculation using different " two examples are shown
below. ) AEP is the ratio between the calculated AEP at hub height and the calculated AEP at the
measuring height.
A: Height of measurement = 10 m. Hub-height = 50 m.
" = 0.1
" = 0.14
" = 0.2
)AEP = 1.34
)AEP = 1.52
)AEP = 1.76
The example shows that if " = 0.2 is used in an AEP calculation instead of the real value of " = 0.1 the
result will be an overestimate of 1.76/1.34 = 1.31 or 31 %
B: Height of measurement = 20 m. Hub-height = 50 m.
" = 0.1
" = 0.14
" = 0.2
)AEP = 1.20
)AEP = 1.28
)AEP = 1.40
The example shows that if " = 0.2 is used in an AEP calculation instead of the real value of " = 0.1 the
result will be an overestimate of 1.40/1.20 = 1.17 or 17 %
The example shows that if the measuring height is increased the uncertainty due to the estimate of the
wind shear is decreased.
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Guidelines for Preparation and Evaluation of Investments in Wind Farms
Volume 2: Analysis of Technical Wind Farm Issues
Page 41
Appendix 4
Economic benefit from micro-siting
An example of the economic benefit of an improved micro siting.
Conditions:
Micro siting:
2 % increase of the annual energy production.
Project size:
Wind turbine size:
Number of wind turbines:
Mean annual wind speed:
AEPgross per wind turbine:
Combined loss and correction factor:
Price or value per kWh:
Project lifetime:
24 MW
600 kW
40
7 m/s (Weibull distribution)
1,55 GWh
0.85
0.55 RMB/kWh
20 years
Increased economic benefit per year:
Total increased economic benefit in project lifetime:
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11.6 mill RMB