In the technical specification of a wind turbine, the power curve is given for normal conditions, in particular, ambient temperature t0 = °15 C and air density
0 0 3
( , ) 1.225kg t h = m
ρ . In turn, the air density depends on temperature and height above sea level and can be determined by the ideal gas law [14]
( , ) 3.4837 P h
( )
t h = ⋅ t
ρ (4.9) where
t
is ambient air temperature[
K] andР
is atmosphere pressure [kPa] depending on the height above a sea levelh2
( ) 101, 29 0, 011837 4, 793 10 7
P h = − ⋅ +h ⋅ − ⋅h
(4.10) In accordance with GOST R 54418.12.1—2012 or IEC 61400-12, if the average air density at the site differs by more than 0.05 kg/m3 from1.225 kg/m3, then the power curve provided by wind turbine manufacturer should be corrected [24].Thus, it was necessary to determine
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the air density at the tower height subject to the average annual air temperature at MS Chavanga (t=1.2 C°
)
for the previously selected wind turbine. The results were obtained by equation (4.10) and presented in Table 4.4.Table 4.4 Assessment of the need to correct the power curve taking into account the actual air density
Wind turbine SWT-3.6-120
WT,m
H 90
( ), kPa
P h 100.229
WT 3
( , ),kg t H m
ρ 1.2727
0
WT 0 3
( , ) ( , ) ,kg t H − t h m
ρ ρ 0.0477
The deviation of air density at the tower height at the offshore WPP site from the value at the mean sea level does not exceed the normalized value (0.05 kg/m3).Therefore, for the pre-selected wind turbine placed near MS Chavanga, the correction of the power curve for the actual air density was not required.
4.4.2 The assessment of ice impact on the power production
Atmospheric icing significantly reduces aerodynamic characteristics of wind turbines, since the blades aerodynamics are sensitive to the additional surface roughness and shape change caused by ice. As a consequence, the lifting force decreases and the drag increases, which leads to a reduction of output power and, ultimately, to the shutdown of the wind turbine.
Basically, the icing losses are determined according to the intensity, duration and frequency of icing, the maximum ice load, the type of ice and their change over time [25].
Assuming the rough assessment, icing losses might be determined in accordance with the power curves of the wind turbine for different icing conditions, which are shown on Figure 4.2. For the wind turbine SWT-3.6-120 with pitch regulation, the dependence of the losses percentage on wind speed for “rime ice” meaning white-colored formation of ice with air gaps between the frozen particles and “frost” conditions characterized by soft low-density snow-like formations was established and presented on Figure 4.3 [26].
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(a) (b)
Figure 4.2 Impact of icing conditions on power curves for wind turbines with stall-control (a) and pitch-control (b) [26]
Figure 4.3 Dependence of wind turbine power loss on wind speed for “rime ice” and “frost”
conditions
Further, the annual power production of the wind turbine was calculated provided that “rime ice” conditions exist in the temperature range from 0 to -5 °C and “frost” conditions occur in the temperature range from -5 °C to +5 °C assuming the humidity is permanently high [14]. Depending on the wind speed at the tower height, the percentage of power loss was determined according to Figure 4.3. The results of calculations of icing losses for the one year period are presented in Table 4.5 and on Figure 4.4.
Table 4.5 Icing losses estimation for SWT-3.6-120 at the site of the offshore WPPin Murmansk region
Month 1 2 3 4 5 6
Full energy, MWh 1653 1165 1464 1728 1420 1222
Energy subject to losses, MWh 468 727 1090 1514 1385 1222
Icing losses, % 71.7 37.6 25.6 12.4 2.5 0
Month 7 8 9 10 11 12 Annual
Full energy, MWh 1129 1280 1383 1474 1784 1795 17498
Energy subject to losses, MWh 1129 1280 1379 1314 1579 1271 14358
Icing losses, % 0 0 0.3 10.9 11.5 29.2 17.9
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Figure 4.4 Icing losses of energy for SWT-3.6-120 located in Murmansk region
Based on Table 4.5, the highest losses (up 71.7%) are observed in January. Summarizing, icing losses exist during 9 months and amount 17.9% on average per year for the wind turbine SWT-3.6-120 without anti-icing devices.
4.4.3 The assessment of losses relating to high wind speeds and airflow deviations
Hysteresis losses at high wind speeds, losses at high turbulence and due to the air flow deviations cause the actual power production of wind turbines differs from the values corresponding to the power curve declared by the manufacturer [14]. One of the main reasons is distinction between normal conditions corresponding the power curve and conditions at the offshore WPP site, which are characterized by low temperatures, high humidity.
In order to estimate the considering losses, the annual output was calculated assuming the shutdown of a wind turbine at a wind speed of 25 m/s and subsequent start-up at a speed of 20 m/s. The numerical evaluation is presented in Table 4.6.
Table 4.6 Hysteresis losses at high wind speeds, losses at high turbulence and due to the air flow deflection at the site of the offshore WPP between the mouths of the Varzuga and Chavanga rivers in Murmansk region
Wind
turbine Characteristic
SWT-3.6-120
Full energy, GWh 17242
Energy subject to losses, GWh 16856
Losses relating to high wind speed and airflow deviations, % 2.24
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0
1 2 3 4 5 6 7 8 9 10 11 12
dE icing, %
Т, month
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Thus, percentage of annual energy losses relating to high wind speed and airflow deviations obtains 2.24% for the wind turbine SWT-3.6-120.
4.4.4 The determination of annual energy output with regard of losses
Summary results of different losses estimation for the offshore WPP between the mouths of the Varzuga and Chavanga rivers in Murmansk region is given in Table 4.7. Here values of losses due to downtime caused by the breakdown of the wind turbine, losses for internal energy consumption and electrical losses within the offshore WPP was approximately set as assumptions [14].
Table 4.7 Assessment of energy losses for offshore WPP between the mouths of the rivers Varzuga and Chavanga in Murmansk region
Type of losses SWT-3.6-120
Losses of energy, %
Wind vane losses 0
Losses due to downtime caused by the
breakdown of the wind turbine 3
Losses for internal energy consumption 7
Icing losses 17.9
Losses relating to high wind speeds and airflow
deviations 2.24
Electrical losses 3
Losses due to administrative and operational
restrictions 0.1
Losses as a percentage of annual energy,% 33.24 Total energy losses, MWh 5731.2 Annual electricity production subject to total
losses, MWh 11510.75
The greatest contribution to the total losses is made by icing losses. In order to reduce these losses icing-preventing systems of the blades might be provided. The total losses for a single wind turbine SWT-3.6-120, installed at the considering site of the offshore WPP, are 33.24%
or 5731.2 MWh.
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5 ELECTRICAL DESIGN OF THE OFFSHORE WIND POWER PLANT
The design of the electrical part of the offshore WPP is based on infield power connections within the WPP and the grid or shore connection. In addition, the selection of power equipment has to be done [16].
Theoretically, an offshore WPP can be connected to the coastal infrastructure by AC or DC cable lines. High-voltage DC cable lines are used to connect a WPP, that is remote from the coast, to reduce power losses during the transmission of electricity over long distances.
However, the DC transmission system requires the construction of expensive converter stations: a marine step-up (from the WPP) with rectifier converters and a coastal step-down (from the energy system) with inverters. Therefore, for stations with a capacity of up to 100 MW, located at a distance of less than 100 km from the coastline, it is preferable to use AC transmission, and for offshore WPP that are no more than 10 km from the coast, connection is carried out at an average voltage [27].
According to the offshore WPP site, the shortest distance from the wind turbine to the coastline is about 2.5 km. Thus, a connection of the WPP to the onshore facilities at 10 kV voltage level was proposed. Hence, it was necessary to increase the output generator voltage, which is equal to 0.69 kV for model SWT-3.6-120. Since the construction of sea-based transformer substations increases the complexity and, as a consequence, capital costs of an offshore WPP, a step-up transformer 10/0.69 kV was suggested to be installed in each wind turbine tower by the manufacturer.