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Aerodynamic & Load Calculation for the Design of 2 MW Wind Energy Convert
System with Low speed Gearbox
Young Chan Kim, Younguk Sohn*, Yong Whan Kim, Eung chae Lee, In Soo Park, Kyung Ryul Kim,
Kyehwan Gil, Chinwha Chung
PoWER Center, Pohang University of Science & Technology
790-784, Hyojadong San 31, Pohang, Korea;
(*) Younguk Sohn, TEL: (82) 54 279-1815, Fax: (82) 54 279-1799, younguk@postech.edu
J. Y. Ryu, J. I. Park, C. J. Byun
UNISON Co. Ltd.,
330-882, Jangsan-ri, Susin-myun, Cheonen-si, Chung-nam,
TEL:(82) 41 620-333, Fax: (82) 41-551-5611
Abstract
Under the national project for the development of 2 MW wind energy convert system, we are under development of
the prototype of 2 MW wind turbine with low speed gearbox. This system adopts low speed gear box with planetary
and spur gear and is pitch regulated variable speed type with the synchronous permanent magnet generator. The
compromised size of generator in diameter and width are adopted to meet the structural design requirements. In this
paper, the concept study for the type, the aerodynamic design for the blade and the details of load calculation will be
presented. The detailed characteristics of the system will also be introduced.
Key word: KBP-200M, blade, load calculation, drive train
1. Introduction
To cope against coming energy crisis and to save environments, the policies for the development of renewable
energy have been driven eagerly by Korea government since 2001. As a part of its effort, the standard for design
evaluation of wind turbine system was established in 2003 and two types of 750 kW class wind turbines - one was
geared and other was gearless directive drive type, they are the first wind turbines developed in Korea – were
developed in 2004. Now, they are under on-site demonstration in the test field for commercialization.
Futher to the above project, we commenced the development of 2 MW wind turbine with low speed gearbox,
which is named as KBP-2000M (Korean Baram Project) under the national project for the development of large scale
wind energy convert system at the mid of 2004 with the target date of completion of the mid of 2007. Besides this
project, the prototypes of 1 MW class wind turbine which is dual - rotor blade type and 2 MW class wind turbine with
3-stage gear box are also under development in Korea with the same financial support from the government with the
target date of completion of the end of 2006. On the other hand, the conceptual design of 3 MW class turbine for
offshore installation, which will be followed by the detail design in near future, has been finished by other parties
with the same government financial support at the beginning of 2006.
As described above, wind turbines from 750 KW class to 3 MW class of offshore installation will be lined up at
the test field and/or site in near future in Korea.
This paper describes the characteristics of 2 MW wind turbine with low gear ratio type - which will be the
second wind turbine to be developed in Korea by the consortium of PoWER CENTER and UNISON Co. Ltd. - blade
design, and load calculation.
2. Characteristics of KBP-2000M
The wind turbine of model “KBP-2000M”, has adopted the low speed gearbox with low gear ratio and is variable
speed type. This is expected to be more reliable in operation than conventional 3-stage gear box type. It has
advantage too with respect to the weight over the direct drive wind turbines of the same capacity due to the smaller
generator. Hence the tower top can be more compact in dimension and weight than the usual direct-drive wind
turbines.
A low speed gearbox with low gear ration was adopted in the concept of this system. At the front end of the gear
box an input main shaft is located. The main shaft has a through hole with an opening at the rear end for the cable of
pitch system. The input shaft of the gear box is connected to a flange by a bolt connection. At the rear end of the
gearbox housing a mounting surface shall be provided for the brake caliper support. The torque of the gear housing
will be carried with two torque arms to the main frame structure. The rated capacity of KBP-2000M is 2000 kW with
3-blades at wind speed of 11.5 m/s and rotational speed of generator, 15.3 rpm. The power is to be produced in the
range of wind speed of 3 to 25 m/s. The tip speed ratio (TSR) of rotor blade was selected as 8, in which power
coefficient is 0.482, by compromising the dimensions of blades and generator with the problems related to rotational
speed. The optimal TSR is maintained by torque control to get maximum wind energy under the rated wind speed. In
partial load conditions under rated speed, the torque will be controlled by a predefined torque-speed curve as shown
in Fig. 1. This torque-speed curve is chosen to achieve operation at the optimum tip speed ratio of the rotor. Over the
rated speed, the power is controlled by blade pitch controller and torque controller simultaneously to be operated in
rated speed within ±10% of error.
The pitch system is the main braking system of the wind turbine generating system (WTGS) (primary and
secondary). It is designed to be functioning with independent electrical drives for each rotor blade and to be battery
back up to ensure redundancy and fault tolerance. An additional mechanical brake is integrated in the high speed side
of the gearbox in drive-train. This mechanical braking system is the third braking system of the WTGS and is
designed to keep the wind turbine within permissible limits under all wind conditions if the first and second braking
systems fail. The primary function of the mechanical brake is to bring the rotor to a complete stand-still after it was
decelerated to idling operation by the pitch system. The mechanical brake is also used in emergency stops to
decelerate the WTGS as quickly as possible. The pitch system as well as the mechanical brake are controlled by both
the control system and the safety system of the WTGS. By design the safety system will always take precedence over
the control system. The safety system is fail-safe, meaning that due to the wiring it can not cause any blades to pitch
to a lower pitch angle or otherwise cause an acceleration of the WTGS.
The nacelle on the tower is supported by a four point bearing with internal toothing. The nacelle is being
yawed into the prevailing wind direction by means of four electrical gear motors. During normal operation of the
WTGS the nacelle is fixed by ten calipers. For yawing the calipers remain partly closed.
Fig. 1 Control zone of controller
3. Blade design
Rotor blade of KBP-2000M was designed for the annual averaged wind speed of 8.5 m/s. The specifications are
shown in Table 1.
Table 1 Design parameters for KBP-2000M WTGS Table 2 Selected airfoil profile
Rated power 2 MW of KBP-2000M rotor blade
Rated wind speed 11.5 m/s Cylinder
Range of wind speed 3-25 m/s AE02-40
Power control Blade pitch AE02-35
Tip speed ratio 8 AE02-30
Nominal rotor speed 25 m/s AE02-25
Range rotor speed 6-18 rpm NACA 63-421
Rotor diameter 88.0 m NACA 63-618
Hub height 80.0 m NACA 63-618
NACA63 series were selected due to their good performances in several middle scale models of wind turbine and
their wide usage. The profile of NACA63 is distributed between 68% and 100% of blade, in which the most portion
of aerodynamic force is produced. The circular profile is selected at blade root to connect with hub flange smoothly.
The profile of AE02 series is distributed up to 68% of blade to confirm the structural strength. They are thick but give
good performance with large angle of attack. Table 2 shows the profiles selected for KBP-2000M and Fig. 2 and 3
shows 2D and 3D profile shapes of AE02 and NACA63 series airfoils.
The blade is compromised properly between aerodynamic performance and mechanical strength by being sure
that the ratio of thickness to chord length is about 40% at the position at the maximum chord. The coefficients of lift
and drag to design the geometric shape of blade were determined at selected angles of attack with maximum CL/CD.
Fig. 2 Airfoil shapes of blade Fig. 3 Distribution of airfoil sections
Fig. 4 shows the distributions of relative chord length, and thickness of blade, respectively. The theoretically
computed geometric shape was adopted with consideration of mechanical strength, fabrication, material volume and
etc, with minimum sacrifice of aerodynamic performance. The Cp-TSR graph for various pitch angle is shown in Fig.
5. Cp value is maximum at TSR=8 for pitch angle = 0, which is a aerodynamic design condition of blade.
1.0
relative chord
relative thickness
0.8 relative twist
0.6
0.4
0.2
0.0
0.0 0.2 0.4 0.6 0.8 1.0
-0.2 r/R
Fig. 4 Relative chord, thickness, twist distribution
Fig. 5 Cp-TSR characteristics of blade
4. Load calculation
The loads for the KBP-2000M are calculated according to GL regulation (Edition 2003) and IEC61400-1. All
required conditions are based on TC2A of IEC61400-1. The simulations are performed with aeroFLEX developed
by Aerodyn Energiesysteme GmbH. The original program FLEX5 has been developed by Stig Oye. The load are
calculated at
• All three blade roots B1, B2, B3
• Different blade sections
• Flange connection hub/shaft rotating system R1
• Flange connection hub/shaft fixed system N
• Different tower sections
The loads are estimated as three distinct categories as extreme loads, fatigue loads and deflections. The extreme
loads are conversed from dynamic forces due to wind and control actuations to static loads. Meanwhile, the fatigue
loads are computed with repeat loads from all normal operation conditions and estimated number of start and stop
procedures during lifetime, even the loads are low enough not to fail the system. The deflections are results of loads
which WTGS is suffered. The deflections in blade and tower give configuration of tower top parts such as hub and
yaw system. The basic load cases are from regulations and bear many different sub cases according to wind,
operation and environment conditions. So all simulated loadcases in KBP-2000M are more than 300.
Fig. 6 is the typical section loads of blade root from loadcase DLC1.2E2, the condition is normal power
production. Table 3 and 4 are extreme loads and fatigue loads in blade bearing respectively, in which the extreme
loads contain the partial safety factors. In fatigue loads the load spectra for different operational and ambient
conditions have been converted to a damage equivalent rectangular load spectrum. The values in Table 4 are given
for a Wohler curve slope m = 4.
Table 3 Extreme loads on blade bearing
blade Fx-B Fy-B Fz-B Mx-B My-B Mz-B
load case
No. [kN] [kN] [kN] [kNm] [kNm] [kNm]
DLC1.6b1-4 3 928 11 92 -19 -487 -100
DLC6.1-3-4 3 -109 368 -48 -29 1037 7292
DLC1.2h1 3 334 113 271 -33 -6554 1717
DLC1.3b1-3 1 224 85 -219 172 5773 1200
DLC1.2h1 3 334 113 271 -33 -6554 1717
DLC6.1-3-4 3 -112 321 -55 -76 1616 7380
Table 4 Fatigue load on blade bearing
Fx-B [kN] Fy-B [kN] Fz-B [kN] My-B [kNm] Mz-B [kNm]
316.1±85.7 -0.15±86.1 123.2±37.5 -3015±910.5 60.73±1264
No. of load cycles n = 1.0×108
(a) Force in direction pitch axis of blade (b) Moment in chordwise direction
(c) Flapwise deflection (d) Pitch angle
Fig. 6 Section forces at the root of rotor blade in the nominal wind condition [DLC1.2E2]
5. Further works
The KBP-2000M is still in process of design. All basic configurations and parameters are determined, but a lot
of detail designs are floating according load calculation and engineering. The engineering design will be completed
August in 2006 and then the prototype of KBP-2000M will be manufactured during another one year.
Acknowledgment
This project is funded by Korea Energy Management Corporation [KEMCO]. We appreciate their support for the
development of 2 MW wind turbine generator system.
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