International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 5- March 2016
Hydraulic Wind Power Transfer System
Modeling
Anjitha.K#1, B.Kavitha*2
#
Final year PG student, Power System Engineering
*Asst.Professor, Electrical and electronics department, Sri Krishna College of Technology, Coimbatore, India
Abstract—A
variable
speed
gearbox
is
conventionally used to transmit power from the wind
turbine to the generator placed at the top of the
tower. A hydraulic circuit replacing the gearbox
system can remove much weight from the tower.
Various wind turbines are connected to a centralized
generator at the other end. A mathematical model of
the hydraulic wind power transfer system is obtained
and its several speed-step responses are made and
compared with test results for accuracy. These
simulations are carried out by MATLAB/SIMULINK
software package.
Keywords– Hydraulic wind power transfer system
(HWPTS), mathematical modeling, variable speed
gearbox, wind turbine
I. INTRODUCTION
Utilizing potential sources of renewable energy
available around the world, we can fulfill all power
demands and eliminate the negative effects of fossil
fuels in power generation. The production costs of
the wind energy harvesting units has been reduced
following the recent advancements in wind turbine
manufacturing and have resulted in the expansion of
the application of wind power plants by 30%.
Consequently, wind turbines can become one of the
major power sources contributing to the world’s
energy demands. However, the harvesting technology
has remained in its traditional topology. Typical
horizontal axis wind turbines include a rotor to
convert the wind energy into the shaft momentum.
This rotor is connected to a drivetrain, a gearbox, and
an electric generator, which are integrated in a nacelle
located at the top of the tower. These components,
specifically the variable speed gearbox, are
expensive, bulky, and require regular maintenance,
which keeps wind energy production expensive. In
addition, since the gearbox and generator are located
on the top of the tower, its installation and
maintenance are time consuming and expensive.
Accumulation of the wind energy from several
wind turbines in one central unit at the ground level is
an innovative solution to address the above
deficiencies. In this novel system, each wind tower
harvests wind energy and converts it to a highpressure fluid. The energy flows from several wind
turbine towers are combined and fed to the central
ISSN: 2231-5381
unit. At this unit, the combined fluids are split
between a main generator and an auxiliary generator.
This technology will eliminate the weight from the
tower which reduces the maintenance time and cost.
Moreover, instead of having one generator and one
variable gearbox for each wind tower, multiple wind
turbines are integrated to ultimately reduce the capital
cost.
A hydraulic transmission system is identified as
an exceptional means of power transmission in
applications with variable input or output velocities
such as manufacturing, automation, and heavy-duty
vehicles. It offers fast response time, maintains
precise velocity under variable input and load
conditions, and is capable of producing high forces at
high speeds. Moreover, HTS offers decoupled
dynamics, allowing for multiple-input, single-output
drivetrain energy transfer configurations. Earlier
research has shown the possibility of using this type
of power transfer technology in a wind power plant,
even though it is not feasible in its electrical
counterpart
II.GEARBOX DRIVE SYSTEM
A gearbox is typically used in a wind turbine to
increase rotational speed from a low-speed rotor to a
higher speed electrical generator. A common ratio is
about 90:1, with a rate of 16.7 rpm input from the
rotor to 1,500 rpm output for the generator. Some
multi megawatt wind turbines have dispensed with a
gearbox. In these so-called direct-drive machines, the
generator rotor turns at the same speed as the turbine
rotor. This requires a large and expensive generator.
Other wind turbines on the market sit in-between,
with gearbox ratios of about 30:1, dispensing with the
highest speed stage in a typical gearbox. There is a
trade-off between the reliability of gearboxes and
gear stages and the cost of slower, higher torque
generators.
The design of a wind turbine gearbox is
challenging due to the loading and environmental
conditions in which the gearbox must operate. Torque
from the rotor generates power, but the turbine rotor
also applies large moments and forces to the windturbine drivetrain. It is important to ensure that the
drivetrain effectively isolates the gearbox, or to
ensure that the gearbox is designed to support these
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 5- March 2016
loads, otherwise internal gearbox components can
become severely misaligned. This can lead to stress
concentrations and failures.
Wind-turbine drivetrains undergo severe transient
loading during start-ups, shut-downs, emergency
stops, and during grid connections. Loads that result
in torque reversals may be particularly damaging to
bearings, as rollers may be skidding during the
sudden relocation of the loaded zone. Seals and
lubrication systems must work reliably over a wide
temperature variation to prevent the ingress of dirt
and moisture, and perform effectively at all rotational
speeds in the gearbox.
Gear and bearing fatigue standards by AGMA and
ISO are used for design; these only capture a subset
of the potential failure modes of the components. For
instance, the ISO 6336 gear standard provides an
established method for calculating resistance to
subsurface contact failure and for tooth root breakage.
The standards are doing their job, but these are not
the most common failure modes observed in wind
turbine gearboxes. More common causes of failure
are manufacturing errors such as grind temper or
material inclusions, surface related problems, such as
scuffing or micro pitting, and fretting problems from
small vibratory motions, such as may occur when a
machine is parked. Scuffing is adhesive wear and
subsequent detachment and transfer of particles from
one or both of the meshing teeth. It can happen
quickly and is generally considered to be associated
with an absence or breakdown of the lubricant film
under high loads. Micro pitting is a surface fatigue
resulting from generation or numerous surface cracks,
and is associated with insufficient film thickness.
Film thickness is affected by sliding speed, load,
temperature, surface roughness, and chemical
composition of the lubricant.
Many wind-turbine gearboxes have also suffered
from fundamental design issues such as ineffective
interference fits that result in unintended motion and
wear, ineffectiveness of internal lubrication paths and
problems with sealing. Improving the resistance of
future gearbox designs to all these issues is a key for
the future cost of energy generated by wind turbines.
III.HYDRAULIC WIND POWER TRANSFER
SYSTEM
The hydraulic wind power transfer system
consists of a fixed displacement pump driven by the
prime mover (wind turbine) and one or more fixed
displacement hydraulic motors. The hydraulic
transmission uses a hydraulic pump to convert the
mechanical input energy into pressurized fluid.
Hydraulic hoses and steel pipes are used to transfer
the harvested energy to the hydraulic motors.
A schematic diagram of a wind energy HTS is
illustrated in Fig.1. As the figure demonstrates, a
fixed displacement pump is mechanically coupled
with the wind turbine and supplies pressurized
ISSN: 2231-5381
hydraulic fluid to two fixed displacement hydraulic
motors. The hydraulic motors are coupled with
electric generators to produce electric power in a
central power generation unit. Since the wind turbine
generates a large amount of torque at a relatively low
angular velocity, a high displacement hydraulic pump
is required. The pump might also be equipped with a
fixed internal speed-up mechanism. Flexible highpressure pipes/hoses connect the pump to the central
generation unit.
The hydraulic circuit uses check valves to ensure
the unidirectional flow. A pressure relief valve
protects the system components from the destructive
impact of localized high-pressure fluids. The
hydraulic circuit contains a specific volume of
hydraulic fluid, which is distributed between the
hydraulic motors using a proportional valve. Finally,
the governing equations of the hydraulic circuits are
obtained.
Fig:1
IV.MATHEMATICAL MODEL
The dynamic model of the hydraulic system is
obtained by using governing equations of the
hydraulic components in an integrated configuration.
The governing equations of hydraulic motors and
pumps to calculate flow and torque values are utilized
to express the closed-loop hydraulic system behavior.
Note that all parameters are measured in British
Engineering units.
V.FIXED DISPLACEMENT PUMP
In the simulation tests we usually make use of the
formula for the pump delivery. For a variable
delivery pump, an approximate expression:
(1)
3 -1
Qp - pump delivery [m .s ]
αp - displacement angle of either a pump disk or
casing
[°]
3
-1
Kqp- pump delivery coefficient [m per°s ]
ηvp-pump volumetric efficiency coefficient [-]
Respective block diagram is shown:
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 5- March 2016
Δph -pressure fall in the engine [Pa]
ηmh- coefficient of the engine's mechanic-hydraulic
efficiency
Respective block diagram is shown:
Fig 2: Fixed displacement pump model
VI.CHECK VALVE
In simplified considerations one usually analyses
the static characteristics of a valve, arising out of its
catalogue specifications. In such a case one can make
use of two formulas for different operational phases
of the valve when closed and open.
(2)
Fig 4: Hydraulic motor
VIII.PROPORTIONAL VALVE
(3)
Flow rate is given by equation:
-Flow-rate through the valve [m3.s-1]
-Slopecoefficient
of
the
staticcharacteristics
[m5.N-1.s-1]
P -System operational pressure [Pa]
valve's
-Valve opening pressure set while in operation
[Pa]
Respective block diagram is shown:
(6)
-Gap aspect ratio
-Cross sectional area
-Pressure in the supply line
-Pressure in flow controller chamber
- Density of fluid
Respective block diagram is shown below in figure:
Fig 3: Check valve model
Fig 5: Proportional valve model
VII.HYDRAULIC MOTOR
A displacement engine is usually described with
two formulas. One of it regards absorptivity and the
other its torque. According to the engine absorptivity
can be put down as follows:
IX.FLUID COMPRESSIBILITY
Pressure value at a known flow rate is given by the
equation:
(4)
3
(7)
3
-1
Qs - fluid flow rate related to compressibility [m /s]
Qh - engine absorptivity [m .s ]
3
Kqh- engine absorptivity coefficient [m ]
3
Vs - fluid volume subject to pressure effects[m ]
-1
ωh- angular velocity of the engine shaft [s ]ηvhcoefficient of the engine's volumetric efficiency
Es - fluid bulk modules [Pa], p - fluid pressure [Pa]
D - Differentiating operator
The torque developed by the engine amounts to:
(5)
Mh - torque [N⋅m]
3
Kmh-engine's torque coefficient [m ]
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 5- March 2016
Respective block diagram is shown below:
Fig 6: Fluid compressibility model
Fig 8: SIMULINK diagram of wind HTS
X.HYDRAULIC LOAD
The angular velocity of hydraulic engine shaft is
given by equation:
(8)
Ih- moment of inertia of the engine and machine parts,
2
reduced upon the engine shaft [N⋅m⋅s ]
Bh - resistance coefficient of viscous friction in the
engine and machine parts, reduced upon the engine
shaft
[N⋅m⋅s]
Mo - moment of technological resistance, resulting
from the machine operation [N⋅m]
The system model was simulated using
MATLAB/Simulink package. A pulse width
modulation (PWM) signal of 100 Hz with 10% duty
cycle was used to control the proportional valve to
direct the flow toward the auxiliary motor. The speed
step response of the system was generated by
applying a step voltage to the dc motor to accelerate
the hydraulic pump from 0 to 300 rpm. After reaching
a steady state, a second step voltage was applied to
speed up the system from 300 to 400 rpm, followed
by a step down back to 300 rpm to analyse the
undershoots.
The simulation parameters for the prototype
model is given below in the table
-1
TABLE 1: Prototype parameters
ωh- angular velocity of the engine shaft[s ]
SYMBO
The block diagram is given below:
QUANTITY
VALUE
UNIT
L
Pump
0.517
displacement
Primary motor
0.097
displacement
Auxiliary
0.097
motor
displacement
Fig.7: Hydraulic load model
Primary motor
9.6
inertia
X1.SIMULATION RESULTS
The basic SIMULINK/MATLAB diagram to
obtain the angular velocity of the system based on
given parameters of a prototype is given below in
figure:
Auxiliary
4.8
motor inertia
Primary motor
0.01416
damping
Auxiliary
0.01150
moor damping
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Pump leakage
0.046-
coefficient
0.068
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 5- March 2016
Primary motor
0.06
pump leakage
coefficient
Auxiliary
motor
0.01
pump
leakage
coefficient
Pump/motor
0.90
Dimension
total
less
efficiency
Pump/motor
0.95
Dimension
volumetric
less
efficiency
β
Fluid
bulk
183695
psi
modulus
ρ
Fluid density
0.0305
υ
Fluid
1.105
Fig 10: SIMULINK diagram for mathematical
model
The system model was simulated using
MATLAB/Simulink package. A pulse width
modulation (PWM) signal of 100 Hz with 10% duty
cycle was used to control the proportional valve to
direct the flow toward the auxiliary motor. The speed
step response of the system was generated by
applying a step voltage to the dc motor to accelerate
the hydraulic pump from 0 to 300 rpm. After reaching
a steady state, a second step voltage was applied to
speed up the system from 300 to 400 rpm, followed
by a step down back to 300 rpm to analyze the
undershoots.
The simulation results obtained from the
MATLAB/SIMULINK for velocity profile for
mathematical model based on figure is given below:
viscosity
The simulation results obtained from the
MATLAB/SIMULINK for velocity profile for a
prototype model based on figure is given below:
Fig 11: Velocity profile from mathematical model
Fig 9: Velocity profile from prototype model
The velocity profile for the prototype model is
given below in figure .For a zero input the output
velocity is also zero. When input is increased to 400
rpm the output increased to a higher value of
1100rpm and when it was decreased to 300rpm the
output deceased to value of 820rpm. So for a given
input the output velocity is of a higher value that can
rotate the generator to get required electrical output.
The SIMULINK/MATLAB block diagram for the
hydraulic transfer system based on mathematical
modelingis given below in figure 10:
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The velocity profile for the mathematical model is
given below in figure .For a zero input the output
velocity is also zero. When input is increased to 400
rpm the output increased to a higher value of
1000rpm and when it was decreased to 300rpm the
output deceased to value of 780rpm. So for a given
input the output velocity is of a higher value that can
rotate the generator to get required electrical output.
A slight difference is obtained in results between
the prototype and mathematical model due to the
geometrical difference between both models. The
experimental results demonstrate the accuracy and
performance of the mathematical model of the
hydraulic wind energy transfer system.
XI1.CONCLUSION
This project introduced a gearless wind power
transfer system as an alternative to traditional wind
turbine drivetrains. A mathematical model of the
hydraulically driven energy harvesting system was
obtained. The dynamic behavior of the mathematical
model to a velocity step input demonstrated a close
agreement with the results obtained from the
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 5- March 2016
experimental setup. The mathematical model could
be used to scale the system for an industrial-level
wind power plant. Exclusive benefits of
implementing such a system based on the proposed
model include 1) eliminating the variable speed
gearbox using a hydraulic circuit; 2) having one
central generator instead of multiple generators,
which decreases the capital cost of the wind power
plant; and 3) displacing most of the equipment from
the nacelle to the ground to obtain better accessibility
to the generation unit and to reduce the maintenance
costs and time.
ACKNOWLEDGEMENT
The authors would like to thank the
Principal, the HoD and to all faculty members of EEE
Department, Friends who have render their valuable
help in completing this paper successful.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
Z. Jedrzykiewicz, J. Pluta, and J. Stojek, ―Application
of the MATLAB— Simulink package in the simulation
tests on hydrostatic systems,‖ ActaMontan, vol. 1, pp.
29–36, 1998.
G. A. Sohl and J. E. Bobrow, ―Experiments and
simulations on the nonlinear control of a hydraulic
servosystem,‖ IEEE Trans. Control Syst. Technol., vol.
7, no. 2, pp. 238–247, Mar. 1999.
S. Habibi and A. Goldenberg, ―Design of a new high
performance electrohydraulic actuator,‖ in Proc. 1999
IEEE/ASME Int. Conf. Adv. Intell. Mechatron.,
Atlanta, GA, USA, Sep. 19–23, 1999.
S. Habibi and A. Goldenberg, ―Design of a new highperformance electrohydraulic actuator,‖ IEEE/ASME
Trans. Mechatron., vol. 5, no. 2, pp. 158–164, Jun.
2000.
K. Dasgupta, ―Analysis of a hydrostatic transmission
system using low speed high torque motor,‖ Mech.
Mach. Theory, vol. 35, pp. 1481–1499, Oct. 2000.
J. G. Slootweg, H. Polinder, and W. L. Kling,
―Dynamic modeling of a wind turbine with doubly fed
induction generator,‖ in Proc. IEEE Power Eng. Society
Summer Meeting, Vancouver, Canada, 2001.
J. G. Slootweg, S. W. H. de Haan, H. Polinder, and W.
L. Klimg, ―General model for representing variable
speed wind turbine in power system dynamics
simulations,‖ IEEE Trans. Power Syst., vol. 18, no. 1,
pp. 144–151, Feb. 2003.
S. G. Kim, J. H. Kim, and W. S. Lee, ―Hydraulic
system design and vehicle dynamic modeling for
development of a tire roller,‖ Int. J. Contr. Autom.
Syst., vol. 1, no. 4, pp. 89–94, Dec. 2003.
K. Wu et al., ―Modelling and identification of a
hydrostatic
transmission
hardware-in-the-loop
simulator,‖ Int. J. Veh. Des., vol. 34, pp. 52–64, 2004.
T. Senjyu, R. Sakamoto, N. Urasaki, H. Higa, K.
Uezato, and T.
Funabashi, ―Output power control of
wind turbine generator by pitch angle control using
minimumvariance control,‖ Elect. Eng. Jpn., vol. 154,
no. 2, pp. 1–18, Jan. 2006.
Y. Lei, A. Mullane, G. Lightbody, and R. Yacamini,
―Modeling of wind turbine with a doubly fed induction
generator for grid integration studies,‖ IEEE Trans.
Energy Convers., vol. 21, no. 1, pp. 257–264, Mar.
2006.
A. V. Akkaya, ―Effect of bulk modulus on performance
of a hydrostatic transmission control system,‖ SadhanaAcad. Proc. Eng. Sci., vol. 31, pp. 543–556, Oct. 2006.
U.S. Department of Energy, (2008, May). Wind energy
by 2030–Increasing wind energy’s contribution to U.S.
ISSN: 2231-5381
[14]
[15]
[16]
[17]
[18]
[19]
[20]
electricity
supply.DOE/GO2567.[Online].Available:http://www1.e
ere.energy.gov/windandhydro/pdfs/41869.pdf.
S. Eriksson, H. Bernhoff, and M. Leijon, ―Evaluation of
different turbine concepts for wind power,‖ Renew.
Sustain. Energy Rev., vol. 12, no. 5, pp. 1419–1434,
Jun. 2008.
A. Ragheb and M. Ragheb, ―Wind turbine gearbox
technologies,‖ in Proc.1st Int. Nucl. Renew. Energy
Conf. (INREC10), Amman, Jordan, Mar. 2010.
B. Ji-Zhong, X. Ai-Guo, Y. Xin-Hua, and Z. Li-Kun,
―Simulation model of hydraulic turbine speed control
system and its parameters identification based on
resilient adaptive particle swarm optimization
algorithm,‖ in Proc. Power and Energy Eng. Conf.
(APPEEC), Mar. 2010, pp. 1–4.
A. V. Akkaya, ―Effect of bulk modulus on performance
of a hydrostatic transmission control system,‖ Sadhana,
vol. 31, no. 5, pp. 543–556, Oct. 2006. [20] S.
Hamzehlouia, A. Izadian, A. Pusha, and S. Anwar,
―Controls of hydraulic wind power transfer,‖ Proc.
IECON, Melbourne, Australia, 2011.
A. Pusha, A. Izadian, S. Hamzehlouia, N. Girrens, and
S. Anwar, ―Modeling of gearless wind power transfer,‖
in Proc. IECON, Melbourne, Australia, 2011.
S. Hamzehlouia and A. Izadian, “Modeling of hydraulic
wind power transfers,” in Proc. Power Energy Conf.
(PECI), Champaign, IL, USA, 2012.
A. Izadian, ―Central Wind Turbine Power Generation,‖
U.S. PatentApplication US2013/0127166 A1, May 23,
2013.
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