MDA AUG-SEP 2018 FINAL AUG-SEP 2018 DIGITAL | Page 44

The Future
Pioneers
Hydrostatic transmission
for wind turbine application
Driven by climate change and the need to decrease carbon dioxide emissions, the installation of
wind turbines for electricity generation has expanded rapidly over the last decade.
T
he US generates more than
6.3% of its total electricity
from wind and has a goal
of harnessing 20% of the nation’s
energy from wind by 2030 [1].
Globally,more than 52GW of
clean, emissions-free wind power
was added in 2017, bringing total
installations to 539 GW. With
new records set in Europe, India
and in the offshore sector, annual
markets will resume rapid growth
after 2018.Most utility scale wind
turbines (greater than 1 MW) are
installed far away from the point
of use, increasing transmission
cost while incurring about 5%
power loss in transmission lines.
In contrast, for distributed wind,
small (less than 100 kW) and
midsize (100 kW to 1 MW)turbines
can satisfy local demand and make
the electrical grid more reliable
and stable. At present, distributed
wind turbines are growing slowly
because of high installation,
operation
and
maintenance
costs[2].
captured by rotor blades. The low
speed rotor power is transmitted
to high speed generator through
a multi-stage fixed ratio gearbox.
In a fixed-ratio gearbox turbine,
the generator speed changes with
wind speed.An expensive power
converter is required between
the generator and the grid to
compensate for the mismatch in
voltage and frequency. Studies
conducted by the National
Renewable Energy Laboratory
among others, document failure
frequency and downtime for wind
turbines [3]. The studies show that
gearboxes and generators failure
accounts for 95% of the turbine
downtime because of the difficulty
in replacing them. The failure of
gearboxes and generators is due
to unsteady wind, causing impact
loading which in turn reduces
the life of the components. This
failure not only decreases the
annual energy production of the
turbine, but also increases the
maintenance cost. So, there is a
need for a reliable transmission to
replace the existing gearbox.
Proposed Design:
A hydrostatic transmission (HST) is
a reliable and continuously variable
Figure1. Conventional wind turbine with cost
breakdown
In a conventional turbine (Figure
1), power from the wind is
44 | August-September 2018 | Global MDA Journal
Figure2. Schematic of HST wind
transmission (CVT). It consists of a
hydraulic pump drivinga variable
displacement
motor
(Figure
2). For a continuously variable
transmission, at least one unit
must have variable displacement.
In a wind turbine, the rotordrives
the fixed displacement pump
creating the hydraulic flow. The
pressurized hydraulic fluid is fed to
the variable displacement motor
driving the generator. Hydraulic
pumps and motors have a power
density that is ten times higher than
electric motors and generators, making
the transmission more compact [4].The
slight compressibility of the hydraulic
fluid in an HST reduces the impact
loading on the mechanical components
and increases their life. As a CVT, an
HST can adjust to varying wind speed.
It decouples the generator speed from
the rotor speed, rotates the generator
at a constant speed and eliminates
the expensive power convertor (about
7% of total turbine cost). A hydrostatic
transmission has lower transmission
efficiency than a mechanical gearbox,
but the overall system efficiency is
still competitive with a conventional
gearbox turbine since there is no need
for a power converter.Hydraulics are
necessary for high power and high
load applications such as construction
equipment. Commercial hydraulic
components for HSTs in the required
power range are readily available at a
reasonable cost.
The use of a hydrostatic transmission
in a wind turbine creates the possibility
of adding energy storage to the turbine
using a hydraulic accumulator. When
the wind speed is above the rated