Low-cost small-scale wind power generation.
Date
2009
Authors
Whaley, David Michael
Editors
Advisors
Soong, Wen
Ertugrul, Nesimi
Ertugrul, Nesimi
Journal Title
Journal ISSN
Volume Title
Type:
Thesis
Citation
Statement of Responsibility
Conference Name
Abstract
This research investigates a low-cost generator and power electronics unit for smallscale
(<10kW) wind turbines, for both standalone and grid-connected applications. The
proposed system uses a high-inductance permanent magnet generator together with a
switched-mode rectifier (SMR) to produce a variable magnitude output current. The
high inductance characteristic allows the generator to operate as a current source, which
has the following advantages over conventional low-inductance generator (voltage source)
systems: it offers simple control, and avoids the need for bulky / costly energy storage
elements, such as capacitors and inductors.
The SMR duty-cycle is controlled in an open-loop manner such that 1) maximum
power is obtained for wind speeds below rated, and 2) the output power and turbine
speed is limited to safe values above rated wind speed. This topology also has the ability
to extract power at low wind speeds, which is well suited to small-scale wind turbines,
as there is often limited flexibility in their location and these commonly see low average
wind speeds.
The thesis is divided into two parts; the first part examines the use of the SMR as a
DC-DC converter, for use in standalone applications. The duty-cycle is essentially kept
constant, and is only varied for maximum power tracking and turbine speed / power
limiting purposes. The SMR operates in to a fixed voltage source load, and has the
ability to allow current and hence power to be drawn from the generator even at low
wind and hence turbine speeds, making it ideal for battery charging applications. Initial
dynamometer testing and limited wind-tunnel testing of a commercially available wind
turbine show that turbine power can be maximised and its speed can be limited by
adjusting the SMR duty-cycle in an open-loop manner.
The second part of the thesis examines the use of the SMR as a DC-AC converter
for grid-connected applications. The duty-cycle is now modulated sinusoidally at the
mains frequency such that the SMR produces an output current that resembles a fullwave
rectified sinewave that is synchronised to the mains voltage. An additional H-bridge inverter circuit and low-pass filter is used to unfold, filter and feed the sinusoidal
output current in to the utility grid. Simulation and initial resistive load and preliminary
grid-connected tests were used to prove the inverter concept, however, the permanent
magnet generator current source is identified as non-ideal and causes unwanted harmonic
distortion.
The generator harmonics are analysed, and the system performance is compared with
the Australian Standard THD requirement. It is concluded that the harmonics are caused
by 1) the low-cost single-phase output design, 2) the use of an uncontrolled rectifier, and
3) the finite back-EMF voltage. The extent of these harmonics can be predicted based on
the inverter operating conditions. A feed-forward current compensation control algorithm
is investigated, and shown to be effective at removing the harmonics caused by the nonideal
current source. In addition, the unipolar PWM switching scheme, and its harmonic
components are analysed. The low-pass filter design is discussed, with an emphasis on
power factor and THD grid requirements. A normalised filter design approach is used
that shows how design aspects, such as cutoff frequency and quality factor, affect the
filter performance. The filter design is shown to be a trade-off between the output current
THD, power loss, and quality factor.
The final chapter summarises the thesis with the design and simulation of a 1kW
single-phase grid-connected inverter. The inverter is designed based on the low-pass filter
and feed-forward compensation analysis, and is shown to deliver an output current to the
utility grid that adheres to the Australian Standards.
School/Discipline
School of Electrical and Electronic Engineering
Dissertation Note
Thesis (Ph.D.) - University of Adelaide, School of Electrical and Electronic Engineering, 2009
Provenance
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.