Download Stability research of a local wind/diesel power system

Converter control design for Battery Energy Storage Systems applied in
autonomous wind/diesel systems
by Magni Þ. Pálsson
1.
operating hours and the number of start/stop
cycles of the diesel engine.
Initiation
I graduated from the University of Iceland,
Reykjavík, Department of Electrical Engineering,
in June 1990.
The BESS consists of battery bank connected to a
forced commutated converter.
In September 1994 I started my studies towards a
dr. ing. degree here at NTNU. My main field of
research is design of converter controllers for
Battery Energy Storage Systems (BESS) for use
in autonomous wind/diesel systems. The main
purpose of using a BESS in such applications, is
to enhance power quality and increase the
efficiency of the system, by reducing the number
of operating hours of the diesel engine.
3.
Modelling
The configuration of the wind/diesel model is
based on the second generation wind/diesel
system on Frøya, pictured below [1].
Wind turbine
AG
Customer loads
Diesel generator
set
This work was suggested by my supervisor
professor dr. ing. Hans H. Faanes and originated
from a project that EFI (The Norwegian Electric
Power Research Institute) has had going for
several years on the island of Frøya off the coast
of Sør-Trøndelag. My scientific advisor is dr. ing.
Kjetil Uhlen, researcher at EFI.
DE
SG
Converter and control unit
Battery storage
Converter
Dumpload
DC
AC
CONTROL SYSTEM
I completed the obligatory courses in the spring
1996 and since then I have been working on the
research part on a full time basis
2.
Fig. 1: The Frøya wind-diesel system [1]
The computer program PSCAD/EMTDCTM[2]
has been used for the modelling and simulation. I
have also developed reduced-order models in
SIMULINK[5]. The EMTDC model is more
detailed than the SIMULINK model, which
enables me to study more closely the switching
phenomena of the converter. In the SIMULINK
model I use a static model of the converter, but in
PSCAD/EMTDC I am able to control each
transistor. The SIMULINK model has been used
as a basis for deriving linear models. The
purpose, in this work, of linearising is to be able
to use eigenvalue analysis to study the behaviour
Description
Using wind energy to produce electricity exposes
power systems to special problems. The
fluctuating nature of the wind power induces
fluctuations in the system voltage and frequency
of an autonomous wind/diesel system. The
voltage fluctuations may cause annoying flicker
in incandescent light bulbs. The need for reducing
the fluctuations is therefore present. Effective use
of a BESS also increases the efficiency of the
wind/diesel system, by reducing the number of
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of the two converter control techniques developed
and try and reveal any differences. The linearised
models are also an ideal starting point for further
control design.
BESS ”lifts” the voltage level. This may be
achieved by other means, e.g. by installing a
capacitor bank. This solution though might cause
problems when shifting between low load and
high load conditions.
4.
The models have also been validated against
measurements on the Frøya wind/diesel system.
The simulated and measured results proved to
correspond quite well.
Status of the work
I have developed two control strategies for the
converter. The first one uses a sinusoidal pulse
width modulation technique (sinusoidal PWM)[3]
and is based on controlling the amplitude and
phase of the converter output voltage. The latter
method is a hysteresis type current control
method[3], based on controlling the converter
output current.
5.
I have now written my thesis and will defend it
this spring.
Both control techniques show good simulation
results both in wind/diesel applications and in a
simulated weak power grid (e.g. distribution
grid). Although this latter application has not
been a central part of this work, the simulation
results are very promising and should encourage
to further studies. Fig. 2 shows the system voltage
when wind power is integrated into a weak power
grid. The three trajectories in the figure are (from
top to bottom): using BESS with current
controlled converter; using BESS with voltage
controlled converter and using no BESS.
6.
[1]
[2]
[3]
System voltage with wind integration into a weak power grid
230
[4]
BESS w/CCC
BESS w/VCC
226
222
V
218
[5]
214
No BESS
210
0
1
2
3
4
Further work
5
sec
Fig. 2: System voltage in a weak grid with wind power
integration.
Using a BESS obviously enhances the voltage
quality in this case. The most significant
difference is how the BESS is able to damp the
voltage fluctuations almost completely. Another
clear difference is the voltage level, i.e. how the
42
References
Uhlen, K.: Modelling and robust control of
autonomous hybrid power systems.
Division of Engineering Cybernetics NTH,
1994.
PSCAD/EMTDCTM: Manitoba HVDC
Research Centre, 400-1619 Pembina Hwy.,
Winnipeg, Manitoba, Canada.
Mohan, N.; Undeland, T. M.; Robbins, W.
P.: 1995, Power electronics. Converters,
applications and design, John Wiley &
Sons, Inc., USA.
Pálsson, M.; Uhlen, K.; Toftevaag, T: 1997,
Modelling
and
simulation
of
an
autonomous wind/diesel system equipped
with forced commutated converter,
Proceedings of the 7th European
Conference on Power Electronics and
Applications
(EPE’97),
Trondheim,
Norway.
SIMULINKTM: The MathWorks, Inc.,
Cochituate Place, 24 Prime Park Way,
Natick, Massachusetts 01760, USA.