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Schemes for Engineers in Research and Development
Global Research Awards: Profiles
Dr Dragan Jovcic –University of Aberdeen
Advanced converter systems for thyristor-based multiterminal HVDC
Introduction
Dr Jovcic is a lecturer at School of Engineering,
University of Aberdeen. The Award supported a 6
month secondment to McGill University in Montreal
for the period July to December in 2008. His
collaborator was Professor B.T. Ooi from Electrical
and Computer Engineering at McGill University.
The Award enabled Dr Jovcic to take sabbatical leave
and fully dedicate his time to the research project.
The close collaboration with McGill University has
brought significant benefits in terms of immediate
research outputs, development of links with leading
international research centres and building of new
research skills and knowledge. This sabbatical
project has been of great benefit to all
participants.
Technical Activities
The research project was
concerned with the novel high-power converter
systems that would enable development of
multiterminal High Voltage Direct Current (HVDC)
transmission, and ultimately high-power DC
transmission networks.
HVDC has been in operation
worldwide for over 50 years because it offers some
advantages over conventional AC transmission in
particular application areas. All the HVDC
installations operate as two-terminal systems for
point-to-point transmission. There are two HVDC
installations which have operated as 3-terminal
systems for a limited time, but this topology has
not been widely accepted for various techno-economic
reasons.
It is well documented that
there is great demand and incentive for developing
technologies for tapping on HVDC lines and for
multiterminal HVDC. A small, isolated community in
northern Canada located close to major HVDC routes,
may gain significant benefits from connecting to the
national transmission grid. Multiterminal topologies
would increase flexibility of HVDC, improve overall
grid reliability and enhance power trading.
The DC networks are different
from multiterminal HVDC since differing DC voltage
levels are used at various network segments. Some of
the immediate application areas for DC networks
would be with the offshore renewable power parks and
the proposed HVDC (North Sea) supergrid. The
recently developed HVDC light is a very suitable
interconnection solution for renewable power parks.
Because of distributed nature of renewable power
sources, the DC networks with 2-3 DC voltage levels
and multiterminal HVDC would offer significant
benefits for power collection.
The development of
multiterminal HVDC and DC networks will require
significant further advances in the two key
high-power components: DC Circuit Breaker (CB) and
DC transformer. A DC circuit breaker enables
isolation of a faulted line or a unit which
increases system reliability. A DC transformer can
transform DC voltage levels to maintain optimum
costs and losses.
A large part of the studies on
this project has been concerned with development of
DC fault current limiting component. These studies
have produced excellent results, since a complete
design procedure for new converter systems is
proposed to enable satisfactory responses for even
most severe faults at any of the terminals.
As a challenging demonstration
system, a 200MW DC/DC converter model is developed
which would connect 88kV DC system with a 500kV DC
transmission. Figure 1 below shows the final
simulation results for the most severe fault on low
voltage circuit (V1 reduces from 88kV to 0kV). The
converter controller is inactive and only the
inherent converter responses are studied. The
important conclusions are:
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The current on faulted
side (I1) stays approximately constant, or shows
modest increase,
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The current on unfaulted
side (I2) reduces to zero, implying that fault
is not propagated to the opposite side,
-
There is no loss of
converter control during the fault,
-
The internal voltage (Vc)
does not undergo overshooting.
While traditional protection
with AC grids reacts to fault currents (detect high
current and open circuit), a suitable DC/DC
converter can prevent fault current levels
altogether.
Multiple models for
multiterminal HVDC and DC networks were developed in
the last stages of the project. Figure 2 shows one
of the 4-terminal DC network models under study.
These studied have confirmed that a suitable DC
transformer can facilitate development of DC
networks. It is postulated that the costs of a DC/DC
converter could be justified since it can
simultaneously achieve three functions: 1) voltage
stepping, 2) voltage/power regulation, and 3) fault
current limitation.
Fig 1. 200MW, 88/500kV converter. Transient response for a zero-impedance fault on V1.
Fig 2. High voltage, 1.8GW, DC transmission network.
Research Deliverables, Dissemination and Collaborations
In terms of immediate
outcomes, the project has generated two journal
articles and one conference paper. Further papers
are under preparation. Dr Jovcic attended two IEEE
conferences and made presentations.
During the sabbatical, Dr
Jovcic, gave four seminars at leading research
centres: University of Toronto (Canada), Ryerson
University (Canada), McGill University (Canada) and
ABB Power Electronics (Sweden).
A collaborative research
project with Ryerson University was developed in
February 2009, in order to further the studies on DC
networks and to build a DC network laboratory
prototype at University of Aberdeen. This research
proposal was submitted for consideration for funding
to EPSRC.
Acknowledgements
Dr Jovcic is grateful to the
Royal Academy of Engineering for the support of this
project. He kindly acknowledges the contribution of
McGill University and University of Aberdeen as
important facilitators of this project. Special
thanks to Professor Ooi for outstanding backing in
all aspects of the project and invaluable assistance
in personal matters.
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