03-05-2011, 04:31 PM
STATCOM-controlled HVDC Power Transmission for Large
Offshore Wind Farms: Engineering Issues
Abstract –
The paper considers a solution for integration of
large offshore DFIG-based wind farms with a common collection
bus controlled by a STATCOM into the main onshore grid
using line-commutated HVDC connection. A design procedure
is described and the controlled system is validated using
PSCAD/EMTDC simulations confirming high performance of
the proposed control strategy in both normal operation conditions
and faults. Engineering issues related to STATCOM capacitor
sizing and reduction of STATCOM rating are considered
and their effectiveness is confirmed.
I. INTRODUCTION
It is predicted [1] that by 2020 up to 12% of the world’s
electricity will be supplied from wind power. Due to environmental
concerns, the majority of wind farms are planned
to be offshore where wind conditions are generally better
whilst planning restrictions are reduced. For example, several
wind farms with registered capacities around 1000MW are
planned off the North-West Coast of Great Britain with distances
over 100km to the nearest grid connection point. To
transmit such bulk power over such distances creates challenges
for both system operators and wind farm developers.
Currently two alternative connection methods are available
for connection of remote wind farms to the grid: High Voltage
DC (HVDC) and High Voltage AC (HVSC). It is proven
[2,3,4] that HVDC transmission offers many technical, economic
and environmental advantages for large wind farm
with long distances to the main grid.
HVDC transmission can be based on two alternative technologies:
Voltage Source Converter (VSC) using IGBTs, and
Line-Commutated Converter (LCC) using thyristors. The
VSC technology has the advantage of forced-commutation so
that an external voltage source is not required, but it has
higher overall losses than thyristor based LCC HVDC.
Moreover, the present power rating for a single VSC is limited
at the 330MW level and the reliability of VSCs in service
has yet to be fully proved while LCC HVDC technology
has been operated with high reliability and little maintenance
for more than 30 years. However, the LCC requires a commutation
voltage supplied by a synchronous compensator [3].
Alternatively a STATCOM can be used that can typically
provide much faster control than a synchronous compensator
[5,6] and with lower losses.
For reasons of energy capture and reduced drive train
stresses, variable speed turbines are becoming the norm for
new wind farm installations. The direct-drive synchronous
generator (optionally with permanent magnet excitation) and
Doubly Fed Induction Generator (DFIG) have become the
two generator alternatives. The former has the disadvantages
of cost and a power converter rated for the full power. Although
requiring a gearbox, the DFIG requires a converter of
only 25% of the generator rating for an operating speed range
of 0.75 to1.25pu and is considered a lower cost, proven technology
solution. The ratings of commercial DFIG-based
wind turbines now reach 5MW [7].
The converter-controlled DFIG, STATCOM and LCC
HVDC have all been well studied as separate components. In
[4-6], the overall power system concept and possible control
paradigms have been described, but these studies have not
included system analysis or a formal procedure of control
system design. A detailed mathematical study of the system
that resulted in the plant model appropriate for a formal control
design had been represented by the authors in [9]. The
present paper is a logical continuation of our previous study
[9]: the proposed control algorithm is applied to the more realistic
power system configuration, the model for simulation
studies includes lumped DFIG model under converter control
[8] to represent 1000MW wind farm, the mechanical dynamics
is taken into account, the HVDC model is according to
CIGRE [11] and includes detailed switching models for both
rectifier and inverter together with their harmonic filters, and
the impedances of all off-shore cables and transformers are
considered as well. The performance of the power system
during normal operation and faults is studied, and engineering
issue of STATCOM rating is discussed.
II. THE POWER SYSTEM STUDIED
The power system studied is shown in Fig.1. It comprises a
DFIG-based offshore wind farm, an island platform for
HVDC rectifier station and auxiliary equipment, AC filters,
and a STATCOM unit. Whereas capacitive AC filters can
compensate for the HVDC converter reactive absorption in
lumped amounts, the STATCOM provides fine reactive
power control. In addition the STATCOM being a voltage
source can provide the commutation voltage required by the
HVDC page link and dynamic reactive power compensation for
the network during disturbances and fault conditions. Commutation
voltage and reactive power control can also be
sourced from DFIGs, but high reactive power compensation
by the DFIGs will add to losses in the wind farm connection
cables. The offshore platform is linked to the main onshore
inverter station via submarine HVDC cable.
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