01-01-2013, 11:21 AM
New Voltage Source Converter Topology for HVDC Grid Connection
of Offshore Wind Farms
Voltage Source.pdf (Size: 348.6 KB / Downloads: 46)
Abstract
Large offshore wind farms are recently emerging as
promising alternative power sources. Long distances between
offshore generation and onshore distribution grid demand new
solutions for their connection to the AC network. HVDC systems
based on voltage source converters (VSC) are a promising
alternative to conventional AC transmission above a certain cable
length. This paper presents a new VSC transmission topology for
HVDC grid connection of offshore wind farms.
INTRODUCTION
The recent emergence of larger and more efficient wind
turbines establishes bright prospects in wind power
generation. The first 5 MW wind turbine will be erected in
Brunsbüttel near Hamburg in summer 2004 by REpower
Systems AG [1]. These latest developments extend the
potential of large-scale offshore wind generation, which
becomes a rapidly growing worldwide alternative power
source.
Today, especially large offshore wind farms in the power
range of several hundred megawatts are getting into focus.
Limited availability of onshore sites and better offshore wind
conditions are driving the wind turbines offshore.
Environmental requirements regarding noise pollution and
the visible impact as well as colliding interests in the nearshore
areas (recreation, military, coastal shipping, fishing
etc.) lead to increasing distances between offshore wind
farms and onshore distribution grids. Remote locations,
however, often imply deep water depths, complicating the
foundation of the wind turbines in the seabed. Recent
improvements in submarine foundations (i.e. tripod,
quadropod or lattice structures) allow deeper water depths,
whereas the current economic limit of such installations lies
in the range of 30 to 35 m [2]. Another important factor that
causes prolonged transmission distances is the necessity of a
strong grid connection point with a significant short-circuit
capacity. Reaching a suitable AC network connection point
requires often a long onshore transmission line.
DESCRIPTION OF PROPOSED TOPOLOGY
The topology of the proposed AC/DC converter is shown in
Fig. 2. It incorporates a VSC and cycloconverters (direct
converters) connected via a medium frequency (MF) AC bus.
Every wind turbine is equipped with a passive line filter, a 3-
by-2 cycloconverter, an MF transformer and a circuit breaker.
This enables the individual wind turbine to operate as an
adjustable-speed generator (ASG), offering multiple
advantages compared to fixed-speed operation, as i.e.
increased efficiency [5]. The valves of the cycloconverter do
not need any turn-off capability and can be realized by fast
thyristors connected in anti-parallel. The MF transformer
increases the generator voltage from 690 V to 33 kV. The
high-voltage side of the transformer is connected to the MF
AC bus via a circuit breaker allowing the disconnection of
the wind turbine.
Principle of operation
By alternately commutating the cycloconverters and the VSC
it is possible to achieve soft commutations for all the
semiconductor valves [6]. The cycloconverter can be solely
operated by line commutation (natural commutation) whereas
snubbered or zero-voltage commutation is always enabled for
the VSC. The operation principle during a commutation
sequence is described below. A more detailed description
including a carrier-based modulation scheme can be found in
[5]. The basic waveforms can be found later in this paper.
The VSC is commutated at fixed time instants with constant
intervals (switching frequency mf = 500 Hz), thus generating
an MF square-wave voltage on the AC bus. When the main
transformer current and voltage have the same sign
(instantaneous power flow is directed from the DC-side to the
AC-side), the conditions are set for a snubbered commutation
of the VSC. The process is initiated by turning off the
conducting valve at zero-voltage conditions. The current is
thereby diverted to the snubber capacitors. The antiparallel
diodes of the incoming valve take over the current once the
potential of the phase terminal has fully swung to the
opposite. At this stage, the IGBTs that are antiparallel to the
diodes can be gated on at zero-voltage zero-current
conditions. Reversing the transformer voltage during the
VSC commutation establishes the possibility for natural
commutation of the cycloconverters. The commutation of a
cycloconverter phase leg is initiated by turning on the nonconducting
valve in the direction of the respective phase
current. The VSC voltage and the leakage inductances of the
transformers govern the natural commutation.
DESCRIPTION OF REFERENCE TOPOLOGY
Today, the predominant solution for adjustable-speed wind
turbines is the doubly-fed induction generator (DFIG) ASG.
Its configuration is shown in the lower part of Fig. 3. The
stator of the induction generator is directly connected to the
wind farm grid whereas the rotor windings are connected to a
frequency converter (back-to-back VSC) over slip rings. This
allows the wind turbine to operate over a wide speed range,
depending on the rating of the back-to-back VSC [5]. Unlike
the new proposed topology, however, the solution with a
DFIG requires slip rings (costly and maintenance intensive in
an offshore environment) and does not enable full adjustablespeed
operation. Nevertheless, DFIG is currently the
preferable solution when the wind turbines are directly
connected to the main AC grid.
SYSTEM PARAMETERS
Both compared VSC transmission topologies are rated at 200
MW. The wind farm consists of one hundred commercially
available 2 MW wind turbines equipped with induction
generators. The power factor for this type of generator is in
the range of 0.9, thus consuming reactive power. The
converters have to supply this reactive power to compensate
the magnetising currents of the induction generators. The
total power rating of the wind farm is thus 222 MVA. The
stator voltage of the induction generators is 690 V, a common
choice for wind turbine generators. The HVDC voltage level
is ± 150 kV. Table 1 gives an overview of system parameters.
Voltage rating
The rated SSOA (switching safe operating area) voltage VSSOA
combined with the long-term stability against cosmic
radiation defines the IGBT voltage rating. For improved
reliability and to avoid false triggering due to cosmic
radiation the maximum allowed SSOA voltage VSSOA,max is
generally derated by approximately 40% from the maximum
device voltage Vce,max. The margin between the maximum and
the rated SSOA voltage is due to voltage spikes caused by
diode reverse recovery currents. In a soft-switching
environment this margin is considerably smaller. Table 1
shows the voltage ratings of the applied 5.2 kV soft-switched
and 2.5 kV hard-switched IGBTs [8]. The higher voltage
capability of the soft-switched IGBT can be explained by its
snubbered commutation. The limited di/dt capability and the
consequently longer turn-off time allow a higher blocking
voltage capability in trade-off, at an acceptably low on-state
voltage drop.
ADDITIONAL LOSSES
This section covers the converter losses that are not related to
the IGBTs (refer to section V for IGBT losses), namely diode
losses in the different VSCs and thyristor losses in the
cycloconverters. Additional system losses as filter, transformer
or cable losses are not included in this comparative
study, focussing on the converter losses and IGBT ratings.
Diode losses in the VSCs
The diode losses are an essential part of the total losses in the
VSCs. In particular when the VSC operates in a rectifier
mode, as it is the case in both the main VSCs. Thereby the
diodes are conducting during the major part of a commutation
cycle. This increases the system efficiency as the diode
conduction losses are lower compared to the IGBT
conduction losses. The allowed current density for the diodes
is approximately twice the one for the IGBTs. This results in
approximately half the silicon area for a diode compared to
an IGBT.
CONCLUSION
The emergence of larger and more efficient offshore wind
farms has opened new challenges in their grid connection. A
number of commercial VSC transmission schemes are now in
operation and show their suitability and potential. But despite
the range of advantages that VSC transmission offers, the
high initial costs and the switching losses limit the area of
application.
The concept of a novel soft-switching AC/DC converter is
presented in this paper. A single-phase VSC with capacitive
snubbers connected to cycloconverters via an MF AC bus
promises substantial benefits both in efficiency and initial
costs. Table 3 presents an overview of the IGBT power rating
and the different converter losses. It includes detailed results
about the proposed soft-switched (SS) topology and the hardswitched
(HS) reference topology for different frequency
modulation ratios p. A high frequency modulation ratio
causes extensive switching losses but reduces the harmonic
frequency distortion. The switching frequency for a
frequency modulation ratio p = 9 is 450 Hz, which makes a
comparison with the proposed topology (mf = 500 Hz) more
significant. Table 3 also includes the power ratings and losses
of the cycloconverters and the back-to-back converters (B2B)
for different frequency modulation ratios p.