New topology for more efficient AC/DC converters
for future offshore wind farms
Stephan Meier
Staffan Norrga
Hans-Peter Nee
Royal Institute of Technology
Stockholm, Sweden
Email: stephan@ekc.kth.se
Royal Institute of Technology
Stockholm, Sweden
Email: staffan@ekc.kth.se
Royal Institute of Technology
Stockholm, Sweden
Email: hansi@ekc.kth.se
they are not affected by cable charging currents. Above a
critical cable length, a compensation unit is required in the
AC transmission system, whereas DC cables can have any
length required without charging current problems. In the DC
case, the cable capacitance is instead an advantage since it
can store energy. Furthermore, the higher power transmission
capability per cable makes the DC cable even less expensive
than an AC transmission cable with the same capacity.
I. I NTRODUCTION
Drawbacks of the VSC transmission compared to the tradiDuring recent years the international community has shown tional AC transmission are:
a greater environmental responsibility. This has created excel- - The costs for the VSC transmission are quite high. Existing
lent opportunities within the field of windpower generation. VSC transmission technology cannot offer an economically
Due to the development of larger and more efficient wind- competitive solution compared to AC transmission mainly due
power farms, windpower has become an important alternative to the high cost of the complex converters and cables that are
power generation worldwide. Thus, an increasing number of required [2].
large offshore and remote onshore windpower farms will get - The high-frequency pulse width modulation (PWM) switchconnected to the AC transmission network.
ing leads to high switching losses. During periods of low
High voltage direct current (HVDC) systems based on power generation (i.e. during low wind speed) the switching
voltage source converters (VSC) have lately emerged as a losses may consume a significant portion of the generated
competitive alternative to the conventional AC transmission. power. In the Tjæreborg VSC transmission project [3] for
The principle of an HVDC system based on VSCs, called VSC example, the power transmission during low power generation
Transmission, is shown in Figure 1. It provides an efficient and is operated via a parallel 10 kV AC cable connection due to
reliable solution, overcoming the major technical challenges economic reasons.
facing traditional AC solutions. These are namely problems
This paper proposes a novel topology promising benefits
with high capacitive cable currents and the drawback that the in both costs and efficiency. Its principle of operation is
frequency of the windfarm network ff arm must be equal to the discussed with respect to the commutation of the valves and a
grid frequency fgrid . VSC transmission particularly provides suitable modulation method. The benefits and challenges of the
isolation between the offshore installation and the mainland proposed topology are outlined and discussed in comparison
distribution network. VSCs behave like autonomous AC volt- with other state-of-the-art solutions.
age sources [1]. This provides the following PSfrag
advantages:
replacements
II. T OPOLOGY
- Sending and receiving end frequencies are independent and
allow frequency control for the wind farm generators. Thus
The topology of the proposed AC/DC converter is shown
they can be operated at their optimal efficiency operation point in Figure 2. It incorporates a VSC and cycloconverters (direct
over a wide range of wind speeds.
- The power flow is fully defined and controllable. The active
AC grid
Wind farm network
power flow between the wind farm and the grid can be
ff arm , Vf arm
fgrid , Vgrid
controlled precisely allowing high flexibility in cases such as
P,
−P ,
start-ups, shutdowns or faults and disturbances. The possibility
Qf arm
Qgrid
P
DC cable
to generate a controlled reactive power provides grid voltage
control supporting weak networks or even passive loads. The
Vdc
VSC
VSC
VSC located at the wind farm side supplies the AC voltage and
the reactive power required to operate induction generators.
Fig. 1. Principle of a VSC transmission.
The advantage of DC cables compared to AC cables is that
Abstract— The concept of a novel soft-switching AC/DC converter is applied for use in future offshore wind farms. A voltage
source converter with capacitive snubbers and cycloconverters,
connected via a medium frequency AC bus, promises substantial
benefits. This paper presents this new topology and describes its
principle of operation. Furthermore, a comparison with state-ofthe-art adjustable speed generators reveals the potential of this
new concept.
PSfrag replacements
(1)
converters) connected via a medium frequency (MF) AC bus.
Every wind turbine is equipped with a passive line filter
(shown as an inductor in the schematic), a 3-by-2 cycloconverter, an MF transformer and a circuit breaker. The valves of
the cycloconverter do not need any turn-off capability and can
be realised by thyristors connected in anti-parallel. The MF
transformer increases the generator voltage from e.g. 690 V to
33 kV. The connection to the MF AC bus is done via a circuit
breaker allowing disconnection of individual wind turbines.
The MF AC bus connects the wind turbines to a single-phase
VSC via the main circuit breaker and the main transformer.
This MF transformer increases the bus voltage from e.g. 33 kV
to 150 kV. The high voltage side of the transformer is connected to a single-phase VSC, whereas one of the transformer
terminals is connected to the midpoint in the DC link created
by bus-splitting capacitors. These DC capacitors provide the
DC voltage source necessary for the dynamics of the system
and govern the voltage ripple on the DC line. The VSC
valves consist of IGBTs (e.g. 5,2 kV IGBT designed for softswitching from ABB) connected in series to support the high
voltage DC. Additionally, the VSC is equipped with snubber
capacitors connected in parallel to each of the semiconductor
switches. The capacitors should be sufficiently large to allow
zero-voltage turn-off and turn-on of the IGBTs. The ground
reference of the VSC can be made at the midpoint in the DC
link.
ii
VSC with one phase leg
MF
AC bus
replacements
HVDC
output
MF
transformer
G
~
Wind turbine
generators
Fig. 2.
Lf ilter
(3)
Lλ
ui
N1 N2 v d i i
Lλ
Lf ilter
ui
N1 N2 v d
Commutation of a cycloconverter phase leg.
simplicity the assumption is made that the transformer in the
wind turbine and the main transformer can be represented by
their total leakage inductance (Lλ ) whereas the voltage source
converter is represented by a constant voltage (vd ) during the
commutation of the cycloconverter. Thus, the cycloconverter
sees a voltage N1 N2 vd where N1 and N2 are the winding
ratios of the transformers. In order to achieve a natural
commutation of a cycloconverter phase legs, the polarity of
the VSC output voltage has to be right. In this example, vd
has to be negative as indicated in Figure 3.1. The commutation
is initiated by turning on the non-conducting valve in the
direction of the current through the phase terminal (ii for phase
i). The voltage supplied by the VSC appears across the leakage
inductance and the incoming valve gradually takes over the
current (Figure 3.2). Finally, the initially conducting valve
turns off as the current through it goes to zero (Figure 3.3).
The current derivatives dii /dt during both turn-on and turnoff are determined by the total transformer leakage inductance
Lλ and are thus relatively low.
2) Commutation of the VSC: Successive commutations of
the cycloconverter phase legs, as mentioned above, eventually
leads to a reversal of the current through the main MF
transformer. Now the main transformer voltage utr and current
itr have the same sign and the instantaneous power flow is
directed from the DC-side to the AC-side. Thereby the conditions are set for a snubbered commutation of the VSC. Figure 4
shows the stages of a VSC commutation where itr is positive.
The process is initiated by turning off the conducting valve at
zero-voltage conditions. The current is thereby diverted to the
snubber capacitors which are getting recharged (Figure 4.2).
When the potential of the phase terminal has fully swung to
the opposite, the diodes in the incoming valve take over the
current (Figure 4.3). Finally, the switches that are anti-parallel
to these diodes are turned on at zero-voltage and zero-current
conditions (Figure 4.4). Thereby the VSC is prepared for a
subsequent reversal of the current.
The reversal of the transformer voltage utr by the VSC
commutation establishes the possibility for natural commutation of the cycloconverters. Thus the cycle can be repeated.
At low currents itr the commutation of the VSC may become
unduly lengthy as the recharging of the snubber capacitors
becomes slower. In the case of zero load, it is impossible to
commutate the VSC in the fashion described above. However,
a quasi-resonant commutation mode is proposed to solve this
problem [4]. By short-circuiting the low voltage windings
of the wind turbine transformers using the cycloconverters
Cycloconverters
Line filters
ui
N1 N2 v d i i
Fig. 3.
By alternately commutating the cycloconverters and the
VSC it is possible to achieve soft commutations for all
the semiconductor valves [4]. The cycloconverters can be
solely operated by source commutation (natural commutation)
whereas snubbered or zero-voltage commutation is always
enabled for the VSC.
1) Commutation of the cycloconverter: Figure 3 shows an
example of the commutation of a cycloconverter phase leg. For
G
~
Lλ
Lf ilter
A. Principle of operation
G
~
(2)
The schematic of the proposed topology.
2
(1)
utr
Id
itr
(2)
utr
Sawtooth carrier for ii > 0
Sawtooth carrier for ii < 0
Id
u∗a
itr
u∗b
Sfrag replacements
(3)
utr
Id
itr
(4)
utr
Id
u∗c
itr
utr
Fig. 4.
Commutation of the VSC.
itr
it is possible to initiate a resonant process during the VSC
commutation, governed by the snubber capacitors and the
leakage inductances of the transformers. This resonant process
can be utilised for recharging the snubber capacitors.
PSfrag
replacements
3) Modulation: Apart from maintaining soft
commutation
ua
as outlined before, the control system for the proposed converter should fulfill two main requirements. Firstly, proper
operation of the transformers should be ensured by avoiding
low frequency or DC components in the transformer voltages.
This is achieved by constant VSC commutation intervals, thus
ub
applying a square-wave voltage to the MF AC bus. Secondly,
it should be possible to obtain the desired PWM patterns
for the cycloconverters. By making the commutations of the
cycloconverter phase legs at appropriate instants in the interval
between two VSC commutations, the width of the PWM pulses
uc
can be chosen freely. This may be achieved in multiple ways,
e.g. with a carrier-based modulation method.
B. Waveforms during commutation
AD
C AD
CA
B
B
D: VSC commutation
A,B,C: Commutation of the cycloconverter phase legs
Figure 5 shows the basic waveforms under one period of
the cycloconverter. The upper graph shows the sqare-wave
main transformer voltage utr , the two lower graphs show the
Fig. 6. Waveforms during commutation sequence enabling constant soft
commutation.
utr
voltage ui and current ii of an arbitrary cycloconverter phase
leg. For simplicity, the commutation effects on the waveforms
are not included in Figure 5. Therefore, Figure 6 shows the
basic voltage and current waveforms during a commutation
sequence. To simplify matters, the main transformer current
itr is plotted for the case where only one cycloconverter
is connected to the MF AC bus. The uppermost graph in
Figure 6 illustrates the generation of the switching signals by
comparison of the control signals (u∗i for phase i) with two
repetitive sawtooth carriers. This can be interpreted as a carrier
based method where sawtooth carriers with either positive or
negative slope are used, depending on the current direction.
ui
ii
Fig. 5.
Basic waveforms under one period of the cycloconverter.
3
conventional solutions. Thereby, the cycloconverters allow
adjustable speed operation of the wind turbine generators.
Different generators are suitable for application, preferably
a squirrel cage induction generator which is a cheap and
low-maintenance solution.
This modulation scheme is chosen with respect to the fact that
the sign of each phase potential (ui for phase i) after a VSC
commutation is opposite to the sign of the current through
the corresponding phase (ii for phase i). This implies that the
instants for commutating the cycloconverter phase legs have
to be determined in different ways depending on the direction
of the line currents ii [4].
For clarity, the durations of the commutation processes have
been greatly exaggerated in Figure 6. In practice they occupy
only a minor fraction of the commutation cycle. Furthermore,
the impact of the commutation processes on the curveforms
is not shown in the figure. In practice it may be necessary to
adjust the timing of the switchings for the voltage-time area
lost or gained during the commutations.
III. C OMPARISON
The main benefits are summarized below:
- Single-phase MF transformers replace the three-phase
transformers. This reduces not only the cost but also the
weight and volume of the transformers, thus increasing the
flexibility in their placement.
- The number of series-connected IGBTs in the VSC
decreases very significantly due to the reduction to one
phase leg. The IGBT switches tend to be expensive, as they
require complex gate drives and voltage-sharing circuitries.
These assure that all series-connected devices of each valve
are switched simultaneously with a minimum stress on the
individual device. A reduction in their number is highly
desirable.
- The switching losses can be reduced significantly due
to soft-switching. Soft commutation is achieved for all the
semiconductor switches by alternately commutating the
cycloconverter and the VSC.
- The valves of the cycloconverters can be realised by
comparably cheap and well-established fast thyristors with
low losses compared to IGBTs.
OF ADJUSTABLE SPEED GENERATORS
This section presents a discussion of different relevant
topologies of adjustable speed generators (ASG) using HVDC
transmission. A typical load curve of a wind turbine generator
is given in Figure 7. It shows the electrical output power as
a function of the turbine speed as well as the power curves
relative to the turbine speed at different wind speeds. Based
on this load curve, the key advantages of adjustable speed
operation [5] are discussed:
- The acoustic noise is reduced significantly at low power
conditions due to the possibility of low-speed operation (range
a in Figure 7).
- The system efficiency is improved because the turbine speed
can be adjusted to the maximum output power over a wide
range of wind speeds (range b in Figure 7).
- It allows simple and cost effective pitch control. Pitch angle
control is only applied to limit the maximum output power at
high wind speeds (range c in Figure 7). The wind turbine is
stopped above the cut-off wind speed it was designed for (see
d in Figure 7).
- Mechanical stresses are reduced and the power quality is
improved. The wind turbine system gets more elastic (e.g.
wind gusts are absorbed by storing energy in the mechanical
inertia of the turbine) which significantly reduces torque and
power pulsations.
The new topology implies several challenges that have to
be studied carefully:
- The design of the MF transformers is crucial. Especially the
choice of the core material and the winding design have to be
adapted to the specific characteristics of the proposed topology.
In particular the large step changes in voltage (determined by
the snubber capacitors of the VSC) expose the transformer to
a high dv/dt. Thus, the design of the transformer insulation
needs to comply with the demanding specification.
- The effect of the square-wave MF voltage on the AC cables
between the cycloconverters and the VSC has to be investigated. It is an important factor determining the configuration
of the MF network as well as the performance of the complete
Today, variable speed wind turbines are mainly built as
direct-in-line or doubly-fed induction generator ASG systems
(see Figure 8). All considered topologies are based on the
same technology for the connection of the wind farm DC
output to the local transmission grid. A VSC that is located
onshore links the AC transmission grid to the submarine
DC cable. Different VSC topologies are suitable for DC
power transmission, e.g. two-level, multilevel diode-clamped
or multilevel floating capacitor converters [6]. Subsequently,
the considered topologies are described and their advantages
and disadvantages discussed.
Electrical power P [p.u.]
1.25
1
c
0.75
0.5
b
0.25
0
A. Proposed topology
The topology proposed in this paper (Figure 8.1) offers
multiple advantages, providing the same functionality as
d
a
0
0.25
0.5 0.75 1 1.25 1.5
Turbine speed n [p.u.]
1.75
Fig. 7. Electrical output power as a function of turbine speed for adjustable
speed generators
4
C. Doubly-fed induction generator ASG
The doubly-fed induction generator (DFIG) ASG configuration is shown in Figure 8.3. It differs from the direct-inline ASG in the point, that the stator of the DFIG is directly
connected to the windfarm grid wheras the rotor windings
are connected to a frequency converter (back-to-back VSC)
over slip rings. The rotational speed of the wind turbine is
proportional to the frequency difference between the stator
(grid) and rotor (converter) frequency. For a typical converter
rating of 25 % of nominal generator power, the speed range
of the DFIG ASG is ±33% around the synchronous speed
[5]. This limited speed range is the main disadvantage of the
DFIG ASG. In addition, the necessary slip rings prove to be
delicate in an offshore environment and cause considerable
maintenance costs. Advantages of the DFIG ASG are the
reduced costs for the frequency converter and the improved
system efficiency within the operational speed range due to
lower converter losses.
The DFIG ASG is widely used nowadays and plays an
important role in the planning of larger wind turbines (e.g.
the 5 MW REpower 5M [9].
(1)
Cycloconv
3Ø
VSC
VSC
1Ø
VSC
(2)
3Ø
VSC
VSC
3Ø
VSC
3Ø
VSC
VSC
3Ø
VSC
VSC
3Ø
VSC
VSC
3Ø
VSC
VSC
(3)
3Ø
VSC
VSC
3Ø
VSC
(4)
3Ø
VSC
VSC
3Ø
VSC
3Ø
VSC
Fig. 8. Different topologies of adjustable speed generators. (1) Proposed
topology, (2) Direct-In-Line ASG, (3) Doubly-fed induction generator ASG,
(4) Individual connection of wind turbines to HVDC link.
D. Individual connection to HVDC link
Another possible configuration is the individual connection
of the wind turbines to the HVDC link (see Figure 8.4).
Thereby, the wind turbines are provided with their own VSC
offering all advantages of adjustable speed operation. Such a
configuration puts high requirements on the system stability
and the control of the common DC voltage, which must be
controlled with compromise between all the wind turbines [7].
ABB has proposed a new Windformer concept based on
individual AC/DC converters in the year 2000 [7]. The concept
is based on a multipole (gearless), high voltage permanent
magnet synchronous generator, which can be directly connected to the VSC without a transformer. Today, the realisation
of this concept is uncertain.
system.
- An appropriate control system has to be designed on two
levels: On the system level, the wind park as a whole,
every individual wind turbine and the grid connection are
controlled. This covers aspects such as normal operation, fault
handling, protection or communication within the wind park.
On the converter level, the control system has to supply the
modulation signals to the thyristor and IGBT valves.
B. Direct-In-Line ASG
The direct-in-line ASG configuration is shown in Figure 8.2.
The connection between the wind park AC grid and the
HVDC transmission cable is realised by a three-phase VSC.
A technique based on VSC transmission is HVDC Light [3],
which has proven well for connecting wind farms to the AC
grid (i.e. Tjæreborg VSC transmission project [3]). In the
direct-in-line ASG configuration, every wind turbine generator
is connected to the wind park grid by a full-size frequency
converter (back-to-back VSC). This solution allows the use of
different wind turbine generators, i.e. squirrel cage induction
generator or synchronous generator.
The direct-in-line ASG offers all advantages of adjustable
speed operation. The only disadvantage is the fact that the
frequency converter is rated at the nominal generator power.
This makes the converter large and expensive. Moreover, the
efficiency of the back-to-back VSCs is an important factor for
the total system efficiency as the nominal power flows through
it. Despite this drawback, the direct-in-line ASG configuration
gets more into the focus for larger windturbines nowadays (e.g.
the 4,5 MW Enercon E-112 [8]).
IV. C ONCLUSION
The emergence of larger and more efficient offshore windfarms has opened new challenges with respect to their
grid connection. A number of commercial VSC transmission
schemes are now in operation which 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 VSC with capacitive snubbers
connected to cycloconverters via an MF AC bus promises
substantial benefits both in efficiency and initial costs. A comparison with state-of-the-art ASG solutions shows the potential
of this new topology: The number of series-connected IGBTs
in the VSC is reduced, cheaper and lighter MF transformers
are applicable, the switching losses are reduced due to softswitching and full adjustable speed operation of the wind
turbines is enabled by the cycloconverters. As a consequence,
5
the application of VSC transmission in the grid connection of
windfarms becomes far more attractive.
ACKNOWLEDGMENT
The authors would like to express their gratitude to VindForsk and the Swedish Energy Agency for financial support.
R EFERENCES
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[8] Reference webpage: www.enercon.de
[9] Reference webpage: www.repower.de
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