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Electric Power Systems Research 78 (2008) 747–755
Offshore wind farm with a series multiterminal CSI HVDC
Dragan Jovcic ∗
Department of Engineering, King’s College, University of Aberdeen, Aberdeen AB24 3UE, UK
Received 23 October 2006; received in revised form 17 May 2007; accepted 29 May 2007
Available online 17 July 2007
Abstract
This paper presents an integrated design of an offshore wind farm and an interconnection circuit based on a series multiterminal HVDC link with
current source inverters (CSI). The transmission converters are used to achieve variable speed operation for a group of generators, and this enables
use of very simple generators. The series converter connection eliminates offshore transformers. The paper discusses the control systems for both,
generator side and grid side converters. A 200 MW wind farm is simulated on PSCAD/EMTDC platform and the responses confirm satisfactory
operation for a range of wind speed changes. It is shown that each generator group can operate with a different and optimal frequency and that
wind variations on individual units cannot jeopardize system stability. The main challenges for the proposed topology are system insulation and
management of transmission line losses, and the paper studies some possible solutions.
© 2007 Elsevier B.V. All rights reserved.
Keywords: HVDC transmission; HVDC control; Current source inverters; Wind energy; Synchronous generators
1. Introduction
The UK government aims to achieve 20% energy production from renewable energy sources by 2020. Such ambitious
increase in renewable energy share from the present 3% will be
largely based on increase in wind energy generation. Considering economies of scale, environmental and other issues, it is
recognized that large size offshore wind farms are well placed to
accommodate the future wind energy generation [1]. The existing small-scale European offshore wind farms are located in
shallow waters close to the shore, but because of the environmental and social aspects and the increased energy yield, the
future wind farms might be located at larger distances, some
approaching 100–150 km from the connection point onshore.
Theoretically, the offshore farms at distances below 60 km can
be connected either using ac or dc link, while at greater distances
only dc can be used because of the issues with long ac cables.
Most of the existing wind farms are of small size and they
do not have significant influence on the host grid in terms of
stability and control support. However, as the power share from
wind farms increase, it is necessary that wind farms should take
more active role in the ac systems regulation and support. Many
∗
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E-mail address: [email protected].
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doi:10.1016/j.epsr.2007.05.023
countries have already introduced special connection requirements for large wind farms [2]. A large offshore wind farm with
ac interconnection would need an additional converter system
(e.g. STATCOM or Static Var Compensator) in order to provide
adequate power quality at the grid connection point [3]. If an
HVDC connection is used, the onshore HVDC converters can
provide both voltage control and stabilisation, in particular if
self-commutated converters [4] are used. The above technical
arguments are in strong favor of HVDC interconnection however the additional costs of HVDC converters have made the ac
interconnection more attractive with wind farm developers.
Because of numerous technical benefits, (higher energy yield,
reduced dynamic loads, reduced noise) the modern MW-size
wind turbines always use variable speed operation, which is
achieved with a system of converters. These converters are typically associated with individual generators and they contribute
significantly (around 30%) to the costs of the wind generators.
With an HVDC interconnection link, there is potential to use
the transmission converters to provide variable speed operation [5,6] and thus eliminate the generator converters. Reference
[5] proposes a parallel-connected multiterminal HVDC system
using Voltage Source Converters (VSC). The parallel connection achieves some savings in components but it has limitations
including the need for special transformers and the use of wind
generators with gearboxes, as further studied in the sections
below.
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D. Jovcic / Electric Power Systems Research 78 (2008) 747–755
The weight on offshore platforms has significant influence
on the sizing of offshore platforms, and potential elimination of transformers would imply significant savings in costs.
This research studies a series, multiterminal HVDC based on
Current Source Inverters (CSI), which can build the transmission voltage without transformers. Such new interconnection
topology calls for an integrated approach in designing the
wind farm and the interconnecting links where unconventional wind generator designs are needed. It is postulated that
with a properly developed HVDC link, some other “standard”
wind farm components could be eliminated thus balancing the
costs.
The CSIs which use self-commutating switches have not been
widely utilized in HVDC/FACTS because of some practical disadvantages compared to VSCs [7,8]. These “traditional” CSI
issues (like the need for series connected diodes with asymmetric switches, terminal capacitor and tri-level control) can
be resolved with some cost increase, and the other advantages
of current source connections might offer further cost incentives
[7]. Assuming no technical obstacles are present, this paper studies the system-level benefits and challenges when using a large
number of CSIs in a multiterminal HVDC link.
This paper aims investigating electrical circuits and control
principles for a 200 MW test wind farm, assuming that multiterminal HVDC connection is used.
The Section 2 of this paper presents the model for the study
system and compares various alternative HVDC electrical circuits. In Sections 3 and 4, the control system is studied for
generator side and grid side converters, respectively. Section
5 presents simulation results and some open challenges are discussed in Section 6.
2. CSI based multiterminal HVDC for wind farms
2.1. System topology
Fig. 1 shows the electrical circuit for the considered wind
farm. It presents a 200 MW offshore wind farm consisting of 100
individual 2 MW, 4 kV wind generators. The wind generators are
the presently largest commercially available units based on permanent magnet (PM) direct-drive synchronous generators (SG).
This generation concept is becoming very attractive for developers because reliability issues with gearboxes are eliminated
[9].
The commercial PMSG units would employ a fully rated set
of converters to enable variable speed operation. In the proposed
connection in Fig. 1, the generator converters are not needed,
since the transmission converters facilitate variable speed operation. The wind turbines operate at a variable speed which is
however common for all generators in the same group. Each
group can operate at the most optimal speed. The generator
speed is regulated using the CSI controls, and the reference
speed/frequency is derived as the optimal average speed for the
group. The inability to operate individual machines at the most
optimum speeds is not considered to be a great loss in efficiency,
since it is likely that the wind profile will be largely similar
for a group of closely located turbines. The loading on individ-
Table 1
System rating depending on offshore topology
Ng
Nc
Idc [kA]
Vdc [kV]
Vnom [kV]
2
3
4
5
50
34
25
20
0.86
1.3
1.72
2.25
245
163
122
98
123
82
61
49
ual machines therefore can be expected not to be significantly
different.
The use of multiple HVDC terminals ensures that a relatively
small number of generators are connected to a single converter,
implying better adjustment of individual turbine speeds. The
number of units in a group is determined considering efficiency
of the turbine group and transmission system ratings. Since CSIs
are used, the generator ac current Ig is directly linked to the dc
(transmission) current [7] as:
Mmg Idcg
Ig = √
2Ng
(1)
where Ig is the rated RMS generator current; Idcg the rated dc
current; Mmg ≈ 0.95 the nominal value for the modulation index;
Ng is the number of generators in a group.
In the above formula all generators in a group operate
at the same power, for the simplicity in calculations. The
above formula is used to study topology of the electrical system, as shown in Table 1, assuming the wind farm from
Fig. 1. The following notation is used: Nc the number of
offshore converters, Vdc , total dc transmission voltage, Vnom ,
the largest neutral–earth voltage on generators (for insulation
purpose).
The downside of choosing smaller group size (small Ng ) is
that a large number of converters are needed, and also there will
be high insulation level on generators. On the other extreme,
choosing large number of generators in a group, results in high
dc current rating (high losses) and poor wind speed tracking
(poor coefficient of performance) on individual turbines. To give
good overall performance it is decided to use four generators
within a group (bold data in Table 1). The offshore transformers are not needed since the required dc transmission voltage
is achieved by connecting the converters in series. There are
25 self-commutated converters, which can develop the total dc
voltage of over 120 kV; which is adequate for long distance
transmission.
Since the generator converters and transformers are eliminated, the proposed concept gives potential for large capital cost
savings. The costs of these wind farm components would otherwise be required with ac interconnection, and they might offset
high HVDC converter costs.
2.2. Comparison with alternative HVDC connections
Fig. 2 shows the two alternative HVDC-based connections,
which are considered for comparison. In Fig. 2(a), a comparable
offshore wind farm with multiterminal parallel HVDC connection is shown, which is also based on 4 generator groups and
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Fig. 1. 200 MW wind farm with series multiterminal HVDC interconnection.
25 converters. Such connection requires transformers, which
would be prohibitively large, if a low-frequency directly coupled machine is used. A typical 22-pole 2 MW PMSG operating
with the optimal tip speed ratio of around 7 [10] delivers ac
power in the frequency range of 1–3 Hz for common wind
speeds. In order to increase the frequency, such interconnection
would likely be based on generators with gearboxes in which
case induction machines might be most suitable option. Because
of reliability reasons gearboxes are undesirable on offshore
generators.
Fig. 2(b) shows a 200 MW wind farm with a conventional 2terminal VSC HVDC operating at constant ac frequency. The
standard wind generators (based on permanent magnet synchronous machines) are assumed, which include local converters
enabling variable speed operation. The overall system has 100%
increased total converter rating compared with Fig. 1. Table 2
compares the three topologies in terms of required wind farm
components, where the series multiterminal connection appears
the most attractive. It is also mentioned that from the reliability
point of view the schemes in Figs. 1 and 2(b) might have some
Fig. 2. Two alternative offshore connections. (a) Parallel, multiterminal VSC-HVDC; (b) two terminal VSC-HVDC.
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Table 2
Comparison of HVDC topologies
HVDC
connection
Generator
converters
Transformers
required
High insulation
level
Gearbox
required
Series
Parallel
2 Terminal
No
No
Yes
No
Yes
Yes
Yes
No
No
No
Yes
No
The ac grid at the inverter side is represented as a conventional
LC circuit where the Short Circuit Ratio is 3.8 with X/R = 10.
At the CSI ac terminals typically small capacitors are needed to
enable harmonic filtering and to improve stability [7]. The simulation proves that with adequately fast controls this capacitance
can be eliminated.
3. Generator side converter control system
disadvantage since failure of a single converter implies that four
generators are out of operation.
2.3. System model
The studied 100-machine, 26-converter system is overly complex to be simulated in detail. A simplified model is sought
that would be practical for simulation on the commercial
PSCAD/EMTDC platform and yet it should maintain the important features of the actual system.
Fig. 3 shows the system model, which is used in all simulation. It includes four, series-connected and appropriately scaled,
CSIs at the rectifier side. Three of the converters are each connected to a single 50 MW generator to simulate adequate power
level. In order to study dynamics within the offshore ac grids,
one generator group is represented with a detailed, four-machine
model appropriately scaled. All system parameters, as well as
nominal values for main variables, are shown in the figure.
The synchronous machine parameters are given in Appendix
A (Table A1).
The standard CSI converter models are used [7], however
with multiterminal connection a bypass switch is required to
enable dc current flow if a converter is out of service.
3.1. Speed controller
The controllers for generator side converters combine multiterminal HVDC control principles and PM synchronous
machine control strategy. They utilize the current source converter operating methods assuming that the dc current is constant
[7]. According to the series multiterminal HVDC theory [11],
one converter maintains dc current at the nominal level (the
onshore inverter in this case), and all other terminals (rectifiers) control the dc voltage. By varying the individual dc
voltage the power on a particular rectifier converter is controlled. Fig. 4 shows the controller for each of the offshore
converters.
The generator speed wg is controlled by changing the generator electrical torque [9,12], and with permanent machines the
electrical torque is:
Te = 23 Pψsg iqgi
(2)
where Ψ sg is the stator flux, and iqgi is stator current of individual machines (q index for q axis and d index for d axis
component). The above formula requires that the converter
coordinate frame is linked with the rotor d-magnetic axis.
The transmission converters are therefore synchronized with
Fig. 3. PSCAD/EMTDC model for the considered system. CSI: current source inverter (with integral by-pass switch).
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A decision is made to control the dc voltage in the inner
speed loop, rather than the machine current isq , for the following
reasons:
Fig. 4. Generator side CSI controller. Θg , rotor position (average for the generator group); ωg , rotor speed (average for the generator group); ωg1 –ωg4 , rotor
speed for individual machines; vw , wind speed (average for the generator group);
Idg , d-axis component of generator current (sum for the generator group); Cf ,
rated stator flux; Vdcg , dc voltage on the corresponding CSI; Pacg , generator ac
power (sum for the generator group); MPPT, maximum power point tracking
algorithm.
the generator rotor axis using position encoders, as shown in
Fig. 4.
The two components of the stator flux are:
ψsd = Lsd idgi + ψmm ,
ψsq = Lsq iqgi
(3)
where Ψ mm is magnetizing flux (i.e. PM flux which is constant),
Ls is synchronous reactance. From (2) and (3) we conclude that
by keeping idgi constant, typically idgi = 0, we can achieve good
torque control with iqgi . Nevertheless, contrary to the vector
control principles with conventional drives, current iqgi is not
directly controlled in this system, as discussed below.
The dynamic equations for the machine stator windings are:
vdg = [Rs + pLsd ]idgi − ωg Lsq iqgi
(4)
vqg = [Rs + pLsq ]iqgi + ωg [Lsd idgi ] + Ea
(5)
where Ea is the excitation voltage, vg is the generator terminal
voltage, and Rs is the stator resistance.
The basic CSI converter control equations are [7]:
Igd = Mdg Idcg ,
Iqg = Mqg Idcg
(6)
where Mdg and Mqg are the two components of the converter
modulation index and Idg , Iqg are converter currents. Note that
converter
current is the sum of generator currents in a group
Ig = 4i=1 Igi . Using the converter power balance equation and
assuming idg = 0, we obtain:
Vqg Iqg = Vdcg Idcg
(7)
Combining (5)–(7), assuming that currents for generators
in the same group are identical, we derive expression linking
converter dc voltage and generator q current:
Mqg Vdcg = [Rs + pLsq ]4iqgi + Ea
(8)
This equation demonstrates that the generator current q component (and therefore generator speed) can be regulated by
manipulating the converter dc voltage, but a first order lag with
the time constant Lsq /Rs is introduced.
• The fast dc voltage control is also required for transmission
system stability and component rating,
• Fast control of generators is not important since wind turbines
have large inertia (2 s < H < 5 s in p.u.) which is comparable to
the inertia of conventional generators of hundreds MW rating
[13]. The control lag (time constant Lsq /Rs ) is much smaller
than machine inertia.
• As seen in (8), the dc voltage is proportional to derivative of
machine current implying possible large dc voltage variations,
which are undesirable.
3.2. Maximum power point tracking
The maximum power point tracking (MPPT) algorithm, that
is based on power measurement [9], is employed. It enables
operation at best turbine coefficient of performance for a wide
range of wind speeds, without measuring wind speeds. The converters regulate machine speed in this case and the blade angle
controllers are equipped with power feedback loops to prevent
excessive machine loadings at high winds.
3.3. Flux controller
The Mdg control input with vector-controlled drives is frequently used to regulate the d-current to zero thus enabling
maximum flux, good torque response and minimal reactive
power flow [9,12]. In the proposed circuit we have a small ac
grid connected to each of the CSI, and it is therefore meaningful to also have some form of ac voltage (Vg ) control on this
grid. As shown in Fig. 4, the inner loop on the flux controller
uses ac voltage regulation in order to prevent voltage excursions
during disturbances. The fast feedforward speed signal is also
introduced since the SG ac voltage depends primarily on the
machine speed.
4. Controller for the grid side converter
4.1. Controllability and control structure
A controllability analysis is given to explain main control
principles for a current source self-commutated converter. Referring to Fig. 3, and neglecting losses, the fundamental frequency
vector equation for the inverter current Ii is:
Īi =
V̄oi − V̄aci
jXt
(9)
where Vaci is the terminal ac voltage, Voi is the inverter ac voltage
and Xt is transformer reactance. Assuming that the coordinate
frame is linked with voltage d component Vdaci (PLL is tracking
Vaci ) we have that Vqaci = 0. Therefore using (9) the d and q
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Fig. 5. Grid side CSI controller. Idci , dc current; Vaci , inverter grid ac voltage;
Vacia − Vacic , inverter grid phase voltages; Pdci , dc power at inverter; Vdci inverter
dc voltage; PLL, phase locked loop.
current components are:
Idi =
Vqci
Xt
(10)
Iqi =
Vdoi + Vdaci
Xt
(11)
Considering that Vqaci = 0, the steady-state active power balance equation is Paci = Pdci :
Vdaci Idi = Vdci Idci
(12)
Replacing in (12) the expression for inverter d current
(inverter side equivalent to (6)) we obtain:
Vdci = Mdi Vaci
(13)
From the above equation we conclude that Mdi control channel is suitable for regulating dc voltage, which enables direct
control of dc current.
Using (11) we conclude that Mqi control input is suitable for
regulating ac voltage. A simplified inverter controller schematic
is shown in Fig. 5. Note that the current source inverters have
inherently good regulation of (fault) converter current and therefore no inner control loops are required.
4.2. Management of transmission losses
With the multiterminal series connected HVDC systems the
dc current theoretically stays at the rated value for all power
transfer levels, and consequently they suffer from high I2 R losses
at a reduced power transfer. The inverter current reference reduction technique may be applied during low power transfer, but this
implies that the potential maximum power on individual rectifier
stations becomes limited. Fortunately, all rectifier converters in
the wind farm can be expected to have similar power level, since
they have the same power generation aim (maximising power
output) and the wind speed at each turbine group will be within
a certain band.
An additional control level is therefore added to reduce the
current reference when lower power transfer level (Pdci ) is
detected, as seen in Fig. 5. This is a very slow control function, which uses only the local power measurement at inverter,
and therefore no communication with rectifiers is needed. The
current reference reduces proportionally with dc Power, but it is
Fig. 6. Current reference look-up table for grid converter.
well above the curve corresponding to maximum voltage operation, as seen in Fig. 6. This current reference margin enables
increased generation of 40 MW at all power levels as seen in
Fig. 6. Such margin level implies that rectifiers normally operate
somewhat below the maximum voltage, and it would allow each
8 MW rectifier to increase power by 1.6 MW above the average
converter power across the wind farm. The margin caters for difference in wind speeds and it provides room for transient power
adjustments at individual rectifier stations.
5. Simulation results
Fig. 7 shows the PSCAD/EMTDC simulation with variations
in wind speeds. To demonstrate the control concept, the wind
speeds at each converter group are made largely different, and
also wind gusts, represented by steep 3 m/s pulses, are introduced as seen in Fig. 7(a). Fig. 7(b) confirms good stability
and it illustrates that each generator group operates at different frequency, obtained by following the best turbine coefficient
of performance for the local wind speed. Note that the group
3 power (Pac3 ) is around two times larger than group 2 power
(Pac2 ) in interval 30–35 s illustrating adequate margin on dc current control. The operation of dc current reference runback for
low power generation is demonstrated in Fig. 7(d). Fig. 7(e)
shows that the grid facing HVDC converter enables excellent
control of grid voltage, which stays within 0.05% of the rated
value.
The generators in the proposed wind farm have no local
controls (neither exciter nor local converters) since they are controlled as a group from a joint converter. With such circuit, it is
important to study the dynamics within the four-generator group
to investigate possibility for the instability. Fig. 8 shows the tests
with widely different wind speeds one each of the turbines in the
same group, although such case is unlikely in practice. It is seen
that the transient responses are fast and well damped. Fig. 8(b)
illustrates that the machines operate at the same speed and transient deviations are minimal. It is also observed in Fig. 8(c) that
torques on individual machines can exceed rated values since
the machine speed is not regulated at optimum values for indi-
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Fig. 7. Simulation responses for wind speed changes.
Fig. 8. System response following wind change within a generator group.
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vidual machines. It is therefore thought that the turbine blade
angle regulator, which protects the rating limits of individual
machines using power feedback, might in addition include a
torque feedback. Also the machines in this concept might be
slightly overrated. The coefficient of performance for individual turbines is shown in Fig. 8(e), indicating optimal power
generation.
It is stressed here that the machine damper windings (data are
given in Appendix A) play important role in the system stability,
and in preventing that machines in a group oscillate against one
another. The stability of the generator groups is influenced by
the HVDC controls.
6. Discussion
Fig. 10. Relative transmission line losses as the function of power transfer.
This section analyses in more depth some of the open challenges in the proposed wind farm topology.
(3) Using an isolating transformer at each CSI, with secondary
star point grounded.
6.1. System insulation
6.2. Transmission losses
Since the converters are connected in series, those converters
coupling directly to the dc cable (furthest from the grounding point) will have largest line–earth potential. Note however
that all converters have same voltage and current rating but the
insulation level is different.
Fig. 9 shows the line–neutral (V1an ) and line–line (V1ab ) voltages at the generator terminals of group 1. It is seen that line–line
voltage is low, i.e. the rated voltage is equal at all stations (25 kV
rms in the simulated model), but the neutral–earth voltage is over
40 kV and therefore insulation will need special considerations.
The following options are proposed:
Fig. 10 compares the percentage dc cable losses (Ploss /Pdc )
as the function of power transfer level for different HVDC control strategies. The curve “dc current reduction” represents the
proposed controller with transmission losses management in
Fig. 6. Evidently, a significant reduction in losses is achieved
when compared to the case of constant dc current operation.
The losses at lower power are at a similar relative level as with
rated power transfer (3%).
Although constant dc voltage (parallel HVDC) gives lowest losses, such topology requires much higher capital costs
and necessitates machines with gearboxes, as shown in
Fig. 2(a).
(1) Insulating generator windings with respect to generator frame. Such insulation is technically feasible since
it is considered much simpler than the case of high
line voltage generators, like those installed recently at
Troll A platform which operate at 56 kV line–line voltage
[14].
(2) Insulating nacelle with respect to tower. Such option would
enable use of conventional low voltage generators.
Fig. 9. Line–neutral (V1an ) and line–line (V1ab ) voltages at a generator in group
1.
7. Conclusions
The use of series multiterminal HVDC for connecting a large
wind farm enables very simple wind farm topology where offshore transformers are eliminated and simple generators are
sufficient. It requires operating several machines as a group,
but the wind farms are suitable for such topology since the wind
power across the machines in a farm will be comparable.
The control system for each HVDC CSI is based on synchronous machine control concepts and it must also consider
the HVDC transmission system control tasks. The HVDC converters are capable of achieving variable speed wind turbine
control.
The PSCAD simulation results with a 200 MW wind farm test
system demonstrate satisfactory operation for a range of wind
speeds, which can be different at each generator group. Further
tests with wind variations on individual turbines within a group,
do not indicate stability problems.
It is concluded that the main open challenges are the insulation of generators and optimal management of transmission
cable losses. The proposed higher-level control function enables
that the relative transmission losses are kept at the same low level
over a wide range of power transfer.
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Appendix A. System parameters
Table A1
Generator parameters (unscaled)
Parameter value
Description
S = 2 MVA
U = 4 kV
Rs = 0.0208 Lsl = 0.0014H
Ld = 0.0048H
Lq = 0.0031H
Rdd = 0.099 Rdq = 0.099 ωm = 19 rpm
P = 22
J = 2.62e6 kg m2
Rated power
Rated voltage
Stator resistance
Stator leakage inductance
d axis magnetizing inductance
q axis magnetizing inductance
d axis damper resistance
q axis damper resistance
Rated shaft speed
Number of poles
Rotor and turbine inertia (H = 2.57s at 19 rpm)
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Dragan Jovcic (B.Sc., Ph.D., SMIEEE) obtained a diploma engineer degree in
control engineering from the University of Belgrade, Yugoslavia in 1993 and a
Ph.D. degree in electrical engineering from the University of Auckland, New
Zealand in 1999.
He is currently a lecturer with the University of Aberdeen, Scotland, where
he has been since 2004. He also worked as a lecturer with University of Ulster, in
period 2000–2004 and as a design engineer in the New Zealand power industry
in period 1999–2000. His research interests lie in the areas of FACTS, HVDC
and control systems.