Download Offshore wind farm with centralised power conversion and DC

Offshore wind farm with centralised power conversion
and DC interconnection
Dr D. Jovcic and N. Strachan
Dr D. Jovcic and N. Strachan are with University of Aberdeen, School of Engineering, King’s college, Aberdeen,
AB24 3UE, Scotland. E-mail: [email protected], [email protected]
Abstract: This paper presents the study of a hypothetical large offshore wind farm based on centralised power
conversion and interconnected to the grid using a multiterminal parallel HVDC link. The 300MW wind farm
consists of 60 squirrel-cage based 5MW generators connected to a common DC bus using 10 VSC (Voltage
Source Converter) converters. The transmission system converters provide variable speed generator control,
and therefore individual converters are not required for each wind generator, implying savings in wind farm
costs.
The paper studies the technical and economical benefits of the proposed topology, as well as the selection
of the main components. A detailed analysis of the control circuits for both generator and grid facing converters,
with respect to primary control functions, is also given.
PSCAD/EMTDC simulation of the proposed concept is presented for realistic wind signals. The results
confirm operation at an average optimum coefficient of performance at each respective generator group, as well
as satisfactory stability even for severe wind speed changes. The proposed concept reduces the costs
associated with DC interconnection and may simplify integration of large offshore wind farms at substantial
distances.
Key Words: HVDC transmission, Wind power generation, AC machines, Pulse width modulated power
converters.
1
Introduction
There is currently significant interest in offshore wind farm development because of a range of techno-economic
and environmental benefits, and the recent granting of a number of licenses for large offshore wind farms by the
UK government.
Theoretically, future offshore wind farms at distances less than 60km from the shore could be connected to
the grid using either an AC or DC link, whereas at a greater distance only DC links are applicable [1,2]. There
are several main advantages of using DC over AC transmission for wind farms:
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•
For the same power transfer level, DC cables are smaller with lower losses and have no limitations in
length. The reactive power demand by AC cables varies proportionally to the voltage difference across
the cable, and very stable voltages are needed with large wind farms and long cables.
•
Grid facing VSC HVDC converters can meet and exceed all interconnection voltage/frequency control
requirements since they have the ability to independently and rapidly control active and reactive power
at the connection point. Also as a result, HVDC has the ability to connect to much weaker networks
than comparable AC interconnections, and therefore either larger wind farm ratings or shorter
connecting distances are possible [1,2]. It is certain that any larger AC cable installation will need an
additional converter system at the grid connection point in order to reduce losses, ensure stability and
to meet the latest grid operator requirements. This would likely be from the Static Var Compensator
(SVC) or Static Compensator (STATCOM) families [3], with an additional cost penalty.
•
The DC bus of an HVDC link is a convenient connection point for a range of storage units. DC
capacitors, supercapacitors or super conducting magnetic energy storage (SMES) can be readily
connected to the DC bus, and can possibly employ the same HVDC interfacing converter.
DC interconnections have not been hitherto used with offshore wind farms primarily because of the
substantial capital costs associated with converter stations. However a thorough recent study by the UK
government [4] concluded that DC transmission based on the latest Voltage Source Converter (VSC)
technology [5] is the most suitable option for offshore submarine networks.
This paper assumes that a DC interconnection is techno-economically justifiable, and goes a step further in
optimising the cost of HVDC connection with wind generators. The VSC transmission converters facing the
generators can potentially take over many of the wind generator control functions. Consequently, it may be
possible to employ simpler wind generators with less costly local converters. ABB has recently demonstrated a
pioneering concept of a VSC HVDC directly driving (and providing full control functions of) a high voltage motor
(50kV, 40MW) at the Troll-A offshore platform [6]. Some other research projects on wind farm HVDC
interconnection have also proposed that high voltage converters could provide variable speed control of wind
generators [7]. This concept potentially implies capital cost savings since individual wind generator converters,
and their associated controls, may account for a significant proportion of the total wind generator cost.
The parallel connected multiterminal DC system studied in [7] considers transmission converters controlling
individual wind generators. Such topology is likely to be uneconomical in practice since the converter currents
would be well below their ratings. The use of DC/DC transformers on individual machines may offer a suitable
solution, but there are many challenges in developing a suitable DC/DC boost converter [8]. Because of
limitations in the rating of present wind generators, it is proposed to connect each converter to a group of wind
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generators, which leads to the concept of centralised power conversion [9]. The research in [9] uses series
connection of converters, which shows potential for transformer cost savings, however insulation of the offshore
equipment may prove to be complex and current source converters are also more expensive.
This paper aims to explore the integrated design of a large wind farm with DC interconnection, assuming
centralised power conversion and a parallel multiterminal HVDC. The goal is to develop a detailed simulation
model of a 300MW system, and to study the associated technical and financial trade-offs. Additionally,
centralised power conversion requires specialised control methods that will be investigated in detail.
The paper is organized as follows. The proposed wind farm topology is introduced in Section II, followed in
Sections III and IV by a more detailed technical and financial appraisal of centralised power conversion. Section
V examines the control strategies, and PSCAD simulation results are presented in Section VI.
2
Wind farm topology
2.1 The proposed wind farm topology
Figure 1 shows the electrical circuit for the proposed wind farm topology. The 300MW offshore wind farm
consists of 60 individual 5MW, 4kV wind generators. It is assumed that the wind farm distance from the shore is
approximately 100km.
The wind generators resemble the commercially available 5MW units. However, the wind generators do not
need individual converter systems since the transmission converters enable variable speed operation of each
respective generator group.
The offshore electrical network consists of 10 generator groups each connected via a dedicated 160kV
Voltage Source Converter (VSC). Each 30MW group comprises of 6 generators and a single 4kV/99kV
transformer operating at variable frequency.
The wind turbines operate at variable speeds in order to maximize energy capture, reduce stresses and
reduce noise. Generator speed is controlled using the VSC converters, with all the generators within a group
operating at the same speed. The speed of generators in the group is derived as the average optimum speed.
Note that each respective generator group can operate at the most suitable speed independently of the speed
of other groups. The inability to operate individual machines within a group at the entirely optimized speed
results in an energy efficiency loss that is dependent on the number of machines within the group, which is
studied in Section III.
The 10 VSC converters are connected in parallel to a common DC bus, thus operating in a parallel
multiterminal HVDC connection. The common DC voltage is maintained at the nominal level (160kV) by the
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single VSC inverter located on-shore. The selected voltage and power levels correspond to the standard VSC
modules for HVDC-light.
Figure 2 shows two conventional interconnection topologies that might be contenders for a similar wind farm,
with Figure 2a employing AC interconnection and Figure 2b employing DC interconnection. Compared with
Figure 2a, the circuit in Figure 1 has the advantage of the DC transmission and the presence of an on-shore
converter. However, the wind farm in Figure 1 will suffer “Cp losses” associated with inadequate speed control
of individual turbines within a group.
The topology in Figure 2b uses DC interconnection but it has twice the total converter rating compared with
Figure 1, owing to the presence of both: individual generator converters, and the offshore transmission
converter. Therefore, any financial benefits arising from individual wind generator simplifications in Figure 1 are
regarded as capital cost savings. Table 1 summarizes the major cost implications for the three respective
topologies. The particular technical challenges and trade offs that have cost significance are studied below,
including: selection of machine types, optimum calculation of group size, and control strategies.
2.2 Selection of generators and transformers
It is postulated that simple 4-pole squirrel-cage induction machines are the most suitable generators for the
proposed offshore wind farm. The caged rotor is simplest and least expensive compared with commonly used
permanent magnet machines or wound-rotor machines. The nominal operating frequency of the offshore
network is 50Hz (at full power), and gearboxes (approximately 120 ratio) will be needed. Also, induction
machines are better suited for parallel connection, since slip variations improve stability and reduce shaft
stresses.
The directly coupled (no gearboxes) synchronous generators are not suitable in the proposed concept since
they would require operation at very low offshore electrical frequency. Typical 20-30 pole 5MW permanent
magnet machines would operate at a rated frequency of approx. 4-9Hz for a rated windspeed of 11.5m/s. In
order to transmit rated power at such a low frequency, transformers would require very large cores with a
resulting significant weight penalty.
The proposed system can be operated at an even higher offshore grid frequency which might be beneficial
considering the resulting reduction in transformer weight. However, the gearboxes would increase (2-3 stage)
and core losses would also have to be estimated.
The transformer and induction machine magnetic circuits behave similarly for changes in frequency.
Assuming that machines operate in the linear region of B-H curve and neglecting losses, we can conclude that
the flux is proportional to Vt/f ratio (Vt is the transformer voltage and f is the frequency). If flux is kept at rated
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value, the transformer rated power is directly proportional to the frequency as shown in Figure 3. The electrical
off-shore frequency will be lower at low wind speeds, and also linearly proportional to wind speed for maximum
efficiency [9,10]. Considering that the mechanical power of a wind turbine is proportional to a cube of the wind
speed [9,10], we conclude that there is no danger of transformer overloading as seen in Figure 3. Standard
50Hz transformers and induction machines are therefore recommended.
3
Selection of group size
3.1 Study of cp losses
The number of machines connected to a single converter is a primary design parameter in the proposed
topology. The main concern is the resulting relative loss of efficiency for individual turbines since all machines
within a group operate at the same speed, which may not be the optimal speed for each individual machine.
In order to study the relative Cp losses between different group sizes, a range of tests were performed on the
PSCAD wind farm model for 2, 4, 6 and 8 machine group sizes. A wind front passing a row of ‘grouped’ turbines
is simulated by using a representative test wind speed signal with a delay element. The delay interval is linearly
increased at neighbouring turbines. Considering that the angle of the wind front can change relative to the row
of ‘grouped’ turbines, the basic delay interval is further varied within the study, as is also the average
windspeed.
Figure 4 depicts the test wind speed signal on two adjacent ‘grouped’ turbines for a 5s signal delay with a
base wind speed of 11 m/s. The depicted signal is created using the wind speed model described in [11] (and
also implemented within the PSCAD library), and which can be considered as extreme in noise levels. The 5s
delay would correspond to a wind front angle of 84deg with a 500m turbine spacing.
The energy produced by a generator group is compared with the energy of another group of the same size,
under the same wind speed signals, but operated with converters at each individual machine. The resulting
relative percentage loss in energy for differing group sizes, and differing delay intervals, is depicted in Figure 5a
for the rated base wind speed (approximately 11m/s). It is seen, perhaps surprisingly, that the delay (angle of
the wind front relative to the grouped turbine row) has a very limited influence on the resulting energy loss. It is
also seen that losses increase as the number of machines in the group increases (approximately 5% for 2
machines and 14.5% for eight machines), as expected. Furthermore, as the number of machines increase, the
incremental energy loss is lower. Figure 5b depicts the resultant average (over different time delays) energy
loss according to group size assuming rated wind speed.
Figure 5c presents the resultant energy loss over the range of different wind speeds, assuming a 2s delay
interval (Note the 2s delay values depicted in Figure 5a best match the average energy loss values shown in
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Figure 5b). It is seen that the percentage energy loss falls as the average wind speed falls. This is explained by
the reduced magnitude of the noise content on the wind signal, which is causing speed variations on
generators. It is likewise seen that above the rated wind speed (11.5m/s) the % energy loss also falls owing to
the action of the Blade Pitch Angle (BPA) controller which acts on the individual machines (described in Section
V).
In order to realistically represent the losses over a longer period, the variation in base wind speed is
modelled using the Weibull distribution, as given by [12]:
(
f (V ; k , λ ) =(k c ) V c
w
w
)k − 1 exp − (Vw c )k 
(1)
Where f(Vw) is the probability of observing wind speed Vw , k is the dimensionless Weibull shape parameter, and
c is the Weibull scale parameter. The commonly used special case of the Weibull distribution, where k=2
(Rayleigh distribution), is employed along with a scale parameter c=8.88m/s. This corresponds with an average
farm wind speed of 8m/s, and a resulting wind farm capacity factor (ratio of actual energy produced to the
hypothetically maximum possible assuming operation at rated wind speed) of 40%. The resulting base wind
speed frequency distribution, along with the corresponding power generation levels for a single 5MW generator,
is depicted in Figure 6a.
By combining the energy loss according to the group size and base wind speed, and Weibull wind speed
distribution shown in Figure 6a), the average wind farm energy losses and capacity factor are obtained as
depicted in Figure 6b. An overall annual energy loss of around 6%, and a 2% drop in capacity factor, can be
expected when operating six machines in parallel.
3.2 Transmission losses and converter sizing
Single IGBT switches at their highest power (5-6kV) typically have a 200A minimum rating. If a small number
of machines in a group is selected, then either the electronic switches will be underutilized (current below 200A),
or the transmission voltage should be reduced. Because of the significant cost of electronics we presume that
the offshore converters will be operated at their rated currents. Considering this limitation for a 300MW wind
farm and 5MW generators, the corresponding transmission DC voltage and total system losses are depicted in
Figure 7. Note that the losses include cable losses and converter losses, assuming power generation according
to same Weibull wind speed distribution. It is clear that it would not be economical to operate with less than 4
machines in a group, since transmission losses (or converter costs) would escalate considerably.
If generators over 5MW become available in the future, they will further favourably influence the results
shown in Figures 6 and 7.
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4
Capital investment benefits
The centralised power conversion brings capital cost savings since fewer components are required, as seen
by comparing Figure 1 and Figure 2b. It is further assumed that the total converter rating has a primary
influence on the cost of electronics, although the number of converters will also have some influence. Therefore,
the capital investment is assumed largely independent on the number of machines in a group.
Table 2 summarizes the financial assumptions which are based on the study of 3MW offshore wind
generators in [13] with appropriate scaling for the generator and wind farm size employed in this study.
The total savings in the components, which are given in table 2, include: elimination of converters on
individual turbines, simpler generator rotors and simplified controls. These savings amount to around 13% of the
turbine costs. When the overall offshore wind farm costs are considered (including 13.5% costs for marinisation
[13]), the total initial cost saving with centralised power conversion is expected to be approximately 5-6%.
Considering Figures 6 and 7, it is concluded that a 6-machine group is the optimal configuration for the
considered wind farm, and this topology is studied in further sections. If a greater grouping size is chosen then
financial penalties increase because of the loss in efficiency. If however a smaller number of generators within a
group is chosen the implication is the underutilization of the converters or greater system losses.
A similar study performed with a 100MW/50kV wind farm found that a 2-generator grouping provided the
optimal configuration. With smaller farms the transmission voltage is lower and the generator groupings can be
smaller. It is acknowledged that many factors are not included in [13] and in the above analysis, like voltagelevel dependent costs and maintenance implications, which may influence these results.
In conclusion, the following design aspects will maximize the benefits of the proposed centralised power
conversion:
•
Increased generator size, which will imply a smaller number of generators in a single group.
•
Lowering transmission voltage (as losses would permit), since this enables better utilization of
converters even with a small generator group size.
•
5
Wind farms where generator capital costs contribute significantly to the overall costs.
Control system
5.1 Controller for a generator-side VSC converter
The principle of controlling a parallel multiterminal HVDC is based on a common and constant DC voltage.
The inverter station is most suited for regulating the DC voltage since offshore stations may be periodically offline. The power through each of the offshore converters is therefore proportional to the DC current in its own
branch.
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The main challenge with the generator-facing VSC converter control is the need to control both the
transmission system and the generators. Machine speed control is best achieved using the principles of flux
oriented vector control, as employed with typical industrial drives [14]. A form of vector control suitable for
controlling a group of parallel machines is developed in this work.
The proposed control system for the offshore converters is shown in Figure 8. The machine power, and
consequently machine torque, is varied by changing the angle (Mθgx) of the VSC converter voltage. The rotating
coordinate frame position (θg) is determined using a transvector-type Phase Locked Loop (PLL), which
synchronises with the 4kV offshore grid voltage. This coordinate frame will have average position for the group
of machines and therefore this will slightly deteriorate the quality of control. The generator speed is regulated
indirectly, by controlling power transfer through the transformer as with vector control principles.
As shown by the lower control diagram in Figure 8, the generator speed control is based on two series
connected controllers. The inner loop regulates the DC current (in the VSC transmission converter) which
improves performance of the outer speed control loop. The inner DC current loop also contributes to the stability
of the DC transmission system and prevents converter overcurrents.
The speed reference is calculated using the Maximum Power Point Tracking (MPPT) method [15]. By
knowing cp-λ curves for the particular turbine (which can also be determined experimentally), a look-up table is
created which links the generator power output with the ideal rotating speed. This curve ensures that the turbine
is operated at the maximum coefficient of performance for all wind speeds. The speed feedback signal is the
average for the six machines within the group, and therefore a communication link between generators and the
VSC is required. A generator trip signal should also be supplied to the VSC converter. Tripping a generator
would require that the MPPT algorithm adopts a different look-up table that corresponds to a reduced number of
machines.
The VSC converter control signal magnitude (Mmg) is used to regulate the generator terminal voltage as
shown in the upper control diagram in Figure 8. The terminal voltage is regulated to be proportional to the
generator speed (V/f control) in order to prevent generator/transformer flux saturation, and to enable good gain
in the torque control loop. The ratio between the voltage and speed reference is the rated stator flux.
Although with individual vector controlled drives voltage feedback is not necessary, in the proposed topology
this control prevents transient overvoltages on the offshore AC networks. In the proposed topology, the model
for a group of machines can not be accurately known at all times, and this may lead to temporary flux
deviations. The voltage feedback provides additional protection against flux saturation and overvoltages. All
controller parameters are given in the Appendix.
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5.2 Blade angle controller
Normally, the blade angle controller maintains machine power/speed within rated values at high winds. In
centralised power conversion wind farms, the blade angle regulator plays a more important role since this is the
only controller acting on individual generators.
The studies have concluded that torque feedback control is required in addition to the conventional power
feedback as shown in Figure 9. A machine can experience high torque when the group operates at low speed,
but the power on an individual machine is higher (because of high wind). The torque feedback (Tacxy) is provided
by measuring individual machine electrical power (Pacxy) and speed (ωgxy), where the subscript x refers to the
generator group number (1-10), whilst subscript y refers to the individual machine within the group (1-6).
5.3 Controller for the grid-facing VSC converter
Figure 10 shows a schematic of the controller for the inverter VSC, which resembles conventional VSC
transmission control. The DC voltage is regulated by the exchange of active power through the converter, using
variation in q component of the converter voltage (Mqi) [16]. The 110kV AC voltage is regulated in a feedback
manner using the converter AC voltage d component (Mdi) as shown in the upper control diagram. The position
of the rotating coordinate frame (θi) is determined using the PLL and AC voltage measurements. The inner
current control loops with VSC transmission control significantly contribute to reducing overcurrents during
faults. The controller gains are determined using PSCAD simulations.
6
Simulation results
6.1 Simulation model
A suitable model for the system in Figure 1, assuming a 6-generator grouping, is developed on PSCAD/EMTDC
platform [17]. It would be extremely difficult to model such a complex system in detail, and some simplifications
are adopted. The model has full rated power (300MW), however the number of the off-shore converters is
reduced to three. Two offshore groups are modelled in detail to include six 5MW induction generators and a
30MW rated converter. The third group is represented by a simplified 240MW equivalent system. Such a model
facilitates: detailed studies of the dynamics within a single generator group, interactions between generator
groups and their converters, and also study of the fully rated transmission system.
All the machine models are based on a 5MW double cage induction machine. All machine parameters are
given in the Appendix. The 240MW generator is based on the same 5MW machine model, which is represented
in a single model as 48 coherent machines.
The converters are standard IGBT based three-level neutral point clamped PWM controlled converters. The
turbine model uses standard mathematical expressions [9,10], assuming a 120m diameter, 12.5rpm rated
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speed and typical cp-λ curves [15]. Turbine parameters are given in the Appendix. The grid model represents a
relatively weak 110kV system with typical equivalent impedance assuming a Short Circuit Ratio (SCR) of 4
(SCR=MVA/Pdc), and X/R=10.
6.2 Wind speed changes
Figure 11 shows the simulation results for changes in base wind speed. The wind signal is based on the model
given in Figure 4 with a 2s incremental delay on individual turbines. Although it is unlikely that the base wind
speed will vary significantly across a wind farm, largely different base wind speeds are applied at each turbine
group, demonstrating the concept of different operating frequencies at each VSC converter. Figure 11a shows
the base wind speed variations on respective generator groups. As the wind speed reduces, each group
converter is capable of adjusting machine speed in order to maintain the best average coefficient of
performance as shown in Figure 11b-d.
Figure 11e-h presents the response of grid-side variables. It is seen that the grid AC voltage regulation is
excellent (within 0.2%) despite significant variations in wind speed. The stability and fast response of the
inverter-side DC voltage control, which is confirmed in Figure 11f, is crucial for the successful control of the
offshore converters. In addition, at 40.5s, a 100ms, 10% grid voltage sag (fault) is simulated, and the responses
are indicated for critical variables: DC voltage and AC voltage. It is demonstrated that this wind farm will have
good fault ride-through similar to conventional VSC transmission systems.
Figure 12 presents detailed studies of the variables for two machines within a single group (Group 2). The six
generators in a group operate at the same frequency but their slip is different according to the torque on
individual machines. The measurements shown indicate widely varying instantaneous powers and torques on
individual machines (Figure 12a and 12c), however stability is not compromised despite the use of low-slip highefficiency machines. The transient blade pitch angle responses on turbines 22 and 26 (Figure 12b) highlight the
independent control of pitch angle on individual turbines in order to decrease respective performance
coefficients and curtail generated power/torque above rated wind speeds.
Figure 13 compares the speed and torque for the case of a fixed speed machine (no converters), individual
variable speed machine and a machine operating within a 6-machine grouping, for the same wind speed signal.
As expected, it is seen that individual machine control allows for the largest speed variation, implying the best
tracking of wind speed. In Figure 13b) it is seen that group control will imply torque stresses which are larger
than with individually controlled machines, but smaller than in case of fixed speed machines. Therefore
structural stress will increase with the proposed wind farm topology, which will resultantly have financial
penalties.
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7
Conclusions
The DC interconnection of wind farms is suitable in the context of greater offshore distances, grid connection
requirements and lower cable costs. The results in this paper further demonstrate that by adopting an integrated
design of the wind generators and interconnection system, some of the costs associated with the HVDC
converters can be offset by simplifying the offshore wind generators. By using centralised power conversion, the
HVDC transmission converters take over turbine speed control and expected losses in efficiency are in the
order of 2-6%, depending on the generator group size. It is further concluded that if VSC converters are
employed to control a group of generators we can expect approximately a 5-6% reduction in the capital costs of
the offshore components.
The PSCAD/EMTDC simulation confirms the capability of the 300MW wind farm to operate satisfactory
under realistic wind speed inputs. It is demonstrated that each VSC converter can operate at a different
frequency, enabling averaged optimum speed regulation at corresponding turbine groups. The simulation also
indicates that there is no danger of instability or excessive machine loading even if the wind speed significantly
differs at each wind turbine, although an increase in torque stress is observed. The regulation of AC variables
onshore proves excellent performance which strengthens the case for using DC links with weak AC
connections.
The control systems for the offshore wind generators and blade angle actuators employ new approaches
which indicate good performance in the simulation models.
Page 11 of 19
8
References
[1]
N.M.Kirby, M.J. Luckett, L.Xu, W.Siepmann, “HVDC Transmission for large off shore wind farms” IEE ACDC Power Transmission, November 2001, London, Conference publication no 485, pp 162-168.
[2]
K. Søbrink, P.L.Sørensen, E. Joncquel and D. Woodford, "Feasibility Study Regarding Integration of the
LÆSØ SYD 160 MW Wind Farm Using VSC Transmission", CIGRE SC14 Colloquium, Three Gorges Dam,
August, 2001.
[3]
L.Holdsworth, N.Jenkins, G.Strbac “Electrical Stability of Large, Offshore Wind Farms. IEE AC-DC Power
Transmission, November 2001, London, Conference publication no 485, pp 156-161.
[4]
The UK Crown Estate, “East coast transmission network technical feasibility study”
http://www.thecrownestate.co.uk/, January 2008.
[5]
Kjell Ericsson "Operational Experience of HVDC Light" Seventh International Conference on AC-DC Power
Transmission. IEE. 2001, pp.205-210. London, UK.
[6]
M.Hyttinen, J.O.Lamell, T,F,Nestli, “New Application of Voltage Source Converter (VSC) HVDC to be
Installed on gas Platform Troll A” CIGRE, General Meeting, 2004, Paris, paper B4-210.
[7]
W.Lu, B.T. Ooi, “Optimal Acquisition and Aggregation of Offshore Wind Power by Multi-Terminal Voltage–
Source HVDC, IEEE Transactions on Power Delivery, Vol 18, no 1, January 2003, pp 201-206.
[8]
Carlson, O.; Lundberg, S.; “Integration of wind power by DC-power systems” IEEE PowerTech, Russia, 2730 June 2005 Page(s):1 – 4,
[9]
D. Jovcic, J. Milanovic “Offshore Wind Farm Based on Variable Frequency Mini-Grids with Multiterminal DC
th
Interconnection” IEE 8 International Conference on AC-DC Transmission, London, March 2006, pp 215219.
[10] V.Akhmatov, “Analysis of Dynamic Behavior of Electric Power Systems with Large Amount of Wind Power”,
PhD Thesis, Electric power Engineering, Technical University of Denmark, 2003.
[11] J.G. Slootweg, S.W.H. De Haan, H. Polinder, and W.L. Kling, "General model for representing variable
speed wind turbines in power system dynamics simulations," IEEE Trans. Power Sys., vol. 8, no. 1, pp.
144-151, Feb. 2003.
[12] E. Akpinar, S. Akpinar, “Statistical analysis of wind energy potential on the basis of the Weibull and
Rayleigh distributions for Agin-Elazig, Turkey” in Proc. IMechE, Part A: Journal of Power and Energy,
Vol218, no 8, 2004, pp 557-565.
[13] L. Fingersh, M. Hand, A. Laxson. (2006, Dec). Wind Turbine Design Cost and Scaling Model, National
Renewable
Energy
Laboratory
(NREL),
Colorado,
Tech.
Rep.
TP-500-40566.
Available:
http://www.nrel.gov/wind/pdfs/40566.pdf
[14] B.K. Bose “Modern Power Electronics and AC drives” Prentice Hall 2002.
[15] M. Chinchilla, S. Arnaltes, J.C. Burgos, “Control of Permanent-magnet Generators Applied to variable
Speed Wind-Energy Systems Connected to the Grid” IEEE Transactions on Energy Conversion, Vol 21, no
1, march 2006, pp 130-135.
Page 12 of 19
[16] D.Jovcic, K.Kahle, “Compensation of particle accelerator load using converter controlled pulse
compensator”, IEEE Transactions on Power Delivery, Vol2, no 2, April 2006, pp 801-808.
[17] Manitoba
HVDC
Research
Centre,
“PSCAD/EMTDC
User
Manual,”
Tutorial
Manual,
1994.
Page 13 of 19
Main body figures
Fig. 1 300MW off-shore wind farm with centralised power conversion and parallel multiterminal HVDC
connection.
Fig. 3 Operating modes and rating curves for the
offshore machines.
Fig. 2 Conventional wind farm topologies.
Fig. 4 Wind speed signal on two adjacent turbines
with a 5s delay, and with base wind speed of 11m/s.
Page 14 of 19
Fig. 8 Controller for a generator-side VSC.
Fig. 5 Energy loss analysis dependent on the number
of machines within a group a) increasing delay for rated
wind speed (11m/s), b) average energy loss, c)
increasing base wind speed for delay of 2s.
Fig. 9 Blade angle controller.
Fig. 6 Average wind farm energy loss analysis. a)
Weibull coefficients and machine power, b) percentage
energy loss and capacity factor.
Fig. 10 Controller for the grid-side VSC.
Fig. 7 Average energy loss in DC cables and
converters dependent on the number of machines
within a group.
Page 15 of 19
Fig. 11 Wind farm simulation for changes in wind speed. a) group average wind speed, b) group 1 and 2 AC
power, c) total grid-side DC power and group 3 AC power, d) group 1 and group 2 average generator speed, e)
group 1 and group 2 DC current, f) grid-side DC voltage, g) grid AC voltage, h) grid dq components of AC currents.
Page 16 of 19
Fig. 13 Machine speed and torque comparisons for
various group sizes. a) speed, b) electrical torque.
Fig. 12 System variables for generators 2 and 6 within
generator group 2. a) AC power, b) blade pitch angle, c)
electrical torque.
Page 17 of 19
Main body tables
Table 1: Major cost implications with studied topologies (- issue, + no issue)
Topology Cp loss Cable Converters Grid connection
+
+
+
1)
+
+
2a)
+
+
+
2b)
Table 2: Capital investment study
Total cost of 300MW offshore wind farm
Wind turbine capital cost per installed MW
Savings
Generator converters capital cost per turbine
Control system capital cost per turbine
Wound rotor (or magnets) cost per turbine
Total capital cost saving on wind farm
% cost
100
42
M£
639
0.9
10
2.2
1
5.5
0.09
0.0197
0.009
35.43
Appendix tables
Table A.1: Induction machine and turbine parameters
Parameter
Value
5 MW
Rated Power
4 kV
Rated Voltage
50 Hz
Rated frequency
4
Pole number
0.005 p.u.
Stator resistance
0.015 p.u.
Stator reactance
Magnetizing reactance 3.8 p.u.
Rotor cage 1 resistance 0.01 p.u.
Parameter
Rotor cage 1 reactance
Rotor cage 2 resistance
Rotor cage 2 reactance
Moment of inertia
Mechanical damping
Optimum tip speed ratio
Rated windspeed
Turbine radius
Value
0.0033 p.u.
0.002 p.u.
0.017 p.u.
3.5 s
0.008 p.u.
6.8
11.5 m/s
60 m
Table A.2: Controller parameters
Notation
KP1
KI1
KP2
KI2
KP3
KI3
KP4
KI4
KP5
Value
0.2 1/kV
3 1/kVs
4 1/kA
1000 1/kAs
0.002 kAs/rad
0.04 kA/rad
0.02 1/kA
0.3 1/kAs
0.01 kA/kV
Notation
KI5
KP6
KI6
KP7
KI7
KP8
KI8
KP9
KI9
Value
0.5 kA/kVs
0.02 1/kA
0.3 1/kA
0.2 kA/kV
2 kA/kVs
5 deg/MW
18.2 deg/MWs
5 deg/kN
5 deg/kNs
Page 18 of 19
Author biographies
Dr Dragan Jovcic obtained a B.Sc. 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 HVDC, FACTS,
renewable energy and control systems.
Nicholas Strachan studied Electronic Engineering at the University of Aberdeen, Scotland where he received a
Master’s degree with Honours in 2003.
He is currently studying towards his Ph.D. degree at the University of Aberdeen, Scotland. He previously
worked as a Control and Instrumentation Engineer in the UK Oil and Gas industry in the period 2003-2005. His
current research interests are in wind power generation, electrical energy storage and control systems.
Page 19 of 19