Title
Operation and Control of HVDC-Connected Offshor
e Wind Farm
Author(s)
Muyeen, S.M., Takahashi, R., Murata, T., Tamura
, J.
Citation
IEEE Transactions on Sustainable Energy, 1(1): 3
0-37
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URL
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Type
Text Version
2010-04
http://hdl.handle.net/10213/1840
© 2010 IEEE. Personal use of this material is p
ermitted. Permission from IEEE must be obtained
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Journal Article
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Additional
information
http://kitir.lib.kitami-it.ac.jp/dspace/
1
Operation and Control of HVDC-connected
Offshore Wind Farm
S.M. Muyeen, Member, IEEE, R. Takahashi, Member, IEEE, T.Murata, and
J. Tamura, Senior Member, IEEE
Abstract— This paper presents operation and control strategies
of offshore wind farm interconnected to a high-voltage DC
(HVDC) system. The offshore wind farm composed of variable
speed wind turbines driving permanent magnet synchronous
generators is considered in this study, based on DC-bus concept.
The HVDC transmission system based on three-level (3L) neutral
point clamped (NPC) voltage source converter (VSC) is used for
the interconnection between offshore wind farm and onshore grid.
Detailed modeling and control strategies are developed for the
individual component of the overall system. A simple fuzzy logic
controller (FLC) is adopted in offshore VSC station, which is one
of the salient features of this study. Real wind speed data is used in
the simulation study to obtain realistic response. Both dynamic
and transient analyses of the proposed system are carried out
using laboratory standard power system software package,
PSCAD/EMTDC.
Index Terms— fuzzy logic controller (FLC), high voltage DC
(HVDC), offshore wind farm, permanent magnet synchronous
generator (PMSG), transient stability, variable speed wind
turbine (VSWT), and voltage source converter (VSC).
T
I. INTRODUCTION
HE conventional energy sources such as oil, natural gas,
coal, or nuclear are finite and generate pollution.
Alternatively, the renewable energy sources like wind, fuel cell,
solar, biogas/biomass, tidal, geothermal, etc. are clean and
abundantly available in nature. Among those the wind energy
has the huge potential of becoming a major source of renewable
energy for this modern world. In 2008, 27 GW wind power has
been installed all over the world, bringing world-wide installed
capacity to 120.8 GW. This is an increase of 36% compared
with the 2007 market, and represents an overall increase in the
global installed capacity of about 28.8% [1].
As well as the onshore trend, offshore wind farms have also
been continuing its growth rapidly. Some leading countries in
wind energy arena are focusing more on offshore technology.
The main reasons for adopting offshore are lack of the suitable
onshore sites and much better wind conditions of offshore sites
(wind is much stronger and more constant). By 2007, the
industry had developed 25 projects with a total capacity of
Manuscript received March 30, 2009.
S. M. Muyeen, R. Takahashi, T. Murata, and J. Tamura are with the
Electrical and Electronic Engineering Department, Kitami Institute of
Technology, 165 Koen-Cho, Kitami, Hokkaido, 090-8507, Japan (e-mail:
muyeen@pullout.elec.kitami-it.ac.jp).
around 1,100 MW in five countries in Europe [2], many of
which are large-scale and fully commercial.
The offshore wind farm is located, in general, far away from
onshore grid point of common coupling (PCC). If the distance
is long or if the grid to which the offshore wind farm is
connected is weak, high voltage DC (HVDC) transmission
system might be a suitable and feasible solution than the high
voltage AC transmission. This paper focuses on HVDC system
for offshore wind farm interconnection with the onshore grid,
where the distance between the two systems is assumed to be
100 km. Two types of HVDC transmission topologies, i.e.,
HVDC with voltage source converter using IGBTs
(VSC-HVDC) and line-commutated converter HVDC
(LCC-HVDC) are used nowadays for offshore wind farm
connectivity [3-11]. However, in this study, VSC based HVDC
transmission system is considered for offshore wind farm
connectivity to the grid.
The HVDC transmission using VSC came in service for the
first time in Sweden in 1997 as a trial 3 MW scheme [12]. At
the present hundred MW class VSC based HVDC systems are
in service. They have become possible due to the synchronized
development in a number of technical areas, including
semiconductor switches, DC transmission lines (especially the
DC cables), main circuit of converter station, and system
controls [13]. The use of VSC-HVDC for offshore wind farm
connectivity has been reported already in some literatures
[6-11]. In [6-8], the wind farms considered are composed of
fixed speed induction generators. In [9], synchronous
generators are used in the offshore wind farm. Doubly fed
induction generator (DFIG) is chosen as wind generator for
offshore wind farm in [10]. However, PMSG is becoming very
popular nowadays as variable speed wind generator. In PMSG,
the excitation is provided by permanent magnets instead of
field winding. Permanent magnet machines are characterized as
having large air gaps, which reduce flux linkage even in
machines with multi magnetic poles [14, 15]. As a result, low
rotational speed generators can be manufactured with relatively
small sizes with respect to its power rating. Moreover, gearbox
can be omitted due to low rotational speed in PMSG wind
generation system, resulting in low cost. In a recent survey,
gearbox is found to be the most critical component, since its
downtime per failure is high in comparison with other
components in a wind turbine generator system (WTGS) [16].
2
Therefore, in our previous work [11], we reported
VSWT-PMSG as offshore wind generator, where each PMSG
is connected to the AC-bus using frequency converter
composed of AC-DC converter, DC-link, and DC-AC inverter.
Time averaged model is considered therein for the frequency
converter used in VSWT-PMSG. Moreover, hydrogen
generation scheme is emphasized therein instead of controller
designing for the individual component of HVDC
interconnected offshore wind farm. In this study, the focus is
given to develop the suitable control strategy for the individual
component of HVDC connected offshore wind farm during
normal and grid fault conditions.
In this study, each VSWT-PMSG of the wind farm is
connected to the common DC-bus through an AC-DC
converter. The DC-bus is connected to the wind farm AC-bus
through only one DC-AC inverter system. In this way, the
number of DC-AC inverters and transformers necessary for
AC-bus connected wind farm explained in [11] can be
decreased and as a result the cost of overall system can also be
reduced. The DC-AC converter of each VSWT-PMSG ensures
the maximum power transfer to the DC-bus by using maximum
power point tracking (MPPT) controller. The DC voltage of the
DC-bus is converted to AC voltage of the wind farm AC-bus
using a DC-AC inverter. In both offshore and onshore HVDC
stations, three-level (3L) neutral point clamped (NPC) VSCs
are considered. Suitable control strategies are developed for
both offshore and onshore VSC stations. A simple fuzzy logic
controller (FLC) is adopted for the offshore VSC station
control which is one of the salient features of this study. This
reduces the intricacy of the control for offshore VSC station.
For dynamic analysis real wind speed data measured in
Hokkaido Island, Japan, is used in the simulation analyses to
obtain the realistic responses. Moreover, transient stability
analysis is also performed considering severe line to ground
fault, in this study. Finally, it is concluded that the proposed
HVDC interconnected offshore wind farm is dynamically and
transiently stable under the developed control schemes.
II. MODEL SYSTEM
The single line diagram of the proposed system composed of
offshore wind farm, two 3L VSC-HVDC stations, and two DC
cross-linked polyethylene (XLPE) cables is shown in Fig. 1.
The offshore wind farm is composed of VSWTs driving
PMSGs. Each VSWT-PMSG is connected to DC-bus through
AC-DC
3L-Offshore VSC
Offshore Wind Farm (300 MW)
AC/DC
DC-bus
DC/AC
AC-bus
Sw1
converter and then DC voltage of the DC-bus is converted to
AC voltage using only one DC-AC inverter. To speed up the
simulation aggregated model of wind farm is considered in the
simulation analyses, where several small size wind generators
are represented with a large capacity wind generator. Therefore,
though offshore wind farm power capacity is assumed as 300
MW, two 150 MW wind generators are considered to be
connected to the DC-bus, in this study. The parameters of
PMSG are shown in Table. I.
Rated Power
Rated Voltage
Frequency
Number of Poles
H
III. WIND TURBINE MODELING
The mathematical relation for the mechanical power
extraction from the wind can be expressed as follows [17]:
PM = 0.5 ρ C p ( λ , β ) π R 2VW3 (1)
[W ]
Where, PM is the extracted power from the wind, ρ is the air
density [kg/m3], R is the blade radius [m], Vw is the wind speed
[m/s], and Cp is the power coefficient which is a function of
both tip speed ratio, λ, and blade pitch angle, β [deg].
Cp is expressed in the next equations [18].
C p ( λ , β ) = 0.5 ( Γ − 0.02 β 2 − 5.6 ) e −0.17 Γ
(2)
λ=
ωm R
VW
, Γ =
R 3600
⋅
λ 1609
(3)
ωm_opt=0.0775Vw [pu]
(4)
Where ωm is the rotational speed [rad/s].
(300 MW
±150 kV, 1000 A
3L-Onshore VSC
Sw1
Main Network
(500 kV, 50 Hz)
DC Cable
Sw2
Sw2
V0
DC Cable
Fig. 1. Single line diagram of the proposed system
CB
CB
Sw3
HFF
HFF
Sw4
0.01[pu]
1.0[pu]
0.7[pu]
1.4[pu]
The proposed system is connected to the main grid through
transmission lines. Hence, the active power from the offshore
wind farm is transmitted to the main grid via two DC XLPE
cables. The cable model available in PSCAD software is used
in the simulation. The ABB 150 kV XLPE cable parameters
shown in the Appendix are used in this study. The cable length
is considered as 100 km. A high-pass filter (HFF) is connected
at both offshore and onshore VSC stations to absorb the
well-defined harmonics for three-level converter/inverter
systems. The individual component modeling is demonstrated
below.
Sw3
VSWT-PMSG
TABLE I
PARAMETERS OF PMSG
150 [MW]
Stator Resistance
3.0 [kV]
d-axis Reactance
20 [Hz]
q-axis Reactance
150
Field Flux
3.0 [sec]
Sw4
F
3
Fig. 3. Block diagram for MPPT controller
Turbine Input Power[pu]
When the rotor speed ωr goes beyond the rated speed of
PMSG, then the pitch controller presented in [19] is considered
to control the rotational speed. Therefore, output power will not
exceed the rated power of the PMSG.
Locus of maximum
captured power
1.4
1.2
13m/s
1.0
0.8
12m/s
0.6
11m/s
10m/s
9m/s
8m/s
0.4
0.2
0.0
7m/s
6m/s
0.6
0.8
1.0
Rotor Speed[pu]
0.4
IV. MODELING AND CONTROL STRATEGY OF WIND FARM
Detailed switching model is considered in this study instead
of time averaged model described as follows.
A. Generator Side AC-DC Converter Connected to DC-bus
In this study, the direct drive VSWT-PMSG connected to the
wind farm DC-bus through an AC-DC converter is considered.
In the simulation analyses, PMSG model available in the
package software PSCAD/EMTDC [20] is used. The electrical
scheme of the VSWT-PMSG adopted in the study is shown in
Fig. 4.
1.2
Fig. 2. Wind turbine characteristics for variable speed operation
Pref_1=0.197Vw-1.451 [pu]
Pref_2=0.127Vw-0.749 [pu]
Pref_3=0.068Vw-0.282 [pu]
(5A)
(5B)
(5C)
Generator-Side Converter
The characteristic of Cp changes depending on the wind
speed. Eq.(4) is a relation between the optimal rotational speed,
ωm_opt, corresponding to the optimum power coefficient Cp_opt,
and the wind speed, VW. This expression is obtained by
differentiating Cp with respect to ωm, assuming that β equals
zero. Fig. 2 shows characteristics between the wind turbine
power PM and the rotor speed ωm for various wind speed, in
which the locus of the maximum output power Pmax is shown by
a dashed line. The Pmax is chosen as the reference power, Pref,
for AC-DC converter for each VSWT-PMSG, so that
maximum power can be transferred to the common DC-Bus
and it is calculated using maximum power point tracking
(MPPT) controller as shown in Fig. 3. The MPPT controller is
designed based on linear functions as expressed in Eq.(5). At
the same time, wind turbine should be operated at ωm_opt as
expressed in Eq.(4) to achieve the power capture corresponding
to Pref. In addition, it is ensured that the rotor speed ωr does not
go below 0.4 pu during the low wind speed, which is
considered the minimum speed, in this study.
Eq. (5A)
Eq. (5B)
Vw
Pref_1
1
Pref_2
0
1
0
Vw>10Î1
8<Vw<10Î0
10
8
+
S6n
+
0
Vw>8Î1
Vw<8Î0
Pref
0.9
Q
8
Eq. (1)
1
>
ωopt
ωr
+
PI
0
*
pmsg_n
+
Qpmsg_n+
-
S5n
S4n
>0.9Î1
<0.9Î0
1
Pref (Normal operation)
0
Pref×Vonshore_grid (Fault condition)
1+0.01s
I*q
PI-1
Kp=0.15
Ti=0.15
+
Iq
-
*
d
I
PI-3 -
Kp=0.15
Ti=0.15
×
0.01
0.01
Id
+
1+0.002s
1+0.01s
1+0.002s
Id Iq
6.0
Ipmsg a,b,c
+100
-100
C
Each converter is a standard 3-phase two-level unit,
composed of six IGBTs and antiparallel diodes. The
well-known cascaded control technique shown in Fig. 5 is
adopted in this study for VSWT-PMSG operation. As the
converter is directly connected to the PMSG, its q-axis current
is proportional to the active power. The active power reference,
Pref, is determined in such a way to provide the maximum
power to the DC-bus with the help of MPPT controller as
shown in Fig. 3. However, when a network disturbance occurs,
Pref set-point is changed according to the command signal from
onshore VSC station determined from the onshore grid terminal
voltage as
Pref_n
Ppmsg_n
Pref_3
PPMSG-DC
Fig. 4. Electrical scheme of VSWT-PMSG
+
Eq. (5C)
0.4
1
S1n
Vdc
Vonshore_grid
+
S2n
S3n
DC-bus
wr
∫
dq
abc
PI-2
Vd*
Kp=2.0
Ti=0.025
PI-4
Vq*
Kp=2.0
Ti=0.025
S1n
*
a,b,c
abc V
dq
θr
Vd* Vq*
.
.
.
S6n
Carrier
Signal
4
Fig. 5. Control block for generator side AC-DC converter
shown in Fig. 5. It gives excellent transient performance as
demonstrated in Sect. VI.B. On the other hand, the d-axis stator
current is proportional to the reactive power. The reactive
power reference is set to zero to perform unity power factor
operation. In Fig. 5, the superscript n represents the individual
number for each wind generator.
B. DC-AC Inverter Connected to Wind Farm AC-bus
The well-known cascaded control scheme shown in Fig. 6 is
used as a control methodology for the grid side inverter of wind
farm. The grid side voltage phasor is synchronized with the
controller reference frame by using the phase locked loop
(PLL) as shown in Fig. 7 [21]. The d-axis current can control
the DC voltage of the DC-bus. On the other hand, the q-axis
current can control the reactive power of wind farm AC-bus
and hence the terminal voltage at high voltage side of the
transformer can be kept constant at desired reference level.
V*cq
V*DC-bus
TABLE II
THE SWITCHING TABLE
+Vdc
0
-Vdc
V0
SW1
1
0
0
SW2
1
1
0
SW3
0
1
1
SW4
0
0
1
C
V*a,b,c
dq
A. Onshore VSC Station
The objective of the onshore 3L NPC VSC station is to
maintain constant voltages at HVDC line and onshore grid. The
insulated gate bi-polar transistor (IGBT) switching table for 3L
NPC based onshore VSC station is shown in Table II. The gate
signals generation scheme for the IGBT devices used in
onshore VSC station is shown in Fig. 8. The cascaded control
scheme explained in Sect. IV.B is considered for the onshore
VSC station, which is used in the control block shown in Fig. 8
by the doted line. The onshore grid voltage and HVDC line DC
voltage are the control inputs for the cascaded control of VSC.
However, in this case, the reference signals are compared with
double carrier wave signal to generate the switching signals for
3L NPC VSC according to the switching rule shown in Table
II.
VSC
+
−
PI-1
V DC-bus
−
G1
I*d
+
1+T1s
PI-2
1+T2s
Id
PI-3
+
+
Q WF_Grid
+
−
PI-4
−
G2
Carrier Wave
1+T3s
−
PI-5
jX
1+T4s
θe
Iq
abc
V WF_Grid
dq
Va,b,c
Ia,b,c
3φ
Vα
Wind Farm
AC Grid
( )
cos
Vb
Vc
2φ
×
ε
+
+
Vβ
-cosθ
ω0
PI
+
+
ω
Double
Carrier Wave
Generated
Switching
Reference
Switching
Pattern
Pulse
Generation
Fig. 8. Gate signal generation scheme
Fig. 6 Control block diagram for wind farm DC-AC inverter
Va
V*a,b,c
PLL
V*WF_Grid
sinθ
Controller
R
V*cd
I*q
Q* WF_Grid
abc
1/s
θ
×
sin
Fig. 7. Block diagram of PLL
V. MODELING AND CONTROL OF HVDC STATIONS
Multi-level converter/inverter topology is more suitable to
high voltage application considering the factors of better
harmonic performance and capability to withstand high phase
currents. Three-level (3L) neutral point clamped (NPC) VSC is
preferred in this study, both for offshore and onshore stations.
The one pole structure of three-level NPC based IGBT
converter is shown in Fig. 1.The detailed control strategies for
offshore and onshore stations are presented in the following
section.
B. Off Onshore VSC Station
In the proposed scheme, the wind farm DC-AC inverter can
maintain constant voltage and frequency of the AC-grid side.
Therefore, the intricacy of vector control can be avoided in
offshore VSC station adopting a simple control strategy based
on modulation control. The offshore VSC is controlled and
operated as a voltage source with constant AC frequency and
phase angle. Control strategy including the modulation index,
M, used in this study is shown in Fig. 9 where a simple fuzzy
logic controller is adopted. This gives excellent transient and
dynamic performances as shown in Sec. VI. The modulated
angle, θ′, which equals zero, is applied to the PWM reference
generator in phase A as shown in Fig. 9. The angles for phases
B and C will be shifted by 120 and 240 degrees, respectively.
The insulated gate bi-polar transistor (IGBT) switching table
shown in Table II is also considered for 3L offshore VSC
station. Double triangular carrier signal is used to generate gate
C forCIGBT switches.
pulses
Gate Signals to the
Offshore VSC Station
e
Vref =1.0
+
en
Ke
-
Vt
+
Z-1
PWM
-
Δen
Δe
KΔe
Fuzzy
Logic
Controller
f
Reference
Generator
θ′
Μ
δn
Κδ
+
Z-1
+
5
Fig. 9. Control block for FLC controlled offshore VSC station
The proposed FLC shown in Fig. 9 is used to determine the
modulation index, M. The FLC is explained in the following:
1) Fuzzification
To design the proposed FLC, the error signal, e(k), and
change of error signal, Δe(k), are considered as the controller
inputs. M is considered as the controller output, which is
actually modulated index. For convenience, the inputs and
output of the FLC are scaled with coefficients Ke, KΔe, and
KΜ respectively. These scaling factors can be constants or
variables and play an important role for FLC design in order to
achieve a good response in both transient and steady states. In
this work, these scaling factors are considered as constant for
the simplicity of controller design, and are selected by
trial-and-error in order to obtain the best system performance.
In Fig. 9, Z-1 represents one sampling time delay. The triangular
membership functions with overlap used for the input and
output fuzzy sets are shown in Fig. 10 in which the linguistic
variables are represented by NB (Negative Big), NS (Negative
Small), Z (Zero), PS (Positive Small), and PB (Positive Big).
The grade of input membership functions can be obtained from
the following equation [22].
μ(x ) = [w − 2 x − m ]/w
(6)
Where, μ (x) is the value of grade of membership, w is the
width, m is the coordinate of the point at which the grade of
membership is 1, and x is the value of the input variable.
2) Rule Base
The fuzzy mapping of the input variables to the output is
represented by IF-THEN rules of the following forms:
IF <en is NB> and <Δen is NB> THEN < Mn is NB>
IF <en is PB> and <Δen is PB> THEN < Mn is PB>
NB
NS
PS
ZO
PB
NB
NS Z PS
PB
1.0
1.0
3) Inference & Defuzzification
In this work, for the inference mechanism, Mamdani’s
max-min (or sum-product) [22] method is used. The center of
gravity method [22] is used for defuzzification to obtain Mn,
which is given by the following equation:
N
N
M n = ∑ μiCi / ∑ μi
i=1
i=1
Where, N is the total number of rules, μi is the membership
grade for i-th rule and Ci is the coordinate corresponding to the
respective output or consequent membership function [Ci∈
{-0.1,-0.01,0,0.01,0.1}. The actual modulated index, M, can be
found by multiplying Mn by the scaling factor KM.
VI. SIMULATION RESULTS
In this study, detailed switching models are used for all
converter/inverter models instead of time averaged one. Both
dynamic and transient characteristics are analyzed using the
proposed system. The simulation time step is chosen as
0.00002 sec. The simulation time for dynamic and transient
analyses are chosen as 600 sec and 10 sec, respectively.
A. Dynamic Characteristics Analysis
The real wind speed data used in the simulation are shown in
Fig. 11, which were measured in Hokkaido Island, Japan. The
MPPT controller determines the reference power set-point
using wind and rotor speeds. The responses of reference
powers and rotor speeds for the two wind turbine generator
systems are shown in Figs. 12 and 13, respectively. The
DC-AC inverter of the wind farm can maintain the wind farm
DC-bus and AC-bus voltages at constant level as shown in Figs.
14 and 15, respectively. The DC-bus topology based wind farm
makes the system less costly as explained in Sect. I. On the
other hand, the simple fuzzy logic controlled offshore VSC
station can transfer the wind farm real power efficiently to the
onshore grid as shown in Fig. 16. The onshore VSC-station can
maintain
U s e d in V S W T - P M S G :1
U s e d in V S W T - P M S G :2
-0.1 -0.01 0 0.01
0.25 0.5
Input (en, Δen)
0.1
Output (Mn)
Fig. 10. Fuzzy sets and their corresponding membership functions
The entire rule base is given in Table III. There are total 25
rules to achieve desired index, M.
Wind Speed [m/sec]
16
-0.5 -0.25 0
TABLE III
15
14
13
12
11
10
9
8
FUZZY RULE TABLE
100
NB
NS
NB
NB
NB
NS
ZO
PS
PB
NB
NS
NS
ZO
NS
NS
ZO
PS
ZO
200
300
400
500
600
T im e [ s e c ]
NS
PB
NS
PS
ZO
NS
ZO
PS
PS
ZO
PS
PS
PB
PS
PS
PB
PB
Fig. 11. Wind speeds used in the variable speed wind turbines
1 .2
Reference Powers [pu]
en
0
Δen
Mn
(7)
V S W T - P M S G :1
V S W T - P M S G :2
1 .0
0 .8
0 .6
0 .4
0 .2
0 .0
0
100
200
300
400
T im e [ s e c ]
500
600
6
voltages at the HVDC line and onshore grid constant as shown
in Figs. 17 and 18, respectively. Therefore, it is seen that the
developed control strategies are well suitable for the normal
operation of wind farm under randomly varying wind
conditions.
P M S G :1
P M S G :2
1 .0
0 .8
0 .6
200
150
100
50
0
0
100
200
300
400
500
600
Fig. 17. DC voltage at the onshore VSC station
0 .2
100
200
300
400
500
600
T im e [ s e c ]
Fig. 13. Rotor speeds of PMSGs
DC Voltage at DC-bus [kV]
250
0 .4
0
8
6
4
2
1 .2
1 .0
0 .8
0 .6
0 .4
0 .2
0 .0
0
100
200
300
400
500
600
T im e [ s e c ]
Fig. 18. AC voltage at onshore grid
0
0
100
200
300
400
500
600
T im e [ s e c ]
Fig. 14. DC-bus voltage of offshore wind farm
Wind Farm Terminal Voltage [pu]
300
T im e [ s e c ]
0 .0
1 .2
1 .0
0 .8
0 .6
0 .4
0 .2
0 .0
0
100
200
300
400
500
600
T im e [ s e c ]
Fig. 15. AC-bus voltage of Wind farm AC terminal
350
B. Transient Characteristics Analysis
For transient stability analysis, the most severe symmetrical
three-line-to-ground fault (3LG) is considered as a network
disturbance, which occurs at the grid side of the onshore VSC
station (fault points F in Fig. 1). The fault occurs at 0.1 sec, the
circuit breakers (CB) on the faulted lines are opened at 0.2 sec,
and at 1.0 sec the circuit breakers are reclosed. In this analysis,
it is assumed that wind speed is constant and equivalent to the
rated speed for the wind turbines. This is because it may be
considered that wind speed does not change dramatically
during the short time interval of the transient stability
simulation. When the onshore grid voltage goes lower than a
certain value during network disturbance (in this study 0.9 pu),
the wind farm power command signal is varied according to the
grid voltage at the onshore grid as shown in Fig. 5. The
responses of AC voltages at both VSC stations are shown
together in Fig. 19.
1 .2
A t W in d F a r m A C - b u s
A t O n s h o r e G r id
AC RMS Voltage[pu]
Real Powers [MW]
T o ta l V o lt a g e A c r o s s T w o C a p a c ito r s
V o lt a g e A c r o s s U p p e r L e g
V o lt a g e A c r o s s L o w e r L e g
350
AC voltage at Onshore Grid[pu]
Rotor Speeds [pu]
1 .2
Fig. 16. Real powers at offshore wind farm and onshore grid
DC Voltage at the Onshore
VSC Station [kV]
Fig. 12. Reference powers for AC-DC converters used in VSWT-PMSGs
300
250
200
150
100
A t O n s h o r e S ta t io n
A t W in d F a r m T e r m in a l
1 .0
0 .8
0 .6
0 .4
0 .2
0 .0
50
0
100
200
300
400
T im e [ s e c ]
500
600
0
2
4
6
T im e [ s e c ]
8
10
7
Fig. 23. DC voltage at the offshore VSC station (3LG)
The reference powers used in wind farm AC-DC converters are
shown in Fig. 20. The real and reactive power responses at
onshore grid are shown in Fig. 21. The pitch controller controls
the mechanical power of wind turbine in order for the PMSG to
become stable. Therefore, the PMSG rotor speeds become
Reference Powers [pu]
1 .2
V S W T - P M S G :1
V S W T - P M S G :2
1 .0
0 .8
0 .6
300
250
200
150
100
50
0
0
2
4
6
8
10
T im e [ s e c ]
0 .4
Fig. 24. DC voltage at the onshore VSC station (3LG)
0 .2
stable as shown in Fig. 22. The DC voltage at both offshore and
onshore VSC stations are shown in Figs. 23 and 24,
respectively. Simulation results clearly show that the proposed
system can overcome severe 3LG fault under the developed
control strategies.
0 .0
0
2
4
6
8
10
T im e [ s e c ]
Fig. 20. Reference power commands used in offshore AC-DC converters
Real and Reactive Powers
at Onshore Station [MW]
350
Real P ow er
R e a c t iv e P o w e r
300
VII. CONCLUSIONS
250
200
150
100
50
0
-50
0
2
4
6
8
10
T im e [ s e c ]
Fig. 21. Real and reactive powers at onshore VSC station (3LG)
1 .2
Rotor Speeds [pu]
350
DC Voltage at the Onshore
VSC Station [kV]
Fig. 19. AC-side voltages at offshore and onshore VSC stations (3LG)
P M S G :1
P M S G :2
1 .0
0 .8
0 .6
0 .4
0 .2
0 .0
0
2
4
6
8
10
T im e [ s e c ]
In this paper, the operation and control of DC-bus based
offshore wind farm topology is investigated which is connected
with a HVDC system through the 3L NPC based VSC stations
and XLPE cables. The maximum power is transferred from
variable speed wind turbine generator systems to the DC-bus
through the AC-DC converters due to the MPPT controllers.
Only one DC-AC inverter converts the DC-bus voltage to a
suitable AC voltage of wind farm AC-bus. It is seen that the
FLC controlled offshore 3L-NPC VSC station can transfer the
real power of offshore wind farm to the onshore grid
successfully and efficiently. The onshore 3L VSC station can
ensures the constant HVDC line voltage as well as supplies
necessary reactive power to the onshore grid. As a result, the
onshore grid voltage can be maintained constant even in the
case of network disturbance. The control strategies developed
for the HVDC connected DC-bus based offshore wind farm are
verified by both dynamic and transient characteristics analyses.
Finally, it is concluded that the proposed system performs
sufficiently well for HVDC interconnected DC-bus based
offshore wind farm under both normal and network fault
conditions.
DC Voltage at the Offshore
VSC Station [kV]
Fig. 22. Rotor speed of PMSG (3LG)
350
VIII. ACKNOWLEDGMENT
300
This work was supported by the Grant-in-Aid for JSPS
Fellows from Japan Society for the Promotion of Science
(JSPS). The authors also acknowledge contributions from
Thomas Ackermann and Stephan Meier for providing some
valuable technical data.
250
200
150
100
50
0
0
2
4
6
T im e [ s e c ]
8
10
8
IX. APPENDIX
The ABB 150 kV XLPE cable parameters are shown below
[23].
CABLE PARAMETERS
Conductor Radius
Insulator Outer radius
Copper Resistivity
Permittivity
Resistance at 20°C
0.0178 m
0.0370 m
1.724e-8
2.3
0.0172 Ω/km
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