Dynamic Modelling and Implementation of VSC-HVDC System
The Grid Connected Large Offshore Wind Power Plant Application
Muhammad Raza
∗
and Oriol Gomis-Bellmunt
†
Centre d’Innovaci
´
o Tecnol
`
ogica en Convertidors Est
`
atics i Accionaments, Departament d’Enginyeria El
`
ectrica,
Universitat Polit
`
ecnica de Catalunya, 08028 Barcelona, Spain
Keywords:
VSC-HVDC, Power System Analysis, Wind Farm Integration, DC Transmission System, Power System
Modelling.
Abstract:
This research contribution, investigates and analyse the operational characteristics of a voltage source con-
verter (VSC), High-Voltage Direct Current (HVDC) transmission system. The main objective of this research
endeavor is to evaluate the implementation of a HVDC transmission system for integrating offshore wind en-
ergy with the grid. Dynamic model of the system is developed in the Simulink environment. Stability analysis
has been performed through three case studies namely the two active grid interconnection, active and pas-
sive grid interconnection and the offshore wind farm interconnection with the grid. Results are analysed and
compared according to E.ON grid code requirements for offshore grid connection.
NOMENCLATURE
i, u complex current, voltages
p, q active power, reactive power
l, c, r inductance, capacitance, resistance
P
m
modulation index
K, T gain constant, time constant
ω angular speed
Subscripts
r, i real, imaginary component
d, q direct, quadrature axis component
Sign Convention: load oriented; consumed active and
inductive reactive power are considered positive.
1 INTRODUCTION
Wind energy is one of the leading and foremost
sources of energy in reducing the greenhouse-gas
emission. According to the European Wind Energy
Association, 9,616MW of wind energy was installed
∗
The research leading to these results has received funding
from the People Programme (Marie Curie Actions) of the Euro-
pean Unions Seventh Framework Programme (FP7/2007-2013) un-
der REA grant agreement n 317221.
†
The project is also supported by the Ministerio de Econom
´
ıa y
Competitividad, Plan Nacional de I+D+i under Project ENE2012-
33043.
in the European Union during 2011, among which
8,750MW was onshore and 866MW offshore(Wilkes,
2010).
According to wind distribution theory, roughness
class at sea level is lower than at ground. Lower
roughness class means less obstacles and low wind
speed variation. This predict that wind farm at off-
shore could have higher capacity as compare to on-
shore site. For short distance offshore site from land,
HVAC transmission system is suitable as, it is least
expensive, evolve and conventional technology for
grid integration. However, at longer transmission dis-
tance due to cable capacitance, reactive losses in-
creases significantly. Typically, HVAC transmission
length is limited to 120km for offshore connection.
HVDC system has some advantages over HVAC but
it has higher development cost(Hulle and Gardner,
2010).
Nowadays, transmission and distribution sys-
tem operators are giving significant consideration to
HVDC transmission system. Past researches has es-
tablished an idea that in comparison to HVAC for
longer distance, HVDC transmission system is a suit-
able solution for transferring a large amount of en-
ergy, but beside that it is also very important to ensure
that HVDC system should fulfills the grid compli-
ances and perform similar as conventional transmis-
sion system.
General grid code defines the operational limits
53
Raza M. and Gomis-Bellmunt O..
Dynamic Modelling and Implementation of VSC-HVDC System - The Grid Connected Large Offshore Wind Power Plant Application.
DOI: 10.5220/0004717900530062
In Proceedings of the 3rd International Conference on Smart Grids and Green IT Systems (SMARTGREENS-2014), pages 53-62
ISBN: 978-989-758-025-3
Copyright
c
2014 SCITEPRESS (Science and Technology Publications, Lda.)
Figure 1: Point-to-Point Voltage-Source-Converter High Voltage Direct Current Transmission System Configuration.
and characteristic that a system must exhibit at point
of common coupling (PCC). These limits are defines
in term of voltage levels, frequency deviation and du-
ration limits, reactive power capability, fault condi-
tion, and active power control. The E.ON grid code
compliances for offshore wind farm integration with
the grid summarized as follows(E.ON, 2008)(E.ON,
2006).
1. Voltage Stability: The nominal voltage (U
n
) level
of the grid is 155kV and continuous operational
voltage range is 140-170kV. Wind generators
should not be disconnected if voltage decreases
0.8U
n
up to 3s, and voltage increase 1.2U
n
up to
200ms.
2. Active Power Control and Frequency Limits: If
frequency at PCC bus drop to 46.5Hz or rise above
53.5Hz then all wind turbines must be discon-
nected within 300ms. If grid frequency is in the
range of 46.5-47.5Hz or 51.5-53.5Hz then tur-
bines are only allowed to disconnect after 10s.
After fault recovery in order to stabilize the volt-
age level, increase in active power per minute is
permissible at maximum rate of 10% of P
n
. If
grid frequency exists within the range of 50.1Hz
to 51.5Hz, active power must be decrease at the
rate of 98% per Hz and 25% per second.
3. Reactive Power Control: Within the voltage range
of +/-0.05U
n
, and at power generation from max-
imum to 0.2P
n
, wind turbines must be capable of
supplying reactive power up to 0.4P
n
in overex-
cited and -0.3P
n
in under-excited condition. At
PCC bus power factor must be control within the
range of 0.925 lagging to 0.95 leading.
4. Fault-Ride-Through Capability: During fault,
wind turbines must inject addition reactive current
into PCC bus in order to support the voltage level.
Voltage support must be provided when voltage
drop is equal or greater than 5% of generator rated
voltages. Voltage support control system should
be activated within 20ms after fault detection. The
amount of addition reactive power injection is de-
pend on nominal voltage and rated current of the
wind generator. Wind turbine must not be discon-
nected if voltage drops to 0.15U
n
for 600ms.
Although, Voltage Source Converter (VSC) trans-
mission system is new as compared to Line Commu-
tator Converter (LCC) but it provides some advan-
tages such as, option to control the active and reac-
tive power flow independently, low harmonics, black
or passive start, low or no reactive power requirement
at no-load and bi-directional power flow.
In VSC, principle of a converter control systems
is based on (1). Power flow between converter and
network is controlled by varying voltage magnitude
and phase angle.
P =
U
pcc
U
pwm
sinδ
ωl
Q =
1
ωl
U
2
pwm
−U
pcc
U
pwm
cosδ
(1)
2 VSC-HVDC SYSTEM
MODELLING
The architecture of the VSC-HVDC transmission sys-
tem developed is shown in Fig. 1. In this config-
uration, two points (defined as PCC bus) are con-
nected via two VSC system and DC cables. PCC
bus could be a connection point of a grid, island net-
work (passive load), or wind farm. Bi-direction power
among these two buses flows according to VSC con-
trol mode.
System is divided into three sections for mod-
elling; VSC (converter/inverter), DC transmission
line, and AC network. PWM-Converter is connected
with the grid through coupling reactance. Coupling
reactance stabilize the AC current, reduce the fault
current, helps to reduce the harmonic current content
SMARTGREENS2014-3rdInternationalConferenceonSmartGridsandGreenITSystems
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and enables the control of power flows from the volt-
age source converter. Typically, AC filter and con-
verter reactor transform the non-sinusoidal output of
converter into sinusoidal output.
The transformer is an ordinary single phase or
three-phase power transformer, with a tap changer.
The secondary voltage (filter bus) is control with the
tap changer to achieve the maximum active and reac-
tive power from the converter (both consumption and
generation). In order to maximize the power trans-
fer, low frequency zero-sequence voltage generated
by the converter, is block by un-grounding the trans-
former secondary winding(Wang et al., 2009). More-
over, ability of VSC system to support the grid stabil-
ity depends on three factors:
• Rated Current of IGBTs: According to (3), DC
power is equal to AC active power. Since, maxi-
mum current flow is limited by IGBTs, therefore
at rate power, MVA capability decreases if AC
voltage drops.
• DC Voltage Level: Maximum AC voltage gener-
ated by VSC depends upon maximum DC voltage.
Also, the amount of reactive power flow is propor-
tional to the voltage difference of PCC and con-
verter bus voltage. The reactive power capability
of a converter is low if the difference between DC
level and grid voltage is low.
• Rated DC Current: Maximum active power flow
depends on the current carrying capacity of DC
cable.
2.1 Voltage Source Converter (VSC)
Model
In Simulink for RMS simulation, converter is model
using DC voltage controlled AC voltage source and
output signal is depend on modulation index. Com-
plex AC voltage generated by the converter is calcu-
lated using (2). For simplicity, it is assume that there
are no losses in the converter thus, power balancing
equation is completed using (3). Converter can act as
rectifier or inverter depending upon operation mode
and set-points.
Figure 2: Equivalent Model of DC Cable.
Figure 3: Simulink Control Diagram of DC Cable.
U
r
= K
0
P
mr
U
dc
U
i
= K
0
P
mi
U
dc
(2)
P
ac
= ℜ (u
ac
· i
∗
ac
) = U
dc
I
dc
= P
dc
(3)
2.2 DC Transmission Line
DC cable is modelled as a π-link as shown in Fig.
2. Cable capacitance is divided into two parts and
added with capacitance of the converter capacitance.
In DC transmission only active power flows, there-
fore, line inductance is small enough to neglect, but it
is included in model to complete the differential equa-
tion(Bao et al., 2011). Control system diagram of the
transmission system is shown in Fig. 3.
2.3 AC Network System
AC network is consist of transformer, capacitor bank
and series reactance. The system is represented as
LCL network (inductance-capacitance-inductance).
Characteristically, this configuration act as low pass
T-filer as shown in Fig. 4. Transformer impedance
and voltages are referred to the filter bus side. Us-
ing (4) and (5), we can represent AC system in dq0
-coordinate system.
u
∠u
pcc
pwm d
= u
∠u
pcc
pcc d
+ ωl
T
i
∠u
pcc
pcc q
+ ωl
L
i
∠u
pcc
pcc q
− ωcu
∠u
pcc
pcc d
(4)
u
∠u
pcc
pwm q
= u
∠u
pcc
pcc q
− ωl
T
i
∠u
pcc
pcc d
− ωl
L
i
∠u
pcc
pcc d
+ ωcu
∠u
pcc
pcc q
(5)
DynamicModellingandImplementationofVSC-HVDCSystem-TheGridConnectedLargeOffshoreWindPowerPlant
Application
55
3 HVDC CONVERTER CONTROL
SYSTEM
Control system has five operational modes i.e. pas-
sive network control, active power control (P), re-
active power control (Q), DC voltage control (U
dc
),
and AC voltage control (U
ac
). The flow diagram of
HVDC control system is shown in Fig. 5. Control
modes are defined according to the operational re-
quirement(Yuan and Wang, 2012).
1. U
dc
− Q: In this mode, DC-voltage and reactive
power set-points are given. Usually, this mode is
defined at the grid side converter to integrate pas-
sive load with active grid.
2. U
ac
− P: In this mode, AC-voltage magnitude and
active power set-points are given. This mode is
set to integrate two active grids.
3. P − Q: This mode specifies active and reactive
power set-points. Through, this mode PQ char-
acteristic of a system is determined.
4. U
ac
−U
dc
: This mode specifies AC and DC volt-
age set-points.
5. Passive Network Control: In this mode, converter
operates as a slack node (magnitude and phase
control). Set at passive network or wind farm side
converter control.
3.1 Active Power Control
In this mode, active power flow is control at PCC bus
at desire value. An adaptive closed-loop proportional-
plus-integral control system is developed for active
power control. In voltage oriented reference system,
for the condition u
pcc
d
= 1.0p.u, the active current
reference can be obtained using control configuration
shown in Fig. 7. Active power flow from AC to DC
network raises the DC voltage. If there is large im-
balance in power at both side then DC voltage will
Figure 4: Phasor Representation of Transformer
Impedance, Series Reactance and Filter Capacitor as
LCL Network.
Figure 5: Voltage Source Converter Control Architecture.
Figure 6: Dynamic Limits for Active Power Control.
Figure 7: Proportional Plus Integral Controller for Active
Power Control.
exceed the maximum operating voltage limit of ca-
ble. Furthermore, when transferring large power over
a long distance, DC voltage difference between send-
ing end and receiving end will be high due to line re-
sistive losses.
Therefore, it is needed to limit the power flow ac-
cording to maximum permissible DC voltage. Dy-
namic saturation limits shown in Fig. 6, are applied at
the output of proportional-plus-integral controller.
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3.2 Reactive Power Control
In this mode, reactive power flow is control at PCC
bus at desire value. In voltage oriented reference sys-
tem, reactive power can be controlled by reactive cur-
rent reference using (6). PI control configuration is
calculated using (7).
q
set
= i
m
sinδ = −i
∠u
pcc
pcc q re f
(6)
Reactive power demand depends on the PCC bus
voltage. Often, it is defined in grid code to have a
power factor within the range of 0.925 overexcited
and 0.95 in under-excited operation. Thus, accord-
ing to PQ characteristic maximum permissible reac-
tive power flow is approx. 55% of rated power.
i
∠pcc
pcc q re f
=
K
p q
1 +
1
s · T
i q
· (−q
set
+ q
meas
)
(7)
3.3 DC Voltage Control
In this mode, converter control the DC voltage at de-
sire value. Active AC and DC power transmitted over
a DC cable using voltage source converter is calcu-
lated using (8) and (9).
P
ac
= U
d
I
d
+U
q
I
q
(8)
P
dc
= U
dc
I
dc
(9)
In balance rotating, synchronous frame in voltage
oriented system, dq0 voltages are constant. Such that
U
q
and U
0
are zero and U
d
is rated voltage. Imbalance
in AC and DC power leads to a change in DC voltage
level across DC capacitors. Voltage across the capac-
itors is calculated using (10).
C
∂U
dc
∂t
=
1
U
dc
{
P
ac
− P
dc
}
(10)
To maintain the constant DC level at rated voltage,
system must fulfill condition (11).
P
ac
= P
dc
U
d
I
d
= U
dc
I
dc
(11)
From (11), it is obvious that DC voltage can be
controlled through the d-component of reference cur-
rent. The PI control system implementation for DC
voltage control is shown in Fig. 8. Maximum and
minimum permissible voltage levels are 120% and
90% of rated DC cable voltage, respectively.
Figure 8: Proportional Plus Integral Control for DC Voltage
Control.
3.4 AC Voltage Control
In this mode, AC voltage of PCC bus is control at
desire value. It is obvious from (1) that reactive power
flow can be control by controlling voltage difference
between converter and PCC bus. Thus, voltage drop
across transformer and line reactance together can be
calculated using (12). In dq0 balance voltage oriented
synchronous frame, reactive power is controlled by
q-component of the reference current, consequently
controlling AC voltage level.
Close loop PI control system is implemented to
control the AC voltage. In addition to that, washout
filter is also incorporated to block the high frequency
noise in input signals. The control diagram of AC
voltage control is shown in Fig. 9.
∆u = u
∠0
pcc
− u
∠0
pwm
≈ (r · i
d
− x · i
q
) + j (x · i
d
+ r · i
q
)
≈ − x · i
∠u
pcc
q
re f
+ jx · i
∠u
pcc
d re f
(12)
3.5 Passive Network Control
Passive network control is designed for operating
wind farm and consumer load. In this mode, PCC
bus acts as an active node i.e. voltage magnitude of
converter bus is adapted according to load impedance,
such that voltage level at PCC bus controlled to
1.0p.u. Passive network control equations is derived
using (13).
Figure 9: AC Voltage Control of Voltage Source Converter
with Washout Filter.
DynamicModellingandImplementationofVSC-HVDCSystem-TheGridConnectedLargeOffshoreWindPowerPlant
Application
57
u
∠u
pcc
pwm d
= u
∠u
pcc
pcc d
+ x
T
· i
∠u
pcc
pcc q
+ x
L
· i
∠u
pcc
pwm q
u
∠u
pcc
pwm q
= −x
T
· i
∠u
pcc
pcc d
− x
L
· i
∠u
pcc
pwm q
(13)
Voltage stability depends upon the demand of re-
active power. The droop Characteristics for voltage
set point is implemented to generate or absorb the re-
active power. A droop characteristic for the voltage
controller is demonstrated in Fig. 10. Since, reac-
tive power flows from higher voltage level to lower
voltage level, therefore, set-point of voltage should be
increased, when the demand for the reactive power
increase. Droop control system is useful for sup-
plying power to inductive load or wind farm with
DFIG(Zhang and Lennart, 2011).
Figure 10: Droop Characteristic of Passive Network Con-
trol.
The voltage reference value is calculated using (14):
u
∠u
pcc
pcc d
= u
∠u
pcc
pcc d set
+ K · q (14)
4 DYNAMIC PERFORMANCE
ANALYSIS
Dynamic response of HVDC system is evaluated us-
ing three case studies. Analysis are performed to eval-
uate the system ability to remain stable due to small
disturbance during operation. Since, VSC-HVDC
transmission will be used as an alternative of the con-
vention AC transmission system, therefore, it is im-
portant to assess the performance of the system com-
patibility with the existing system. And, it is required
to perform the operational capability tests such as,
ability to transfer large power at long distance while
maintaining voltage stability, independent active and
reactive power control capability, ability to integrate
weak grid, ability to supply power to island or pas-
sive network, and ability to integrate wind farm. For
all case studies, approx. 100km transmission length
is considered.
Figure 11: Power Flow Response of a HVDC System Inter-
connecting Two Active Grid.
Figure 12: P-Q Diagram of ’Side-A’ Converter with 100km
Transmission Line.
4.1 Interconnection of Two Active Grids
In this study, transmission system describes in Fig. 1.
is connected with two active grids each at both sides.
Capacity of converter is approx. 200MW. ’Side-A’
converter is set in P − Q mode and ’Side-B’ converter
is set in U
dc
− Q mode. Initially, P − Q set-points
are zero, therefore, no power flows. Step change in
power demand from zero to full load at 0.3s is applied
by changing P − Q set-points at ’Side-A’ converter,
which lead power to flow from side A to B. Due to
sudden in-feed of active power into transmission sys-
tem, DC voltage rises up to 1.16p.u. Increase in DC
voltage level is proportional to the active power avail-
able to ’Side-B’ converter to supply into network.
Power and voltage response of the network are
shown in Fig. 11. and Fig. 13. respectively. It is to be
notice that at set active power at ’Side-A’, in order to
stabilize the voltage level at PCC bus, 20% of P
n
re-
SMARTGREENS2014-3rdInternationalConferenceonSmartGridsandGreenITSystems
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active power injection is required. This is achieved by
setting the reactive power set-point to −0.2p.u. This
demonstrates VSC control scheme capability to con-
trol active and reactive power independently.
Losses and voltage drop over the DC transmis-
sion line depend only on the resistance of the ca-
ble. At given cable parameters, voltage drops over
the cable according to simulation results are approx.
0.04U
dc rtd
.
Figure 13: DC and AC Voltage Response Interconnecting
Two Active Grid.
PQ diagram provides information about the range
and limits of operating point of the system for stable
operation. Fig. 12. shows the PQ characteristic of
’Side-B’ converter, connected to the capacitor bank
of 0.15P
n
capacity.
Voltage strength of the network also depends on
the short circuit capacity of the grid. A high short
circuit capacity means, network is strong and stiff i.e.
switching on/off a load, or shunt capacitors/reactors
will not affect the voltage magnitude, significantly.
The effect of grid strength is directly reflected by
the grid frequency. Strength of the network is defined
by a short circuit ratio (SCR) i.e. it is the ratio of grid
short circuit power to the converter rated power(Egea-
Table 1: Strength of Network.
Network Strength SCR
High > 5
Moderate 3 − 5
Low 2 − 3
Very Low < 2
Alvarez et al., 2012).
SCR =
S
“
k grid
P
r con
(15)
The comparison of VSC-HVDC system ability to
maintain the system frequency at nominal value with
both strong and weak network connection can be vi-
sualized from Fig. 14 and Fig. 15. At lower SCR,
change in grid frequency during transition period is
approx. 0.7Hz, which lead grid frequency drop to
49.3Hz at ’Side-A’ and rise to 50.7Hz at ’Side-B’. In
addition, weak grid has high internal impedance as
compare to strong grid, thus it cause the higher volt-
age drop at PCC bus. Therefore, in order to stabilize
the bus voltage 20% of P
n
at ’Side-A’ and 5% of P
n
at ’Side-B’ capacitive reactive power injection is re-
quired.
Figure 14: Effect of change in grid frequency at both sides
PCC bus on changing load demand at SCR ratio of 5.0.
4.2 Interconnection of Passive Network
In this case study, analysis is made to evaluate the per-
formance of developed control system for black start
and to supply energy to passive network. For simula-
tion, network configuration is same as shown in Fig.
1. Here, converter at load side ’Side-A’ is set into pas-
sive mode and grid side converter ’Side-B’ is set into
U
dc
−U
ac
mode.
Load side converter act as an active source to the
load. The passive control system tries to maintain the
PCC bus voltage at 1.0p.u, and voltage magnitude and
phase angle at the converter bus adopt itself, accord-
ingly. The response of basic operation of consumer
connected to the grid through HVDC system is shown
in Fig. 16. Consumer load of 0.7p.u active power and
0.1p.u inductive reactive power is set for simulation
purpose.
DynamicModellingandImplementationofVSC-HVDCSystem-TheGridConnectedLargeOffshoreWindPowerPlant
Application
59
Figure 15: Grid frequency variation at both sides PCC bus
at 2.5 SCR.
AC signal generated by VSC is dependent on DC
voltage. Sufficient DC voltage is required to generate
the desired AC magnitude and phase angle to fulfill
the demand. Converter model generates 1.0p.u AC
magnitude at 1.0p.u DC voltage level at modulation
index (P
m
= 1). At the given parameter, voltage drop
across the transmission line is 5% of U
dc
. In order
to deliver the rated power, it is required to compen-
sate the voltage drop and the power losses into the
transmission system. This is done by adjusting the set
point for DC voltage control at the grid side converter
to higher level. Fig. 17. shows the voltage response
at passive load PCC bus.
In Table. 2, capability of VSC-HVDC system to
operate at different power factor is shown. As ex-
plained previously, that the passive network control
system tries to maintain the PCC bus voltage at the
load side to 1.0p.u. The effect of capacitive and in-
ductive load can be seen from converter bus voltage.
It is to be seen that at u
pcc
, nominal voltage re-
Table 2: Simulation Results of ’Side-A’ System at Different
Capacitive and Inductive Power Factors.
Passive Load
DC
Voltage
Across
Capaci-
tors
Power
Factor
u
pcc
u
pwm
Active
Power
Reactive
Power
Side-
A
Side-
B
Capacitive
0.90 1.00 0.87 1.00 0.48 1.00 1.05
0.925 1.00 0.89 1.00 0.41 1.00 1.05
0.95 1.00 0.93 1.00 0.33 1.00 1.05
0.98 1.00 0.97 1.00 0.20 1.00 1.05
1.00 1.00 1.05 1.00 0.00 1.05 1.10
Inductive
0.98 1.00 1.12 1.00 0.20 1.07 1.12
0.95 1.00 1.17 1.00 0.33 1.11 1.16
0.925 1.00 1.19 1.00 0.41 1.14 1.18
0.90 1.00 1.20 1.00 0.48 1.15 1.20
Figure 16: Active and reactive power at PCC bus of ’Side-
A’ connected with passive load.
mains 1.0p.u at all power factor and converter control
system adapt its voltage to generate or consume reac-
tive power. Now, if we observe the simulation results
at 0.9 capacitive power factor, converter bus voltages
(u
pwm
) reduce to 0.87p.u. And, at 0.9 inductive power
factor, u
pwm
increases to 1.2p.u. Moreover, DC volt-
age across the capacitor at ’Side-B’ is rise to its max-
imum permissible voltage. Theoretically, voltage col-
lapse phenomena occur, when bus voltage does not
remain within the following limits;
0.8 ≤ u
n
≤ 1.2
Since, internal bus voltages are within the above
design criteria. Thus, it can be concluded that the sys-
tem is stable for 0.9 leading to 0.9 lagging power fac-
tor.
Figure 17: Response of AC voltage and voltage phase angle
at PCC bus connect to passive load
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Figure 18: Doubly-Fed-Induction generator torque speed
characteristic
4.3 Interconnection of Wind Farm
In this case study, system response connecting wind
farm with the grid over long distance is analysed.
MATLAB standard DFIG model is used for simula-
tion purpose. The behavior of wind turbine output
power with respect to turbine speed and wind velocity
is shown in Fig. 18. At ’Side-A’ in Fig. 1. an equiv-
alent model of wind farm array is connected with the
overall capacity is 45MWatt. To simulate the model,
converter connected with wind farm is set into pas-
sive control mode and grid side converter is set into
the U
dc
−U
ac
control mode.
The response of wind farm power in-feed into the
grid via HVDC transmission system can be observe
from Fig. 20. It can be notice that power generation
increases due to change in wind speed from 3m/s to
14m/s. As a result, active power received from wind
Figure 19: Dynamic AC voltage response due to wind en-
ergy injection into the grid
Figure 20: Wind farm active power generation and in-feed
into the grid
farm is transferred to grid with minimum losses, and
HVDC system provides the stable operation. Fig. 19
demonstrate the ability of VSC-HVDC system to ful-
fil the DFIG reactive power demand and control sys-
tem is able to maintain the voltage level within the
design limits.
Despite the higher investment cost of HVDC
transmission system as compare to AC transmis-
sion system for interfacing wind turbines with the
grid(Bresesti et al., 2007), HVDC system provide
benefit of reducing the impact of varying wind gen-
eration and gives ability to control the reactive power,
consequently, supports the grid voltage.
5 CONCLUSION AND
RECOMMENDATION
The aim of the work resulting in this research is to de-
velop the VSC-HVDC control system and study the
feasibility of using it as an alternative of conventional
HVAC system. ’Case-A’ study concluded that with
independent control of the active and reactive power,
it is possible to transfer large power between strong
and weak grid and achieved improve voltage and fre-
quency stability. Results have shown that the reac-
tive power generation capability of VSC-HVDC sys-
tem increases by increasing capacitor bank capacity
and, ability to operate at full active power at maxi-
mum over-excited condition is increased by decreas-
ing the transmission length. The design system is ca-
pable of operating at 0.9 under and over excited power
factors. The simulation results of ’Case-B’ show that
the change in power factor at load side does not af-
fect the voltage stability at grid side system. Also,
DynamicModellingandImplementationofVSC-HVDCSystem-TheGridConnectedLargeOffshoreWindPowerPlant
Application
61
leading and lagging reactive power generation capa-
bility of converter can be improved by incorporating
transformer tap control into converter control system.
Results from ’Case-C’ study conclude that the devel-
oped control system is capable of integrating wind
farm with the grid over long distance and fulfil reac-
tive power demand of wind turbine without addition
compensation.
In addition, VSC-HVDC system is model for sim-
ulation of small signal disturbance. The developed
control system can be extended for critical case in-
vestigation such as fault-ride-through capability and
multi-terminal HVDC system. This research outcome
is useful for pre-designing electrical power system,
power system modelling and simulation and to study
the impact of wind energy on system dynamics.
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