Overview Strategy of Wind Farm in
VSC-HVDC Power Transmission
S.C.Gupta
Tarun Shrivastava
A.M.Shandilya
Electrical Engg. Department
MANIT Bhopal MP
tarunmitsmanit08@gmail.com
Electrical Engg. Department
MANIT Bhopal MP
akmpshandilya@gmail.com
Abstract—The VSC-HVDC system performance has the most
competitive and new feasible solution for integration of high wind
penetration at offshore sites for efficient power transmission.
This paper presents an overview of various control strategy for
power converters and the plan of services in VSC-HVDC
transmission. For long distance transmission line, reactance
limits the transfer capability of high voltage alternating current
(HVAC) system. With the advancement of VSC-HVDC
technology reactance can be smoothly controlled, hence transfer
capability can be enhanced. VSC-HVDC (voltage source
converter- high voltage direct current) technology also meets the
challenges that are faced by 2-level and multilevel converters.
Rapid developments in IGBT open new gate for controlled
strategy in converters. Due to this there is a paradigms shift in
interfacing renewable energy with electric power system. The
real and reactive power can be controlled indepedently by using
VSC-HVDC system and black start capability is restored by an
AC grid. This paper lays focus on VSC-HVDC transmission
technology with integration offshore wind power. It will help
operating engineers, designers, and research scholars to focus on
this work and decision making. VSC-HVDC transmissions are
giving ease of belongingness to energy transition.
Keywords: VSC-HVDC transmission system, voltage source
converter, offshore wind farms, Black start.
I.
INTRODUCTION
In developing countries, increased demand of electricity
has forced the power utility to improve their transmission
system along with reduced CO2 emissions worldwide. On the
other hand non conventional energy sources like wind, solar
energy has been growing fast and has the potential to reduced
CO2 emissions [1]. Large scale wind energy is highly
penetrative and an attractive renewable energy all over the
world in the power system. Consequently HVAC transmission
system compared with HVDC transmission technology is
more economical with enhanced long distance power transfer
capability. The global climatic advantages facilitate the
integration of long distance large wind farms. Lots of research
work has been done in HVAC and HVDC system for the
power transfer capabilities; voltage deviation, economic
feasibility, design manufacturing, and conversion [2,3,5].
HVAC system generally concern for increasing the
transmission power losses which affects the cost with respect
to long distance and reactive power compensation [4, 6, 8]. So
HVAC system is not favored for grid connected large and
remote offshore wind farms [7]. HVDC transmission systems
978-1-4673-8962-4/16/$31.00 ©2016 IEEE
Electrical Engg. Department
MANIT Bhopal MP
scg.nit.09@gmail.com
are more feasible and reliable solution for improving the
power stability, power quality and reducing transmission
losses. Two types of HVDC converters, Line-Commutated
Converters (LCCs) known as Current Source Converters
(CSCs) and Voltage Source Converters(VSCs) as shown in
Fig.1. The advancement in power electronic technology, new
VSC-HVDC transmission system led to development of IGBT
has an promising advantages over of LCC HVDC such as low
cost, compact size and reliable operation of weak AC system
[9,26].
Performance and dynamic stability of AC systems can be
enhanced by integrating off shore power grid to high capacity
offshore wind farms. Independent controllability of VSC –
HVDC technology has increased its application in
transmission for controlling real and reactive power [1]. Such
system employs IGBT, IGCT and GTO for self commutation.
In AC side the disadvantages of using power converters are
increase in power losses, THD, phasor difference shift in
voltage and current can be overcome with advent of flexible
VSC-HVDC technology regarding the flexibility of converters
in the AC system, the short circuit capacity is insignificant.
Using the new switching methods and new topologies
significantly reduces the converter loss, low order harmonics
and controls the phase shift between output voltage and
current on the AC side [10, 11]. Power synchronization
controls are investigated for connection of two weak AC
systems, improving the technical features of the VSC-HVDC
system offshore wind power integration, power grid support
and present relevant topologies.
Receving End
Sending End
AC
AC
Reactive power
Real power
Controller
-2
Controller
-1
Wind
Farm
Reactive power
AC Grid
AC
Filter
Filter
Fig. 1 CSC-HVDC and VSC-HVDC
II.
VSC-HVDC SYSTEM
With the continuously ongoing research work in the field
of high power electronics HVDC system, lead to paradigms
shift in AC system. In electric power transmission system,
overhead lines or underground/submarine cables transmit
large amount of electric power by HVDC, in an efficient
manner. Application of HVDC transmission system is
increasing day by day all over the world as it offers high
power transmission capability for long distance transmission
[12-14]. HVDC technology is a milestone for major evolution,
regarding power Electronics devices and control systems
[15,16,17-20]. The fully controlled semiconductor IGBT with
VSC operates high switching frequencies up to (1-2 kHz)
utilizing pulse width modulation (PWM).The list of fully
control high switching frequency semiconductors as shown in
Table I.
Monopolar system
Back-to-back system
Bipolar system
Multiterminal system
Fig. 2.1 VSC-HVDC Configure Diagrams
TABLE I
Fully controlled high switching frequency semiconductors[26]
Acronym
IGBT
IECT
GTO
Type
Transistor
Transistor
Thyristor
IGCT
Thyristor
GCT
Thyristor
Full Name
Insulated Gate Bipolar Transistor
Injection Enhanced Gate transistor
Gate Turn-off Thyristors
Integrated
Gate
Commutated
thyristors
GateCommutated Turn-off Thyristors
A. Configurations of VSC-HVDC
Following configurations are available in VSC-HVDC:
Monopolar VSC-HVDC System: Two converters are
connected by positive or negative DC voltage with a
single pole line is used.
Bipolar VSC-HVDC system: This is most applicable
configuration where two insulated converters connected as
positive and negative poles. These are work as
independently if both poles neutrals are grounded. If one
pole is failed to transmit power, respective other pole can
be transmit power which enhance the reliability [21, 23].
Back-to-back VSC-HVDC
system: In this
configuration two independent neighboring systems with
different and incompatible electrical parameters are
connected via a DC link. Two AC system interconnected
with same or different frequencies i.e. 50Hz and 60Hz.
Ex- configuration can be found in Japan and South
America [22, 23].
Multi-terminal VSC-HVDC system: There are
combinations of more than HVDC converters are
interconnected through transmission lines or cables. A
Multi-terminal HVDC system can be either parallel or series
connected to the same voltage source. Ex- The SardiniaCorsica-Italy connection, the Pacific Intertie in Unites State
of America and the Hydro Quebec-New England Hydro
from Canada to Unites State of America [24, 25]. All VSCHVDC configurations as shown in Fig. 2.1.
B. Components of Classic VSC-HVDC
VSC-HVDC technology has certain attributes such as
converters, transformers, AC filters, phase reactors, DC
cable, breakers, and DC shunt capacitors by which
performance of all over system can be improved in Fig.
2.2. The role and attributes of components are shortly
described in the following subsections.
Fig. 2.2 Overall VSC-HVDC system
a) Converters:
The HVDC converters have a significant role to play in
HVDC system. These converters are used for transformation
of AC into DC (rectifier) and DC into AC (inverter) at the
sending end and receiving end respectively. For converting
AC lines in to DC lines, the requirement of DC must be fitted
in AC. Power converters uses either transistor or SCR, which
are available as fully controlled semiconductor devices in
present days. Significant accuracy and high reliability with
efficient design manner can be achieved with them. LCCs is
driven by the AC voltage connection with minimum short
circuit power. This is impossible for LCCs based thyristors.
All limitation of LCC is addressed by VSC-HVDC system.
As compare to SCR based system, VSC based converter have
higher working frequency whose corresponding frequency are
determined by switching losses. Switching losses are most
attention seeking and trouble creating issues in VSC based
applications which are directly related to high frequency
PWM operation. Switching frequency of IGBT based PWM
are high, which create any fundamental voltage and current
waveform whose phasor and magnitude matches frequency
components [27]. Such Waveforms also have high order
harmonics in addition to fundamental frequency components,
these harmonics consists of carrier frequency component and
its multiple of the PWM. Higher the rate of
IGBT
commutations, higher will be the switching losses.
b) Filters:
In electrical system, reduction of higher order harmonics
can be achieve by many passive technique associated with
series line reactors, tuned harmonics filters and high pulse
number converters. Reducing the low harmonics distortion
and reactive power compensation consisting of certain filter
technique such as tuned Harmonic Filters, Series Induction
Filters, Parallel Connected Resonant Filter, Series Connected
Resonant Filter, Series Active Filters are applicable [28].
Passive, active and hybrid harmonics filter is traditionally
used to consume harmonics with low cost and simple robust
structure.
c) Transformers:
The transformation of bus bar input voltage to the AC
voltage by 3-phase single converter transformer in the
converter. Large value of voltage control requirements at
converter transformer having Tapping range is large (25 ~
30%) with small steps to give necessary adjustments in supply
voltage.
d) Smoothing Reactors:
The aim of smoothing reactors is to prevent the
intermittent current with minimum load. DC fault current and
resonance in DC circuit also reduces high frequency
harmonics caused by interference from the overhead lines.
e) HVDC Cables:
HVDC systems have plenty of fascinating properties that
make the connection in or between networks by extruded low
weight cables. HVDC cables are generally used for submarine
and long distance transmission at high megawatt (MW) levels.
f) HVDC Breaker:
HVDC breaker can smoothly minimize the restriction of
operating speed, which generates more transfer losses in VSC
station. To overcome these limitation, a new hybrid HVDC
breaker is proposed [27], having negligible conduction loss,
fast, efficient and reliable. Therefore, the working of HVDC
breaker is to isolate the faults and avoiding the disruption of
HVDC grid voltage which is very fast and reliable.
g) HVDC Capacitors:
HVDC Capacitors do not only enhance the transmission
capacity or system stability, but also contribute to economical
growth of the system. The Capacitance minimizes the DC
voltage ripples which affect the switching action in VSCHVDC. Two same size capacitor are connected to DC side.
III.
MULTI-TERMINALVSC-HVDC TECHNOLOGIES
HVDC system is a feasible and most adoptable technology
related with flexible AC transmission system (FACTS)
continue
to advance for commercial and industary
applications [29]. The advantages of VSC-HVDC in the AC
grid system include independent controlled of real and
reactive power, connection of weak AC system, fast control
output, black start capability and improvement in the multiterminal system or complete DC grids. PWM VSC based
HVDC transmission reduces the size and cost of harmonics
filter, and commutation failure. In the modern power system
HVDC and FACTS system having important technologies are
fully or partially deregulated in many countries such as China,
India, South America, Middle East require infrastructures to
power growth and interconnection of island grid and
renewable sources [30-32].
A. Multi-terminal VSC-HVDC control:
To achieve high voltage level with reduced harmonics
(nearest to sinusoidal), a Modular Multi-Level Converter
(MMC) topology has been developed in [36]. Multi-terminal
VSC-HVDC systems are emphasize for large-scale renewable
energy sources such as offshore wind farms and
interconnected with regional AC grids [33] focusing on
various control approaches. Each of these are explained
below.
a) Master-Slave Control:
In a DC grid, only one converter is connected to constant
DC voltage mode is knows as master terminal and all others
are connected to constant power mode is known as slave
terminal. Power flow balance is maintained by the master
terminal, they will balance DC over and under voltage
problem of entire DC grid. Master slave control is not
recommended for application in multi-terminal VSC-HVDC,
because of unavailability of N-1 security. So that advance
modified version of this controller, called, Voltage margin
control have been recommended to solve the problem of N-1
security by various authors [34-36].
b) DC Droop Control:
There are two or more converter in a DC grid, appointed as
DC voltage droop controller and all others are operated in
power control mode. These controllers are having capability to
balance the instantaneous power in DC grid. This control is
known as frequency droop control of Multi-terminal VSCHVDC system [35, 36].
c) Master-Slave with Droop Control:
This control is combination of master and droop control (i.e.
constant DC voltage–master terminal, one and more converter
with DC voltage droop). In this control, power balance is
controlled by master terminal in DC grid. Any master
terminals outage results in the other terminals to follow voltage
droop control.
B. Multi-terminal VSC-HVDC Topology:
The comparison of different Multi-terminal VSC-HVDC
topologies for transferring power from offshore wind forms to
AC grid have been dipicted in Table II [37].
x Point to point topology (PPT): Multiple point to point
links connected to offshore wind farm to onshore grids.
x The general ring topology (GRT): Each line connected
to all the nodes composing a ring.
x Star topology (ST): Each line connected to either wind
farm or a substation connected to a central star node.
x Star with a central switching ring topology (SGRT):
Larger ring or a concentrated switching ring used in
star configuration.
x Wind farms ring topology (WFRT): Can deal to
connect a wind farm to an onshore substation.
x Sub-Station ring topology (SSRT): An onshore substation ring topology which connected to offshore wind
farm with each sub-station.
IV.
x
VSC-HVDC CONTROL SYSTEM
Fast and bidirectional real power flow and absorbing
or delivering reactive power flow can be controlled
by VSC-HVDC transmission network [1]. Rectifier
side is controlled by DC voltage, inverter side energy
is controlled by real power and classical HVDC
power flow can be adjusted by PWM scheme in
IGBT, to control the real and reactive power
independently. Regulating the real power flow can be
controlled by DC voltage on DC side and at AC side
with frequency variation or real power.
TABLE II
Comparison of Multi-terminal HVDC topologies for large offshore wind farm [37]
PPT
Offshore
plate form
No
GRT
Topology
Communication
Flexibility
Redundancy
Comments
No
No
No
No
Yes
Good
Good
Yes
Yes
Yes
No
No
No
Good
Bad
Poor
Yes
Yes
Yes
SGRT
Yes
No
Poor
Yes
Simple but it lacks flexibility
Flexible but some circuits have to be rated
for the full system power
The circuits rating equal to the rating of the
wind farm or substation to which the circuit
is connected, but it has a weak point at the
central node
Has the advantages of both GRT & ST, but
still needs full power in the central ring
WFRT
No
Yes
Good
No
No
No
Yes
Yes
Yes
Good
Bad
Poor
ST
SSRT
SS side
WF side
Total
Ring
Line
Total
The reactive power control can be adjust by AC
voltage, where the reactive power flow or reference
value without changing DC voltage. So this controlling
system can also consider that real and reactive power
by DC voltage side, frequency variation on AC side,
and their respective reference value.
x VSC–HVDC consisting of fast inner and outer current
control loop. The AC current is controlled by inner
current control loop. And outer controller controlled
the DC voltage controller, the reactive power
controller, the AC voltage controller or the frequency
controller as relatively slow. Since both loop
controllers cannot be represented at same time, the
choice was depended on reference value of current
converter for the application in power system [38, 39].
x Inner current control operates in the dq rotating
reference frame with PI regulator (controller). In case
of balanced conditions dq voltage and current controller
are constant in steady state for unbalance voltage
condition i.e. unsymmetrical voltage, + ve and - ve
sequence of AC quantities have to be considered in
control the operation of converter. Therefore design of
inner current controller may be divided into a + ve and ve sequence current controller. They can be determined
by DC components and AC components respectively.
Outer controllers are implemented by PI controller,
with corresponding to a reference value. Therefore, one
converter can control DC voltage for balancing power
and another converter adjusting the real power within
the range.
A. Complete-Independent Control Strategy
VSC-HVDC system partially depends on different control
strategy. The converter control strategies can be broadly
classified as:
x DC voltage control or real power control
x Reactive power control or AC voltage control
DC voltage control can be categories: Master-Slave
control and DC voltage droop control, Master-Slave
controllers maintain the fast communication where the DC
voltages are fixed between the terminals. But when using
voltage droop controller, no DC regulation and
Good
Yes
Yes
Yes
Allow the isolation of a faulty circuit as in
the case of point to point topology without
needing full system power rating of the ring
circuits
communication at the terminals are needed, for a more
reliable transmission system. The main advantages of DC
voltage droop control is frequency power control in AC
system and compensating the losses in a DC network [40]. As
both above control strategies required coordation of either real
and/or reactive power reference or values of slopes, will
change the power flow coordination, not only in the AC
system but in DC network. Overall outer control system
shown in Fig.3.0, benefits of VSC-HVDC control system are
explain below.
Grid
VSC
Station
VSC
Station
AC
DC voltage control mode
+
AC voltage or reactive
power control mode
Active power control mode
+
AC voltage or reactive
power control mode
Fig. 3.0 Overall Control System in VSC-HVDC Transmission Network
a) Frequency Control:
VSC–HVDC system can provide the frequency regulation
for under frequency due to unexpected loss of generating unit
and for over frequency due to unexpected load rejection at AC
system. Grid code require wind farm with voltage and
frequency operated at normal conditions [41].The real power
reduction in the grid must be controlled by the frequency
deviation from its nominal value. Large frequency range wind
farms are isolated with weak interconnection, where the
systems are more stable.
b) Real Power Control:
Wind farms regulate the real power output with desire
level and ramp rate. Transient stability during faults is very
important in real power control. Some grid codes regulate the
real power injection or absorbing from high and low ramp-up
and ramp-down rates. The real power is uniformly transmitted
with wind farm timely, which can be detected by deviation of
wind farm frequency to support the reference value of real
power [42].
c) Reactive Power and Voltage Control:
In power system the Voltage levels for utility is maintained
constant and for consumers operate within voltage range. In
normal operation, voltage level at the point of common
coupling can be increased and decreased by injecting or
absorbing the reactive power to the grid. So Wind turbine
must be adequate supplying reactive power for the power
factor lagging to leading range. The reactive power capability
is high when grid voltage is low to conventional shunt
compensation of VSC–HVDC system, particularly useful in
preventing voltage collapse [43].
d) Black start capability:
Restoring the power without using the relay on external
energy sources is known as Black start process. Black start
capability operated with the large reactive load for long
transmission line and will be required to synchronies to other
power plants. Black start capability is an important aspects for
stability assessment in long power systems [44, 45].
e) Capabilities of Transmission System Operators (TS0):
Several technical and economical aspects of TSO is to
identify necessary grid process of encouraging or establishing
short circuit current to acceptable levels by avoiding
widespread disturbances and overcoming grid bottlenecks. So
that VSC-HVDC transmission system are a viable option, for
removal of local bottlenecks in a grid and wide range
reactive power control capability with undesirable load flow
path[46].
V. GRID INTEGRATION OF OFFSHORE WIND FARMS
The grid code technical requirements for the large wind farms
should be applied to the power plants integrated with HVDC
systems as follows. The nature of these requirements are
discussed in the Table 3 [47, 48].
x Wind farms should provide active power output
irrespective of frequency deviations.
x The operating voltage and frequency of wind farm
should be within the limits.
x The reactive power control requirements allows the
P/Q capabilities of each network.
x The installation of FACTS device to control the
voltage regulation and enhance Low Voltage Ride
through (LVRT)capability may provide grid code for
wind farms.
VI.
METHODS FOR VSC-HVDC GRID CONNECTED
OFFSHORE WIND POWER SYSTEMS OF LVRT:
The abrupt change in a large power system due to short circuit
or any other disturbances may result in power stability
problems. Therefore LVRT capability require in the grid code
of VSC-HVDC systems [43]. The control schemes are:
x
x
DC chopper in DC link installation in grid side: DC
chopper rise in over-voltage above in certain threshold
value with excess energy in DC link may attain high
DC overvoltage. So power generation of wind farm is
completely interrupted and consequently mechanical
forces will reduce turbine efficiency.
Use of communication system in wind farm, with
move the data at grid side: The most important
demands for wind power farms are to design a reliable
communication infrastructure
for monitoring,
managing and controlling. [40]Wind farms are
x
x
communicating directly and sharing the data with each
individual turbine to maximize the power generation.
Using DC link for voltage control between the
converters: when wind side converter initiates a DC
voltage control, the DC voltage exceeds a pre defined
threshold value, but during the faults, DC voltage
decreases and converter requires the fault-ride through
(FRT) mode to operate the wind farm in that low
voltage [48].
AC voltage regulation as importance of DC voltage by
offshore converter: The rise in DC voltage is
monitored with reference to ramp down AC voltage
by the offshore converter. However, abrupt voltage
drops at the AC system due to fault condition, the
reloading droop controller reduces the modulation
index with voltage reduction, to avoid the mechanical
and electrical stress.
VII. FREQUENCY REGULATION BY VSC-HVDC GRID
CONNECTED OFF SHORE WIND FARMS
There are some control strategies identified for complete this
task.
x Frequency modulation and voltage deviations on shore
side: The receiving end controller increases DC voltage
and sending end controller responded the frequency
capabilities of wind turbine. If the frequency response
is increases, the DC voltage and corresponding
offshore wind farm frequency is also increase. So
frequency response can be detect by converters at both
sides [40].
x The coordination of on shore grid frequency directed
by wind turbine. Wind turbine frequency can be
adjusted by onshore grid frequency. The offshore
frequency generally remains constant when the voltage
deviation is required to approach a successful response.
This is applicable for communication process without
delay for calculations.
VIII. CONCLUSION
The overview of VSC-HVDC are represented in following
areas such as renewable resources generation , system supply,
weak and passive grid system and power distribution with
economically feasible local grid. This paper presents HVDC
system topology with large number of converters can be
arbitrarily selected in generalized module. And its application
for integration with VSC-HVDC offshore wind farm. The
development and functionality of CSC HVDC, VSC-HVDC
and MMC HVDC technologies are presented for enhancement
of power delivery and combination with wind farm according
to the grid code requirements. The control concepts for multiterminal based VSC-HVDC system has been explained in
great detail. The decoupling with AC grid side is provided for
additional controllability in transmission system with more
emphasis on voltage deviation in the transmission line is
demonistrated. The WFRT are the most fascinating multiterminal topologies. A comprehensive literature survey on
improvement in the control methods of the VSC-HVDC
connected offshore wind farm systems for satisfying the
LVRT and frequency regulation requirements are presented in
this paper.
IX.
[1]
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