Download II. Components of Vsc - Hvdc

International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Voltage Source Converter Based HVDC
Overhead Transmission System
Mar Mar Win, Thet Tin

Abstract— With the development of VSC technology and pulse
width modulation (PWM), the VSC-HVDC can provide a
number of potential advantages as compared with classic
HVDC, such as short circuit current reduction, rapid and
independent control of active and reactive power, etc. With
those advantages VSC-HVDC are widely used in transmission
and distribution systems. It is an efficient and flexible method to
transmit large amounts of electric power over long distances by
overhead, underground and underwater or submarine
transmission lines.
One of the advantages of applying a
voltage-source converter (VSC) based HVDC system is its
potential to be connected to very weak AC systems where the
conventional line-commutated converter (LCC) based HVDC
system has difficulties. In this paper, the typical transmission
system consisting of VSC-HVDC connecting two AC grids is
studied. In order to analyze the steady state and dynamic
behavior of the developed VSC-based HVDC transmission
system, various operation conditions are created.
Thyristor) or the IGBT (Insulated Gate Bipolar Transistor).
Overhead lines or underground / submarine cables can be
used as transmission path. Based on the functions and the
locations of the converter station, four main HVDC system
configurations are used in power system transmissions. These
four HVDC system configurations can be used for both
voltage source converter (VSC) and current source converter
(CSC) topologies.
Monopolar HVDC system
Bipolar HVDC system
Homopolar HVDC System
Back-to-back HVDC system
Multiterminal HVDC system
Keywords— Active and reactive power control, HVDC system,
LCC, PWM converter, VSC.
I. INTRODUCTION
Today high voltage direct current (HVDC) transmissions
are competitive to conventional HVAC transmission and the
advanced flexible. But, DC power at low voltage could not be
transmitted over long distances, thus giving rise to high
voltage alternating current (AC) electrical systems. However,
high voltage AC transmission links have disadvantages and
engineers were engaged in the development of technology for
DC transmission as a supplement to the AC transmissions. For
the long transmission, DC is still more favorable than AC.
The invention of mercury arc rectifiers and the thyristors
valves, made the design and development of linecommutated current sourced converters possible. The first
commercial HVDC line built in 1954 was a 98km submarine
cable with ground return between the island of Gotland and
the Sweden mainland. The first capacitor commutated
converter (CCC) build in 1998. An improved in the
thyristor-based commutation, the CCC concept is
characterized by the use of commutation capacitors inserted
in series between the converter transformers
and the
thyristors valves. The valves of these converters are built up
with semiconductors with the ability not only to turn-on but
also to turn-off. They are known as VSC (Voltage Source
Converters). Two types of semiconductors are normally used
in the voltage source converters: the GTO (Gate Turn-Off
Figure-1 (a) Back-to-back (b) Monopolar (c) Bipolar (d) Parallel
Multi-terminal and (e) Series Multi-terminal
II. COMPONENTS OF VSC - HVDC
Manuscript received Oct 15, 2011.
First Author name, Electrical Power Engineering Department,
Mandalay Technological University, ., (e-mail: [email protected]).
Mandalay, Myanmar, 09-401569606,
A typical VSC-HVDC system, shown in Fig 2, consists of AC
filters, transformers, converters, phase reactors, DC capacitors and
DC cables.
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
power balance during transients and reduce the voltage ripple on the
DC side.
F. DC Cable
The cable used in the VSC-HVDC applications is a new developed
type where the insulation is made of an extruded polymer that is
Figure-2 . VSC based HVDC system
particularly resistant to DC voltage. Polymeric cables are the
preferred choice for HVDC mainly because of their mechanical
strength, flexibility and low weight.
A. Converter
The converters are VSCs employing IGBT power
semiconductors, one operating as a rectifier and the other as an
inverter. The two converters are connected either back-to-back or
through a DC cable, depending on the application.
Figure-3 (a) Two-level VSC (b) Three-level VSC
B. Transformer
Normally, the converters are connected to the AC system
via transformers. The most important function of the
transformers is to transform the voltage of the AC system to a
level suitable for the converter.
C. Phase Reactor
The phase reactors are used for controlling both the active
and the reactive power flow by regulating currents through
them. The reactors also function as AC filters to reduce the
high frequency harmonic contents of the AC currents which
are caused by the switching operation of the VSCs.
Figure -4 (a) Single-core cable with lead sheath and wire armour (b)
Three-core cable with optic fibers, lead sheath and wire armour
III. TRANSMISSION SYSTEM
HVDC transmission systems have three basic parts as
shown in Figure-5:
1) converter station to convert AC to DC
2) transmission line
3) second converter station to convert back to AC.
D. AC Filter
The AC voltage output contains harmonic components,
caused by the switching of the IGBTs. The harmonics emitted
into the AC system have to be limited to prevent them from
causing malfunction of AC system equipment or radio and
telecommunication disturbances. High-pass filter branches
are installed to mitigate these high order harmonics. With
VSC converters there is no need to compensate any reactive
power and the current harmonics on the AC side are related
directly to the PWM frequency. Therefore, the amount of
filters in this type of converters is reduced as compared with
line commutated converters.
Figure-5 HVDC Transmission System from source to consumers
IV. SIMULATIONS AND RESULTS
Figure-4 Filters used in HVDC transmission;(a) Single-tuned;(b)
Double-tuned;(c) High pass (d) C type high-pass
E. DC Capacitor
On the DC side there are two capacitor stacks with the same size.
The size of these capacitors depends on the required DC voltage. The
objective of the DC capacitors is to provide an energy buffer to keep the
A. Simulink Model Construction for VSC-HVDC
For the simulation of voltage source converter used
in HVDC system, HVDC transmission system is constructed
as shown in Figure-7. The HVDC transmission system
consists of four main portions as the AC grid 1, the AC grid 2,
the HVDC link between the AC systems and the converter
stations. The HVDC overhead line is connected to transmit
the power from AC grid 1 to AC grid 2. Thus, rectifier station
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
is set up at AC grid 1 side and inverter station is at AC grid 2
side. The VSCs are located at both stations and the rectifier or
inverter mode operation is executed by the firing angle
control. The data for HVDC transmission system under
construction is shown in Table 1.
Rectifier Station
HVDC Transmission
frequency harmonics, shunt filtering is therefore relatively
small compared to the converter rating. The 40 Mvar shunt
AC filters are 27th and 54th high-pass tuned around the two
dominating harmonics.
Inverter Station
Wind Power
Plant
AC Grid
VSC
Controller
VSC
Controller
Figure-7 HVDC Transmission System for VSC Simulation
Table 1. Data for System Model
Name
Specification
AC Grid
1
AC Grid
2
Rectifier
Station
Inverter
Station
HVDC
line
25kV,200MW,50 Hz
132kV,50Hz
Three Level Bridge
IGBT/Diode
Three Level Bridge
IGBT/Diode
±150kV,100km Bipolar
HVDC Line
Remark
Three Phase
AC Source
Three Phase
AC Source
VSC
Control
VSC
Control
π Section
Lines
B. System Descriptions
A 200 MW (± 150 kV) forced-commutated
voltage-sourced converter (VSC) interconnection is used to
transmit DC power from a 132 kV, 2000 MVA, and 50 Hz
system to another identical AC system. The AC systems (1
and 2) are modeled by damped L-R equivalents with an angle
of 80 degrees at fundamental frequency and at the third
harmonic. The rectifier and the inverter are constructed with
three-level Neutral Point Clamped (NPC) VSC converters
using close IGBT/Diodes.
The rectifier and the inverter are interconnected through a
100 km cable (i.e. 2 pi sections) and two 8 mH smoothing
reactors. The sinusoidal pulse width modulation (SPWM)
switching uses a single-phase triangular carrier wave with a
frequency of 27 time’s fundamental frequency (1350
Hz).Along with converter, the station includes on the AC side:
step down Y-Δ transformer, AC filters, converter reactor; the
system on DC side has: the capacitors and DC filters.
A converter transformer (Wye grounded /Delta) is used to
permit the optimal voltage transformation. The present
winding arrangement blocks triples harmonics produced by
the converter. The 0.15 pu phase reactor with the 0.15 p.u
transformer leakage reactance permits the VSC output
voltage to shift in phase and amplitude with respect to the AC
system Point of Common Coupling (PCC) and allows control
of converter active and reactive power output. The tap
position is rather at a fixed position determined by a
multiplication factor applied to the primary nominal voltage
of the converter transformers. The multiplication factors are
chosen to have a modulation index around 0.85 (transformer
ratios of 0.915 on the rectifier side and 1.015 on the inverter
side).
To meet AC system harmonic specifications, AC filters
form an essential part of the scheme. They can be connected
as shunt elements on the AC system side or the converter side
of the converter transformer. Since there are only high
Figure-8 VSC-Based HVDC Transmission Link 200 MVA (+/- 150kV)
C. Design Procedure
In the present work, the rectifier/inverter are three levels
VSC that use the IGBT/diode module available in the
MATLAB/Simulink/Simpower system. The case study is
done for a VSC based HVDC transmission link rated 200
MVA (200MW, 0.95), ±150kV.
The system on AC side has: step down Y-Δ transformer, AC
filters, Converter reactor.
The system on DC side has: Capacitors and DC filters.
The design of the components on AC and DC side are
shown below.
DC voltage rating: ±150kV System frequency: 50Hz
Source AC voltage: 230kV line voltage
Rated DC current=Rated DC power/Rated DC voltage
D. AC System Modelling
AC system is modeled as a simple three phase AC source
with internal resistance and inductance that is calculated from
short circuit level MVA calculations.
(MVA)B = 200MVA
(KV)B = 230 kV (Phase to Phase rms), 50 Hz
E. Transformer Design
Y grounded /Δ Transformer is used to permit the optimal
voltage transformation. It also blocks the triplen harmonics
produced by the converter. The following data for the
transformer is considered:
Nominal Power =200MVA (total for three phases)
Nominal frequency=50Hz.
Winding1 specifications: Y connected,
Nominal voltage = 230kV rms (Line to Line) X 0.915 (to
simulate a fixed tap ratio) = 210.45kV
Resistance = 0.0025pu, Leakage reactance = 0.0075pu
Winding 2 specifications: Δ connected, nominal voltage =
150kV rms (Line to Line),
Resistance = 0.0025pu, Leakage reactance = 0.075pu
Magnetizing losses at nominal voltage in % of nominal
current: Resistive 5 %( =500pu).
F. AC Filter
Nominal voltage: 150kV
Nominal frequency: 50Hz
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Nominal reactive power: 20% of real power (200MW) =
78.5Mvar Tuning frequency= 27*50 and 54*50.
Quality factor= 15.
G. Simulations Results
Figure-16. Three phase Current at receiving end
Figure-9. DC Voltage at receiving end
Figure-17. DC voltage at sending end (X)
Figure-10. DC Voltage in p.u at receiving end
Figure-18. DC voltage in p.u at sending end (X)
Figure-11. DC real power in p.u at receiving end (Y)
Figure-19. DC real power in p.u at sending end (X)
Figure-12. Receiving end (Y) AC voltage measured in p.u
Figure-20. Sending end (X) AC voltage measured in p.u
Figure-13. Active power in p.u at receiving end (Y)
Figure-21. Active power in p.u at sending end (X)
Figure-14. Reactive power in p.u at receiving end (Y)
Figure-22. Reactive power in p.u at sending end (X)
Figure-15.Three phase Voltage at receiving end
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Figure-23. Three phase Voltage in p.u at sending end (X)
Figure-24. Three phase Current in p.u at sending end (X)
From the above simulation results it can be observed that by
VSC HVDC transmission line, the reactive power at Y end
can be minimized as well as the receiving end voltage can be
maintained at 1pu without using any compensation.
V. CONCLUSION
This Paper presents the steady-state performance of AC
Transmission System and VSC based HVDC transmission
system. The modeling details of HVDC system with three
levels VSC are discussed. From the simulation results, it is
concluded that the system response is fast; high quality AC
voltages and AC currents can be obtained; and that the active
power and the reactive power can be controlled independently
and are bi-directional. The proposed scheme also ensures that,
the receiving end voltage is maintained at 1 pu without any
compensation.
ACKNOWLEDGMENT
The author is deeply grateful to her supervisor, U Thet Tin,
lecturer, Department of Electrical Power Engineering,
Technological University (Mandalay), for help, permission
and suggestions. The author would also like to thank to Dr.
Khin Thuzar Soe, Associated Professor and Head of
Department, Department of Electrical Power Engineering,
Technological University (Mandalay), to her helpful
suggestions, valuable advice and permission to carry out of
this paper. The author appreciates and thanks all her
teachers for their support, and guidance during theoretical
study and thesis preparation.
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“Voltage Source Converter Based HVDC Transmission”, International
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Volume 1, Issue 1, September 2012
Stan, A.I., 2010, “Control of VSC-Based HVDC Transmission System
for Offshore Wind Power Plants”, Master Thesis, Aalborg University,
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Shire, T. W., 2009, “VSC-HVDC based Network Reinforcement”, M.
Sc. Thesis, Delft University of Technology, U. S. A.
The Math Works Inc, 2010a, MATLAB 7.0,
http://www.mathworks.com.
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