HVDC transmission using voltage source converters - DOC

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ABSTRACT                                       transmission based on Voltage Source
       Rapid developments in the field         Converters (VSCs). The VSC based
of power electronic devices with turn off      HVDC       installations    has   several
capability like insulated gate bipolar         advantages compared to conventional
transistors (IGBT) and gate turn off           HVDC such as, independent control of
transistors (GTO), makes the voltage           active and reactive power, dynamic
source converters (VSC) getting more           voltage support at the converter bus for
and more attractive for High voltage           enhancing stability possibility to feed to
direct current transmission                    weak AC systems or even passive loads,
(HVDC).This new innovative                     reversal of power without changing the
technology provides substantial                polarity of dc voltage (advantageous in
technical and economical advantages for        multi terminal dc systems) and no
direct applications compared to                requirement   of   fast    communication
conventional HVDC transmission                 between the two converter stations .Each
systems based on thyristor technology.         converter station is composed of a VSC.
VSC Application for HVDC systems of            The amplitude and phase angle of the
high power rating (up to 200MW) which          converter AC output voltage can be
are currently in discussion for several        controlled simultaneously to achieve
projects are mentioned. The underlying         rapid, independent control of active and
technology of VSC based HVDC                   reactive power in all four quadrants. The
systems, its Characteristics and the           control of both active and reactive power
working principle of VSC based HVDC            is bi-directional and continuous across
system are also presented. This paper          the operating range. For active power
concludes with a brief set of guidelines       balance, one of the converters operates
for choosing VSC based HVDC systems            on dc voltage control and other converter
in today’s electricity system                  on active power control. When dc line
development.                                   power is zero, the two converters can
INTRODUCTION                                   function as independent STATCOMs.
       The       development    of     power   Each VSC has a minimum of three
semiconductors, especially IGBT's has          controllers for regulating active and
led   to   the    small   power      HVDC
reactive power outputs of individual

VOLTAGE                            SOURCE
        The world of converters may be
divided in to two groups that are to be
distinguished     by     their     operational
                                                 BASIC WORKING PRINCIPLE
        One group needs an AC system
                                                           The basic function of a VSC is
to operate and called as line commutated
                                                 to convert the DC voltage of the
coverters.Conventional HVDC systems
                                                 capacitor into AC voltages. Fig 2
employ line commutated converters.
                                                 illustrates the basic operating principle.
        The second group of converters
                                                 The polarity of the DC
does not need an AC system to operate
                                                 voltage of the converter is defined by the
and    is    therefore    called     as   self
                                                 polarity of the diode rectifier. The IGBT
commutated converters. Depending on
                                                 can be switched on at any time by
the design of the DC circuits this group
                                                 appropriate gate voltages. However if
can be further divided in to current
                                                 one IGBT of a branch is switched on, the
source converters and voltage source
                                                 other IGBT must have been switched off
converters. A current source converter
                                                 before to prevent a short circuit of
operates with a smooth DC current
                                                 storage     capacitor.   Reliable   storage
provided by a reactor, while a VSC
                                                 converter     inter   lock   function   will
operates with a smooth DC voltage
                                                 preclude unwanted switching IGBT.
provided by storage capacitor.        Among
                                                 Alternating switching the IGBT’s of one
the self commutated converters it is
                                                 phase module as shown successively
especially the VSC that has big history
                                                 connects the AC terminals of the VSC to
in the lower power range for industrial
                                                 the positive tapping and negative tapping
drive applications.
                                                 of the DC capacitor. This results in a
Diagrammatic Representation of
                                                 stair stepped AC voltage comprising two
                                                 voltage levels +Vdc/2 and -Vdc/2. A
    VSC as shown is there fore called a 2         complex converter layout resulting in the
    level converter.                              larger footprint and higher investment

The VSC based HVDC transmission system            costs makes 2 level technology the

mainly consists of two converter stations         preferred solution for HVDC from

connected by a dc cable. Usually the magnitude    today’s point of view.

of AC output voltage of converter is controlled
by Pulse width modulation (PWM) without           PULSE WIDTH MODULATION

changing the magnitude of DC voltage.             A converter for interconnecting two
                                                  electric networks to transmit electric
                                                  power from one network to the other,
                                                  each network being coupled to a
                                                  respective power generator station. The
                                                  converter, having an AC side and a DC
                                                  side, includes a bridge of semiconductor
                                                  switches with gate turn-off capability
                                                  coupled to a control system to produce a
                                                  bridge voltage waveform having a
                                                  fundamental Fourier component at the
                                                  frequency   of   the     electric   network
    Due to switching frequency, that is
                                                  coupled to the AC side of the converter.
    considerably higher than the AC system
                                                  The control system includes three inputs
    power frequency the wave shape of the
                                                  for receiving reference signals allowing
    converter AC current will be controlled
                                                  to control the frequency, the amplitude
    to vary sinusoidal. This is achieved by
                                                  and the phase angle of the fundamental
    special   Pulse    Width   Modulation.
                                                  Fourier component with respect to the
    Besides the 2 level converters, so called
                                                  alternating voltage of the network
    3 level converters have been used for
                                                  coupled to the AC side of the converter.
    high power applications.
                                                  Through appropriate feedback loops, the
                  A    three   level    VSC
                                                  converter may be used to maintain at a
    provides significant better performance
                                                  predetermined level the power flowing
    regarding the total harmonic voltage
                                                  therethrough or to keep at a preset value
    distortion (THD).However, the more
the voltage across the DC terminals of      contrast to line-commutated HVDC
the converter and, in both cases, to        transmission, the polarity of the DC link
maintain the frequency synchronism          voltage remains the same with the DC
between    the   fundamental     Fourier    current being reversed to change the
component and the alternating voltage of    direction of power flow.
the network coupled to the DC side of
the converter.                              VSC-HVDC        Transmission        System

                                                    The 230 kV, 2000 MVA AC
                                            systems (AC system1 and AC system2
                                            subsystems) are modeled by damped L-
                                            R equivalents with an angle of 80
                                            degrees at fundamental frequency (50
                                            Hz) and at the third harmonic. The VSC

CHARACTERISTICS            OF      VSC-     converters are three-level bridge blocks

HVDC                                        using close to ideal switching device

       The principal characteristic of      model of IGBT/diodes. The relative ease

VSC-HVDC transmission is its ability        with which the IGBT can be controlled

to independently control the reactive and   and its suitability for high-frequency

real power flow at each of the AC           switching has made this device the better

systems to which it is connected, at the    choice over GTO and thyristors. Open

Point of Common Coupling (PCC). In
the Station 1 and Station 2 subsystems to          All harmonics will be cancelled out
see how they are built.                            under ideal conditions.
                                                   Due      to     its    inherent        harmonic
HARMONICS IN VOLTAGE                               elimination capability, the harmonic
SOURCE CONVERTERS (VSC)                            interface of VSC converter is rather
         Like      all    power    electronic      small     in      comparison            to    the
converters, VSC’s generate harmonic                conventional            line         commutated
voltages and currents in the AC and DC             converters.However, harmonic filters
systems connected. In a simplified                 might be necessary on the AC and
manner, from the AC system, a VSC can              DC sides depending on the harmonic
be considered a harmonic current source            performance requirements both for
connected in parallel to the storage               AC and DC sides, AC system
capacitor .This behavior is just opposite          harmonic impedance, DC line/cable
to    those      of      conventional    line      impedance and loss evaluation.
commutated converters.                          VSC      HVDC            has      the    following
Harmonics generated depends on                  advantages
            the station topology (e.g. 6           No need for short circuit power
         pulse or 12 pulse)                           for        commutation.           Can     even
            switching      frequency      of         operate against black Networks.
         IGBT’S                                     Can                 operate           without
            pulse pattern applied                    communication between stations.
Using 12 pulse configuration instead of             Can operate to control the power
6    pulse      will     improve     harmonic         continuously in one direction.
conditions both on AC and DC side.                  No change of Voltage polarity
Characteristic AC side harmonics will                 when the power direction is
have the ordinal numbers                              changed. This makes easier to
              Vac =12n+1; n=1, 2………                   make multi-terminal schemes.
     Characteristic DC harmonics will               Possibility to use robust and
     have the ordinal numbers                         economically extruded cables for
              Vdc=12n; n=1, 2………..                    both land and sea.
    Small converters that reduce the                                           -voltage dc
       requirement for space.                            capacitors
    VSC based HVDC does not add                     
       short circuit power, so there is a                In     the     HVDC           Light
       great freedom in choice of                 transmission schemes, the switching of
       topology      and    interconnection       the IGBT valves follows a pulse width
       points.                                    modulation     (PWM)      pattern.    This
    A substantial reduction in system            switching control allows simultaneous
       losses,    mainly     due       to   the   adjustment of the amplitude and phase
       elimination of the transformer             angle of the converter AC output voltage
       and related equipment. Losses              with constant dc, PWM pattern and the
       could be reduced by up to 25%.             fundamental frequency voltage in a
    Other environmental benefit, e.g.            Voltage Source Converter. With these
       the new motor is epoxy-free and            two    independent   control     variables,
       therefore easy to recycle.                 separate active and reactive power
                                                  control loops can be used for regulation.
APPLICATION’S OF HVDC                             With these two independent control
TRANSMISSION USING VSC                            variables, separate active and reactive
       HVDC         Light   is     a    recent    power control loops can be used for
technology that utilizes Voltage Source           regulation.
Converters (VSC) rather than line                 MAIN DIFFERENCES BETWEEN
commutated converters. HVDC Light
offers advantages due to the possibility
to independently control both active and
reactive power HVDC Light employs
Insulated   Gate      Bipolar      transistors
(IGBTs),     plus      other       important
technological developments:
                                             -
       connected IGBTs
          In this paper, we have presented
the analysis of High voltage DC
transmission using VSC, the number of
advantages          associated               with
implementing VSC-based designs for
HVDC        applications    that    result     in
systems      with   high    reliability      and
superior operating performance; these
benefits        including           economic,
environmental or technical aspects. Of
particular note today is the ability to
control     power    flow     and     prevent
propagation of severe disturbances, thus
limiting blackout extension. This ability
to   maintain       in     dependence          of
interconnected networks can be of prime
importance when the two systems have
different regulatory procedures, notably
if two counties, and also technically if
the load frequency control regimes are
not compatible .These properties are
further enhanced by using HVDC Light
which gives independent control of
reactive power at both stations, in
addition to active power flow control.

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