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					   Hvdc Transmission Using Voltage Source Converters (Vsc)

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


VOLTAGE SOURCE CONVERTERS FOR HVDC
         The world of converters may be divided in to two groups that are to be distinguished by their
operational principle.
         One group needs an AC system to operate and called as line commutated coverters.Conventional
HVDC systems employ line commutated converters.
         The second group of converters does not need an AC system to operate and is therefore called as
self commutated converters. Depending on the design of the DC circuits this group can be further divided
in to current source converters and voltage source converters. A current source converter operates with
a smooth DC current provided by a reactor, while a VSC operates with a smooth DC voltage provided by

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storage capacitor.   Among the self commutated converters it is especially the VSC that has big history in
the lower power range for industrial drive applications.
Diagrammatic Representation of VSC-HVDC




BASIC WORKING PRINCIPLE
         The basic function of a VSC is to convert the DC voltage of the capacitor into AC voltages. Fig 2
illustrates the basic operating principle. The polarity of the DC
voltage of the converter is defined by the polarity of the diode rectifier. The IGBT can be switched on at
any time by appropriate gate voltages. However if one IGBT of a branch is switched on, the other IGBT
must have been switched off before to prevent a short circuit of storage capacitor. Reliable storage
converter inter lock function will preclude unwanted switching IGBT. Alternating switching the IGBT’s of
one phase module as shown successively connects the AC terminals of the VSC to the positive tapping and
negative tapping of the DC capacitor. This results in a stair stepped AC voltage comprising two voltage
levels +Vdc/2 and -Vdc/2. A VSC as shown is there fore called a 2 level converter.


  The VSC based HVDC transmission system mainly
  consists of two converter stations connected by a dc
  cable. Usually the magnitude of AC output voltage of
  converter is controlled by Pulse width modulation
  (PWM) without changing the magnitude of DC voltage.




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Due to switching frequency, that is considerably higher than the AC system power frequency the wave
shape of the converter AC current will be controlled to vary sinusoidal. This is achieved by special Pulse
Width Modulation. Besides the 2 level converters, so called 3 level converters have been used for high
power applications.
              A three level VSC provides significant better performance regarding the total harmonic
voltage distortion (THD).However, the more complex converter layout resulting in the larger footprint and
higher investment costs makes 2 level technology the preferred solution for HVDC from today’s point of
view.


PULSE WIDTH MODULATION
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 coupled to the AC side of the converter. The control system includes
three inputs for receiving reference signals allowing to control the frequency, the amplitude and the phase
angle of the fundamental Fourier component with respect to the alternating voltage of the network coupled
to the AC side of the converter. Through appropriate feedback loops, the converter may be used to maintain
at a predetermined level the power flowing therethrough or to keep at a preset value the voltage across the
DC terminals of the converter and, in both cases, to maintain the frequency synchronism between the
fundamental Fourier component and the alternating voltage of the network coupled to the DC side of the
converter.




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CHARACTERISTICS OF VSC-HVDC
         The principal characteristic of VSC-HVDC transmission is its ability to independently control
the reactive and real power flow at each of the AC systems to which it is connected, at the Point of
Common Coupling (PCC). In contrast to line-commutated HVDC transmission, the polarity of the DC
link voltage remains the same with the DC current being reversed to change the direction of power flow.


VSC-HVDC Transmission System Model




      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 converters are three-level bridge blocks using close to ideal switching device model of
IGBT/diodes. The relative ease with which the IGBT can be controlled and its suitability for high-
frequency switching has made this device the better choice over GTO and thyristors. Open the Station 1
and Station 2 subsystems to see how they are built.


HARMONICS IN VOLTAGE SOURCE CONVERTERS (VSC)
         Like all power electronic converters, VSC’s generate harmonic voltages and currents in the AC
and DC systems connected. In a simplified manner, from the AC system, a VSC can be considered a
harmonic current source connected in parallel to the storage capacitor .This behavior is just opposite to
those of conventional line commutated converters.
Harmonics generated depends on
           the station topology (e.g. 6 pulse or 12 pulse)
           switching frequency of IGBT’S
           pulse pattern applied



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Using 12 pulse configuration instead of 6 pulse will improve harmonic conditions both on AC and DC
side. Characteristic AC side harmonics will have the ordinal numbers
           Vac =12n+1; n=1, 2………
    Characteristic DC harmonics will have the ordinal numbers
           Vdc=12n; n=1, 2………..
    All harmonics will be cancelled out under ideal conditions.
    Due to its inherent harmonic elimination capability, the harmonic interface of VSC converter is rather
    small in comparison to the conventional line commutated converters.However, harmonic filters might
    be necessary on the AC and DC sides depending on the harmonic performance requirements both for
    AC and DC sides, AC system harmonic impedance, DC line/cable impedance and loss evaluation.
VSC HVDC has the following advantages
     No need for short circuit power for commutation. Can even operate against black Networks.
     Can operate without communication between stations.
     Can operate to control the power continuously in one direction.
     No change of Voltage polarity when the power direction is changed. This makes easier to make
        multi-terminal schemes.
     Possibility to use robust and economically extruded cables for both land and sea.
     Small converters that reduce the requirement for space.
     VSC based HVDC does not add short circuit power, so there is a great freedom in choice of
        topology and interconnection points.
     A substantial reduction in system losses, mainly due to the elimination of the transformer and
        related equipment. Losses could be reduced by up to 25%.
     Other environmental benefit, e.g. the new motor is epoxy-free and therefore easy to recycle.


APPLICATION’S OF HVDC TRANSMISSION USING VSC
        HVDC Light is a recent technology that utilizes Voltage Source Converters (VSC) rather than
line 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
                          -voltage dc capacitors
    
    
        In the HVDC Light transmission schemes, the switching of the IGBT valves follows a pulse
width modulation (PWM) pattern. This switching control allows simultaneous adjustment of the
amplitude and phase angle of the converter AC output voltage with constant dc, PWM pattern and the
fundamental frequency voltage in a Voltage Source Converter. With these two independent control



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variables, separate active and reactive power control loops can be used for regulation. With these two
independent control variables, separate active and reactive power control loops can be used for regulation.
MAIN DIFFERENCES BETWEEN HVDC LIGHT AND CONVENTIONAL
MAIN       DIFFERENCES            BETWEEN           HVDC        LIGHT        AND       CONVENTIONAL




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CONCLUSIONS
         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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posted:12/2/2011
language:English
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