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restmushroomsElectronics - Devices

Oct 7, 2013 (4 years and 1 month ago)

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Hvdc Transmission Using Voltage Source Converters (Vsc)


ABSTRACT

R
apid 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 s
ource 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 transm
ission 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 sem
iconductors, 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 requ
irement of fast communication between the two converter stations
.Each converter station
is
composed of
a
VSC.
The
ampli
tude 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 acti
ve 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 w
orld 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.

Th
e second group of converters does not need an
AC

system to operate and is there
fore 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 conver
ters
. 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 p
rinciple. 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 bee
n 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 terminal
s 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 tr
ansmission 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 fre
quency, 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 c
onverters 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
othe
r, 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 wavefor
m 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 pha
se
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 flowi
ng 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

T
he 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




T
he 230 kV, 2000 MVA AC systems (AC system1 and AC system2 subsystems) are modeled by
damped L
-
R equivalents with an angle of 80 degree
s 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 h
armonic 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 conve
ntional 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 c
onditions.

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 depend
ing on the harmonic performance requirements both for
AC and DC sides, AC system harmonic impedance, DC line/cable impedance and loss evaluation.

VSC HV
DC has the following advantages



No need for short circuit power for commutation. Can even operate agains
t 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.



Poss
ibility 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.


A
PPLICATION’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







I
n the
HVDC Light transmission

schemes, the switching of the IGBT valves follows a
pulse
width modulation (P
WM
) 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 indepen
dent 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 ass
ociated 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 abilit
y 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 po
wer flow control.