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HVDC Transmission
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In the past, transmission systems were conservatively
designed with large stability margins and the
then-availabledynamic compensators, ... , were rarely required.
In recent years, energy, environment, right-of-way, and cost
problems delayed the construction of both generationfacilities
and new transmission lines.
This has necessitated a change in the traditional
power system concepts and practices; betterutilization of
existing power systems has become imperative
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Overhead AC transmission is the standard technology for
electricpower transportation. Traditionally it was preferred over
any
other transmission technology, especially in rural areas.
No other technology can compete with overhead AC transmission
interms of cost. Technically and economically, overhead AC
transmissionprovides an advantageous solution.
In today electricity industry, in view of the liberalization and
increasedeffects to conserve the environment, HVDC solutions have
becomemore desirable for the following reasons:
Environmental advantages
Economical (cheapest solution)
Asynchronous interconnections Power flow control
Added benefits to the transmission (stability, power quality
etc.)
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The HVDC technology
The fundamental process that occurs in an HVDC system is the
conversion of
electrical current from AC to DC (rectifier) at the transmitting
end, and from
DC to AC (inverter) at the receiving end.
There are three ways of achieving conversion:
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HVDC can carry more power for a given size of conductor The need
for ROW (Right of Way) is much smaller for HVDC thanfor HVAC for
the same transmitted power. The environmental impact issmaller with
HVDC. Power losses are less with DC transmission, in addition the
absence of
skin effect is also beneficial in reducing the losses If cables
are preferred, dielectric losses are also less in DC transmission
The Corona effects significantly less in DC conductors than for AC
thisalso leads the choice of economic size of the conductors. The
costs of the compensation is also minimum Other investment costs
transmission towers, conductors, insulators,terminal equipment and
etc
Eco
nomicasp
ects
COMPARISON OF AC AND DC TRANSMISSION
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Cos
t
Distance (km)
Variation of costs with Line lengths
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Very fast control of power flow, which implies
stabilityimprovements, not only for the HVDC link but also for the
surroundingAC system.
Full control over the power transmitted The ability to enhance
transient and dynamic stability in associated AC
networks Direction of power flow can be changed very quickly
(bi-directionality). An HVDC link dont increase the short-circuit
power in the connectingpoint. This means that it will not be
necessary to change the circuit breakers in
the existing network. No limits in transmitted distance. This is
valid for both OH linesand sea or underground cables.
Tec
hnicalper
formance
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VSC technology allows controlling active and reactive
powerindependently without any needs for extra compensating
equipment. VSC technology gives a good opportunity to alternative
energy sourcesto be economically and technically efficient. HVDC
transmissions have a high availability and reliabilityrate, shown
by more than 30 years of operation.
Line compensation --- For maintenance of constant voltage at
both endsAC system requires reactive power control, which increases
with increase inlength.
Although DC converters require reactive power related to the
lineloadings, but the line itself does not require any reactive
power
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Problems of AC inter connection The presence of large power
oscillations which can lead to frequenttripping Increase in fault
level Transmission disturbances
Controllability of power flow in DC lines eliminates all above
problems Ground impedance -- the existence of ground current can
not bepermitted in AC, ground impedance negligible in DC
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Reliability of DC transmission is quite good.Reliability in DC
transmission due to the use of high power fastacting semiconductor
devices over mercury arc rectifiersThere are 2 measures of overall
system reliability1. Energy availability2. Transient
reliability.
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MODERN TRENDS
Power semiconductors and valves
Converter control
DC breakers
Conversion of existing AC lines
Operation with weak AC systems
