hvdc Chp 1
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Transcript of hvdc Chp 1
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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