Distinguish between HVDC and HVAC transmission?

Answer 1

DC versus AC.

The vast majority of electric power transmissions use threephase

alternating current. The reasons behind a choice of

HVDC instead of AC to transmit power in a specific case

are often numerous and complex. Each individual

transmission project will display its own set of reasons

justifying the choice.

A. General characteristics

The most common arguments favouring HVDC are:

1) Investment cost. A HVDC transmission line costs

less than an AC line for the same transmission

capacity. However, the terminal stations are more

expensive in the HVDC case due to the fact that

they must perform the conversion from AC to DC

and vice versa. On the other hand, the costs of

transmission medium (overhead lines and cables),

land acquisition/right-of-way costs are lower in the

HVDC case. Moreover, the operation and

maintenance costs are lower in the HVDC case.

Initial loss levels are higher in the HVDC system,

but they do not vary with distance. In contrast, loss

levels increase with distance in a high voltage AC

system

Above a certain distance, the so called "break-even

distance", the HVDC alternative will always give

the lowest cost. The break-even-distance is much

smaller for submarine cables (typically about 50

km) than for an overhead line transmission. The

distance depends on several factors, as transmission

medium, different local aspects (permits, cost of

local labour etc.) and an analysis must be made for

each individual case (Fig. 3).

2) Long distance water crossing. In a long AC cable

transmission, the reactive power flow due to the

large cable capacitance will limit the maximum

transmission distance. With HVDC there is no such

limitation, why, for long cable links, HVDC is the

only viable technical alternative.

3) Lower losses. An optimized HVDC transmission line

has lower losses than AC lines for the same power

capacity. The losses in the converter stations have of

course to be added, but since they are only about 0.6

% of the transmitted power in each station, the total

HVDC transmission losses come out lower than the

AC losses in practically all cases. HVDC cables also

have lower losses than AC cables.

4) Asynchronous connection. It is sometimes difficult or

impossible to connect two AC networks due to

stability reasons. In such cases HVDC is the only

way to make an exchange of power between the two

networks possible. There are also HVDC links

between networks with different nominal

frequencies (50 and 60 Hz) in Japan and South

America.

5) Controllability. One of the fundamental advantages

with HVDC is that it is very easy to control the

active power in the link

6) Limit short circuit currents. A HVDC transmission

does not contribute to the short circuit current of the

interconnected AC system.

7) Environment. Improved energy transmission

possibilities contribute to a more efficient utilization

of existing power plants. The land coverage and the

associated right-of-way cost for a HVDC overhead

transmission line is not as high as for an AC line.

This reduces the visual impact. It is also possible to

increase the power transmission capacity for

existing rights of way. There are, however, some

environmental issues which must be considered for

the converter stations, such as: audible noise, visual

impact, electromagnetic compatibility and use of

ground or sea return path in monopolar operation.

In general, it can be said that a HVDC system is

highly compatible with any environment and can be

integrated into it without the need to compromise on

any environmentally important issues of today.

Answer 2

p[o-0

HVDC refers to High Voltage Direct Current - power transmission lines.

HVAC refers to Heating Ventillation and Air Conditioning.