Introduction to HVDC transmission

The invention of effective dc air blast and SF6 circuit breakers and lightning arresters made the use of HVDC transmission possible. Today there are over 40 HVDC lines throughout the world.
Both ac and dc lines of same voltage level have the same transmission capability and can transmit the same amount of power. However, the dc line has two conductors rather than three in the case of ac line and thus requires two-thirds as many insulators. Therefore, the required towers and right-of-way are narrower in the dc line than the ac line. Also the power losses due to corona phenomena are smaller for dc than that for ac lines. Underground cables used for ac transmission can also be used for dc and they can normally carry more dc power than ac due to the absence of capacitive charging current and better utilization of insulation and less dielectric wear.

The major advantages are:

If the cost of converter station is excluded, the dc overhead lines and cables are less expensive than ac lines and cables.
A dc link is asynchronous.
The corona loss and radio interference are less.
For dc line, reactive power compensation is not needed. However, reactive power support will be required at both ends as explained later.
The line length is not restricted by stability.
The interconnection of two separate ac systems via a dc link does not increase the short-circuit capacity, and thus the circuit breaker ratings of either system.
The dc line loss is smaller than for the comparable ac line.

The major disadvantages are:

The converters generate harmonic voltages and currents on both dc and ac sides and therefore filters are needed.
The converter consumes reactive power.
The dc converter stations are expensive.
The dc circuit breakers are difficult to design.

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