HIGH VOLTAGE DIRECT CURRENT TRANSMISSION LINES (HVDC)


SPOTLIGHT: HIGH VOLTAGE DIRECT CURRENT (HVDC) TRANSMISSION LINES

During the current wars (AC vs. DC) days, it was the economics of transmitting power in high voltage and low through power transformers sealed the fate in favor of AC power transmission. But nowadays, Direct Current is fighting back. Or that least, it is covering what AC power transmission cannot.

Most if not all of the bulk transmission of powers are in AC. That was up until 1954, the firstHVDC (10MW) transmission system was commissioned in Gotland. It is of important to note that as early as 1941; a proposed 60 MW HVDC link in Germany did not fully materialize due to war.

HVDC is favorable to AC transmission on the following reasons:

. No technical limit to the length of a submarine cable connection.
. No requirement that the linked systems run in synchronism.
. No increase to the short circuit capacity imposed on AC switchgear.
. Immunity from impedance, phase angle, frequency or voltage fluctuations.
. Preserves independent management of frequency and generator control.
. Improves both the AC system’s stability and, therefore, improves the internal power carrying capacity, by modulation of power in response to frequency, power swing or line
rating.

The advantages of using HVDC Transmission:

1.The cost of d.c. transmission line is less than 3 - phase a.c. line because only two
conductors are necessary for D.C. line.
2.Tower designs are simple.
3.The dielectric strength of cable is high .
4.The dielectric loss is low.
5.For D.C. overhead transmission lines length is unlimited.
6.Power transmission capacity is higher than a.c.
7.Corona & radio frequency interference losses are less.
8.HVDC link has accurate & quick control of power in the required direction

The Limitations of HVDC Transmission:

1. Transformer for step up – step down voltages are not available in case of HVDC.
2. The terminal equipment is costly.
3. Reliable d.c. ckt. Breakers for higher ratings are not available. (yet)
4. Earth current may cause some side effects.
5. Reactive MVA cannot be transferred over a HVDC link.
6. Although inverters are used, the wave farm of output a.c. is not exactly sinusoidal and
it contains harmonic distortion

One of the most important considerations in any project is the cost. The best of to explain the relationship or comparison between AC and HVDC is this;

HVDC has high const in the construction of its terminal points compared to ac. But as the transmission lines approaches infinite length, which hypothetically means the longer the transmission lines are, its cost would become lesser compared to ac.

Considerable amount or research and development are still being done to improve transmission of bulk power in dc as of the moment. Me for one hopes to be involved in an HVDC project someday.

For details on the topic HVDC Transmission of power, LINKS and RESOURCES of the topic can be seen below.

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