Comparison Of HVAC And HVDC Transmission Network

HVDC Power Cable

HVAC vs HVDC

Electrical power needs to transmit a distance from it's generation station to transmition and distribution between users. Initially transmission system born as DC system, but finally stopped it and grew as AC system. Now again rethining and developing HVDC for power transmission.

The feature comparisor between HVDC and HVAC is as below:


Investment cost:
Considering the power carrying conductors, DC transmission requires fewer conductors than AC transmission - 2 conductors per DC circuit whereas three conductors per 3 phase AC circuit. 

HVDC needs smaller tower for line support thus, HVDC transmission line would cost lesser than an HVAC line. 

But, the terminal converter stations in HVDC are much more expensive which are not required for HVAC transmission. 

Over a certain line length what is called break-even distance, HVDC line becomes cheaper than HVAC. The break-even distance for overhead lines is around 600 km and for submarine lines it is around 50 km.

Losses:
There is no any skin effect losses in DC system. Corona losses also significantly lower in the case of DC system comparing AC system. An HVDC line has considerably lower losses compared to HVAC over longer distances.

Controllability:
There is better voltage regulation in HVDC system due to absence of inductance. Also, HVDC system offers greater controllability compared to HVAC.

Asynchronous interconnection:
We worldwide there are 50 Hz or 60Hz AC system is existed, it is not possible to interconnection between different frequency 50 Hz and 60Hz AC power grids. But in DC system there is no issue about frequency.

Interference with nearby communication lines:
Interference with nearby communication lines is lesser in the case of HVDC overhead line than that for an HVAC line.

Short circuit current:
In longer distance HVAC transmission, short circuit current level in the receiving system is high. An HVDC system does not contribute to the short circuit current of the interconnected AC system.

Most powerful underground power transmission cable system- 640 kV extruded HVDC cable

The world recognized cable manufacturer NKT claims that their 640 kV extruded DC cable system is the latest result of the intensive investment in research and development in the field of HVDC transmission technology. It sets a new world record for extruded HVDC cable technology enabling the integration of renewable power and connecting energy markets.

They said by using cables, efficient power delivery can also be made through densely populated or environmentally sensitive areas. A single pair of 640 kV extruded HVDC cables could for example transmit enough green power to supply three million households.

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