# Deciding Factor for Selection of HVDC over HVAC Transmission

This is a famous question which most of the people ask to an Electrical Engineer that “Suppose we are setting up a power plant so shall we use HVDC or HVAC Transmission?” In this post I am going to answer this question with examples which will help to clear the concept. We have already seen the advantage of HVDC over HVAC in post

The answer to this question lies in Economic Distance for HVDC Transmission. The cost per unit length of HVDC is much less when compared to AC Transmission for the same power capacity but the cost of terminal equipments used in HVDC like converters and Inverters are much higher which makes HVDC uneconomical. Now, as the length of line increases the cost of Transmission line for AC will also increase to a greater extent when compared with HVDC, therefore we can guess that there must be a specific distance beyond which the AC Transmission will be more costly.

This calls for a graph between the cost of AC Transmission Vs Line length and similarly cost of HVDC Transmission and Line length.

As clear from the graph, the vertical intercept of each line (line length = 0) is the cost of Terminal equipments like Transformer for HVAC and Converters & Inverters for HVDC. It is clear that for line length less than 800 km, HVDC is more costly.

It can be seen from the graph that there is a point corresponding to line length= 800 km where the cost of HVAC and HVDC are equal but beyond 800 km the cost of HVDC is less compared to HVAC. This 800 km is called Break even distance. Thus for transmission more than 800 km, it is cheaper and efficient to use HVDC.

Thus it can be seen that length of Transmission decides whether HVDC will be cheaper or HVAC.

Thank you!