When was the first high-voltage direct current power transmission system developed?

As technology advanced, the first high voltage DC transmission system was implemented in the 1950s via the development of rectifier stations with mercury arc values. Now, HVDC power transmission is more efficient than AC transmission after the break-even distance of 600km because it suffers much less voltage drop/line losses than AC systems at the same voltage. Thus, even though HVDC systems are more expensive and complex than AC transmission systems, they make up for their capital cost with lower operating costs due to their efficiency (after the stated break-even distance). Read more about this on our blog.