Advantages and disadvantages of HVDC over AC

HVDC (High Voltage Direct Current) transmission sends bulk power as DC between two converter stations. Compared with HVAC it is cheaper and better for long lines and cables, but its converter stations are costly and complex.

Advantages of HVDC over AC

• Fewer conductors: a bipolar line needs only 2 conductors (3 for AC), so towers are lighter, the right-of-way is narrower and the line costs less.
• No stability limit on distance: there is no reactance or power angle on DC, so power transfer does not fall with length. Any distance can be used.
• No charging current: long cables (submarine or underground) can be used, while AC cables are limited to about 40–60 km.
• Lower losses: no skin effect, no reactive current, and less corona loss for the same voltage.
• Asynchronous link: it joins systems of different frequency (50/60 Hz) or unsynchronised grids, and does not raise their fault level.
• Fast power control: power flow is set by the converter firing angle in milliseconds, which helps damp oscillations.
• Lower insulation level: insulation must withstand the peak voltage. On DC the peak equals the working voltage, while on AC the peak is $\sqrt{2}$ times the rms value, so for the same power DC needs less insulation.

Disadvantages

• Costly converter stations (thyristor/IGBT valves, transformers, filters), so HVDC pays only beyond the break-even distance.
• Harmonics are produced; AC and DC filters are needed.
• Reactive power demand: line-commutated converters absorb 50–60% of the DC power as reactive power.
• DC circuit breakers are difficult to build because DC current has no natural zero.
• No transformers on DC: voltage cannot be stepped up or down easily, and tapping power along the route (multi-terminal) is hard.
• Converters have a low overload capacity.