Advantages and limitations of DC transmission

DC transmission (HVDC) carries power as direct current between converter stations. Compared with AC lines it has the following merits and demerits.

Advantages

1. Fewer conductors and cheaper line: Two conductors (bipole) or one (monopole) instead of three; smaller towers and right-of-way.
2. Lower losses: No skin effect, no reactive current flow, lower corona loss.
3. Less insulation: Insulation is designed for the DC voltage, while AC needs it for the peak ($\sqrt2V_{rms}$).
4. No charging current: Suitable for long submarine and underground cables.
5. No stability limit: No phase angle between ends, so distance is not limited by stability.
6. Asynchronous link: Connects grids of different frequency or not in synchronism.
7. Fast, accurate power flow control by converter firing angle.
8. No increase of fault level of the interconnected AC systems.
9. Better voltage regulation: No reactive drop ($IX$) on the line; no Ferranti effect.

Limitations

1. Costly terminal equipment: Converter transformers, thyristor valves, filters and smoothing reactors.
2. Harmonics: Converters produce harmonics needing AC and DC filters.
3. Reactive power demand: Converters absorb 50–60 % of rated power as reactive power, supplied by capacitors/filters.
4. Voltage cannot be transformed directly; no economical way to tap power along the line.
5. DC circuit breaking is difficult because there is no natural current zero.
6. Economical only beyond break-even distance (~600–800 km overhead, ~50 km cable).
7. Complex control and maintenance; converters have low overload capacity.