DC transmission has some advantages over AC transmission, but it cannot completely replace ac transmission system due to some limitations
High Cost:
- The converters required at both the ends of the line are much more expensive than conventional equipment.
- The converters produce lot of harmonics both on dc and ac sides which may cause interference with the audio frequency communication lines. Filtering and smoothing equipments are provided at the converter station in order to remove ripples from the dc output. It is also possible to provide filters at the ac side to absorb the harmonic current. These considerably increases the cost of converter substation
- HVDC converters need complex cooling system
- Cost of HVDC circuit breaker is higher compared to ac breakers
- Converters have very little over load capacity and absorb considerable amount of reactive power.
Maintenance of insulators in HVDC transmission line is more Circuit breaking in multi-terminal dc systems is difficult and costlier
Therefore because of the cost factors HVDC transmission is economical if bulk amount of power (1500-6000MW) transmitting over large distance (beyond 500kms) compared to HVAC transmission