Investment cost:
DC transmission requires fewer conductors than AC transmission – 2 conductors per DC circuit whereas three conductors per 3 phase AC circuit. HVDC allows line supporting towers to be smaller and, hence, requires lesser right-of-way. Thus, clearly, HVDC transmission line would cost lesser than an HVAC line. However, the terminal converter stations in HVDC are much more expensive which are not required for HVAC transmission. Over a specific distance, called as break-even distance, HVDC line becomes cheaper than HVAC. The break-even distance for overhead lines is around 600 km and for submarine lines, it is around 50 km.
Losses:
Skin effect is absent in DC. Also, corona losses are significantly lower in the case of DC. An HVDC line has considerably lower losses compared to HVAC over longer distances.
Due to the absence of inductance in DC, an HVDC line offers better voltage regulation. Also, HVDC offers greater controllability compared to HVAC.
Asynchronous interconnection:
AC power grids are standardized for 50 Hz in some countries and 60 Hz in other. It is impossible to interconnect two power grids working at different frequencies with the help of an AC interconnection. An HVDC link makes this possible.
Interference with nearby communication lines:
Interference with nearby communication lines is lesser in the case of HVDC overhead line than that for an HVAC line.
Short circuit current:
In longer distance HVAC transmission, short circuit current level in the receiving system is high. An HVDC system does not contribute to the short-circuit current of the interconnected AC system.
Controllability:
Due to the absence of inductance in DC, an HVDC line offers better voltage regulation. Also, HVDC offers greater controllability compared to HVAC.
Asynchronous interconnection:
AC power grids are standardized for 50 Hz in some countries and 60 Hz in other. It is impossible to interconnect two power grids working at different frequencies with the help of an AC interconnection. An HVDC link makes this possible.
Interference with nearby communication lines:
Interference with nearby communication lines is lesser in the case of HVDC overhead line than that for an HVAC line.
Short circuit current:
In longer distance HVAC transmission, short circuit current level in the receiving system is high. An HVDC system does not contribute to the short-circuit current of the interconnected AC system.
Final Conclusion:
HVDC transmission system has many more advantages over HVAC, such as stability, controllability etc.. For distances longer than the break-even distance, HVDC system becomes more cost-effective. Submarine HVDC links can be more suitable for connecting offshore wind farms as they prove to be more efficient and cost-effective than undersea HVAC cables. Hence, there is an increasing interest in HVDC transmission. Still, HVAC system will remain much longer as it has its own advantages in transmission and distribution, such as it can be easily stepped up and stepped down. HVDC is actually a compliment for AC systems rather than a rival.