How many types of DC links are available in a HVDC system?

Monopolar link: It has a single conductor of negative polarity and uses earth or sea for the return path of current. Sometimes the metallic return is also used. In the Monopolar link, two converters are placed at the end of each pole.

Bipolar link:

• The Bipolar link has two conductors one is positive, and the other one is negative to the earth.
• The link has a converter station at each end. The midpoints of the converter stations are earthed through electrodes.
• The voltage of the earthed electrodes is just half the voltage of the conductor used for transmission of the HVDC.
• It has two independent circuits and can be operated as a monopolar link in an emergency.

Homopolar link: It has two conductors of the same polarity usually negative polarity, and always operates with earth or metallic return. In the homopolar link, poles are operated in parallel, which reduces the insulation cost.