DESCRIPTION OF DC TRANSMISSION SYSTEM

DESCRIPTION OF DC TRANSMISSION SYSTEM.

Bipolar link has two conductors, once positive and the other negative.Each may be a double conductor in EHV lines. Each terminal has two sets of converters of identical ratings,in serieson th DC Side. The junction between the two sets of converters is grounded  at one or both ends.Normally,both poles operate at equal currents and hence there is zero ground current flowing under these conditions.

                                 Homopolar link has two or more conductors all having the same polarity (Usually negative) and always operated with ground or metallic return. 

      Because of the desirability of operating a DC link with out ground return, bipolar links are most commonly used. Homopolar link has the advantage of reduced insulation costs, but the disadvantages of earth return outweight the advantages of earth return outweight the advantages. Incidentally, the coronaeffects in a DC line are substantially less with negative polarity of the conductor as compared to the positive polarity. 

                The monopolar operation is used in the first stage of the development of a bipolar line, as the investments on converters can be deferred until the growth of load which requires bipolar operation at double the capacity of a monopolar link.