Different types of dc link in direct current transmission

DIFFERENT TYPE OF DC LINKS:

How many types of dc links? 

Direct-current links are classified as follows

1. Mono polar dc link
2. Bipolar dc link
3. Homo polar dc link

1) Mono polar dc link: The monopolar link has one conductor usually of negative polarity, and ground or sea return.

Fig :Mono polar dc link

2) Bipolar dc link: The bipolar link has two conductors one positive, the other is negative. Each terminal has  two converters of equal rated voltages in series on the dc side.  The midpoint (junction between converter) are  grounded at one or both ends. neutrals are grounded he two poles can operate independently .  Normally they operate at equal current;  then there is no ground current.  In the event of a fault on one conductor,  the other conductor with ground return can carry up to half of the rated load. The rated voltage of a bipolar link is expressed as 100kv or pronounced plus and minus 100 kV

Fig: Bipolar dc link

3)The homopolar link: the homopolar link has two or more conductors all having the same polarity,  usually negative,  and always operates with ground return. 

Fig: homopolar link

In the event of a fault on one conductor,the entire converter is available for connection to the remaining conductor or conductors,  which,  having some overload capability,  can carry more than half of the rated power,  and perhaps the whole rated power,  at the expense of increased line loss.  In a bipolar scheme reconnection of the whole converter to one pole of the line is more complicated and is usually not feasible becausc graded insulation.  In this respect a homopolar of line is preferable to a bipolar line in cases where continual ground current is not deemed objectionable

Reference:direct current transmission by kimbark