Page 83 - Practical Power System and Protective Relays Commissioning

80 Practical Power System and Protective Relays Commissioning

10. No stability problems in DC transmission as there are no constraints on
line length.
11. This system is economic for long transmission lines.
However, there are still some limitations or disadvantages of using DC
transmission:
1. High costs of inverters and converters station at the line ends.
2. Harmonics are generated and these harmonics can interfere with the tele-
communication equipment, which lead to using filters which add to the
cost of a DC transmission system.
3. There is no zero crossing point in DC voltage , which requires advanced
technology to produce the DC circuit breakers. Which means that in
some cases the DC transmissions are done without the use of DC circuit
breakers.

6.5.2 Construction of DC Transmission

DC transmission can be configured in monopolar, bipolar, or homopolar
links. These are discussed below.

6.5.2.1 Monopolar links
Uses one conductor and the return path is provided by ground as shown in
Fig. 6.16.

6.5.2.2 Bipolar links
Each terminal has two converters of equal rated voltage, connected in series
on the DC side. Junctions between the converters are grounded. The

FIGURE 6.16 Monopolar link.