Page 74 - Practical Power System and Protective Relays Commissioning

Transmission Lines Theory Testing and Commissioning Chapter | 6 71

FIGURE 6.3 Ferranti effect voltage rise at line receiving end on open circuit condition.
Several types of high-voltage underground cables are used, for example,
oil field type, cross-linked polyethylene (XLPE) cables, and SF6 Cables.
There are some factors that affect the selection of cable types as follows:
1. Load characteristic of the cable.
2. Voltage rating of the cable which includes:
a. Operating voltage V (line to earth) voltage during normal operation of
the cable.
b. Insulation voltage V 0 (Line to Line) voltage of the cable.
c. In earthed networks V 5 O3 3 V 0 .
d. In nonearthed networks, V should not exceed 1.7 3 O3 3 V 0 .
3. Cable cross-section area should be chosen based on the following factors:
a. Current carrying capacity.
b. The value and the duration of overloading.
c. The permissible voltage drop through the cable.
d. Shortcircuit currents.
e. Requirements of cable termination sealing end.
There is a classification for cable types based upon voltage levels as
follows:

1. Low-voltage cables: up to 1000 V
2. Medium-voltage cables: 1 33 kV
3. High-voltage cables: 66 400 kV
4. Extra-high-voltage cables:400 750 kV
There is also a classification for cable types based on insulation type as
follows:
1. Cables insulated with oil-filled paper.
2. Polyvinyl chloride (PVC) cables.
3. XLPE cables.