Page 73 - Practical Power System and Protective Relays Commissioning

70 Practical Power System and Protective Relays Commissioning

FIGURE 6.1 Common representation of overhead transmission line.

FIGURE 6.2 Loading of overhead transmission lines limit with line length and voltage level.

lines. The Ferranti effect is also associated with long lines; this means the
reactive power in transmission line due to the line capacitance in a no-load
condition; the charging current of this capacitance causes the rise of the volt-
age at the open receiving end of the line which can cause damage of break-
ers, surge arresters, current and voltage transformers, and transformers at the
receiving end of the line. This problem needs to be compensated by using
shunt reactors. The Ferranti effect can also exist in cables and requires shunt
reactors to compensate. Refer to Fig. 6.3.

6.3 UNDERGROUND CABLES

Underground cables are more expensive than overhead lines; they also have
a high capacitance that absorbs charging current that must be supplied by
generators of the system, which is an additional cost. Underground cables
are usually used in very populated areas, or underwater.