Voltage profile along the line is flat only if R= Zc is connected at receiving end (SIL). Under this situation, there is no demand of reactive power from either sending or receiving ends. Loads are decided by consumers and not by a system operator. As such, this exact condition is practically never met although it is ideal.

If voltage at both ends is maintained at 1.0 pu (say, by controlling excitation of generators connected at both ends), the voltage tends to sag as we move towards the midpoint if Ps > SIL. The line absorbs reactive power.

If Ps < SIL, voltage swells and the line generates reactive power.

For a line which has one end maintained at 1.0 pu but kept open-circuited at the other end, overvoltage will be evident at the open end.

Cables have very high SIL (why?). The current at SIL usually exceeds the thermal rating of a cable. Therefore a cable is invariably loaded below its SIL. Cables, therefore, generate reactive power and usually voltages can be very high at low loading levels, especially for long lengths. Besides this, cables are much costlier than overhead lines and are more "unforgiving" towards overloads. Therefore, cables of lengths exceeding 30-40km are rarely used for ac transmission.

Due to voltage problems (and stability problems, which we shall study in the following lecture), underwater cable transmission and bulk transfer over very long distances is carried out using DC transmission technology. We shall study the essentials of this technology later in the course.