In order to prevent over voltages at light loads, it is necessary to have devices for absorbing reactive power (like shunt reactors) not only at either end of a long line but even at intermediate points. Generators connected at the ends of the line have limited reactive power absorption capability as defined by their capability curves. If transmission redundancy exists (i.e., parallel transmission paths exist), then a very lightly loaded long line may be tripped to avoid overvoltage. However this may be detrimental to system security if some additional line trippings take place due to faults. If shunt reactors are permanently connected, they result in large sags in the voltage under heavy loading conditions. Moreover, reactive power demanded by long transmission lines under these situations may be excessive and may lead to system-wide low voltage conditions.
Compensation of a line involves changing the effective line parameters by connecting (lumped) capacitors in series and shunt. These compensating elements effectively reduce the line reactance and increase the shunt susceptance, thereby decreasing the surge impedance. Thus the effective SIL of a capacitor compensated line is higher than an uncompensated line. This increases the loadability of a long line.
Since total conductor cross-sectional area for EHV lines is mainly decided by electric field considerations (corona), these lines have large thermal capabilities, much in excess of the SIL. For long EHV lines, one cannot deviate much from SIL due to voltage constraints. Therefore, the thermal limit of a long EHV line is not the key limiting factor. However, thermal limit is the main limiting factor for short lines (< 100 km) wherein voltage constraints are not violated even for large deviations from SIL.