Series capacitors are used to compensate for line inductive reactance.
When electric power is transmitted over long distances, the transmission lines have inductive reactance due to the inductance of the conductors.
This inductive reactance causes the voltage at the receiving end of the line to be lower than the voltage at the sending end of the line, leading to voltage drop and power loss.
To compensate for this inductive reactance, series capacitors are often used.
Series capacitors are installed in series with the transmission line, and they provide a capacitive reactance that cancels out the inductive reactance of the line.
The capacitive reactance of the series capacitor has the opposite sign of the inductive reactance of the transmission line, and this results in an overall reduction in the total inductive reactance of the line.