Series capacitive compensation on EHV transmission lines is used to improve the stability of the system. This is achieved by reducing the reactive power flow and improving the voltage profile along the transmission line.

1. Purpose of Series Capacitive Compensation:
Series capacitive compensation is used to counteract the inductive reactance of the transmission line. EHV transmission lines have significant inductive reactance due to their length, which causes a drop in voltage and a phase shift in the power flowing through the line. By adding series capacitors, the capacitive reactance can be introduced to cancel out the inductive reactance and improve the power transfer capability of the line.

2. Reduction of Reactive Power Flow:
Series capacitors help to reduce the reactive power flow on the transmission line. Reactive power is the power associated with the exchange of energy between the electric and magnetic fields of the transmission line. By introducing capacitive reactance, the reactive power flow can be reduced, allowing more active power to be transmitted through the line. This improves the overall efficiency of the transmission system.

3. Improvement of Voltage Profile:
Series capacitors also help to improve the voltage profile along the transmission line. Due to the inductive reactance, voltage drop occurs along the line, leading to a lower voltage at the receiving end. By introducing series capacitors, the capacitive reactance cancels out the inductive reactance, reducing the voltage drop and improving the voltage profile. This ensures that the voltage at the receiving end is closer to the voltage at the sending end, maintaining a stable and reliable power supply.

4. Enhancement of System Stability:
By reducing the reactive power flow and improving the voltage profile, series capacitive compensation contributes to the overall stability of the power system. Reactive power flow and voltage fluctuations can lead to instabilities in the system, causing voltage sags, flicker, and other power quality issues. By mitigating these problems, series capacitors improve the stability of the system and ensure the reliable transmission of power.

In conclusion, series capacitive compensation on EHV transmission lines is used to improve the stability of the system by reducing reactive power flow, improving the voltage profile, and enhancing the overall efficiency of the transmission system.