Define Series VAr Compensation?

For very long transmission lines, the inductive reactance of the line becomes so high that not much power can be transmitted through the line, see equation.

P = (V_s V_R)/ X sin δ

Therefore, capacitors are connected in series with the line conductors and are used to reduce the inductive reactance and hence increase the power transmission capability of the line and reduce the voltage drop. Depending on the compensation level (k), the transmitted power given by equation (20) is modified to:

P = (V_S V_R) / X (1-K) sin δ

Where the compensation level  (k)  is  X_capacitor/ X_line

A major drawback is that short-circuit currents cause excessive voltages across the capacitors and special protective devices such as spark gaps are required to protect them. In addition, series capacitors increase the system fault level.