Purpose of providing Reactive Compensation:The purpose of providing reactive compensation is to change the natural electrical characteristics of a transmission line. Shunt connected capacitors are used to maintain the voltage of the transmission system at desired levels during the loaded condition whereas shunt connected reactors are employed to reduce line overvoltages under light load or no-load conditions.
The voltage sag is largest at the midpoint for an uncompensated transmission line. Thus, the optimum location to place a shunt compensator is at the midpoint. Also, the compensator at the midpoint segments the line into two equal sections for each of which the maximum transmittable power is the same. The midpoint shunt compensation can significantly increase the transmittable power but the reactive power demand on the compensator increases rapidly with the increase in transmitted power.
Multiple shunt compensators can be placed on the transmission line located equidistance from each other. With the increase in number of compensators and hence the segments of the line, the voltage profile of the line approaches a flat profile. Theoretically the power which can be transferred over a transmission line doubles with each doubling of the segment. Such a system however would become too complex and costly.
Machine Angle Oscillation in Under-damped Power System:In an under-damped power system, a small disturbance may cause the machine angle to oscillate around its steady-state value at the natural frequency of the entire electro-mechanical system. This oscillation of angle results in a corresponding sustained power oscillation. By appropriate variations in the shunt reactive compensation and hence in the voltage of the transmission line, the accelerating and decelerating swings of the disturbed machine or machines can be neutralized.
When the oscillating generator accelerates and the machine angle increases, the electric power transferred must be increased to compensate for the excess mechanical input power. On the other hand the power transmitted must be reduced to counter the reduced mechanical input when the generator decelerates and the angle decreases.
Thus, with appropriate and fast controls, shunt reactive compensation is able to change the power flow in a system during and following dynamic disturbances. This increases the transient stability and damps the power oscillations. Hence reactive or VAr compensation is used for voltage regulation at the midpoint or at some intermediate locations in the transmission line and at the end of a radial transmission line to avoid voltage instability, to increase transient stability and to damp power oscillations.
Ref: Hingorani and Gyugyi, “Understanding FACTS”, John Wiley & Sons, UK