From Wikipedia -In electrical and electronic systems, reactance is the opposition of a circuit element to a change in current or voltage, due to that element’s inductance or capacitance.
s the transmission lines(AC) posses active and reactive power.
Reactive power which is undesirable and consumed by Capacitance and reactance.
We basically deduce such reactive power,which influenced to power losses in system.
These passive elements form reactance in the system by virtue of reactive power of the system.