If you mean the shunt reactor installed in serise withe capacitors, the role of this reactor is to protect the capacitor by preventing the harmonic to go in the capacitor that called ” Detuned Capacitos “, because the reactor with capacitor in serise connection are composed a ” Serise Resonance Circuit ” where it hase a resonance frequency. So, when we will install the detuned capacitors in a polluated network, we should know all harmonics that we have in this network, then choicing of the detuned capacitors will be done accordingly to the following condition :
The frequency resonance of the detuned capacitor should be absolutly smallest than the smal harmonic frequency
But if you mean the reactor of a filter, this reactor with the capacitor in serise connection composed a ” Filter ” to eliminate a special harmonic, that means if we have in a network the 5th harmonic with a big value of current and we decide to eliminate it, we should install a Filter where his resonance frequency should be approximatly 250Hz. In reality, the resonance frequency for this filter is 240 Hz, because if we use exactly the same value we will have a big short circuit current for this frequency.