Impedance coil is the coil have constant electrical impedance (have no steel core L >> R), used to limit short-circuit current while maintaining a certain voltage value when there is a change in current electricity. 

The impedance coil is used for the compensate capacitor with the function of protecting the capacitor, switch devices, and the relay compensation. The impedance coil  combined with a compensate capacitor eliminates harmonic components to increase the electricity quality for the system. The harmonic filter coil is essential in cases where the system voltage and electric current are greatly distorted.