By hvhipot 
In AC circuits with active resistors, inductors, and capacitors, the current intensity will be maximum when connected in series, limited only by the active resistor. Therefore, in a series circuit where the inductance and capacitance resistance are equal, series resonance (also known as frequency conversion resonance) occurs. Series resonance in power systems can sometimes unexpectedly occur, leading to the fact that overvoltage can occur in certain installations, which can be several times higher than the operating voltage.
The resonance phenomenon is the voltage across the inductor and the voltage across the capacitor, that is, the voltage division in the circuit can obtain very large values, many times higher than the voltage of the current series resonance. If the active resistance of the circuit is very small, the current intensity in the circuit should increase significantly; If there is no active resistor in the circuit, the minimum voltage is sufficient to generate infinite current in resonance. Furthermore, it is evident that the phase angle is zero. Therefore, due to the mutual compensation of inductive and capacitive resistance, the circuit during series resonance exhibits pure active resistance relative to the external environment.
Due to the parallel connection of active resistors, inductive resistors, and capacitive resistors, and the absence of phase shift between current and voltage at circuit terminals, current resonance occurs. Those. Their inductive and capacitive resistance in the circuit are equal, and they reach their minimum value when connected in parallel with the active current in the circuit. Due to the mutual compensation of inductance and capacitance conductivity, resonance occurs. Therefore, in this case, the circuit exhibits pure active conductivity relative to the outside, and the phase angle of current resonance in the main circuit is zero.
It should be noted that in the case of resonance, the current in the inductor and capacitor may exceed the total current of the circuit, and sometimes even exceed the total current. Through current resonance, the energy of the inductive magnetic field is converted into the energy of the capacitor electric field, and vice versa, and the energy from the series resonance is only consumed in the active resistor.