In parallel a resonance, the impedance of a circuit is high. In an ideal resonance (the circuit does not have any resistance) impedance becomes infinitely high, which leads to extremely high overvoltage. At parallel resonance frequency, the voltage obtains its highest possible value at a given current. Parallel resonance can occur when a source of a harmonic current is connected to the electrical circuit that can be simplified as a parallel connection of inductive and capacitive component . In an extreme case, even a relatively small harmonic current can cause destructively high voltage peaks at resonance frequency. Parallel resonance is common when there are capacitor banks or long AC lines connected with large transformers. In this case, large capacitances and inductances start to resonate with each other.