However, as the order of the harmonics (frequency) increases, the inductive reactance increases while the capacitive reactance decreases.
At a particular frequency of harmonics, the inductive and capacitive reactances become equal and resonance sets in. Resonance can cause erratic conditions in the power systems such as transient high or low voltages, unexplained breakdown of equipment such as failure of
transformer windings or failure of capacitor. Transient voltages generated by such harmonics resonance can also result in unexpected operation of relays and breakers.The phenomenon of Harmonic Resonance should be borne in mind when modifying the system to add capacitors to improve the power factor or when adding new inductive loads such as motors, reactors, or transformers to existing systems containing capacitors.
The formula to determine the order of harmonic which may cause resonance is
where MVAsc is the impedance of the source and MVARcap is the reactive power drawn by the capacitors.Thus when a 20 MVAr capacitance is connected across a source of 1000 MVA, there will be a condition of resonance at the 7th Harmonic.
The possibility of harmonic resonance should be explored and eliminated during any modification/addition of loads in the power system.