AC three-phase system can have positive, negative and zero sequence currents when the phases are not balanced. Likewise, the third harmonic currents can have positive, negative and zero sequence currents. When the third harmonic current in each phase is balanced and displaced by 120o, the current is positive sequence. When the third harmonic currents are balanced and in phase with each other, the current is zero sequence. In most three phase systems the third harmonic current produced are closer to being in-phase, but with a mentioned slight unbalanced (%10) in magnitude and phase angle, whole third harmonic of currents are not zero sequence and can not trapped by interposing delta connection completely.