Harmonics in a 3-Phase Load with Open Neutral

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As far as third harmonic currents are concerned, they cannot be present in the line currents if the neutral is not grounded. A neutral connection to the ground is very essential for third harmonic currents to be present in the line.

In star connected three phase system, under balanced conditions, vector sum of three phase currents will be zero i.e, Ir+Iy+Ib=In=0. Also, magnitude of zero sequence current Ir0=Iy0=Ib0=1/3(Ir+Iy+Ib)=1/3*In. In=0 in a three phase star connected system without neutral return path . Hence zero sequence current is equal to zero. Since third harmonic currents have zero phase sequence, they cannot appear in line or phase currents in an unbalanced/balanced 3 phase star connected system.

I've the following system: 240VAC -> DC Power Supply -> 3ph Inverter -> 3ph Permanent Magnet Brushless DC Machine (with star-connected windings). The motor is operated as BLDC drive (i.e. six-step PWM current control). I test the system with two control algorithms, i.e. CA and CB, where CA is conventional PWM current control, and CB is modifed PWM current control. I measure the phase current and perform the FFT of the phase current using Agilent 35670A Dynamic Signal Analyzer. When CA is applied, FFT of the phase current shows 1st, 5th, 7th, 11th, 13th, etc harmonics. However, when CB is applied, some even harmonics (e.g. 2nd, 4th, 8th, etc) and triplen harmonics (e.g. 3rd, 9th, etc) appear. These harmonics are not obvious if the FFT spectrum uses "linear magnitude" scale. When the FFT spectrum is shown in "log magnitude" scale, these harmonics become quite significant. Since the neutral point of the motor is not shorted to ground, I wonder how these triplen harmonics can appear. Kindly advise. Thanks.