Harmonics in 3-phase systems with no neutral

Question

I have an EV charge point that is a 3-phase 4 wire system that comprises of no neutral (L1/L2/L3/PE). As per the datasheet, the third harmonic content of the line current is rated as approx. 8% (THDi &le;8%). 
 Question: with no neutral, where does the current arising from the third harmonic content of the line current flow? My best guess would be through the protective conductor &ndash; which is not ideal as I&rsquo;m trying to prevent unwanted tripping of the upstream 30mA RCD device. 
 thanks for the help.

mapj1 · Accepted Answer

One of the nice or nasty things about 150Hz, is that is looks the same on all 3 phases, so if the 3f zero crossing is say 10 degrees after the 50Hz one, on one phase it will also be on the other 2 phases. So if the loads on all 3 phases were exactly equal, the 3 F, 9F components etc would all cancel. However this does not mean that you need a neutral to have distortion to the line waveform that gives a 150Hz repeating feature. In fact the 3 phase/ 6 diode bridge does it depressingly well because in reality the 6 diodes are never quite equal. So the answer is that the 3 F current in any one phase is imperfectly cancelled by the nearly corresponding 3F currents in the other two, and when the 150Hz current is is 5-10 percent of the 50Hz RMS current then that 3F component is is 30-20 dB down and is about the sort of thing you get with no special precautions taken. I would expect any 3F currents flowing in the CPC to be a lot lower. 
 Mike.

Harmonics in 3-phase systems with no neutral

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