High 3rd harmonic on the neutral

Consider a 3-phase DB with 3 equally-loaded single-phase resistive circuits currently active (say a heater in each of 3 rooms). The individual SP circuit neutrals will of course take the full current of one heater. Where the 3 neutrals join and become a single neutral at the DB's N bar, it becomes more interesting. At the moment in time where L1 is at the peak of the cycle, there is a current I1 say flowing back from the L1 heater. Consider what is happening to L2 at this exact moment. The voltage (and hence current, for a resistive load) are delayed by 120 degrees compared to L1. So when L1 is at its positive peak, L2 is at -0.5 of peak, so I2 = -0.5I1. Similarly, I3 = -0.5I1 at this moment in time. So there is a big current flowing back into the neutral bar from the L1 heater, and two half-sized currents flowing from the neutral bar out to L2 and L3 heaters. So there's a high current on the neutral bar, but zero current flowing in or out of the DB's supply N.


Over time, the voltages and hence currents on L1,L2,L3 vary in a sine wave, but at all times the sum of the 3 neutral currents is zero.

Consider a 3-phase DB with 3 equally-loaded single-phase resistive circuits currently active (say a heater in each of 3 rooms). The individual SP circuit neutrals will of course take the full current of one heater. Where the 3 neutrals join and become a single neutral at the DB's N bar, it becomes more interesting. At the moment in time where L1 is at the peak of the cycle, there is a current I1 say flowing back from the L1 heater. Consider what is happening to L2 at this exact moment. The voltage (and hence current, for a resistive load) are delayed by 120 degrees compared to L1. So when L1 is at its positive peak, L2 is at -0.5 of peak, so I2 = -0.5I1. Similarly, I3 = -0.5I1 at this moment in time. So there is a big current flowing back into the neutral bar from the L1 heater, and two half-sized currents flowing from the neutral bar out to L2 and L3 heaters. So there's a high current on the neutral bar, but zero current flowing in or out of the DB's supply N.


Over time, the voltages and hence currents on L1,L2,L3 vary in a sine wave, but at all times the sum of the 3 neutral currents is zero.