How Triplen Harmonic Currents add in the Neutral

Figure 1 shows currents on the phases of a 3-phase,4-wire system return on the neutral conductor. The 120° phase shift between linear load currents will result in their balanced portions instantaneously canceling in the neutral. With linear loads, the neutral can be the same size as the phase conductors because the neutral current cannot be larger than the largest phase current, even when the load is completely unbalanced.

Figure 1

Zero SQ Filter off-loads the neutral conductor

Zero SQ Filter, is a parallel-connected, electromagnetic, zero sequence filter that removes 3rd and 9th harmonic currents as well as other zero sequence currents (lo) from the neutral conductor. The Zero SQ Filter presents a very low impedance alternate path for the neutral current to return to the phases because the windings of the Zero SQ Filter are configured to cancel the net flux created by the diverted neutral current. Neutral current, neutral-to-ground voltage, voltage distortion, and supply transformer losses are lowered because there is only a residual amount of 3rd harmonic and other zero sequence currents left in the neutral and phase conductors between the transformer and the Zero SQ Filter connection point. Consequently a double ampacity neutral is not required between the supply transformer and the Zero SQ Filter connection point.

Figure 2

Summary

Zero SQ Filter

Parallel connected on 3-ph, 4-wire systems to divert the flow of triplen harmonic currents (3rd,9th, & 15th) away from the neutral conductor and upstream   Best results are achieved when installed at a downstream power panel (fed from a 3 pole branch CB) near the harmonic generating loads   The phase current returned by the Zero SQ Filter will be equal to 1/3 the neutral current drawn and must be protected accordingly. e.g., a 100A 3 pole circuit breaker is used to protect a 300A Zero SQ Filter   On installations where neutral current could be drawn from upstream side of the Zero SQ Filter (i.e. multiple subpanels) or where the downstream load is expected to grow, the Field Adjustable Impedance (FAI) option should be selected.

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