Zero Sequence Current - Homopolar current Relay Transformer Feeder

[LMSANZ]

Hi, we have a 30/0.4kV, 1600KVA Dyn11 transformer. After checking for imbalances in the primary phase currents, we connected relay software and noticed the presence of homopolar (zero sequence) current.

I suspect that our power cable supplying the transformer could be starting to develop a fault to ground. The transformer is fairly new, having been installed in 2020, and it always operates at 50% of its rated load capacity.

The residual homopolar current seems to increase and decrease with changes in load current, which is unclear to me. In the case of a cable fault, why would variations in load current cause variations in the residual current?

In adiition we have an incomer CB for the Switchgear which doesn´t show the unbalance in the power meter. Unfortunately I only have access to the current value not phasor diagram in the Incomer.

I have attached a phasor. Link

Thank you for any insights you can provide.

LMS

 

[RRaghunath]

Transformer draws third harmonics as part of magnetising current from the source. The third harmonic currents are homopolar just like the zero sequence currents. Thus, appearance of homopolar currents on primary of the transformer is explainable.
The magnetising current does vary based on the magnitude of the applied voltage on the primary.

R Raghunath