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Distribution transforme

    • 4 posts
    July 31, 2019 2:37 PM PDT



    I have 2 typical distribution transformer (1500 kVA, 3.3kV / 380 V, Dyn, solidly grounding) which supplied unbalanced 1 or 3 phase load. Normally 1 transformer operate and the other one is standby. When I parallel these transformers, Zero-sequence current will flow circulating between neutral of these transformers which operate OCG. Can anyone give advice about how to reduce this circulating zero-sequence? How this current can harm my equipment? How much acceptable neutral current on the transformer?

    This post was edited by Reed Figueroa at July 31, 2019 2:37 PM PDT
    • 29 posts
    July 31, 2019 4:04 PM PDT

    Technically, these circulating currents are not zero sequence currents but are actually 3rd harmonic currents.  However, they do act like zero sequence currents in that they do not have a phase shift and add arithmetically.  These currents are usually small enough that they do not cause overheating of the transformer, but can cause problems with ground fault relaying which may result in a false GF trip.  To reduce the circulating currents you should install neutral grounding resistors (I believe the terminology in the 50Hz world is neutral earthing resistor).  As I recall, the ground fault relays should have a setpoint of 10% of the rated current the resistors were designed to limit to.  Or, you could install an automatic transfer switch and not let the transformers operate in parallel since each one is rated to supply the load.  

    This post was edited by Derek Nelson at July 31, 2019 4:06 PM PDT