Transformer neutral Grounding 2

[ashbeel6]

Under what conditions the promary and secondary neutrals of a Y Y power transformer can be connectec to a common ground bus that is bonded to main ground grid. I have recently come across this situation at a very old industrial SITE. Any comments, reference & suggestions aare most appreciated.

Morrison Dean

 

[busbar]

You may be hard pressed to get a pat answer. It would almost always be case dependent, and be effected by impedance paths during fault, transient and unbalanced conditions, and reactances of bus/cable component dimensions and spacing, aside from various conductor sizes. A concern could be the case of a significantly higher impedance path from the common bus to the ground grid than the individual impedances from transformer neutral connections to the common bus. A worst-case condition may cause elevation of the lower-voltage neutral to raise significantly above ground potential {with consequent insulation damage} caused by a primary-side/higher-voltage-system ground fault. [Distribution voltage assumed.]

IEEE C57.105 “The YG-YG should not be used except on 4-wire effectively grounded primaries as it merely carries the system ground through the transformation…” “If the connection is such as to permit this condition on the primary side, the transformer will supply part of the unbalanced phase-to-neutral loads and line-to-ground fault currents.”

IEEE C62.92.4 “If a primary-to-secondary fault developed within the transformer and the neutrals were not connected, the resistance of the return current path could be so high that not enough fault current would flow to enable the primary protective device to clear the fault. High voltage would then be impressed on the secondary for an extended period of time, posing a risk to humans, animals, and equipment.”

A hung jury, of sorts? Review the entire text of standards in making your determination.

 

[jbartos]

Suggestion: Ygrd-Ygrd is often used in power distribution network transformers, where there is relatively balanced load and minimal zero sequence component passing through the transformer. Any other winding connections for this type of transformer are considered less advantageous.

 

[jlazucena]

jlazucena
I would like to add balanced voltages in the primary of the transformer to jbartos post, only as a clarification because "minimal zero sequence component passing through the transformer" implies balanced voltages