3-ph, 21kV grd Wye-Wye Transformers Connections
3-ph, 21kV grd Wye-Wye Transformers Connections
(OP)
I would like some information regarding connecting 3 phase, 21 kV grd Wye-Wye transformers in a common neutral system. I contend that the 500’ UG feed from the OH line should be 4-wires, bringing the neutral wire to the Ho lug on the transformer and connecting this to the grounding rod as well (Wye-Wye primary neutral grounded with grounded 4 wire 480/277 secondary). Our construction manager argues that we only have to pull 3 phases to the transformer bushings and connecting the ground rod to the Ho would suffice (3 wire grounded Wye primary with grounded 4 wire 480/277 secondary). The Ho is connected internally to Xo, but does have a disconnecting link. I’ve heard that this type of connection can cause high 3rd harmonics and telephone interference not to mention high ground/neutral current in an unbalance 3 phase secondary load. This transformer is not a shell type and has a 4 legged core that is supposed to help stabilize the neutral. Does anyone have any literature or thoughts on this matter? Please advise.






RE: 3-ph, 21kV grd Wye-Wye Transformers Connections
Applicable material may be found in the standard indexed at http://standards.ieee.org/reading/ieee/std_public/description/surge/C62.92.4-1991_desc.html From there, references can be found.
RE: 3-ph, 21kV grd Wye-Wye Transformers Connections
RE: 3-ph, 21kV grd Wye-Wye Transformers Connections
Cliff
RE: 3-ph, 21kV grd Wye-Wye Transformers Connections
A grd Wye-Wye transformer can produce stray flux which may cause tank heating. A five-legged core should be used to provide a path for the stray flux.
RE: 3-ph, 21kV grd Wye-Wye Transformers Connections
RE: 3-ph, 21kV grd Wye-Wye Transformers Connections
This is the lengthy way of restating what stevenal has said for the reasons that jghrist has provided.