3-ph, 21kV grd Wye-Wye Transformers Connections

[2banee]

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.

 

[busbar]

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.

 

[stevenal]

No cencentric neutral or tape shield on those 21kV cables?

 

[2banee]

Thanks busbar. Steve, we're pulling 25kV, 1/0 full neutral rated concentric XLPE. Does that help?
Cliff

 

[jghrist]

Without a primary neutral, the zero-sequence current from an unbalanced load or ground fault on the secondary would have to flow through the earth, causing stray voltages and overcurrent protection problems. The concentric neutral provides the neutral, though.

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.

 

[stevenal]

The concentric neutrals, ground rod, tank, H0, X0, and 2ndary neutral should all be bonded together.

 

[bertisevil]

You are both partially correct. The manager says, 3 phases, that is correct. You say 4 wires, that is correct, also. The primary you stated is a 2 conductor cable. By pulling in 3 phases you, in reality are pulling in 3 phase conductors and 3 neutrals. The concentric neutrals on the pole end needs to be connected to the system neutral which in turn is grounded. The neutrals on the xfmr end have to be connected to the tank ground that is connected internally to the neutral. Then these neutrals and grounds should be connected to a driven ground rod or rods.
This is the lengthy way of restating what stevenal has said for the reasons that jghrist has provided.