Wye-Wye bank grounding question

[PowerDawg]

I am receiving some conflicting information on a Wye-Wye transformer connection. Concerning a three transformer bank connected wye on the secondary side, should the neutral wire that connects all three transformers be grounded to the transformer tank lug of just the center transformer or should the neutral wire be grounded to the grounding lug of each transformer?

 

[waross]

The lug on the transformer is to provide a means to ground the transformer tank. The lug on the transformer tank is not meant to provide a ground for other purposes.

Bill
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"Why not the best?"
Jimmy Carter

 

[PowerDawg]

You are absolutely right waross. The issue I have is that I have seen two different drawings... one drawing shows the secondary neutral bonded to each of the three transformer lugs and one drawing shows the secondary neutral bonded to only the center transformer lug. I've got a friend that told me a story the other day that when he was a lineman, he went to a wye secondary bank that was having some voltage problems. A co-worker of his at the time told him to cut the bond wire from the transformer grounding lug to the secondary neutral on both outside transformers but to leave the one in the center alone. He told me the problems went away. He asked me to explain this to him and I am grasping to find an explanation. I also want to make sure I am telling my crews the correct way to make this grounding connection.

 

[prc]

Understand we are discussing the grounding of winding neutrals of three single phase transformers. Before common neutral is formed the wires will be carrying phase load current of transformer. Normally these wires are insulated from ground and after shorting together connected to station earth. Tank earth terminal is for grounding the tank case. When you are grounding each phase neutral to tank, there is chance for part of the current to by pass through tank and probably that may be reason for voltage problems. Multiple earthing is not done for the neutral bonding bus.

 

[PowerDawg]

Another piece of information I remembered last night. My friend indicated that when he arrived at the bank, the two outside transformer secondary to tank ground were hot enough to burn high-voltage rubber gloves so there was a significant amount of current flowing through them. It was probably #6 copper wire used for the secondary neutral to tank ground connection. Could a difference in transformer impedance or unbalanced load have caused these issues? Again, I'm looking for which way is the proper way to make this grounding connection and be able to explain why to a group of linemen.

 

[waross]

Well, if the transformers are 50 KVA producing 120/208 Volts, and are fully loaded, the current in the neutral jumpers will be over 400 Amps,- the same as in the hot lines.
Once past the neutral jumper the neutral current is usually much less and the size of the neutral service conductor may be reduced.
However the jumper between the transformers carries the same current as the hot leads and should be the same size as the phase conductors.
I have seen a lot of instances where someone assumed that because the jumper was a neutral it could be reduced in ampacity. Not so.
If the neutral jumper was undersized than I would expect both the jumper and the #6 AWG grounding conductor to get hot.

Bill
--------------------
"Why not the best?"
Jimmy Carter