Single bushing transformers are used in Y connected 3-phase
banks with a grounded neutral or in single phase connections between phase
and ground. The other end of the primary is connected to the tank
internally. There is a ground connection terminal on the outside of the
tank, usually on the bottom third of the tank and toward the back. The
single bushing primary reduces the cost of the transformer in several ways.
Since one side of the primary is grounded the insulation requirements are
less and only one surge arrestor is required. That is actually what the
'second bushing hung outboard on a bracket' is. Transformers with two HV
bushings are used in delta connections and in Y or single phase connections
phase to phase. I
prefer the single bushing transformer for coiling since one side of the TC
circuit is grounded, but this is only my opinion. You can always ground one
side of a two bushing transformer to create the same effect. I have not
seen a difference in cost in the two configurations, at least not from the
standpoint of getting one used/rebuilt from an electric company. Im sure
there is a difference in the cost of the new unit. Also, when powering up a
single bushing transformer, be quite sure the tank is solidly grounded! Hope
>I've been studying pole transformers in my area, and I've got questions.
>Some pole transformers have two identical HV bushings. Others have a
>single HV bushing on top, with a second bushing hung outboard on a bracket.
> I'm guessing that I'm seeing single ended & double ended transformers.
>Is one kind preferable to the other for coiling?
>Why would a power company use both kinds? Why not just use all the same