Re: Pole Transformer Basics
The two types of transformers you are seeing are most likely for single
and three phase operation. The ones with two identical bushings are
somewhat more versatile. They can be used (three at a time) to replace
a single, three phase, transformer. The HV bushings connect across two
of the phases on the high tension side. These transformers can also be
used to provide single phase power from a three phase line (rather than
connecting from HT line to ground).
As for the other type, I think you may be describing a lightening
arrestor. Transformer mounted lightening arrestors are approximately
the same length as HV bushings and mounted on brackets outboard, usually
from the top edge of the transformer, and in a vertical position. There
is a jumper wire from the (single) HV bushing, or open air spark-gap
from the HV bushing to the lightening arrestor. The lightening arrestor
grounds to the case of the transformer at the bottom and has no internal
connection to the transformer. Lightening arrestors can also be mounted
separately, on the poles.
The second type of transformer is more common in residential areas fed
from single phase HT lines. The first type is more common in heavily
populated, or industrial areas, served by three phase lines.
There is a two phase transformer with two HV bushings. It is rare in
the US, but still used in places. The two phase system is usually
served by four overhead HT wires, and is more common in areas with 240
volt LV power mains.
Instrument transformers may also have two identical bushings. Some
instrument transformers approach the size of small residential pole
Some residential transformers bring the ground lead, from the HT
winding, out through a bushing on the side of the transformer. The
extra bushing facilitates testing of the transformer.
I can't see any real advantage to either type for coiling, but someone
on the list may have a good reason for choosing one over the other.