Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
Hi Phil,
Tesla list wrote:
Original poster: FIFTYGUY@xxxxxxx
In a message dated 5/1/06 12:37:55 PM Eastern Daylight Time,
tesla@xxxxxxxxxx writes:
>Yes, but there is no assumption. D.C. said "as the interior side of
>primary is at X1 (grounded) potential". That indicates, connecting X1
>to ground. That is clear.
Sorry, I'm a little slow, but that still doesn't clarify it to
me. If anything, this raises the additional issue of which end of
the *primary* winding is adjacent to the core or secondary
winding. Again, unless I'm missing some conventional labelling of
pole pigs, the worst case could end up:
One end of primary (LV winding), the end furthest from core, at
ground potential
Other end of primary (LV winding), the end closest to core or to
the secondary's (HV winding) HV end, at 240V
Neither end of the LV windings are at ground. They are
floating. 240 Vac would be typical in the US connecting the LV
across X1 and X3. X2 is left unconnected.
I see we got our X's and H's mixed up. Thanks for being persistent.
One end of secondary (HV winding), the end furthest from core, at
ground potential
Other end of secondary (HV winding), the end closest to the core or
to the primary's (LV winding) 240V end, at 15kV
Ideally, the core side should be at RF ground as this is the least
stressful situation. I connect H2 on my pig to RF ground..
Obviously this is the highest-stress situation, and best-case is
the complete opposite. My concern is calling the "H1" or "X1"
labelled ends of the windings the ones closest to the core or other
winding. Is there some convention that pig manufactures must follow
when labelling, or should a pig owner open and verify the connections?
Not a bad idea. Popping the lid and taking a look doesn't hurt. But
there is a standard for all distribution transformers. Looking at
the transformer so that the LV bushings are facing you, the HV
bushing H1 is on the left and H2 is on the right. The LV bushing X1
is on the right. X2 (neutral) is in the center and X3 is on the
left. That is the standard for a single phase distribution transformers.
The transformer core is connected to the case. I have left the core
as built and tie RF ground to the transformer case. Mains ground
stops at the control panel. In other words, I make sure RF ground
and mains ground are not connected (otherwise, you're going to get
little shocks here and there at the control cabinet).
You're concern is certainly valid because we got our X's and H's
mixed up. H2 should be at RF ground. This is the HV side closest to
the core and when looking at the LV bushings, H2 is the HV bushing
on the right.
On an (9, 12, or 15kV) NST, the interior ends of both HV
windings are both grounded to the core. Grounding one "bushing
end" puts 4.5, 6, or 7.5kV stress at the center of the HV windings
to core, where there was zero stress before. If the depotted NST
HV winding fails, and the core isn't still grounded, that puts the
core floating at 4.5 - 7.5kV where it normally would be at ground.
And thus it would put an abnormal stress to the primary winding
adjacent to the core. So in that respect I don't think that
connection scheme minimizes primary to secondary stress.
Yes, the center of the HV side is at the core. I'm talking about
"if" the NST is unpotted, that situation could be removed. Then it
could be setup just like a distribution transformer. But I don't
unpot NST's (too messy). Just a possibility if someone wanted to go
that route. The case of the NST would also be tied to RF ground. I
set my NST under the coil and tie RF ground to the case. Mains
ground is nowhere near and only back at the control.
Hope that helps clarify.
Take care,
Bart
-Phil LaBudde
-Phil LaBudde