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RE: 110KV BIL?

Original poster: "Loudner, Godfrey by way of Terry Fritz <twftesla-at-uswest-dot-net>" <gloudner-at-SINTE.EDU>

Hi Ray

The transformer was insulated so that it could withstand a 110KV pulse of
short duration. Transformers on power lines often experience momentary high
voltage surges and must be able to withstand them to continue in service. It
does not mean that the insulation can withstand continuous stress from
110KV. I am now assuming that both PTs are identical and have two high
voltage bushings. Place a ballast on one of the leads from a variac, and
keep the variac unplugged. Even a setting of zero volt on the variac can
produce enough voltage on the high side of the PTs to ruin your day. Connect
the primaries of the PTs in parallel across the lead from the ballast and
the remaining lead from the variac. Connect two of the HV terminals
together. Set the variac at 60V and switch it on. If you can draw an arc
between the two remaining open HV terminals, you have phased the two PTs
together. If you cannot draw an arc, then unplug the variac an interchange
the primary connections on ONE of the PTs. It does not matter which PT you
make this interchange on. Now turn on the variac. You should now be able to
draw an arc between the two open HV terminals.  You have successfully phased
the two PTs in series. Unplug the variac. Now ground one of the HV terminals
you have previously connected together. This is called center grounding. The
two PTs are ready to receive 120V to produce 28.8KV. If you applied 120V
without the center grounding, there will probably occur an insulation
failure in the PTs. With center grounding, each PT will see only its normal

If your PT has only one HV terminal, then it might be the case that one of
the HV leads is connected to the core of the transformer. You cannot series
such PTs as you are sure to have an insulation failure.

Godfrey Loudner                                          

> -----Original Message-----
> From:	Tesla list [SMTP:tesla-at-pupman-dot-com]
> Sent:	Tuesday, May 15, 2001 9:58 PM
> To:	tesla-at-pupman-dot-com
> Subject:	110KV BIL?
> Original poster: "Ray Haynes by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <ray.haynes-at-home-dot-com>
> On a potential transformer what does the BIL rating mean? My only guess
> it's
> the breakdown voltage from the high voltage winding to either the case or
> low voltage winding. Is that correct?
> If that is true then if I had 2 potential transformers (120 to 14.4KV)
> each
> with 110KV BIL then I could connect the HV sides in series (for 28.8KV)
> and
> not worry about breakdown from either high voltage winding to anywhere
> (case
> or low voltage winding) caused by the transformers themselves (excluding
> any
> tesla related arcs)?
> Ray