Resistance of a pole pig

From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Tuesday, April 07, 1998 2:35 PM
To:  Tesla List
Subject:  Re: Resistance of a pole pig

> >Erik

Gary's explanation (repeate below) is great. You could measure the
temperature rise of the windings by measuring the DC resistance of the HV
winding. Measure the resistance cold. Short the output, put power to it
with a variac of suitable rating (many tens of kVA for your transformer)
for a SHORT time. Disconnect the power, measure the resistance. Knowing the
temperature coefficient of copper, you can figure the temperature. Do this
at a few settings. Also, see how fast it cools down to room temperature
again. From all this you can calculate the time constant.

Then, all you'll need to know is a reasonable max temperature for the
windings. 80 degrees C would be a nice conservative number.

> >
> The short circuit impedance of a transformer is not the same as the
> 'characteristic impedance' (the ratio of rated voltage to rated current),
> except specially built transformer like neons.  It might be listed on the
> name plate as 'percent impedance', with values in the range of 5 to 10
> percent.  What that means is that if you bolt a solid short across the
> secondary and connect the primary to a strong source (a source capable of
> supplying many times the rated current of the pole pig), a current of
> to 1/.05 times the rated current will flow, until something melts or the
> circuit breaker opens.  Electrical engineers for the utility use the
> impedance of all their transformers to size their fuses.  On average,
> lightning strikes distribution lines at a rate of once per mile per year,
> often producing a power arc.  The nearest circuit breaker should open the
> line, putting the minimum number of people out of power.  If the nearest
> breaker doesn't work then another one, further away and with a larger
> will try to open the line.  Every utility of any size will have an
> electrical engineer calculating circuit breaker settings, using percent
> impedance of transformers, so this is a number that someone in your area
> knows even if it is not on the name plate.  Call the local utility and
> to speak with an engineer in the Transmission and Distribution section.
> For Tesla coil purposes, the percent impedance and short circuit current
> not of a lot of interest, however.  If you short your pole pig, you will
> probably blow the fuse on the distribution transformer supplying your
> and maybe supplying several of your neighbors, requiring a visit by the
> utility.  This is not good for public relations!  What you really want to
> know is how much power you can get out for what period of time before you
> damage your pole pig.  These have a long thermal time constant.  Your
> T and D engineer might be able to give you exact specification for your
> transformer, but I would not be surprised by numbers like 150 percent for
> hour, 200 percent for 5 minutes.  That is, your 10 kVA unit would happily
> supply 15 kVA for an hour, or 20 kVA for 5 minutes, with no ill effects.
> Gary Johnson (I used to teach this stuff at Kansas State University)