# Re: Transformers in parallel

```Tesla List wrote:
>
> Original Poster: "Eric Davidson" <edavidson-at-icva.gov>
>
> Matus et al,
>
> Proper phasing is but one concern when connecting transformers in parallel.
>  They must also have the same exactly the same ratio and the same
> impedance.  This may seem obvious, but two transformers with the same
> secondary voltage may have different primary voltages.  I don't mean 120 or
> 240 volts, I mean 110, 115, 117 or 120 volts.  If the secondary current
> ratings are identical, the impedances will be close enough. For the best
> and safest results, I'd shoot for finding identical transformers (make,
> part# etc.) to connect in parallel.

Connecting NSTs in series is not easy.  Conservative, and unnecessary.
Because of the extremely high leakage reactance of NST's (about equal to
(rated voltage(/(rated current), paralleling of units with similar
voltage is quite practical. as no significant circulating currents will
result.  Even such a mismatch as a 12 kV and a 15 kV transformer in
parallel won't be much of a problem, as the leakage current will just be
about equal to (12-3)/(12+3) x rated current.  In the case of 60 ma
transformers this means there will be a circulating current of about 7
ma.  If the transformers are connected to low load resistance, the
currents divide exactly equally, while for higher load resistances the
load division will be unequal but not excessive.  I used a pair of 60 ma
transformers (9 kV and 12 kV) in parallel for a short time, feeding into
a tank circuit with a capacitance of 0.025 ufd, and didn't observe
anything strange happening or any excess heating.

With low-reactance pole pigs the story is TOTALLY different and
commercial practice used to be (when I went to college, a long, long
time ago - don't know what it is now) to use "equalizing reactors" when
connecting two transformers in parallel, even though they had the same
nominal rating.

> or safe.  The secondary midpoint is connected to the case of the
> transformer internally, as this cuts the potential between the secondary
> bushing and the case in half.  In order to connect them in series you have
> to unpot them and remove the connection to the case, or run them with their
> cases ungrounded and floating.  Either of these options makes for an unsafe
> situation and puts extra stress on the insulation (especially where the 120
> volt connection is).  Since voltage is not the sought after quantity, place
> a string of 9kV 30 mA units in parallel and you will be just fine. Hope
> this helps!

Totally agree with this part.
>
> Eric
> edavidson-at-icva.gov

Ed

```