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Is it possible to parallel two power transformers of different step down ratios? The transformers have the same MVA rating, but a different impedance. If done, what will be the circulating current?

RATINGS ARE :

TRANSFORMER 1:

  • 16/20 MVA
  • 33 kV / 6.6 kV
  • DYn11 = 8 % / 10 %

TRANSFORMER 2:

  • 16/20 MVA
  • 11 kV / 6.6 kV
  • YnD11 = 5.66 % / 7.07 %
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    \$\begingroup\$ If they have MVA ratings, please set up a video camera when you try it. And stand well back... You might have a YouTube hit in the making! \$\endgroup\$
    – user16324
    Commented Dec 29, 2012 at 12:05
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    \$\begingroup\$ Seriously!!!!? How is anyone that has to ask this question allowed anywhere near this stuff? You can black out a small city by messing this up. This is one place amateurs simply don't belong. \$\endgroup\$
    – Olin Lathrop
    Commented Dec 29, 2012 at 14:46
  • \$\begingroup\$ I'm hoping it's a homework question. \$\endgroup\$
    – akohlsmith
    Commented Dec 29, 2012 at 15:17
  • \$\begingroup\$ Why would you want to use both, are you trying to power a factory? \$\endgroup\$
    – Gunnish
    Commented Dec 29, 2012 at 15:39

2 Answers 2

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Parallelling two large power transformers with different ratios:

Any questions?

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    \$\begingroup\$ While drama is nice, the result of this cross wiring would almost certainly just result in a trip of the protection eqt. Why would you think that people who deal with MVA sources have a less well developed sense of protection that us low voltage circuit jocks? Protection eqt. is designed precisely to handle this scenario from accidentally happening. When a transformer does let go, it is a spectacular event, but is a long slow build up or caused by lightning or an accident and loss of coolant. \$\endgroup\$
    – placeholder
    Commented Dec 29, 2012 at 15:19
  • \$\begingroup\$ It depends on where exactly the fault occurs. There have been cases where a fault caused a major breaker to trip, which caused a surge, which caused other breakers to trip, etc, eventually resulting in a wide-spread power outage lasting for many hours. This is exactly what happened in August 2003 when the ripple effect of one overloaded transmission line in Ohio shutting down blacked out much of the northeastern US and southeastern Canada. \$\endgroup\$
    – Olin Lathrop
    Commented Dec 29, 2012 at 15:27
  • \$\begingroup\$ What is very interesting about these power distribution grids is that they exhibit chaotic characteristics with limit cycles, so are at certain operational modes unpredictable. What you describe was a cascading fault protection rippling out of control, and while I'm sure there was some damaged eqt. and the subsequent power outage was not fun, the fact that so many protection circuits tripped makes my point. But again this is not the same scenario and is as far away from the MW regime the OP was talking about (GW) vs. what might be KW or less for most of the topics dealt with here. \$\endgroup\$
    – placeholder
    Commented Dec 29, 2012 at 15:36
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One could easily presume that since these transformers are of different primary ratings that this post is just poorly worded and the author should have indicated that there are two sources, one 33KV and the other 11KV.

It is common practice for utilities to prevent line droop on the distribution side by strapping two transformers together. But often at larger separation on the local low voltage side. This is doable because of a common synchronous source, tight tolerances on the plant and of source fuses/breakers etc. I'm had pilot installs for plant control fail because this cross wiring that wasn't documented properly.

WIth different sources (33kV vs. 11Kv) it would be hard to control the situations in which you could guarantee that the input and output conditions are well controlled. Two transformers on the same 33kV or two transformers on a 11 kV line are ensured of operating at the close to the same conditions. This is almost certainly not met with two separate sources.

Ultimately this is probably a bad forum to be asking this if this is more than just an moot question.

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