# Re: 10kva pole pig needed with 29kv primary

Original poster: FIFTYGUY@xxxxxxx In a message dated 10/21/06 10:23:11 A.M. Eastern Daylight Time, tesla@xxxxxxxxxx writes:
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>When placing transformers in either paralell or series ( similar to
>batteries for voltage applications) you get additive properties in
>either voltage or amperage depending on the setup.

OK, with you fine so far...

>Lets consider that we are using just 2 equal types of transformers.
>In paralell, you get the same voltage ( considering the transformers
>are "identical") with twice the amperage draw and amperage out put capacity.
>2 @ 14.4 KV  @ 10KVA   yields 14.4KV @ 20KVA

Again, good pertinent example.

>In Series, you get twice the voltage output but the system will
>output ( in amps) what a single transformer is rated for without damage..
>this setup yields 28.8KV @ 10KVA.

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This is where I see a problem. Twice the voltage at the same amps is twice the VA, almost by definition. To make the numbers easier, say each pig is 1 Amp at 10,000 Volts. Each is 10 kVA. The combination of two pigs with parallel high voltage windings will be rated twice the current, 2 Amps, at the same 10,000 Volts, for 20 kVA total. Each pig still sees its rated 1 Amp/10,000 Volts. Now, the combination of two pigs with high voltage windings in series will be rated at 1 Amp, but at twice the voltage, 20,000 Volts, again for 20 kVA total. Each pig again sees only 1 Amp/10,000 Volts. In each case, combining two identical transformers gives double the individual kVA rating.
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>If  one tries to series 2 transformers of equal voltage but different
>KVA, the system is limited to the lower KVA unit....

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I disagree. The combined rating is *affected* by the lower unit's rating, in either voltage or current, but not necessarily *to* the lower rating. You are correct that combining components with mismatched ratings needs to be approached with caution. Per your example, if one tried to series two transformers of equal voltage but different kVA, then the two transformers must necessarily have different current ratings. Let's say they are both 10,000 Volts, but one is a 10 kVA unit, rated 1 Amp, and the other is a 20 kVA unit, rated 2 Amps. In series the current draw should is limited to 1 Amp, so the 10 kVA unit won't see more than that. This gives a combined rating of 20,000 Volts, 1 Amp, therefore 20 kVA. However, this is "wasting" half of the larger unit's 20 kVA rating, since it will be supplying only 1 Amp at 10,000 Volts. On the other hand, if we take those two same transformers and parallel the windings, the combined rating would be 30 kVA. The combined voltage output would be the same as each transformer's voltage, 10,000 Volts. But the combined output would be 3 Amps - 1 Amp from the 10 kVA unit, and 2 from the 20 kVA unit. So combined output of 3 Amps/10,000 Volts is 30 kVA. In practice, the mismatch *might* cause issues, but the circulating current would be very small due to the relatively large impedance of the high voltage windings, and probably not large enough to worry about. Paralleling the low voltage sides would be a whole different story. Likewise, series combining two transformers of equal current but different kVA can be tricky. Again, this means the two transformers must have different voltage ratings. If one is rated 1 Amp at 10,000 Volts (10 kVA) and the other 1 Amp at 1,000 Volts (1 kVA), the series combination should be fine so far as heating since the same 1 Amp flows through both. But the 1000 Volt transformer may not have sufficient insulation between the high voltage winding and case to prevent a breakdown, since the other transformer will cause the 1 kVA transformer to see at least 5000 volts at one end of its winding. Even if the small transformer was "floated", there would probably not be enough insulation between the high voltage winding and the low voltage windings to prevent breakdown - and it would be tough to float the low voltage windings without a 1 kVA isolation transformer rated at 10,000 Volts insulation. Even if you risked the smaller transformer's insulation issue, the series combination would be rated at 1 Amp, 11,000 Volts, or 11 kVA. The parallel combination would be inadvisable, but probably rated (as you implied) at the lower unit's rating of 1000 Volts. But the combined current would be at first glance somewhere around 2 Amps, giving a combined rating of 2 kVA. Perhaps less in practice, since some of the larger transformer's current would go into the smaller transformer's secondary as wasted "circulating" current, effectively decreasing the current rating of both units. This would be such a serious mismatch of relatively large transformers that I would expect eventual fireworks... I suppose it depends on whether you limited the larger transformer's voltage or not.
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>maybe this example will work...  10 100 ohm resistors in series have
>additive ohms but since they are all rated for 1/4W  we now have a
>"larger ohm value" (1000 ohm) resistor

With you so far...

>but still only capable of 1/4W current

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What's "1/4W current" ? 1/100th Amp through 2500 Ohms? 100 Amps through .000025 Ohms? But I know what you mean. The current allowed through one of your 100 Ohm, 1/4 W resistors - .05 Amps. So putting 10 of those resistors in series still limits you to .05 Amps through the whole string, and each 100 Ohm resistor sees .05 Amps, and dissipates 1/4 Watt. However, the whole string of 10 resistors also sees the .05 Amps. But it's rated at a combined 1000 Ohms. 1000 Ohms with .05 Amps through it gives 2.5 Watts, in other words the 10 resistors together have 10 times the Wattage rating of each individual resistor. Works the same if you parallel them all, for a combined 10 Ohms with .5 Amps distributed through the arrangement.
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> the transformers can be considered "resistors" in the same sense....

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Sure, but keep in mind transformers can either supply or dissipate power, depending on how they're used or abused...
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>and for those new to the list..
>the reason Im getting 30+KV out of this setup is because I have the
>variacs wired for 280 vac total output, which gives the piggie that
>extra boost...

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Sorry to pick on you, but I still don't see how that gets you 30 kVA from two 10 kVA pigs, unless you are putting the 280 Volts into pig primary windings rated at 193 Volts or less. Or you have such a large TC primary capacitor at such high BPS that you're loading down the pigs. Anyhow, I guess for pigs it's just a matter of ballasting to suck twice the kVA out of them.
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-Phil LaBudde

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