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Re: [TCML] PFC reflection back to current control.



Hi Deano, Jim,

Excellent explanation with the pig and pt transformers PFC.

Your 5kva pig at 1.5% is very robust and can deliver in a short circuit 1/1.5% or 66.7 x it's rated output. My 10kva pig is 1/1.9% (so 52.6 x it's rated output). I haven't included anything of this into the calc's. I considered it early on back when Bert Hickman so well explained how pigs are rated for short circuit impedance (balancing transformers in parallel configurations), but this could only be done for pigs. PT's may or may not include this information. Certainly other transformers outside the norm are not. So I just left this out. I could always make changes if it's important enough to coilers (for myself, it's not). Known short circuit current data is easy enough to do (pigs and maybe PT's).

Maybe some check boxes for NST, OBIT, PIG, PT, MOT, etc. would help define the transformer. A requirement for externally ballasted transformers would be a current limit value. Some defining implementation of the above may help get nearer to the ball park, but there are still many issues that require experimentation regardless with PFC.

Oh, and Jim, using Javatc v9.1 doesn't irritate me at all. That was one of my favorites for quite a while. That was back when John Couture and myself use to regularly compare our programs to sync as much as possible. The latest Javatc v12.2 is pretty fast at about 6 seconds on my pc with the latest Firefox (so no longer a big deal regarding speed and certainly more accurate than v9.1 since v9.1 used empirical top load percentages and also could not account for high frequency affects among other things).

Take care,
Bart



David Dean wrote:
Hi Jim

On Wednesday 01 April 2009 07:49:29 pm Jim Mora wrote:
Impedance is reflected through a transformer so that say you are looking into the primary you see the load that is on the secondary divided (in the case of a step up transformer) by the square of the turns ratio. In your case the turns ratio is 14400/240 or 60/1, 60^2 = 3600. So if you have a load of 3600 Ohms on the secondary you will see 1 Ohm looking into the primary.

It works the other way in reverse, let us say (just for grins) you have some inductive ballast that has an inductive reactance of 11.5 Ohms (at 60Hz) in series with the primary. Looking into the secondary you will see 11.5 * 3600 = 41400 Ohms.

Lets say you want a capacitor that is resonant with that ballast at 60Hz.
1/(41400*377) = 64nF. (BTW 377 is omega which is 2*pi*f where f=60Hz)
Xl on the primary side = Xc on the secondary side. Resonance. Power factor is unity.

Do some of these numbers look familiar? Lets look at JavaTC with the inputs you give. It says your transformer impedance is 41499 Ohms. (giving rated mA as 347, 5KVA/14.4KV) If it was a Neon Sign Transformer with a nameplate rating of 14400 Volts open circuit and 347mA short circuit it would be. But you have a Potential Transformer. Apples and oranges. The impedance of your transformer is nowhere near that. 347mA is not the short circuit current. It is the "rated" full load current. You transformer has a nameplate that says something like primary 14400V secondary 120/240V 5KVA and some percent impedance. Since I do not know the percent impedance rating of your transformer, I will use my 5KVA Pole Pig which has a nameplate rating of 1.6 percent impedance as an example.

 Calculating short circuit current:
First we need to find the full load amp rating.
5000VA/14400V = 347.2mA. Then divide FLA by unit impedance (unit impedance is percent impedance divided by 100): 0.3472A/0.016 = 21.7A short circuit. Now find transformer impedance: 14400V/21.7A = 663.6 Ohms. BTW 21.7A * 60 = 1302A at 240V, with no drop (assuming an infinite buss) which is a little bit more than my 150A service can handle.

Now lets take those "real" numbers and put them into JavaTC to see what it gives us for a resonant size cap and for PFC capacitance.
It tells us about 4uF for resonant tank cap and 14390uF for the PFC.

Now lets "fudge" JavaTC a bit. Just put 1mA in the box where it says "Rated mA" just so the JavaScript won't throw an error, and go over to the box where it says "Ballast Current Amps (optional)" and put something reasonable in there, say 20A. Then look at the output called "Transformer Output Current" which is given in Amps, convert that to mA, in this case 500, and stick that number into the box called "Rated mA", hit the calculate button again, and see what happens. Resonant cap size 61nF, PFC, 221uF. Now lets try 30A. Resonant cap size 92nF, PFC 332uF. So adjusting the ballast will not only change the resonant point, but the PFC as well.

Another experiment we can do is to change the breakrate. 60Hz equates to a breakrate of 120BPS. Lets change that to 500BPS by changing the 60Hz to 250Hz. Everything else stays the same. Now we get 22nF resonant cap size and 80uF PFC. So changing the break rate changes both the resonant point and the PFC.

In your case, you have a saturable reactor, so you can adjust the ballast on the fly, and if I remember correctly a variable speed ASRSG, so you can vary the breakrate on the fly. That will allow you to change both parameters at will, run resonant or LTR or STR however you want all with a fixed tank cap size. Cool.
Now as far as PFC goes, weeelllll---
Here is what "I" suggest. Since you are not going to power factor correct the "natural impedance of the transformer" anyway, put the PFC caps where they might do you some good, ahead of the ballast. Assuming all your caps are 100uF and you have enough of them, build strings of different values and put a 20A toggle switch in series with each string. And don't forget to put a 5meg or 10meg bleeder across each one. (cap that is) Make the first string 8 caps. 12.5uF. The next string 4 caps, 25uF. Third string 2 caps, 50uF. Fourth string 1 cap, 100uF. Fifth string, two caps in parallel for 200uF. Thus with 17 caps you can have a range of 0 to 387.5uF in 12.5uF steps at the flip of a few switches, like binary addition. You see where I am going with this?

BTW, take a look again at my page at http://deanostoybox.com/pfc I might have added something since you were there last. Or not.

later
deano



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