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

[David Dean <deano@xxxxxxxxxxxxxxxx>]

Hi Jim

On Wednesday 01 April 2009 07:49:29 pm Jim Mora wrote:
> I have an older version of JavaTC 9.1, which is good for quick input.
> Assuming that part of the program has not been largely redone, a 11400,
> 5kva transformer with 240v rating and 240 applied returns ~ 230uf. 

I guessed something like that just looking at the numbers.

>This should be the honest impedance of the transformer with stock values.

For a NST, OBIT, etc. yes. PT, Pole pig, not even close.

>How does the reflected reactance play out?

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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