# Impedance Voltage of a transformer.

1. Jan 29, 2012

### Bassalisk

I know what impedance voltage is. I will state it just make sure we are talking about the same thing.

Impedance voltage is relative short circuit voltage, when secondary of a transformer is short circuited, and we are slowly raising the primary voltage "Vpr" until the current at the secondary reaches its nominal value. The ratio of the "Vpr" and nominal voltage of the transformer is called impedance voltage. It usually goes from 3% to 20%.

Now, what does this parameter tell me about the transformer? How is this useful?

Is it better to have a lets say 20 ohm or 200 ohm impedance of transformer?

I know its important for 2 transformers when working in parallel to have matched impedance voltages, to prevent one transformer "hugging" more power than the other.

When we have a "rush" and a lot of power is taken from transformer, its voltage at secondary tends to fall. Correct?

To prevent this, regulation transformers slip one ring up, and change the transfer ratio of the transformer, so we can get more voltage at the secondary.

Is this correct?

2. Jan 29, 2012

### jim hardy

Seems unusual name - we just called it the "impedance"..
It tells you how well the transformer "regulates"..
ie at rated current it'll give that much voltage drop;

which is useful when designing a power distribution system so voltages still match several transformers away from power source;
and it tells you how much current the transformer will deliver into a short circuit (rated current X 1/impedance ), useful when selecting downstream breakers..
Now that's interesting- if your pole pig is a 200 amp 3% transformer it'll dleiver over 6,000 amps into a short circuit
and that's why the little breakers in your household electrical panel have numbers on the back like 10,000 amps.

Don't be surprised when you get to "transient" and "subtransient" impedance - same concept except very short term while magnetic fields inside transformer collapse. equations look scary but you'll have no trouble.

"regulating transformers" : - some are motor driven tap-changers as you describe,
some are a slider type (Variac) ;;

others are "ferroresonant" :: employing a tuned circuit resonant near 60 hz but operating near saturation so [strike]load current[/strike] voltage affects inductance;
effectively changing impedance so as to maintain constant output voltage.
No moving parts but kinda inefficient at light load - they run hot of course with such high flux.

3. Jan 29, 2012

### Bassalisk

I know its a strange name, we call it relative short circuit voltage(in my country).
Can you explain this "regulation" property further?
And at rated current, what will give voltage drop?

I don't understand what "pole pig is" :D

4. Jan 29, 2012

### jim hardy

I know its a strange name, we call it relative short circuit voltage(in my country).

we called it just impedance but used units of % .
Primary and secondary are lumped together and treated as if the transformer were a 1::1 with a lumped impedance of a single R and a single L.
Then you work in "% of normal volts" .

Can you explain this "regulation" property further?

yes, the simple ohm's law drop from open circuit to full load on the transformer
but in % of nominal rated rather than absolute number of volts.
transformer has some r +jx that causes drop as its load increases
(Vno load - V full load ) / V no load, X 100% is regulation. That's a definition not the description of an active regulator that it infers.
It's no different from a DC regulator except for the jx's

And at rated current, what will give voltage drop?
I X (r+jx )

when studying AC systems you often work instead of volts and amps in % of some agreed base . In that way you don't have to worry that say 5% of 480 volts is not same as 5% of 120 volts. It's just 5% drop...

(They should soon broach "Per Unit" in your class.)

So at one "per unit" load you'll get one "per unit" voltage drop.
at two per unit load you'll get two per unit drop
at short circuit you''l get (1/impedance) per unit current
a 10% transformer will deliver 10X current, for a short while...
..

I don't understand what "pole pig is" :D
a slang term for the distribution transformer that hangs on a pole and feeds a few households. Linemen named them that because they're heavy and annoying to manhandle into position. When you're dripping with sweat they're " as slippery as a greased pig", which is another slang term.
In old days here at celebrations , the young men would compete to see who could catch and hold a piglet that had been covered in kitchen grease...much to the amusement of the girls .. ahhhh, youth !

You might cross check this with your professor - it is possible you textbook's author uses slightly different notation and monor differences in wording can confuse...
but i think i have got the basic concept out for you .

old jim

5. Jan 29, 2012

### Bassalisk

hahaha interesting anecdote :D I think I understood everything you said. Boy this HV theory isn't so naive as I thought it was.

I did encounter per-unit system and I am quite comfortable with it, and I am handy when converting things, just never saw the point of it. Now I do.

So basically this % thing is telling me how much voltage drop I will have on my transformer. So the lower the better?

Thank you.

6. Jan 29, 2012

### jim hardy

So basically this % thing is telling me how much voltage drop I will have on my transformer. So the lower the better?

In general , yes....
sometimes in a big installation you use the transformer Z to your advantage.
Cost of circuit brakers goes up with interrupting rating so you can use the transformer as an "orifice" to limit fault current, allowing more modest switchgear.

some applications use hi-z on purpose , for example an arc welder where you want maybe 90 volts to strike the arc but really bad regulation so it drops to 30 or so to maintain arc.

Have Fun!

7. Jan 29, 2012

### Bassalisk

Thank you I will! I have my final exam tomorrow, regarding all these topics we were talking about past 5 days. I think I will do good.