calculate transformer impedance

[anfr123]

How to find transformer impedance ( Delta- Y grounded transformer 12000V - 480V)?First , I calculate I2 using this formula -->I2= KVA(load)/Volt_left_side. After that, I calculate I1 using the formula , N1/N2 = I2/I1.
After that , I calculate Z2 = V2/I2. Next , I calculate Z1 using this formula --> N1/N2 = sqrt(Z1/Z2).
After found Z1 and Z2 , how to find the Ztransformer??I would like to get percentage of tranformer impedance.

 

[ScottyUK]

Calculate rated secondary current

Short secondary winding terminals

Apply low voltage to primary and increase until rated current circulates in secondary

Calculate ratio of applied voltage to rated voltage. x100 for percentage impedance

Read text book!





----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[jghrist]

Your calculation of Z2 is a calculation of load impedance, not transformer impedance. As ScottyUK says, you need a test to determine transformer impedance.

 

[GJGerard]

My question is: Why is this voltage ratio (Vp needed to produce rated secondary current with the secondary short circuited, divided by the rated primary voltage) times 100 called an impedance percentage.

Example: If it takes 10 VAC applied to a 120 VAC primary winding to produce the rated secondary current when the secondary is short circuited, the transformer impedance percentage would be 8.3333%.

I understand multiplying by 100 produces a percentage, but how is Vp/Vrated equal to an impedance. In my text books, impedance is always determined by the ratio of ac voltage divided by ac current. Please clear up my confusion over the use of this terminology - which is evidently commonly used in industry. (i.e., a tech note from Saftronics on variable frequency drives makes repetitive use of 2 or 3 % impedance ratings for power transformers.

Thanks for any and all elucidation.

 

[jghrist]

First of all, let's talk about per unit (pu) to start, with the realization that % is 100 times pu.

The voltage ratio Vp/Vr to produce 1 pu current is Vpu (per unit). The impedance in pu (Zpu) is Vpu/Ipu. Since Ipu is 1, Zpu is the same as Vpu.

 

[Skogsgurra]

In fact, this way of expressing impedance is a sloppy way of saying "This transformer has an internal impedance that needs x percent of its rated voltage to produce rated current when the secondary is short-circuited".

The short-circuit test is done during testing in the manufacturer's test lab and the X% impedance is something that established itself long before the ISO and other organisations decided that units shall reflect what we are measuring.

It fits quite well in pu thinking, though.

Gunnar Englund

http://www.gke.org

 

[yuki2]

Expanding on this idea...
Given transformer test data from the manufacturer i.e. load loss, % impedance, no-load loss, % exciting current. How can I calculate a series impedance and shunt admittance for modeling this transformer?

Thanks!