## Transformer %Z and X/R

## Transformer %Z and X/R

(OP)

How a transformer %Z(so called percentage of impedance) and X/R (reactance versus resistance ratio)being controlled and determine during the manufacturing process?

Is there any standard tesing method which can be carried out to ensure the transformer that being sent to site are as per design?

Thanks a lot for your kind reply!

Is there any standard tesing method which can be carried out to ensure the transformer that being sent to site are as per design?

Thanks a lot for your kind reply!

## RE: Transformer %Z and X/R

## RE: Transformer %Z and X/R

## RE: Transformer %Z and X/R

Thanks....

## RE: Transformer %Z and X/R

Since the full-load losses include both the resistance losses and the iron losses, you subtract the no-load losses from the total losses.

The %resistance is 100 * (Full load loss -no load loss)/transformer rating.

Once you know the %R, you can use the %Z derived from the impedance voltage test to determine the %X and the X/R ratio.

## RE: Transformer %Z and X/R

I want to ask some more. if you can answer or show the way to do I'll be glad. How can I calculate maximum continuous power rating of a transformer and duration of overload (for %25, %50 etc.)? I have the other parameters and specifications which may be needed. Thanks a lot...

## RE: Transformer %Z and X/R

IEEE Guide for Loading Mineral-Oil-Immersed Transformers.## RE: Transformer %Z and X/R

do to the none linearity behavior of magnetic cores, hysteresis , Eddy losses, tank magnetic coupling, leakage current and other random factors.

The major factor that affect the impedance are the core volume and shape, the core material, the winding wire size, number of turns on the primary and secondary and the degree of magnetic coupling between the windings.

On the other hand, guides and standards such as ANSI/IEEE Std C57.12 & 12.1 applicable to liquid immersed and dry type transformers, help users to purchase units with impedance with reasonable degree of accuracy from specified values as follow:

DESCRIPTION TOLERANCE

Two Windings:

Z < 2.5% +/- 7.5%

Z > 2.5% +/_ 10%

Three Windings +/- 10%

Zigzag and

Autotransformers +/-10%

For short circuit and other calculations an estimated guide may be found in the enclose material and other similar references.

http://cuky2000.250free.com/XFMR_XoverR.jpg

## RE: Transformer %Z and X/R

## RE: Transformer %Z and X/R

yours

## RE: Transformer %Z and X/R

I am trying to confirm a reactive loss in a load flow calculation for a GSU transformer. I multiplied the line current squared by the reactive ohms. I got the reactive ohms by multiplying the impedance base ohms by the %X. Zbase was found by dividing the phase-to-phase voltage squared by the three phase kVA of the transformer. All values all in three-phase. I'm being told that I then have to multiply my value by three to get the three-phase reactive losses. Why would I have to multiply by three if I am using three phase equivalent values?

## RE: Transformer %Z and X/R

phase-to-phase kilovolts squared divided by the three phase MVA, or

phase-to-phase volts squared times 1000 divided by the three phase kVA.

For single phase values, use phase-to-neutral voltage and single phase kVA or MVA, same formula.

## RE: Transformer %Z and X/R

Use impedance and current to determine the voltage drop. The losses are caused by the resistance. Once you find voltage drop use V^2/R to find the losses. Impedance is per phase so multiply by 3. If you know the current you can use I^2R where R is the resistance. V in the above formula is voltage drop, not applied voltage.

jghrist I'm not familiar with your formula. Maybe I'm going to learn something. Can you work an example or two please?

respectfully.

## RE: Transformer %Z and X/R

I should have noted that my post was in response to Snoopyfoot's question about base impedance, not the main topic of this thread.

Snoopyfoot was calculating reactive loss by as I²X, using a per unit value of X and then multiplying by the base impedance Zbase. He will have to multiply by three to get the three phase reactive loss because as you say, the impedance is per phase.

For example, say you have a 100 MVA, 115 kV (Ø-Ø) transformer, with an impedance of 9%, an X/R ratio of 20, and a line current of 200A:

Zbase = 115²/100 = 132.25 ohms

R% = X%/20

Z%² = X%² + R%² = X%² + X%²/400 = X%²?(1+1/400)

X% = Z%/sqrt(1+1/400) = 8.989%

Xpu = 0.08989

Reactive loss = 3?I²?Xpu?Zbase = 3?200²?0.08989?132.25 = 1,426,554 W = 1,426.554 kW

## RE: Transformer %Z and X/R

respectfully

## RE: Transformer %Z and X/R

In very simplistic terms, both parameters are determined by physical geometry so they are controllable. R is the winding resistance which is proportional to winding conductor resistivity, cross-sectional area and length.

X is inductance or leakage flux, which is determined by the the shape of the chunk of amp-turns (the winding), and the relative location of these amp-turn chunks to each other (i.e. HV to LV windings).

The transformer designer as you can imagine therefore has to specify the dimensions of the windings to the shop, and the shop has to build these windings to the exact dimensions (within tolerances). Manufacturers can normally hit guaranteed impedance and resistance within std tolerance nowadays (7.5% for IX and don't exceed the guaranteed losses for R).

The proof of the pudding is the final transformer test which includes winding resistance, and load loss and impedance tests (check out ANSI C57.12.90 for test descriptions). Failure to meet the guaranteed values in those tests normally indicates something has gone wrong along the way (may involve tear down of the transformer - ouch).

In the field, you're basically limited to resistance test only since the impedance test is normally done at full load current. You need a good size power supply with a bunch of capacitors for tuning since you'd be powering up an inductive load.

Hope this helps,

cheers

## RE: Transformer %Z and X/R