Impedance Voltage

[Luiseng]

Hi All,
we are adquiring two power transformers and the value of impedance voltage was specified as 12.25% +-10 but the value after the tests is 14.1%. the question is what are the implications, advantages or disavantages of having a value higher than specified.
thanks
best regards
Luis

 

[wbd]

The impedance is composed of the primary and secondary ac resistance and the reactance. Since the overall impedance is higher, the resistance and reactance is higher (one or both).
The effect would be your losses are greater but the short circuit will be less due to the higher impedance.
If these are replacement transformers and the originals were 12.25%, you probably want to check the effect on the protection and on voltage drops.

 

[skm]

I don't think there are any advanrages of having higher %Z. At the first place when you have selected the impedance as 12.25%, I presume it must be based on the short circuit requirement and the voltage regulation required for the purpose. Further for a given X/R, this will also mean increase in loss figures (this is not indicated in your question) which will burden you with increased operating cost.

Should you intend to run this transformer in parallel with some other existing transformer of similar rating, you may have to check the application again w.r.t load sharing and circulating current before accepting this transformer. If it is not to be paralleled with any existing trafo and the short circuit current increase is within the limit of the downstream equipment, you may penalise the vendor based on the increased losses and impedance and accept the transformer if it doesnot hurt you much (financially).

Good day gives you happiness, Bad days give you experience. Count Both....

 

[jghrist]

Through fault current will be lower; this could be an advantage. Voltage drop will be higher. This could be a major disadvantage, depending on your system. Var and possibly watt losses will be higher. If you specified losses, any higher watt losses can be handled under that section. Higher var losses may require power factor correction under heavy load.

ANSI standard tolerance for impedance is ±7% of the specified impedance, so you are already allowing more tolerance than standard. Tested values 15% higher than specified are a bit too much.

 

[mpparent]

Not to be overly picky, but I thought the ANSI tolerance was +/-7.5%?

Mike

 

[jghrist]

mpparent,

You're probably right. I wrote that from memory without looking it up. My memory ain't what it used to be. X-)