Transformer impedance

[thinker]

We received information about newly planned transformer parameters. It states: "Transformer... with a nominal 5.75% impedance. The impedance per ANSI C57.12.51 is to be within 7.5% of nominal". I haven't seen this standard, but I assume that 7.5% is not another value of impedance but rather manufacturing tolerance on the value of nominal (or design) impedance. So, I might have a transformer with real impedance of (5.75 +/-0.075)%. Is that true?

 

[Skogsgurra]

Or (5.75 +/- 0.43125) percent. Where 0.43125 is 7.5 % of 5.75 % Not so clear what this percent of percent actually means. But this is how I interpret it.

 

[jghrist]

You are correct about the 7.5% tolerance. A transformer with a nominal impedance of 5.75% must have an impedance between 5.32% (0.925·5.75%) and 6.18% (1.075·5.75%).

 

[cuky2000]

Since is virtually impossible to determine exactly the impedance theorectically, the measured impedance is commonly guaranty by the transformer manufacturer within +/- 7.5% of the specify rated impedance.

Short circuit calc. is commonly do before the transformer is build. Therefore, it is good practice to reduce the specify impedance in 7.5% to obtain the worse case with the larger SC current.

 

[SierraJ]

It is important to note that the statement "with a nominal 5.75% impedance" clearly indicates that the transformer impedance is provided at the transformer nominal capacity. This means that for any other transformer rating (65°C or cooling fans) you will need to calculate the equivalent transformer impedance (based on the nominal value provided).

Most transformers are specified with options to increase the capacity by as much as 40% above the nominal capacity.

 

[wrong100]

thinker
Spec are done at a "nominal temperature,and connection
resistance, as well as load current, input volts, ect".
All must be understood, to make a good evaluation.

 

[cuky2000]

Wrong100,

Could you please elaborate more about the meaning of "nominal temperature,and connection resistance, as well as load current, input volts, ect".

 

[wrong100]

thinker cuky2000

To me nominal means "the expected limited conditions
for operation, as well as the expected results from
other conditions".
In this case the spec is 7.5% and was recived a 5.75%
item.

The point I was trying to make, Is if the operating
conditions are the same then a better unit was supplied.

If the operating conditions are not (anything is different),
than all conditions should be evaluated to ensure that the
"entire required specs are met".

I have under mil specs, been sent laminatins that
did not meet the specs. Mil spec requires a cert with
every shipment. It took 6 mo to resolve.
Well the material was not realy M6 is not an answer.

Get the point.