How to calculate Transformer impedance Voltage?

[WilliamAng]

Hi All,

How to calculate Transformer impedance Voltage without doing short circuit test?

Best regards,
William

 

[charlie b]

Sorry, but your question does not make sense. You are using the word "impedance" and the word "voltage" side-by-side, as though they were the same thing. Are you asking about impedance, or about voltage, or both?

 

[kwired]

I agree with Charlie. Your question does not make sense. You are probably using the wrong terminology for something in that question. Impedance is an opposition to current flow. It will not effect voltage - until there is current flowing.

 

[kingpb]

There is no way to determine series impedance without doing a short circuit test.

The voltage applied during this test is very low. The approximate series impedance will be calculated when the the voltage applied produces the rated current.

 

[AdrianWint]

Sorry, but your question does not make sense. You are using the word "impedance" and the word "voltage" side-by-side, as though they were the same thing. Are you asking about impedance, or about voltage, or both?

The transformer impedance voltage is that voltage, which when applied to a transformer with a shorted secondary winding, will cause rated current to flow in the primary coil. It can be found using the 'short circuit' test. This value is used when calculating the equivalent impedance of a transformer which can then be used in fault current calculations (after allowing for the change in current due to the transformer ratio).

I guess this is probably not something which the average electrician is familiar but is a part of transformer theory.

To answer your original question: I don't think you can calculate it.... it can only be found by experiment.

 

[GoldDigger]

To answer your original question: I don't think you can calculate it.... it can only be found by experiment.

If you somehow know all of the resistive and reactive impedance values (as a function of current if necessary) for the transformer, as well as the turn ratio, then you could calculate the Impedance Voltage.
But if you want to know the impedance voltage so that you can then calculate the other parameters, it seems rather pointless.
The value of the Impedance Voltage is that unlike the other parameters it can be determined directly by a simple experiment as long as you have a variable voltage source available.

 

[Besoeker]

Sorry, but your question does not make sense. You are using the word "impedance" and the word "voltage" side-by-side, as though they were the same thing. Are you asking about impedance, or about voltage, or both?

Impedance voltage is how it is referred to in my 800 page J&P transformer book.

 

[mivey]

Impedance voltage is how it is referred to in my 800 page J&P transformer book.

And it is the result of a test measurement under a specific set of conditions.

We must remember the purpose is to find the transformer equivalent series impedance. To do that, we rig a test that gives us a voltage value that is numerically equivalent to the percent impedance of the transformer.

Kind of like rigging an Archimedes' lever type test where the number of cars on the lever equals a person's mass. We could say the count is given in "mass cars" to indicate how the test was conducted, but what we really mean is the person's mass. We could use some method to calculate the "mass cars" but since "mass cars" is a measurement tool, we really are just calculating the person's mass.

So using that moniker, we CAN calculate the impedance voltage. More correctly, we can calculate the equivalent series impedance (at least reasonably close). You can special-order a transformer with an impedance specification and the manufacturer will build one pretty close. The impedance voltage test confirms the design and gives you the precise value.

The problem with the calculation is that we need a bunch of design criteria about windings, spacing, materials, thickness, grain, orientation, stacking, dimensions, clearances, housings, and I can't remember what all but it is a long list. If the OP does not have the %Z, then it is doubtful the OP will have the design and material specifics to calculate the %Z either.

What the OP may be able to make use of is an impedance range for typical transformers of the same type as the transformer in question.

 

[Besoeker]

And it is the result of a test measurement under a specific set of conditions.

We must remember the purpose is to find the transformer equivalent series impedance. To do that, we rig a test that gives us a voltage value that is numerically equivalent to the percent impedance of the transformer.

Kind of like rigging an Archimedes' lever type test where the number of cars on the lever equals a person's mass. We could say the count is given in "mass cars" to indicate how the test was conducted, but what we really mean is the person's mass. We could use some method to calculate the "mass cars" but since "mass cars" is a measurement tool, we really are just calculating the person's mass.

So using that moniker, we CAN calculate the impedance voltage. More correctly, we can calculate the equivalent series impedance (at least reasonably close). You can special-order a transformer with an impedance specification and the manufacturer will build one pretty close. The impedance voltage test confirms the design and gives you the precise value.

The problem with the calculation is that we need a bunch of design criteria about windings, spacing, materials, thickness, grain, orientation, stacking, dimensions, clearances, housings, and I can't remember what all but it is a long list. If the OP does not have the %Z, then it is doubtful the OP will have the design and material specifics to calculate the %Z either.

What the OP may be able to make use of is an impedance range for typical transformers of the same type as the transformer in question.

One way:

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%.

When I do transformer circuit calculations at the design stage (harmonics or SC) I normally 5% impedance with X/R =4

 

[Sahib]

Hi All,

How to calculate Transformer impedance Voltage without doing short circuit test?

Best regards,
William

Yes. It is possible. Check the transformer name plate. It will show you the percentage impedance. Multiplying its value into the secondary or primary volts and dividing by 100 gives you the impedance voltage on that side of the transformer.

 

[mivey]

When I do transformer circuit calculations at the design stage (harmonics or SC) I normally 5% impedance with X/R =4

That would indicate you anticipate larger transformers on the order of MVA.

 

[Besoeker]

That would indicate you anticipate larger transformers on the order of MVA.

Yes indeed. That's commonly what I deal with.