Voltage impedance

Dear all
Kindly inform me regarding the voltage impedance calculation of 11 kV transformers, is it the important factor to write it down at the name plate???????

 

[Skogsgurra]

Is it the uk in % you are asking about?

If so: Yes, that is a very important piece of information and shall always be included in the transformer nameplate.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

The reason that it is given in %-Voltage and not ohms is that it is the percentage of rated voltage that will cause rated current to flow in a short circuited secondary. This method converts the value to a Per Unit (PU) value that may be used to determine the available short circuit current on either the high voltage side or the low voltage side.
Switch gear fed from a transformer must be rated for equal or greater available short circuit amperes than the value of amperes the transformer can produce as calculated from the percent impedance voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GreatIraq]

Dear Waross

Thanks lot for your assistance, would you please explain to me the practical method to calculate Z% for 11/0.4 KV step down transformer 1000KVA, what is the acceptable limits for Z%!!!!

Cheers

 

[tonyjony]

You don't calculate it. It must be provided from the manufacturer and It's obteined with the Test described by waross.

5% is a typical value for that transformer.

 

[GreatIraq]

Yes , u are right tonyjony it must be provided by manufacturer & they listed at the name plate for each transformers & the value of Z% different from one transformer to another , our typical value in Iraq is 4% greater than that is not acceptable , What I understood from waross that we applied rated voltage 11kv ( our transformer is 11/o.4 KV Delta / Star 50HZ 3Phase) at primary side after shorted secondary side & we measured I short circuit occurred at secondary side & also we can get primary current .!!!!!!!!!! Am I right??? We can get Z% by dividing applied voltage at primary side to( I) short Circuit occurred at secondary side!!!!

My friends, what I need to know the procedure of checking Z% at labs to improve that what the manufacturer wrote at transformer name plate is matching to our practical methods, Hope you got my idea, Sorry for my poor English

 

[racobb]

You need to revisit the waross post. %Z is the level of voltage required on the primary to cause rated secondary current to flow with the secondary shorted....Hence only a few percent of rated. Do NOT apply rated primary voltage with the secondary shorted because it will most likely fail the transformer violently.

To verify the mfg. data, you will need calibrated test equipment and test to the same standards.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[Skogsgurra]

Why do you think that you need to measure uk? Do you think that the manufacturer's data is wrong?

Anyhow, you shall NOT short-circuit with 11 kV applied. Please do not do that. You will have a major accident if you do so.

The procedure to measure uk is as Waross said. You apply a primary voltage gradually until your shorted secondary has rated current. Then you read what primary voltage you have. The ratio between that voltage and rated voltage is uk. If you want it in percent, you multiply by 100.

I do not think that you do have the varieble voltage source needed available. So, please do NOT try with 11 kV primary. That will destroy things. Possibly kill someone.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Are you having trouble verifying the %impedance voltage?
In the test the transformer is short circuited and a variable voltage is applied to the primary. Enough voltage is applied to drive rated current through the secondary. That voltage divided by the rated voltage is the % impedance voltage.
If it takes 4% of rated voltage to cause 100% of rated current to flow in a shorted secondary then your %imedance voltage is 4%.
When you test it your first result will be noticeably less than the nameplate figure. (More current, lower calculated % impedance voltage)
Leave the transformer energized until it reaches operating temperature and adjust your voltage and retest. I have observed noticeable errors testing cold transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[pwrtran]

In terms of pu (or %) value, V%=Z%

 

[juanccga]

hI,

You must energize the transformer by HV side by a regulated souerce, and LV is in short. The current in the short must be at least 25 % of Irated.

You must read the aplied phase-phase voltage (V Vtest) and the aplied current in the HV side (Itest)

so,

Zcc = Vcc = Vtest / Vrated x 100 x Irated / Itest.


If you don`t have a form to carry out the test you can use the M4000 DOBLE, this equipment can measure this value.

Juan C. Castro G.
High Voltage Electrical Engineer