Percentage to per unit conversoin

[kaspor]

Hello

I have the following data for a transformer in per unit which I need to convert to % impedance to enter into ETAP:

MVAbase = 55
Voltage base = 33kV
Positive & negative impedance LV-HV = 0.0030 + j0.1 per unit
Zero sequence impedance LV-HV = 0.002 + j0.06 per unit

To get this into % impedance:
Zpos(%) = sqrt(0.0030^2 + 0.1^2) * 100% = 10%?
Zneg(%) = sqrt(0.002^2 + 0.06^2) * 100% = 6%?

Are these the numbers I enter into ETAP?

 

[davidbeach]

If you need to ask how to convert percentage to per unit...

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[kaspor]

Sorry for my ignorance, I don't understand the response?

One other query I have, for a 3-winding transformer, impedances are provided in terms of Prim-Sec1, Prim-Sec2 and Sec1-Sec2. Assuming Prim-Sec1 and Prim-Sec2 impedances are the same (let's assume 5%), If I make the Sec1-Sec2 impedance the same as this (i.e. 5%), am I going to get a conservative value for fault current?

Let's say there's a fault on secondary winding 1, fault current will flow from Primary winding to Secondary winding 1 and Secondary winding 2 to Secondary winding 1. Typically I've seen Sec1-Sec2 impedances much larger than Prim-Sec impedances. So if I assume a lower impedance for Sec1-Sec2 (i.e 5%), for a fault on secondary winding 1, I am going to get a higher value of fault current from secondary winding 2?

 

[davidbeach]

Why not just use the impedances in the transformer test report? Percent and per unit are first or second year topic. Like the second or third chapter in Blackburn.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

Pri to Sec1 impedance is tested with Sec1 shorted. The contribution from Sec2 will be part of the test results.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

All three should be specifically included in a decent test report.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[kaspor]

Apologies I should've pointed out that I do not have the test report. The transformers have not been manufactured yet. This is a preliminary study.

 

[davidbeach]

Then they can be most anything. What do the specs call for?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Kiribanda]

kaspor,
Do you have the basic knowledge of per unit sytstem in power system calcultions?

 

[kaspor]

I'm not sure why the many questions? If a transformer impedances are represented with the following figures:

Positive & negative impedance LV-HV = 0.0030 + j0.1 per unit
Zero sequence impedance LV-HV = 0.002 + j0.06 per unit
MVAbase = 55MVA

Then these are equivalent to the % impedances in cartesian format? To convert to Z%, you would perform the following:

Zpos_seq(%) = sqrt(0.0030^2 + 0.1^2) * 100% = 10%
Z0_seq(%) = sqrt(0.002^2 + 0.06^2) * 100% = 6%

 

[Palletjack]

I believe the title of the post is what’s creating the questions and confusion.
The title suggest you want help converting the numbers.

I believe the real question is what number you need to enter into ETAP software, correct?

If so I can’t help. Haven’t used ETAP.
But I hope the question is cleared up some.

 

[Parchie]

@kaspor,
Please understand that per unit is the result of dividing an actual impedance with a the base impedance. But if you multiply the per unit value with 100, you get the percent impedance!

Perhaps you are confused with finding equivalent per unit impedances in a different MVA base, are you? Then, it is simple: New Zpu = Old Zpu x (New MVA base/Old MVA base)

 

[davidbeach]

The first rule of per unit club is that we don't talk about per unit club... ;-)

I’ll see your silver lining and raise you two black clouds. - Protection Operations