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# % Xd to Ohms

04/13/2017 8:28 AM

Dear All, good morning

Someone have the formula to change from %Xd to ohms?

Best Regards

Sergio

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#1

### Re: % Xd to ohms

04/13/2017 8:43 AM
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#2

### Re: % Xd to ohms

04/13/2017 9:13 AM

Because a percentage is unitless - it's just a number without dimensions - it cannot be converted to ohms. The ohm (Ω) is an SI derived unit having the base unit equivalent kg⋅m2⋅s−3⋅A−2.

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#3

### Re: % Xd to ohms

04/13/2017 9:27 AM

John, good morning

In order to calculate the generator capability, I have to use the Xs.. As you can see in the picture, none of this values is Xs.. in a paper show that I have to use Xd, but in ohms.. so..

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#4

### Re: % Xd to ohms

04/13/2017 9:42 AM

X (whether it's Xd, Xq or Xs) is a reactance. Units: ohms.

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#5

### Re: % Xd to ohms

04/13/2017 10:03 AM

John, good morning

Are you sure about that? At the upper left side show "reactances %"

Best Regards

Sergio

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#6

### Re: % Xd to ohms

04/13/2017 10:13 AM

I am certain that X (as a reactance) is measured in ohms. I've no idea what those are percentages of. The image you posted is too small to discern, and it's not my field anyway. Where did you find the spreadsheet? Maybe the source can throw some light on it.

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#7

### Re: % Xd to ohms

04/13/2017 10:20 AM

Jhon,

The image belong to a datasheet sent to me by Emerson, at the upper tittle shows the generator model and serial.

For that reason I'm asking the formula to change the %reactance to ohms.

Best Regards

Sergio

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#8

### Re: % Xd to ohms

04/13/2017 10:47 AM

I suggest you e-mail (or better, call) Emerson and ask for some technical assistance.

Good luck.

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#9

### Re: % Xd to ohms

04/13/2017 11:54 AM

John,

I have just found the answer..

Xd% = 1/SCR*100%

Xdohm = Xdpu*base Impedance

Xdpu= 1/SCR

BR

Sergio

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#14

### Re: % Xd to ohms

04/14/2017 12:44 PM

The % is based on the unit rating in VA, and the rated voltage, volts. For 3 phase systems, square the Line to Line voltage, divide that by the VA, then multiply that value, base impedance at voltage, by the per-unit value of your % impedance to get ohms.

Your 1500kVA generator with 5% impedance, rated at 480Vac has 0.1536 base impedance, your ohms are 0.05 x 0.1536 = 0.00768 ohms.

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#11

### Re: % Xd to ohms

04/13/2017 12:22 PM

Click on a cell containing one of the %Xd figures to show the spreadsheet equation used to compute it, and tell us what it says.

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#10

### Re: % Xd to ohms

04/13/2017 12:13 PM

Yup...I think what you're looking for is inductance to impedance calculation....

http://www.uotechnology.edu.iq/dep-eee/lectures/3rd/Electrical/Machines%202/II_SG.pdf

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#12

### Re: % Xd to ohms

04/14/2017 12:31 PM

1) Agreed.

2) The "%" is not an error, it is part of the conversion between the percent system and the per unit system. Any quantity in per unit times 100 = the same quantity in percent; i.e., 1pu = 100%.

For example, the data sheet shows Xd-sat as 176.1%, which is equivalent to 1.761pu. To convert to Ohms, take the base impedance (0.180Ω) times 1.761pu to get 0.317Ω.

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#13

### Re: % Xd to Ohms

04/14/2017 12:35 PM

In order to calculate X"d in ohm you have to multiply xd”% by 0.6^2/1.950 and divide by 100-VLL[kV]^2/S[MVA]*xd"%/100.It is very simply.

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#15

### Re: % Xd to Ohms

04/14/2017 6:30 PM

Somehow I think it's simpler to do it this way => X"dΩ = (X"d%/100)*ZbaseΩ, but to each his own.

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#16

### Re: % Xd to Ohms

04/15/2017 12:43 AM

I agree with you if Vbase=Vrated and Sbase=Srated.

Usually in USA the parameters are presented in p.u.

and Sbase=100 MVA [for instance] and Vbase could be rated transformer secondary voltage or else. It seems in USA the calculation method illustrates the USA democracy.

In IEC standards usually in % [See IEC 60909-1 ch.3.6.1 Synchronous generators- for instance] of Zrg=Vrg^2/Srg.

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