Generator Alternator Reactance

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binwork91

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new york
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electrical engineer
I have a 208V 3P 250KVA generator , and alternator data is in the picture below.

Since my generator is 208V, so I need to convert my alternator reactance from 480v to 208v.

Base on the formula:
1626389223150.png
X0=0.08*((480/208)^2)*(250/250)=0.426
x'd=0.19*((480/208)^2)*(250/250)=1.0118
x"d=0.17*((480/208)^2)*(250/250)=0.9053

x'd is large than 1, it does not make sense. I try to do the research. The formula is right.
But according to number i get, I can tell something is wrong. But I don't know how to make it right.

Anyone know the reason? and provide any article for me to read?
1626389045070.png
 
binwork91 said:
I have a 208V 3P 250KVA generator , and alternator data is in the picture below.

Since my generator is 208V, so I need to convert my alternator reactance from 480v to 208v.

Base on the formula:
View attachment 2557185
X0=0.08*((480/208)^2)*(250/250)=0.426
x'd=0.19*((480/208)^2)*(250/250)=1.0118
x"d=0.17*((480/208)^2)*(250/250)=0.9053

x'd is large than 1, it does not make sense. I try to do the research. The formula is right.
But according to number i get, I can tell something is wrong. But I don't know how to make it right.

Anyone know the reason? and provide any article for me to read?
View attachment 2557184
Click to expand...

Why do you need to convert this? Values are given in p.u.
 
binwork91 said:
The reason is generator is 208V, but the alternator table is base on 480V.
Click to expand...

I see that. If you are modeling with software, your software should be able to handle this. Just enter the base V and base kVA.
 
Xptpcrewx said:
I see that. If you are modeling with software, your software should be able to handle this. Just enter the base V and base kVA.
Click to expand...
I am using EasyPower, I think it only allow to input one voltage for generator, so my generator is 208V, therefore, I need to convert my alternator from 480V to 208V. The software is not smart enough.:(
 
I am using EasyPower, I think it only allow to input one voltage for generator, so my generator is 208V, therefore, I need to convert my alternator from 480V to 208V. The software is not smart enough.:(
Click to expand...

I see. Well, you'll be glad to know your conversion is actually correct. The Z base for 480-V (0.927-Ω) is higher than the Z base for 208-V (0.173-Ω), so if you multiply the p.u. for each case into the respective Z base, you get the same value of actual ohms in both cases. Note: There is nothing abnormal about a X'd p.u. larger than 1. You are converting to a lower V base, so larger p.u. is expected.
 
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Xptpcrewx said:
I see. Well, you'll be glad to know your conversion is actually correct. The Z base for 480-V (0.927-Ω) is higher than the Z base for 208-V (0.173-Ω), so if you multiply the p.u. for each case into the respective Z base, you get the same value of actual ohms in both cases. Note: There is nothing abnormal about a X'd p.u. larger than 1. You are converting to a lower V base, so larger p.u. is expected.
Click to expand...
The problem I faced is the software told me the number should between 0% to 100%. So if my result larger than 1. Something is wrong.
 
Just punch in the figure you find from the datasheet. Understand that the value of X" (subtransient reactance) is in the range of 12% -20%. Then, the transient reactance X' will be a bit larger (19% in your data) and the steady state reactance X even greater than the previous figures! There's no need for you to calculate as the software does those if you punch in the correct base values (kV and MVA bases)
 
So why do you believe it’s wrong?
I see it’s correct

crap. My napkin and pen wasn’t fast enough...
240 would be .76
120 would be 3.04
1.0118 isn’t a far stretch
 
binwork91 said:
The problem I faced is the software told me the number should between 0% to 100%. So if my result larger than 1. Something is wrong.
Click to expand...
Interesting... I just modeled this in EasyPower. It appears the software automatically handles the conversion based on the Generator Rated kV and the connected bus Base kV.

You can model your system as per below. You can see equivalent short-circuit contributions are produced for GEN-1 (480-V) and GEN-2 (208-V) with using the same generator %Z values as given in the datasheet. You can ignore the transformer in this example, as this is only needed to interconnect 480-V and 208-V buses (I set the transformer impedance as low as the software would allow, so its as if its not even there). Hopefully this resolves the confusion...

1626396105058.png

1626395928051.png
 
Xptpcrewx said:
Interesting... I just modeled this in EasyPower. It appears the software automatically handles the conversion based on the Generator Rated kV and the connected bus Base kV.

You can model your system as per below. You can see equivalent short-circuit contributions are produced for GEN-1 (480-V) and GEN-2 (208-V) with using the same generator %Z values as given in the datasheet. You can ignore the transformer in this example, as this is only needed to interconnect 480-V and 208-V buses (I set the transformer impedance as low as the software would allow, so its as if its not even there). Hopefully this resolves the confusion...
Click to expand...
I don't get it, how can software know Voltage of alternator database I use.
If I convert 480V database reactance into 208V (or the data base I get is 208V, sometime I will get a alternator database base on 208V), and if my number are below 1, then I will get a different result.
 
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Hv&Lv said:
So why do you believe it’s wrong?
I see it’s correct

crap. My napkin and pen wasn’t fast enough...
240 would be .76
120 would be 3.04
1.0118 isn’t a far stretch
Click to expand...
The reason is software does not allow me to input number larger than 100%.
So, I must miss something or misunderstand something.
 
Your Gen-2 gives the exact same result as in my example. Not sure what you are doing for Gen-1. What Generator Rated kV are you using for Gen-1?
 
Xptpcrewx said:
Your Gen-2 gives the exact same result as in my example. Not sure what you are doing for Gen-1. What Generator Rated kV are you using for Gen-1?
Click to expand...
sorry for that, i found out that Gen-1, I did not change the Voltage in the gen-1, so I delete the post. the result is same as yours, after i correct it.
 
If your alternator is 208V, why not get the info for the alternator you have?

Doing a conversion might get you in the ballpark, but we can't know exactly how everything is going to change for 208V. The wire on the alternator winding will have to get larger, but it may not be just by the 480/208 ratio. Now the windings will be larger, the wires will be longer, the core the windings are on will be larger - basically everything will have to change, and assuming the reactance will change by some fixed ratio may not be right.
 
steve66 said:
If your alternator is 208V, why not get the info for the alternator you have?

Doing a conversion might get you in the ballpark, but we can't know exactly how everything is going to change for 208V. The wire on the alternator winding will have to get larger, but it may not be just by the 480/208 ratio. Now the windings will be larger, the wires will be longer, the core the windings are on will be larger - basically everything will have to change, and assuming the reactance will stay the same PU value might not be right.
Click to expand...

OP has the correct datasheet. The problem is it uses a 480-V Base and the software doesn’t allow values greater than 100%.
 
binwork91 said:
I don't get it, how can software know Voltage of alternator database I use.
If I convert 480V database reactance into 208V (or the data base I get is 208V, sometime I will get a alternator database base on 208V), and if my number are below 1, then I will get a different result.
Click to expand...

Because the software uses the Generator Rated kV “together” with the bus Base kV to do the conversion.
 
steve66 said:
If your alternator is 208V, why not get the info for the alternator you have?

Doing a conversion might get you in the ballpark, but we can't know exactly how everything is going to change for 208V. The wire on the alternator winding will have to get larger, but it may not be just by the 480/208 ratio. Now the windings will be larger, the wires will be longer, the core the windings are on will be larger - basically everything will have to change, and assuming the reactance will change by some fixed ratio may not be right.
Click to expand...
The alternator data is provide by generator supply house. I tried to get same Voltage alternator data as generator once. The generator manufacture only provide the formula, they asked me to do the calculation by myself. So I think I have no choice.
 
Xptpcrewx said:
Because the software uses the Generator Rated kV “together” with the bus Base kV to do the conversion.
Click to expand...
For example, the generator manufacture do the test for alternator, and provide me two data sheet, one base on 208V, other one base on 480V.
I think, I will use 208V, since my generator is 208V. but OP has 480V alternator.
Do you think the result will be same? base on our hand calculation, we know reactance is different between 208V and 480V.
How can software know the number i input is from 208V datasheet or 480V datasheet.
 
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