1 Per Unit as rated values

[mull982]

When you are given a piece of equipment and told its actual paramaters are those paramaters always taken as 1 per unit for the respective voltage, VA, and current?

For instance if we have a 10MVA generator, rated 13.8kV, 418A, is the rated voltage, current and VA equal to 1.0 where 1.0 pu corrosponds to the rated values listed above?

What about the rated impedance. Lets say we are told that X=15% then do we take rated impedance to be 1pu where 1pu equals 15% or do we take rated impedance to be .15pu?

 

[cableguy]

15% would be .15 "per unit", so that's your "per unit" impedance right there. Typically though, yes, given kV and given MVA are the 1.0 per unit ratings of a transformer. It's rare to see a given 1.0 per unit current in a problem statement though. Keep in mind that if you have different MVA transformers in the same problem (say, one at 10 MVA, one at 20 MVA), you have to make their impedances match by doing a Zpu conversion on one of them. Pick one, doesn't matter, the answers work out the same either way. But you have to convert before working the problem.

1.0 per unit impedance would be when your actual impedance is equal to your impedance base. Zbase = V^2/S (where V is line-to-line and S is 3-phase)

So in this case, Zbase = 19 ohms (take 13800^2/10,000,000). Actual Z = Z per unit * Zbase, so .15 x 19 = 2.85 ohms actual.

You need to make a formula sheet and write all of these per unit equations on it!

 

[mull982]

15% would be .15 "per unit", so that's your "per unit" impedance right there. Typically though, yes, given kV and given MVA are the 1.0 per unit ratings of a transformer. It's rare to see a given 1.0 per unit current in a problem statement though. Keep in mind that if you have different MVA transformers in the same problem (say, one at 10 MVA, one at 20 MVA), you have to make their impedances match by doing a Zpu conversion on one of them. Pick one, doesn't matter, the answers work out the same either way. But you have to convert before working the problem.
1.0 per unit impedance would be when your actual impedance is equal to your impedance base. Zbase = V^2/S (where V is line-to-line and S is 3-phase)

So in this case, Zbase = 19 ohms (take 13800^2/10,000,000). Actual Z = Z per unit * Zbase, so .15 x 19 = 2.85 ohms actual.

You need to make a formula sheet and write all of these per unit equations on it!

Thanks thats what I thought, I just didn't want to get confused at the last minute.

If you have two transformers with different rated MVA's then you can even choose a 3rd arbitrary MVA as the base as long as you convert both transformers to the third base. Isn't that correct?

 

[cableguy]

Yes, but that's time and extra work. This test is a time test, and how well you use it. Don't make extra work for yourself.