# NCEES sample problem 530, power.



## LMAO (Sep 17, 2010)

Can anyone explain what it means in problem 530, when it says "the transformer and the 12.47kV system have them X/R ration"?

I understand how the answer is calculated: basically, first you find the *Z*pu of 12.47 bus, added it to the transformer *Z*pu; then find *S*sc by dividing *S*base by *Z*pu and finally find the *I*sc by dividing *S*sc by (1.73x*V*base).

I just don't understand how "the transformer and the 12.47kV system have them X/R ration" is a hint? What does it mean and what the is use of it?

Thanks you,


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## Flyer_PE (Sep 17, 2010)

The X/R ratio is simply the ratio of reactance to resistance in the transformer. It's extraneous information for this problem.


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## GabeM (Sep 19, 2010)

Flyer_PE said:


> The X/R ratio is simply the ratio of reactance to resistance in the transformer. It's extraneous information for this problem.


I disagree that it doesn't matter. The X/R ratios being the same mean that you can add the impedances without using phasor addition. If the X/R ratios were different, then the phasors would have different directions and you couldn't add the impedances as scalar values.

However, even if the problem didn't state the X/R ratios, it would be reasonable to assume that the X/R ratios are the same.


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## Flyer_PE (Sep 19, 2010)

GabeM said:


> Flyer_PE said:
> 
> 
> > The X/R ratio is simply the ratio of reactance to resistance in the transformer. It's extraneous information for this problem.
> ...


Assuming equal X/R values between a power system and a transformer isn't a safe assumption. In the given problem, the relative X/R values just don't matter. The system fault capability is given in MVA, no angle given or required. The system is the source bus. In order for the X/R values to start to matter, you need an interaction with other fault contributing equipment or another transformer.


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## GabeM (Sep 19, 2010)

Flyer_PE said:


> Assuming equal X/R values between a power system and a transformer isn't a safe assumption. In the given problem, the relative X/R values just don't matter. The system fault capability is given in MVA, no angle given or required. The system is the source bus. In order for the X/R values to start to matter, you need an interaction with other fault contributing equipment or another transformer.


I agree that assuming equal X/R values is not safe; I was referring to making that assumption in the test and not in real life (since, in my opinion, you have to make this assumption to have enough information to come up with a number).

I have been looking at threads here and here to better understand what you mean by how the X/R values don't matter. I understand how to do the problem but I don't understand how the X/R ratios don't matter. There is an impedance associated with the system and an impedance associated with the transformer. You can't just add impedances together like scalar values because they are vectors. However, if the vectors have the same angle, then you can add them together like we do in this problem. See your post on February 24 9:27 PM in the first thread I linked above. You add 1 ohm pu (system impedance) and 1.6 ohm pu (transformer impedance) together like scalar values. Are impedances not vectors?

I also don't see the difference between the source bus and "other fault contributing equipment or another transformer". There could be another transformer in the source bus for all we know, right?

I am new at this so I feel I must be missing something, but I don't know what.


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## Flyer_PE (Sep 20, 2010)

GabeM said:


> I agree that assuming equal X/R values is not safe; I was referring to making that assumption in the test and not in real life (since, in my opinion, you have to make this assumption to have enough information to come up with a number).
> I am new at this so I feel I must be missing something, but I don't know what.


I don't think you're missing anything. I think we are in violent agreement. My statement was basically that the X/R ratios don't matter for this particular sample question. If you're given a problem like this on the test, equivalent X/R ratios is probably a safe assumption to make since they probably won't give you a problem involving that much math. I was just making the point that making that assumption in the real world can get you in trouble. I'm probably a little more sensitive to that one since I do a lot of fault studies for power plants. A lot of assumptions that work just fine for other facilities get thrown right out the window at the power station.


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