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# Omer

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Project Engineer
1. ## zero sequence

Does the zero sequence current flow on the Y connected winding of the XFR?
2. ## NEC/ Overcurrent

@Stephen2awesome Thanks.. This is an interesting one. So for the motor overload I should go for the next lower level. But for the conductor I am allowed to go for the next upper level, right? is it applicable also for the short circuit/ground fault?
3. ## L/L fault question

Yes, it is .5 pu and you will have draw the sequence diagram for LL fault to get the answer.
4. ## NEC/ Overcurrent

is there on the NEC (the part we are usually tested on) a requirement to go for the (next lower level) on the overcurrent protection (overload, ground fault and short circuit) for conductors and motors? (branch circuit, feeders). All I have seen is (next upper level)

6. ## 2 weeks until the exam! Still feeling a little uncomfortable with Per Unit Analysis? Here is a detailed example broken down step by step

I know what you are trying to say, and you are right. Actually, what is going on is that, the secondary voltage of XFR T2 is no longer 13.8 Kv due to the voltage drop of the line and the transformers. and this becomes more evident as high currents flow in the system. The secondary voltage of T2 is even at zero voltage when a 3 phase short circuit occur at the secondary of T2. and the current will only be limited by the series impedances of the system. however, if the voltage at the load was specified at 13.8 KV, I would use my previously mentioned approach.
7. ## 2 weeks until the exam! Still feeling a little uncomfortable with Per Unit Analysis? Here is a detailed example broken down step by step

Of course, if I am not interested in the phase angle.
8. ## 2 weeks until the exam! Still feeling a little uncomfortable with Per Unit Analysis? Here is a detailed example broken down step by step

Actually, transformers Impedances and TL impedance doesn't matter at all. I just need to know the load resistance (which is provided 200 ohm) and the voltage at the load (provided 13.8 Kv) to calculate the current VLL/sqrt(3) / RL =39.8 A (same answer)
9. ## 2 weeks until the exam! Still feeling a little uncomfortable with Per Unit Analysis? Here is a detailed example broken down step by step

Thanks for the question, it is good for practice. However, for this specific problem we can directly calculate the current in the load side and then using the transformers ratio to obtain the current on the other zones, right? Thanks again for the detailed explanation on the link.
10. ## CI test 4 Q2

Thanks @rg1 I just wanted to confirm.
11. ## CI test 4 Q2

Is the solution for this problem correct?
12. ## pu and X/R Ratio

I couldn't see from where he got this relationship. Impedance of the generators could be any value. If I would solve this question is and the impedance of the generator is not given, I would consider it as in finite bus.
13. ## pu and X/R Ratio

Impedance or pu impedance is a complex number having both terms X and R. X/R = tan(theta) .. the impedance angle. if for two equipment, say generator and transformer, having same X/R ratio, then both have the same impedance angle and you can simply add the pu impedance. if X/R ratio is different, then you calculate impedance angle for both and add the pu impedances as vectors. For the above question, I couldn't see how he get the pu of the generator by tis simple division. How was the original question?
14. ## Transformer Losses

The total loss at full load of a single phase transformer is estimated to be 2000 W. The transformer maximum efficiency occurs at 45% of the full load. The estimated no load loss of the transformer is approximately?
15. ## CI-II-54

what you are saying is absolutely right. the only issue with this problem is that, the PF should be taken at the load not at the source. the current should be 34.9 lagging the terminal voltage which have to be calculated. if we assume the PF at the source then what you mention is 100% right.
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