Single line to ground fault

[Mohammed Ahmed]

Hi,
I solved this question in different way by using transformer turns ratio but I got different answer, may I know why the second method is wrong? Thanks

 

[Cram For The PE]

You are mixing up turns ratio and PU. These fault values are well known for protection engineers. No matter what the turns ratio of the transformer is- if you have a line to ground fault on the wye side the 1pu appears as 0.577 pu on the delta secondary side. You are also missing an important point about the 30°. I admit I did not derive it fully and I have been meaning to do this. Here it is below.

 

[tmntjmc]

What's the best way to memorize the fault symmetrical components formulas? the reference guide gives us very basic fault diagrams with no real values.. Any advice would be greatly appreciated.

 

[COJeff]

What's the best way to memorize the fault symmetrical components formulas? the reference guide gives us very basic fault diagrams with no real values.. Any advice would be greatly appreciated.

There are no formulas to memorize. If you look at chapter 5 of the NCEES reference manual the current relation ships are listed. All you need to do is apply Kirkhoff laws!

 

[akyip]

I learned these fault circuit diagrams from Zach Stone's Electrical PE Review course. The way he taught is made it quite easy for me to remember and draw out the circuit analysis diagrams for each type of fault.

If anyone has any questions or thinks you see something wrong with these diagrams, feel free to let me know.

 

[Cram For The PE]

What's the best way to memorize the fault symmetrical components formulas? the reference guide gives us very basic fault diagrams with no real values.. Any advice would be greatly appreciated.

They give it in the reference handbook. The problem listed above is very different though. It is asking this:
"If I have a single line to ground fault occurs on the wye side of the transformer, what does it look like on the delta side"?
This requires a little bit of thinking and is not simply plug in numbers into a formula. I would not stress this problem to much honestly. The chances of them asking such a question are small. But if you get this problem it really means you understand what is happening. It is also important concept to know.
I once worked at a plant and a single line to ground fault occureed . No one could understand the data from the relays. It was only until they asked me and I showed them this diagram that it become clear to all the engineers what exactly happened.