NCEES 532

I guess this seems simple enough the way NCEES presents it as simple OHMs law. My confusion, Line versus System Impedance, hopefully a simple explanation of terminology. I haven't looked much at Transmission lines yet its next on my list.

I understood the Line impedances of 16.75/_ 81 Ohms and 13.4/_81 Ohms as each line of the 3 phase system. The system Z of 13.25/_71 as as the 3 phase and I wanted to divide that by three and add it to the lines to analyze it on a line basis (as they did in their solution 60kV/sqrt 3).

Thanks in advance (Jim?) lol

John

John,

Yep, it's me again. I work from home. This stuff gives me a nice distraction from "real" work.

When they give you the system impedance in the manner listed, the safe assumption is that they are giving you a phase value. The transmission line impedances are also treated as phase values. Therefore, the first thing to do is simply add the three impedances together. In order to solve for the current, the voltage needs to be on a phase basis also. The way I was taught to deal with most 3-phase problems is to first attempt to reduce it to a single phase problem. Once you have it reduced to a single loop, it becomes a simple circuit analysis problem.

Jim

I was always taught:

convert everything to wye (if not already)

then to a single phase equivalent circuit

convert to PU depending on complexity, xfmr's, etc.

line Z is usually per line/phase

load/system Z is usually per phase

As far as what the "system impedance" is: it's just the Thevenin impedance of the entire electric system behind a particular bus--the series/parallel combination of all lines, transformers and generators. System impedance is usually given as the positive-sequence impedance (sometimes along with the zero-sequence), which can be used directly in balanced single-phase calculations (i.e for a three-phase fault).

I'm not sure if anyone has run into this on the PE, but in practice sometimes available short-circuit is given (in Amps or MVA) instead of the system impedance directly. This can be dealt with by assuming a source of 1 per-unit voltage behind the unknown impedance produces the given quantity of Amps or MVA (so you know V and I and just solve for Z).

Thanks guys, that is pretty much what I was able to surmise just wanted to hear the logic. I am all about converting to the single phase analysis.

Man 33 days left, how did you guys ever do this...I'm too old for this crap!

John

mudpuppy said:

I'm not sure if anyone has run into this on the PE, but in practice sometimes available short-circuit is given (in Amps or MVA) instead of the system impedance directly. This can be dealt with by assuming a source of 1 per-unit voltage behind the unknown impedance produces the given quantity of Amps or MVA (so you know V and I and just solve for Z).

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I think the relevant problems on the PE for when I took it had at least one infinite bus problem as well as a couple where you would have to determine the source impedance of a generator or transformer. Either way, I would think you will have to know how to get the fault impedance when given an available MVA.

Jim

hi there - sorry, i'm hijacking your thread b/c i don't know how to post; it's telling me i'm not allowed and i have some questions re: the PE exam! any ideas out there, why i can't post?

so sorry to interrupt.

shellbell500 said:

hi there - sorry, i'm hijacking your thread b/c i don't know how to post; it's telling me i'm not allowed and i have some questions re: the PE exam! any ideas out there, why i can't post?
so sorry to interrupt.

Click to expand...

You have to post twice and then you should be able to post or open a new thread so one more post for you and :bio: