NCEES #536- charging current?

[ElecPwrPEOct11]

I haven't seen a thread on this problems yet. I hadn't heard of charging current before so I think this is an easy answer. Here goes the lob.

For this transmission line problem finding Deq and C are straightforward. Finding Xc you use Xc= 1/(2* pi * f * c). So far so good. Now where does the charging current equation come from: Icharging= Vphase / Xc? I can't find it in Camara's EERM or online. Thanks a ton for any help!

 

[TFT]

PSA Grainger - page 180 has the charging current equation.

 

[ElecPwrPEOct11]

I don't have any books by Grainger. Care to paste it here?

 

[knight1fox3]

I don't have any books by Grainger. Care to paste it here?

Ouch, it's a good book to have for the exam.

 

[DK PE]

+1 on having Grainger/Stevenson or earlier Stevenson. You really need some book on power systems IMHO

Since you listed the equation do you need the book title: Power system analysis by Grainger and Stevenson

I would guess Glover/Sarna or whatever is used more these days has also it but I don't have a copy.

 

[mudpuppy]

Yes, charging current is covered in Glover & Sarma.

 

[ElecPwrPEOct11]

OK, well I may be making a last minute purchase of yet another textbook. I've managed all of the practice problems so far with out it, so I'm a little reluctant to buy another textbook I'll use only once. Votes for Grainger/Stevenson vs Sarma?

In the meantime, can someone please post the transmission line charging equation? Thank you.

 

[DK PE]

You had it written above in 1st post as Icharging= Vphase / Xc I have seen it also written as Icharging= jωC Vl-n

 

[ElecPwrPEOct11]

Thanks DK PE, I didn't know if that was the general form of the equation or not. NCEES solutions often skip steps.

 

[mudpuppy]

From Glover & Sarma:

For a completely transposed three-phase line that has positive-sequence voltages with VAN = VLN/_0°,

Ichg = jωC1VLN

Essentially, it is just Ohm's Law, e.g. I = V/X.

As for buying a new textbook now. . . you wouldn't have a lot of time to become familiar with it before the test, and you'll need to be able to find things in it quickly, so I'm not sure how much good it would do. Just my two cents.

 

[knight1fox3]

From the Sarma book: The current supplied to the transmission-line capacitance is called charging current. For a single-phase circuit operating at line-to-line voltage Vxy = Vxy ∠ 0°, the charging current is Ichg = YxyVxy = jωCxyVxy A

Both the Grainger and Sarma books are good for power systems analysis. I had more topics/eqns. flagged in the Sarma book though. And also as DK mentioned, I purchased the older version of the PSA book by Stevenson only (at DK's suggestion I believe-Oct 2010, LOL) which I found to be less cluttered with heavy details.

 

[dirk2827]

Chapter 9 of the Handbook of Electric Power Calculations has the formula and a few examples. The example on pg 9.28 has the formula written incorrectly (it shows multiplying by Xc rather than dividing), but the calculation is correct.

 

[DK PE]

And also as DK mentioned, I purchased the older version of the PSA book by Stevenson only (at DK's suggestion I believe-Oct 2010, LOL) which I found to be less cluttered with heavy details.

Then for ElectPwrOct11 I'll make the same suggestion as I did for knightfox ..and it worked for him .... A used old version of Elements of Power systems Analysis by William Stevenson maybe 4th edition is a good book and may be $20 if you look around.

 

[ElecPwrPEOct11]

Alright, I ordered the PSA by Glover/Sarna with quick shipping. Hopefully it's more relevant than the >15 year old Stevenson textbooks. Maybe I'll actually use it after the exam? I won't have much time to study from it but if I get any more curveball Qs I have a chance of finding them in the book.

 

[snerts50]

Chapter 9 of the Handbook of Electric Power Calculations has the formula and a few examples. The example on pg 9.28 has the formula written incorrectly (it shows multiplying by Xc rather than dividing), but the calculation is correct.

I recently printed parts of this book out. Filled with useful information. Wish I had it sooner.

 

[ElecPwrPEOct11]

I have another quick question about this problem. The formula calls for Icharging = Vphase/ Xc. In this problem how do you know if the 115kV they give you is a phase or line voltage? If it's a delta system they would be the same and you wouldn't need to divide by sqrt(3) for the answer. How do you know it's a wye?

 

[DK PE]

It would be nice to see explicitly stated but unless otherwise, I would assume all standard transmission line voltages given i.e. 69kV, 115kv, 345kV are line voltages, otherwise known as line-line values.

 

[ElecPwrPEOct11]

^ Hmm, I guess that's fair, and something I assume for motors. But it is assuming wye configuration. Especially for transmission lines it seems delta is more common. And a neutral conductor was not mentioned in the problem statement.

 

[DK PE]

I think you're getting too hung up on delta/wye. If I have a generator hooked to a transformer but keep it hidden in a black box so you can't see anything and hand you the three conductors. Then I tell you this is a balanced three phase source with a line (which means line-line) voltage of say 115kV.

You should be able to tell me the line-neutral voltage... the fact that there isn't a neutral conductor leaving the black box you can touch is not a factor. The fact that there are three phasor voltages and 120° angular displacement describe all you need.

 

[ElecPwrPEOct11]

^ yes, too much thinking on my part. Back to the KISS principle, thanks. DK, do you have time for a quick response for my three phase fault question here:

http://engineerboards.com/index.php?showtopic=16724 ?

I could really use a logic test before tomorrow AM.