Short circuit studies, demand loads, etc.. 9

[oranjeep]

I was working as an electrical engineer for a short time and I was introduced, unpleasantly, to demand load studies and short circuit studies of power distribution systems.
Unfortunatley, I only understood the one-line diagram part of the whole thing. I would like to understand what I was involved in (what the purpose was, how electrical engineers use the studies, when they became useful, who invented the methods, etc.) Obviously, before that job, I was not involved in power very much, so the whold concept was new to me. Can someone simplify the whole thing?

 

[alehman]

These are complex calculations and the safety of the occupants depends on their accuracy. Are you working for a licensed electrical engineer? Consult with your supervisor to be sure you both understand what is expected and your level of knowledge. A good place to start is the IEEE Red Book.

 

[oranjeep]

No, I am not currently working for the consulting engineer.
Part of the reason is because I did not know what a short circuit study was.

 

[rbulsara]

Do you have access to IEEE books? If not go to its website and check out pricing and see if you can afford to buy. You may search libraries.

As alehman suggested best way to beign is reading IEEE books.

The Buff book (not red) deals with Short Circuit Analysis and coordination (SC&C)study. Demand study is relatively easy but SC &C requries close attention and some learnign curve if you are not familiar with it. You need to find a mentor too.

Seminars can also help.

 

[oranjeep]

Thanks. I will refer to the IEEE books. For now, I found a
reference that might help. Uglys electrical references has an example of a short cicuit calculation where it uses an infinite utility, 1500 kva Xfmr 480 pri, 3 phase, 3.5% Z, 3.45% X, 56% R
25'-500kcmil 6/phase service enterance concuctors in steel conduit, a 2000A switch ans a KRP-C-2000SP fuse (not sure what that is-I know it is a fuse, but I am not sure what all the numbers are for)
The example tells you how to hand calculate the short circuit current at the bus after the fuse. Am I on the right track?

 

[rbulsara]

Yes, u are.

KRPCXXXX is just a catalog number (Bussman or Littlefuse or Ferraz) Check out their websites. It means little expcept for its short circuit interrupting rating which is usally 200kA. You need to understand its energy let-through charactersitics as you get more in to this learning.

In fact application brochures of these (specially Bussman) also have basic crash course on SC&C.

 

[alehman]

Also take a look at Cutler-Hammer's consulting application guide (Tab 1) for good coverage of the basics.

http://cutler-hammer.com/NASApp/cs/...DSCLPdfChapt&c=Apubarticles&cid=1047198780225

 

[advidana]

Looks like you new a few courses on short circuit studies, it will take you more time to try self study than a short courses on this subject. You will need to learn the per unit method of calculations before it clicks in. It is not simple, people usually train for years to use and understand power studies.

 

[peebee]

Have you ever heard of a Thevenin Equivalent Circuit? That's basically what you're doing with a fault calc: finding the Thevenin equivalent impedance at a given point in a network, and then calculating the short circuit current that would flow given that impedance.

The point of the whole thing is that when you experience that fault current, a whole lot of power is being released in a very short amount of time, which in general is a really good way to make something blow up. You need to make sure you specify equipment that's rated to handle the fault current to prevent it from blowing up.

It amazes and scares me how many EE's there are out there who have absolutely no idea what any of this means, and who don't realize the potential dangers they create by ignoring it. I'm glad you're interested in it.

 

[jbartos]

Suggestion: IEEE Std 399-1997 IEEE Recommended Practice for Industrial and Commercial Power Systems Analysis
(Brown Book)
This standard tends to describe various power system analyses, includes examples, and provides tables with useful data.

 

[EEIre]

You will find some excellent free technical guides at the following location:

http://www.schneiderelectric.com/en/expert/e2a.htm

Regards

Sean

 

[oranjeep]

Thevenin. Now that explains it. Thanks a lot.

 

[SidiropoulosM]

Oranjeep, by "demand load studies" you probably mean the load forecasting studies we do in order to know how much load each substation will likely experience during the coming years. This allows us to determine what new equipment will be needed and what system reinforcements will be needed to serve future loads. These are determined by power flow studies, using digital computer software.

Short circuit studies, on the other hand, simulate the system during abnormal conditions, typically during three phase and single phase faults, in order to determine the short circuit current levels, which allows us to specify the ratings of circuit breakers.

At the distribution level the above studies are usually sufficient. At the transmission level more studies are required in order to determine the transient (angle)stability and the voltage stability of the system. These studies result in power transfer limitations of the system.

 

[Midos]

Gent's,
I have picked up some very usefull information from your strings above, thank you all very much.

 

[oranjeep]

Thanks again for all the information. It helps to get a "real persons" perspective on the whole thing. This, combined with seminars, and IEE standards, should give me a good grasp on the concept. By the way, I myself was not doing the SC studies, but I was collecting the relevant data. I was working for a PE, who unfortunately, did not have the time to explain the whole thing.

 

[SidiropoulosM]

A good set of lectures on electric power topics is available at:

http://www.ee.sc.edu/classes/fall02/elct751/

 

[caided]

The study objectives, the criteria of the study, description of the results and how it is applied for the systems are well described in the following book:

Ramasamy Natarajan, Computer aided power system analysis, Marcel Dekker, New York, 2002.

You can find this book in the internet also. The short circuit analysis is in chapter 4. There are description of various studies such as load flow, stability, flicker, harmonics, grounding etc.

 

[jbartos]

Suggestion: Visit

http://standards.ieee.org/colorbooks/sampler/Brownbook.pdf

for the Brown Book Overview

http://www.ecmweb.com/ar/electric_basic_shortcircuit_current/

http://oge.apogee.net/pd/dshe.htm

http://www.uscg.mil/hq/msc/PRGuidance/e2-19.pdf

http://www.nepsi.com/formulas.htm#Q) Displacement Power Factor

http://www.nepsi.com/sysstudies.htm

etc. for more info