KVA Method Vs. Per-Unit Method 1

[ThePunisher]

Hi all, I am a big fan of calculating short circuit currents using the per-unit methodology. However, one of our colleagues is down playing this method to be unecessarily cumbersome and stated the KVA method is simple and fast.

I do disagree with him since the KVA method does not provide facility to calculate line-to-ground faults. Furthermore, the KVA method may or may not be conservative on some situations.

I would like to request some feedbacks on the disadvantages of calculating short circuit currents using the KVA method compared to a per-unit method.

Thank you

 

[davidbeach]

What, pray tell, is the kVA method?

 

[umrpwr]

David,

The kVA or MVA method is just a simplified method for performing short-circut calculations by being able to add the equipment ratings in kVA or MVA in order to be able to calculate the equivalent short-circuit current. Nothing that great about it. I think one of the first papers was published in IEEE. See citiation below which is available through IEEE Xplore.

Short Circuit ABC - Learn it in an Hour, Use It Anywhere, Memorize No Formula (Paper TOD-73-132) was approved by the Petroleum and Chemical Industry Committee of the IEEE Industry Application Society for presentation at the 1973 Petroleum and Chemical Industry Conference, Houston, Texas, September 17-19. Manuscript was released for publication October 01, 1973.

There are also several other authors we have written and presented similar methods. I believe EC&M published a book titled Short-Circuit Calculations the Easy Way by John Pascal. Cooper Bussman also has something similar in their SPD.

To answer the original post I agree with the poster that the P.U. method is better. The only thing you are doing with the KVA or MVA method is getting an equivalent value to use for calculating short-circuits. It provides an easy way depending on what you are looking for though. Drawbacks to the simplified method are that that you aren't looking at the X/R ratio and other things like derating of devices. Also per-unit provies a relatively simple way to compare the magnitudes rather than actual units. 1.5 P.U. probably means a lot more than just XX kA.

Link below to EC&M article summarizes the same things.

http://ecmweb.com/mag/electric_beware_simplistic_fault/

 

[PHovnanian]

Manuscript was released for publication October 01, 1973.

Back in the days when everyone was doing SC calculations with a slide rule. Most of these methods are attempts at simplifying the problem so as to make it tractable with the tools of the day. Lots of rules of thumb and other assumptions in order to reduce the number of parameters dealt with.

Now we've got computers (maybe even iPhone apps) that will happily crunch large matrices without complaint. There's still a place for doing back-of-the-envelope calculations. But I'd like to see some disclaimers included about the assumptions made and limitations of the technique in question.

 

[rcw retired EE]

I am a proponent of the MVA method and use it a lot. Many times I am in a discussion or meeting or traveling and do not have a computer with ETAP or EZ power but I can quickly calculate Short Circuit currents or motor starting voltage drops using the MVA method.

Phase-Ground and phase-phase faults can be calculated using the MVA method as long as you can calculate the zero and negative sequence impedances.

Results are accurate enough for most preliminary work. If the short circuit answer is closer than 80% to some critical figure, further work is needed. The answers are within the range of accuracy of the numbers we are using for input: cable lengths, utility short circuit values, generator impedances, etc.

I like being able to look at a power plant one line and in several minutes come up with equipment ratings or areas for further study. The MVA method also provides a good check of our computer program results.

But I am biased. Moon Yuen, the EE who wrote the articles on the MVA method, worked for me in 1974-75 and taught me a lot, so some of my MVA cheerleading is approaching hero worship.

 

[ThePunisher]

Thank you for all your inputs. umrpwr, thank you for the EC&M link.

rcwilson, using the kVA method, did any P.E. approved and certified such calculations in the past? May I get your comments on the EC&M link posted by umrpwr.

david, can elaborate what mean by "What,pray tell"?

 

[davidbeach]

Just an expression. Older english; "What, pray tell, ..." would more likely be something like "What the heck is ..." in modern usage.

 

[Miguel Villarroel]

KVA for preliminar and per-unit when designing, not worth hair-pulling

If there was no electricity there would be no internet. Good point, don´t you?

 

[rcw retired EE]

Punisher- The PE I learned it from was a Chief Engineer at Bechtel Corp. I have stamped calculations using the MVA method and know several other PE's who have done so. (Of course many of us also did a lot of SC calculations by hand with a 6" slide rule.)

The EC&M article by my neighbor brings up good points. As I said above, if the result is close, further analysis is needed. For example, if the MVA method comes up with 32 kA including motor contribution with cable impedance neglected and the MCC is rated 55 kA, I'm not going to do further calculations before ordering the gear. If it was 40 kA or more, I would do more calculations.

However, we will still model the system with ETAP or EZ Power to verify all ratings and to perform the arc flash study.

The MVA method results are not suitable for arc flash calculations except for preliminary rough checks.