Base Apparent Power in a Per-Unit System (Sbase)

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IMissPLa07 Active Contributor Join Date: Aug 2007 Posts: 19	Base Apparent Power in a Per-Unit System (Sbase) 02/19/2010 3:32 PM
	I'm studying for the electrical PE exam in Power. In my study book, one of the example problems is looking for the short circuit current and states (and I quote) "Using an arbitrary base of 100MVA for Sbase..." When studying sample questions from the NCEES, the same type problem came up, and, again they used 100MVA as Sbase (when I looked at the solution...it wasn't stated in the problem statement). My question is, is 100MVA a standard for apparent power? I can't seem to find any precedent in any of my books that states CLEARLY that Sbase in a per unit system is 100MVA. But you cannot solve this problem without knowing that. How do I know which "arbitrary" number they are going to use (or should I ALWAYS assume Sbase is 100MVA??) Thanks for your help!
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msamad Guru Join Date: Feb 2009 Location: Houston, USA Posts: 946 Good Answers: 244	#1 Re: Base Apparent Power in a Per-Unit System (Sbase) 02/19/2010 5:27 PM
	In 'per-unit' method fault calculation, you can select the base MVA any arbitrary value. The ultimate fault current result will be same whatever MVA value you select. In most cases, the largest MVA value mentioned in the problem is considered as the base MVA value. For verification, you can try to solve the same problem using another base MVA value and compare the short circuit current values – you will find the same value. Same thing for the base KV. - MS __________________ "All my technical advices in this forum must be consulted with and approved by a local registered professional engineer before implementation" - Mohammed Samad (Linkedin Profile: http://www.linkedin.com/in/msamad)
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Youngin Active Contributor Join Date: Nov 2007 Posts: 19	#2 Re: Base Apparent Power in a Per-Unit System (Sbase) 02/20/2010 9:05 AM
	The selection of the base VA rating is arbitrary, but be sure you use the base you select consistently. Once you select one, it is locked down. In your problem, 100MVA was selected, probably because it was a convenient base for solution of the problem. '100' is a nice number, because it allows for easy conversion of other values to that base in the per-unit system. For example, 500MVA on that base would be 5.0 pu. Only the base kV value will change, as you cross through transformers.
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