Load Flow in per unit: Putting Matter to rest

[mutimuti]

I just thought of a new on Per Unit Power System Analysis thread to finalise a previous discussion hoping also that other academics can benefit from my experience.

I have just finished doing the simulation of a system example by Stagg & Ei-Abiad,shown in his "Computer Methods in Power Systems", page 284. The example is the most comprensive taking 30 pages of the book. The results correspond almost 100% especially for real power flows (reactive power flows, (0 to 5% difference). The per unit values were derived from a 100MVA base (voltage not given) (I am using a 1kV, 1MVA base and don't need to convert anything).

I am quite happy with the experiment and results and I can proceed now to develop my class tasks with confidence.

mmt

 

[jghrist]

As davidbeach noted in the previous thread, the voltage base has to change where you have transformers. Does the example from Stagg & El-Abiad have any transformers?

 

[mutimuti]

No transformers! and almost 100% of the problems that I am dealing with have no transformers so the problems are greatly simplified.

mmt

 

[davidbeach]

No transformers makes for a pretty dull system; for a real education, your students need to deal with off-nominal voltages and off-nominal phase shifts.

 

[Zogzog]

"No transformers makes for a pretty dull system;"

Not to mention impractical

 

[electricpete]

Not to mention - why even bother with a p.u. system if there are no transformers?

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[mutimuti]

Thank you all for all those that participated in this "dull" topic. "Dull" but not trivial. In some ways I was little understood, especially the object of the exercise. We start our studies at a fairly basic theoretical level and then proceed to more complex situations of the world. So your network can start of with just two or 3 buses and even with these resistances and line susceptances can be ignored, just to get the principle. Now any example that you use in class especially originating from text books should be tested for correctness. Believe me there are many wrong solutions in some reputable books. So I use 3 to 4 books and check everything.

Now coming to my point that there are no transformers in my per unit networks. This so happened to be so in the examples I worked on. We can include transformers with fixed ratios (a line with zero susceptance for practical purposes). Once the students understand the basics, we can proceed to the less "dull" aspects using all kinds of software.

 

[powerjunx]

A per unit calculation can't be appreciated much in abscence of power transformer - it's so dull and not ideal for theoretical calculation.
When i had my power system course, my instructor uses an existing complex power grid diagram for fault calcultation using per unit method - it's challenging - while simulating it using software. The result is pretty similar. But the basic example without using transformer was never dealt.

 

[davidbeach]

The appreciation for per unit calculations comes after solving problems that include transformers using physical units and then doing the same thing again in per unit. Then, and only then, does the work necessary to convert everything into per unit seem like a way of making things easier rather than more difficult.