Load Flow for in per unit
Load Flow for in per unit
(OP)
I am an educator and find that many examples for load flow given in textbooks are in per unit. The software which I use uses physical data (ohms, km MW KV etc)and there is no option in my software for this. I tried to use 1kV, 1Km (impedance is given per km) and 1 MVA so that I would not have to change anything but this only worked for certain cases, for other cases there was no accuracy?. Any advice?






RE: Load Flow for in per unit
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Load Flow for in per unit
RE: Load Flow for in per unit
Is my logic above technically sound?
RE: Load Flow for in per unit
RE: Load Flow for in per unit
Z (pu) =Z(ohm)*(Vbase (kv))/(Sbase (MVA)^2). Clearly, if you make Vbase =1kV and Sbase=1MVA, then Z(pu)=Z(Ohm). Actually may be I exaggerated the extent of the descrepancies in the results, the Real Power flows balance exactly, the reactive powers by about up to 1 to 5%. The voltage solution is correct. I am beginning to think now that in the example which I thought had descrepancies, my (NEPLAN) results are correct and I will continue with my method for more examples and compare. I will continue like this until I am 100% sure since it will help greatly if I do not have to convert literally hundreds of input data by hand and in my opinion for no apparent reason.
mm
RE: Load Flow for in per unit
If you are combining impedances of transformers or machines with line impedances, you will not get the textbook answers by assuming all lines to be 1 km long. Transformer and machine impedances will not be scaled by the same factor.
RE: Load Flow for in per unit
RE: Load Flow for in per unit
I am not sure if i understood you correctly - but I dont think units will ever tie up, eg a 1MVA 1kV system has a per unit current of 0.766A (1000/1000*1.732).
This gives a pu resistance of 1000/0.577 = 1732ohm. To get a 1km line to have such a high resistance would need a small section - I think you would have to many fiddles that the results will not be reliable. Thats why i like the pu system, much simpler to understand.
RE: Load Flow for in per unit
You really need to find the original base information and use that. I believe that most competent system analysis software would allow impedances to be input in PU if the appropriate base values can also be entered. But you can't just make up the base values.
RE: Load Flow for in per unit
mmt