Per Unit Values
Per Unit Values
(OP)
I have to take the PE Electrical test on Friday; I'm having trouble solving problems involving per unit values. If the per unit values are not given and we are asked to solve the problem using per-unit, can we take per unit voltage and current as 1? Also, while taking the transformer voltage as base, sometimes we take the base as primary voltage and sometimes the secondary voltage. Can someone tell me how it works? Your cooperation is highly appreciated.






RE: Per Unit Values
You always find somewhere in the problem statement a V and a VA value that you use as your base. You usually calculate the A base from the V and VA. When you have a large system, a system base is given, but for a small network such as might be on a test, you frequently use the some large power limiting element in the system, such as a xfmr or generator. In the case of a xfmr, there will a Vbase on both sides of the xfmr, but after you convert the network to per unit, there is no xfmr in the network. The xfmr becomes a simple impedance and all voltages are in per unit. You always use the voltage ratio of the xfmr to determine voltage bases on each side, not some red herring voltage thrown at you. e.g. do not be led astray by "Given a 12.47kV/480V xfmr, and a 460V motor..." Your bases are 12.47kV and 480V. Not 460V. If motor impedance is given on a 460V base, you have to convert to 480V via the ratio of (480/460)^2.
RE: Per Unit Values
Alan
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Ben Franklin
RE: Per Unit Values
RE: Per Unit Values
Normally, you select a base voltage corresponding the system voltage you are working with, and a standard base MVA such as 100 MVA or maybe 10 MVA for an industrial system. For dealing with a single transformer, the transformer MVA is often used at the base MVA The base current is computed from the base voltage and base MVA.
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
RE: Per Unit Values
RE: Per Unit Values
RE: Per Unit Values
You ALWAYS choose your own base.