Choose 100MVA as the (apparent power.) For calculation purpose, this value is reasonable and is widely used.
You may alternatively choose the MVA rating of the largest generator or the transformer in the power system. Equipment manufacturers who build these machines, list its MVA rating on the name plate.
Whichever MVA you choose as the base, remember that in per unit system, this value will never change. Be it at generation, transmission, or distribution.
Sample per unit problem solved: http://peguru.com/2011/06/per-unit-system-practice-problem-solved-for-easy-understanding/
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