# Calulating the total thermal power in three phase system

In my book I have following task that actually has a solution. However I do not understand the solution idea and that's why I'm asking you guys to help me understand that if possible.

Why I do not understand are following:

1. Why do we have a $U_a'$, $U_b'$...?
2. Do we in the end use $U_a$,$U_b$,.... or $U_0$?
3. How they calculate the $I_1$ or $I_2$? Why do they use $U_0$ when calculating the current but do not use $Z_{L0}$.