I think the labelling is wrong. Uan is not equal to Udc. Ua = Udc Ub=0 Uc=float
Consider the neutral point voltage at n. The voltage at n can be stated in 3 ways relative to each phase.
Where phase A is connected to Udc, Un= Udc -RIa - L(dIa/dt) - ea.
Where Phase B is connected to ground Un= RIb + L(dIb/dt) + eb.
Where Phase C is unconnected Un=Uc + ec.
Ia=Ib=I so combining 1st and 2nd Un = 0.5*(Udc - ea + eb)
Also must have ea + eb + ec =0
So Uc = 0.5*(Udc - ea + eb) - ec
Added after 2 minutes:
Sorry got my eb and ec signs wrong.
Where phase A is connected to Udc, Un= Udc -RIa - L(dIa/dt) - ea.
Where Phase B is connected to ground Un= RIb + L(dIb/dt) - eb.
Where Phase C is unconnected Un=Uc - ec.
Ia=Ib=I so combining 1st and 2nd Un = 0.5*(Udc - ea + eb)
Also must have ea + eb + ec =0
So Uc = 0.5*(Udc - ea - eb) + ec