change over scheme

i have three conditions of changeover scheme i want a correct written procedure, if total two ut transformer and two st transformer if

1. one ut trips or goes out then one st should supply for that ut

2. if one st goes out then one st or one ut should give supply for that st

3. if two st is out of condition then one ut must be able to supply for this two st

i want a scheme for this three conditions

is this three condition is possible by 2 out of 3 changeover scheme, if yes please explain , if no please explain the correct scheme,

i want the correct answer please, please somebody help me.