The capacity of the unit is different. The voltage after passing through the excitation transformer enters the main transformer. Generally, 135MW unit is 15.7KV, 600MW unit is 20KV. The 660 MW unit corresponds to 22KV. It depends on the capacity of the unit.
The power of the generator is constant, just like a load. It can be equipped with one lamp, or two or more lamps. The output power of different lamps will be different. Similarly, the transformer can change as much V as it can and it has nothing to do with the generator. For another example, cell phone battery charger is 3.7V, battery charger is 12V, three wheeler car battery is 24V, etc. They all use 220V mains power. After going through the charger transformer it becomes several. different output currents.