I have steam generator running in parallel with infinite busbar. the generator operator reduce its load to 0 MW. The generator breaker didn't trip and the continuous to operate with 0MW, and all the load supplied by grid. Is that case have any risk on generator and any protection or not?
If steam was still flowing into the turbine through the stop & control valves then what was happening was that the amount of torque being produced by the steam flow through the turbine was exactly equal to what was required to keep the generator spinning at synchronous speed, which means 0 MW.
Think about it--it takes a certain amount of steam to get the steam turbine and generator up to synchronous speed in order to synchronize it to the grid. To make the synchroscope rotate in the Fast (clockwise) direction, a little additional steam is added to the turbine. This makes the steam turbine and generator spin a little faster than synchronous speed. When the generator breaker is closed, the steam turbine is slowed to synchronous speed by the generator--it's locked into synchronous speed when the generator breaker is closed. BUT, that small amount of extra steam that is flowing into the turbine to make the speed be just a little higher than synchronous speed is extra torque (which was making the turbine-generator spin a little faster than synchronous speed), which the generator converts into amperes which flow out on the grid. That's positive power production--amperes flowing out of the generator on to the grid.
If the operator reduces the steam flowing into the steam turbine when the generator breaker is closed until the amperes flowing out of the steam turbine are at zero, then the MW (power) produced by the steam turbine and generator will be at zero. No amperes are flowing out of the generator, and no amperes are flowing into the generator from the grid, either. The steam turbine is providing just enough torque to keep the generator rotor spinning at synchronous speed--which means no amperes are flowing out of, or into, the generator. 0 MW.
Now, some steam turbines--and it depends on the design of the steam turbine--are not designed to be operated at very low loads (including 0 MW). Steam flowing into the turbine can actually help to cool the rotating turbine blades (if the steam flow isn't sufficient to rotate the turbine blades then the blades will spin in their space and produce heat in a phenomenon called "windage"). Also, steam can actually start to condense in the last stages of the steam turbine which means that water droplets are impinging on the steam turbine blades and nozzles--and that's not generally a good thing for blades or nozzles. Finally, if steam flow into the turbine isn't sufficient then the blades can actually try to "suck" steam into the turbine, which reverses the forces on the turbine blades and they are NOT designed to operate like that for very long.
But, the turbine manufacturer defines how low the power flow (steam flow into the turbine) can go without causing damage to the steam turbine. Some turbines can go all the way down to 0 MW without any ill effects. I've seen some very large steam turbines (600 MW units) that could NOT operate below 200 MW for any extended period of time. They were synchronized and VERY QUICKLY loaded to 200 MW (the boilers were actually fired very hard just prior to synchronization in anticipation of the increased steam flow required to get to 200 MW quickly), and if the load on the turbines ever dropped below 200 MW for less than 45 seconds, if I recall correctly while the units were in operation the "reverse power" relays operated to open the generator breaker and close the steam stop & control valves. All of this to protect the steam turbine from damage caused by insufficient steam flows at low loads. (There wasn't actually any reverse power at 200 MW--but some specially modified reverse power relays were used to protect the steam turbine if the load dropped below 200 MW for any appreciable amount of time.)
Without knowing a LOT about the particular steam turbine at your site, it's impossible to say if any damage occurred while the unit was operated at 0 MW. We also don't know for how long the unit was operated at 0 MW (5 minutes; 30 minutes; 1 hour). And, we don't know how the reverse power relay was set (to open at negative power, when the grid would be supplying amperes to the generator, causing the generator to act as a motor to keep the generator--and steam turbine--spinning at synchronous speed (called "motoring" or "motorizing" the generator). I have seen some small industrial steam turbines in paper mills and lumber mills that will actually run at very small negative power levels without tripping. In other words, the negative power had to exceed some value (-2.0 MW was the most I've ever seen) before the relay operated to close the steam stop & control valves and open the generator breaker. But, those were turbines specially designed to operate that way for special conditions in the mill, so that the operators wouldn't have to re-synchronize the units very often because electrical loads in the mill suddenly dropped.
Finally, reverse power for a synchronous generator doesn't isn't really a problem for the generator. There's very little difference, mechanically or electrically, between motors and generators. The difference is the "flow" of torque and/or amperes. Many of today's modern gas turbines are started using the generator as a motor to spin them up to near synchronous speed, at which time the generator is "switched" back to a generator just prior to synchronization. So, it's not likely the generator suffered any negative consequences while operating at 0 MW.
Hope this helps! If the reverse power relay was working properly and was set per the steam turbine manufacturer's recommendations (the reverse power relay is set to protect the steam turbine--NOT the generator!) then there probably wasn't any damage. Usually, when steam flows through the steam turbine are very low what usually happens is that the last stages of the steam turbine start to get warm (exhaust hood temperatures start to increase). And, usually, if the steam turbine manufacturer thinks this could be a problem there are alarms and trips associated with high exhaust hood temperatures to protect the steam turbine from low steam flows (which occur at low loads).
Good question, though. 0 MW with the generator breaker closed and the steam stop & control valves open just means that the steam turbine isn't providing any more torque to the generator than that required just to maintain synchronous speed. In other words, the generator isn't producing any amperes, nor is it consuming any amperes--the power flow is 0 MW. And, the turbine may or may not be designed to operate for long periods of time (more than a few minutes ???) at 0 MW. You would have to look at the Operations Manual supplied with the steam turbine by the manufacturer to understand what the minimum load the steam turbine is capable of enduring and for how long.