In a thermal power plant turbine overspeed trip is one of the most important protection which saves the turbine to control the overspeed when the unit suddenly trips.
There are various reasons for turbine over speed such as sudden load throwoff, load rejection, boiler trip, turbine trip, generator trip. When the load throwoff and rejection happens on the grid the turbine speed tend to raise to supply the additional load requirement due to load throwoff or rejection. In this this case the load rejection relay (LSR) gets operated and drain the hydraulic oil which closes the HP and IP turbine control valves partially for sometime to control the turbine speed.
When the turbine trips sudden due to boiler trip, turbine protection trip or generator breaker open the turbine tends to get overspeed due to large steam available during this case the HP and IP emergency stop valves full close and reduce speed of the turbine.
The overspeed at which the turbine to be tripped is generally 10 percent above the normal rotating speed. There will be three types of overspeed trip. They are mechanical trip, logical trip and finally electrical first mechanical trip if it fails then goes for logical trip, if it also fails the sure tripping is the electrical trip
For mechanical trip there will be a setting at the first bearing of the turbine where a spring will get acted at overspeed and which drains the oil supply to the stop valves there by resulting in turbine trip.
There are various reasons for turbine over speed such as sudden load throwoff, load rejection, boiler trip, turbine trip, generator trip. When the load throwoff and rejection happens on the grid the turbine speed tend to raise to supply the additional load requirement due to load throwoff or rejection. In this this case the load rejection relay (LSR) gets operated and drain the hydraulic oil which closes the HP and IP turbine control valves partially for sometime to control the turbine speed.
When the turbine trips sudden due to boiler trip, turbine protection trip or generator breaker open the turbine tends to get overspeed due to large steam available during this case the HP and IP emergency stop valves full close and reduce speed of the turbine.
The overspeed at which the turbine to be tripped is generally 10 percent above the normal rotating speed. There will be three types of overspeed trip. They are mechanical trip, logical trip and finally electrical first mechanical trip if it fails then goes for logical trip, if it also fails the sure tripping is the electrical trip
For mechanical trip there will be a setting at the first bearing of the turbine where a spring will get acted at overspeed and which drains the oil supply to the stop valves there by resulting in turbine trip.
At the first bearing there will be trip oil pump which is attached to a spindle and this spindle is attached to a small element which is attached to a spring. When the turbine exceeds the maximum speed then the trip oil pump will push the spindle inside, the element will then pull the spring inside which is connected to a limit switch, which acted as the spring moves inside draining the auxillary oil which closes the stop valves resulting in turbine trip.
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