Abnormal turbine operating conditions will cause damage to the plant and possibly to personnel if allowed to persist.1 The possible dangers include the following:
Overspeeding
Lubricating oil failure
High turbine exhaust pressure (low condenser vacuum)
Governor failure
Water ingress to blading
Thrust bearing failure
Excessive vibration
Excessive temperature differentials
Excessive eccentricity
Supervisory equipment normally monitors the last four items. The remaining dangers have more immediate effects on the plant. They are detected by systems that depressurize the hydraulic pressure of the governing system. This results in shutting the steam valves and disconnecting of the generator.
The preceding list includes dangers that only affect the turbine. Other hazards specific to the boilers, generator, transformers, and high-voltage connections can also initiate a turbine generator trip.
The consequences of overspeed are very serious for the plant and personnel. Therefore, the protective systems have been designed to prevent overspeed. The turbine governing system protects the unit from overspeed. However, if it malfunctions, a separate overspeed tripping system will become activated. When the generator is connected to the grid, the turbine cannot overspeed (the generator is coupled magnetically with the grid). The possibility of overspeed occurs during run-up and when the unit is disconnected suddenly from the load (during a load rejection). The unit is normally disconnected from the grid due to an internal problem such as a loss of lubricating oil. The possibility of overspeed is normally reduced by coordinating the opening of...
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