Turbine Overspeed

It depends a lot on the type of turbine, some such as the impulse (Brown-Curtis) type tend to be more prone to overspeed problems than reactionary types.

In many instances, overspeed is more damaging to the generator than to the turbine, mainly due to rotational mass, they can throw windings and destroy bearings causing a rotor lockup and possible shaft breakage - from that point the turbine is totally unloaded and can easily overspeed.

Other rapid rates of change of loading can lead to overspeed if the governor control is not sufficiently sensitive to prevent it. For example, an open circuit can create a drop in load from maximum to zero in a fraction of a second, this loss of load can easily result in very high overspeed.

Poor choice of components both original and replacement can lead to failure and consequent over speed.

Loss of communication between the turbine speed sensors and the control equipment can be another contributor.

Excess steam flow through the turbine either as a result of boiler overpressure or failure of the turbine control valves to regulate correctly (failure of hydraulic pump, blockage of oil lines, dirty steam, poor maintenance of inlet valves, etc.) can result in overspeed. Typically, turbine overspeed trips need to operate at around 110% of full speed and shut off the steam within a second or two.

Excess vacuum on the turbine outlet can increase steam velocity and turbine speed, failure or non existence of vacuum breakers in condensing boilers can result in secondary overspeed during an initial trip.

The major cause of turbine overspeed is human error, this is evidenced by the fact that a great number of events occur during routine testing where safety controls have been bypassed.