The four main functions of the governing system1 are as follows:
To limit the speed rise to an acceptable limit upon a load rejection (when the unit is suddenly disconnected from the load)
To control the power that is generated by controlling the position of the steam governing valve (or fuel valve in gas turbines)
To control the speed of the turbogenerator during initial run-up and synchronization
To match the power that is generated to the power that is required by the load by responding to frequency changes [only when the generator is operating in islanding mode (i.e., alone), independently from the grid]
The first function is critical to the safety and availability of the plant. If the breaker connecting the generator to the grid opens during normal operation, the shaft speed will increase significantly due to the elimination of the countertorque produced by the generator. The steam flow must be reduced instantly to limit the speed rise. Most machines have a separate overspeed trip to ensure safety of the plant and personnel upon failure of the governing system. It consists of overspeed bolts that protrude out of the shaft. They trip a lever when their overspeed setpoint is reached. This results in depressurization of the hydraulic oil of the governing system leading to closure of the governing valve. Another trip on high acceleration is also included in some plants. Upon a load rejection (opening of the breaker connecting the generator to the grid), acceleration...
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