1.3.1 Acceleration feedback
Acceleration feedback is commonly used to provide a secondary stabilising term to improve the damping of the governor and to assist the prime safety function of the governor in preventing overspeed due to a load rejection.
With a disconnection from the power system, the response of a governor without acceleration feedback would be controlled by the speed error term as modified by the droop law. If the speed droop is set to 4%, the valves would have fully closed by the time the speed was 4% above nominal. A typical value of the initial acceleration rate is 12% per second, so it would be at least 0.33 seconds before the valves had been signalled to the fully-closed state. In addition, inherent delays in the hydraulic system mean that the valves do not achieve the signalled closure response. Taking a linear acceleration rate of 12% per second, the time to reach an overspeed of 10% would be 10/12 = 0.833 seconds. Because of the expansion of steam entrained between the steam valves and the turbine, the turbine will continue to accelerate after the steam valves are closed. To contain the overspeed within the trip setting of 10%, it is generally necessary to incorporate an acceleration term.
A governor incorporating acceleration detection can sense the need to close the steam valves as soon as its measurement circuits are able to respond. The valves can be instructed to close at maximum rate within 30 ms of the load rejection and should be fully closed 100 ms later. Figure 2.6 shows a typical response to a load rejection.
To ensure that this form of acceleration feedback only comes into play during a significant load rejection, a threshold is preset into the sensing system below which there is no output. The acceleration signal must also be present for a preset time before it takes effect.