1.2.1 Simple scheme — boiler on pressure control
In this scheme the governor speed setpoint is controlled by the operator to load the machine by means of the governor droop characteristics. Options for droop adjustment are often incorporated.
For commissioning purposes it may also be an advantage to include a valve position limiter or a load limiter. These would either prevent the turbine valves from opening above a certain proportion of their travel or, by load measurement, would prevent further valve opening above a certain load setpoint. The main disadvantage of the overall scheme is that a load rejection would cause the turbine steam valves to close rapidly, thus losing all flow through the boiler, possibly with maximum firing rate applied. Although the boiler safety valves would lift a few seconds later, there is some likelihood of thermal damage to superheater tubes.
A turbine by-pass system (described in Section 11 of this chapter) is one option which could ameliorate this effect.
Another disadvantage is that any load increase, whether applied to the turbine manually by the operator or automatically because of a fall in frequency, is necessarily matched by the boiler. Because the flow increases, the pressure will fall to a degree dependent on the response of the boiler control system. Additionally, the droop characteristics provided in the basic governor are usually derived by measurement valve position rather than electrical load. This may result in incremental droop deviations at any operating point due to the imperfection of valve linearisation characteristics (see Section 1.4 of this chapter).