1.3.2 Unloading gear
Unloading gear may take several different forms, depending on the plant being controlled. It is usually complementary to a protective function, the operation of which would normally cause a turbine trip. It is incorporated so that the condition likely to cause a trip is sensed early and the turbine load is decreased to reduce the probability of tripping. If a trip does occur, then the transient effect on the plant is reduced because it occurs at a lower load.
Low exhaust pressure unloading-gear is normally provided. The pressure is usually sensed at tapping points on the condenser. During normal operation the unloader has no effect. However, if a condenser or cooling water system fault causes the exhaust pressure to rise above a threshold value, typically 120 mbar, any further rise causes the turbine to progressively unload, the degree of unloading being linearly proportional to the further rise in exhaust pressure. A typical unloading characteristic is shown in Fig 2.7. The purpose of unloading is to reduce the effects or probability of a trip. The reason for providing a trip is discussed separately in Section 3.1 of this chapter. The degree of unloading is usually limited to about 20% load to avoid the possibility of motoring. The rate of unloading is generally consistent with the rate of increase in exhaust pressure. However, if the initiating cause of unloading is removed and the exhaust pressure falls to a normal value, the turbine remains unloaded until the operator or a separate external automatic control function restores the load. This is achieved by resetting the governor speed setpoint which has been independently reduced to track the degree of unloading that has taken place.
On initial start-up and loading, it may be necessary to veto the operation of the loading gear until the exhaust pressure has reduced to a value such that it is unlikely to restrict the initial loading up to full-load. The veto is normally removed by the operator but, if forgotten, the veto will be automatically removed when the exhaust pressure reaches the value which permits full load (about 120 mbar, as shown on Fig 2.7).