A number of methods for isolating a generator once a fault has been detected are commonly being implemented. They fall into four groups:
• Simultaneous tripping involves simultaneously shutting the prime mover down by closing its valves and opening the field and generator breakers. This technique is highly recommended for severe internal generator faults.
• Generator tripping involves simultaneously opening both the field and generator breakers.
• Unit separation involves opening the generator breaker only.
• Sequential tripping is applicable to steam turbines and involves first tripping the turbine valves in order to prevent any over-speeding of the unit. Then, the field and generator breakers are opened.
Figure 9.25 represents a possible logical scheme for the implementation of a sequential tripping function. If the following three conditions are met,
(1) The real power is below a negative pre-set threshold SET_1,
(2) The steam valve or a differential pressure switch is closed (either condition indicating the removal of the prime-mover),
(3) The sequential tripping function is enabled, and then a trip signal will be sent to the generator and field breakers.
• Simultaneous tripping involves simultaneously shutting the prime mover down by closing its valves and opening the field and generator breakers. This technique is highly recommended for severe internal generator faults.
• Generator tripping involves simultaneously opening both the field and generator breakers.
• Unit separation involves opening the generator breaker only.
• Sequential tripping is applicable to steam turbines and involves first tripping the turbine valves in order to prevent any over-speeding of the unit. Then, the field and generator breakers are opened.
Figure 9.25 represents a possible logical scheme for the implementation of a sequential tripping function. If the following three conditions are met,
(1) The real power is below a negative pre-set threshold SET_1,
(2) The steam valve or a differential pressure switch is closed (either condition indicating the removal of the prime-mover),
(3) The sequential tripping function is enabled, and then a trip signal will be sent to the generator and field breakers.
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| FIGURE 9.25 Implementation of a sequential tripping function. |