ABB REG316 4 Series Operating Instructions Manual page 214

ABB Switzerland Ltd
The purpose of the second stage is to guard against excessively
high temperature and possible mechanical damage to the prime
mover. The time delay can be longer in this case, because the
temperature only increases slowly. Should power swings occur
at low load due to the speed regulator or system instability, the
second stage will not be able to trip, because the function re-
peatedly picks up and resets before the time delay can expire. It
is for just such cases that the integrator ("Delay" function) is
needed to ensure reliable tripping.
Block
t >
1
U
P>
I
t >
2
t fast stage interlocked with the main turbine steam valve
1
t slow stage
2
t slow stage with an integrator where power swings are to be expected
3
Fig. 3.5.15.5 Reverse power protection for steam turbines
3-176
REG 316*4 1MRB520049-Uen / Rev. F
Trip
Trip
Start
Integrator
t >
3
Trip