ABB REG316 4 Series Operating Instructions Manual page 195

REG 316*4 1MRB520049-Uen / Rev. F
E. Setting instructions
Settings:
Reactance X
A
Reactance X
B
Phase correction
Delay
Integrator (separate "Delay" function)
Operating principle of the underreactance function
The underreactance or underexcitation function protects the
generator in potentially dangerous operating conditions which
can arise in the event of loss of, or reduced excitation. There is a
danger in such situations of the unit becoming unstable and run-
ning out of synchronism. This causes thermal stress due to in-
duced currents on the one hand, and mechanical stress due to
surges of torque on the other.
It is general knowledge that a synchronous machine may not be
loaded as much capacitively as inductively, because excessive
capacitive load causes it to drop out-of-step. The reason is the
steady-state stability limit as defined by the load angle d = 90°,
which can only be reached when the unit is underexcited, i.e. for
a capacitive power factor j . When the voltage is measured at
the generator terminals, the locus of the stability limit of a gen-
erator/transformer set is a circle as shown in
circle encloses the operating points of an underexcited generator
down to the extreme point X
excitation. The protection has a circular characteristic that does
not normally coincide with the stability limit at the top to avoid
false tripping during voltage dips caused by power system faults.
Operation of the function is delayed to allow for possible recov-
ery of synchronism following dynamic phenomena with brief load
angles of d > 90°. A typical setting for the time delay is 2 s.
The scheme includes an integrator (separate "Delay" function) to
maintain the underexcitation signal in the event of power swings.
This is necessary because the normal "Delay" setting repeatedly
resets during power swings and prevents tripping from taking
place.
ABB Switzerland Ltd
XA-Setting
XB-Setting
Angle
Delay
Trip-Delay
Reset-Delay
Fig.
which represents total loss of
A
3.5.14.3. The
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