ABB REG316 4 Series - Page 196

ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-158
Determining the characteristic
The circular operating characteristic of the protection is defined
by the two points A and B. Point A is given in the case of tur-
boalternators by the unsaturated synchronous reactance x
d
and
in the case of generators with salient poles by the synchronous
reactance x
q
. As can be seen from Fig. 3.5.14.2, the stability of a
generator with salient poles is given by x
q
, because the load
angle
d is also determined by this reactance. The steady-state
stability limit is reach at this point when excitation is lost.
Point B is defined as half the transient reactance x
d
' and deter-
mined by the voltage and current measured at the generator
terminals when the unit is out-of-step and the generator is in
phase opposition to the power system.
a) Turboalternator
HEST 905 018
C
e
x
d
i
u
i
d
j
j - phase-angle
d - load angle
j < u, i
d < u, e
b) Salient pole generator
e
x
d
i
u
q
i
x
i
j
d
HEST 905 018 C
Fig. 3.5.14.2 Vector diagram of an overexcited generator.
Voltages, currents and reactances are in p.u.