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4.2.4.
Application of the underreactance protection function
4.2.4.1.
Introduction
The underreactance protection function can be used for a num-
ber of purposes. It is normally used, however, to detect out-of-
step conditions for load angles d ³ 90°. It is similarly applicable
to monitoring a maximum load angle, e.g. d = 70°.
4.2.4.2.
Out-of-step conditions
The stability limit of a turbo-alternator with or without step-up
transformer is illustrated at the upper left in
tion of the impedance measured at the generator terminals and
at the upper right as a power diagram. The operation of the pro-
tection is set to the circle (lower left of
during a fault or power swings on the power system. The setting
range permits the protection characteristic to be adjusted to the
stability limit curve (see upper left of
ble whether the generator is connected to a step-up transformer
or directly to a busbar.
4.2.4.3.
Monitoring a given load angle
The setting range also facilitates monitoring a given load angle,
e.g. d < 90°, for:
alarm purposes when a certain maximum load angle is
·
reached
fulfilling special requirements, e.g. to take account of the in-
·
fluence of differing values of X
salient pole machines.
·
A load angle of d < 90° represents an offset circle in the imped-
ance plane (see left of
a straight line which is displaced from the R axis by the angle d .
The circle is the locus of the operating points with the load angle
d . The corresponding characteristic in the power diagram is a
straight line with a slope of d .
The value of the load angle d is set by means of the phase-angle
compensation setting which must be increased by the amount
(90 - d ). For d = 70°, the reference voltage R-S and the R phase
current, the following phase-angle compensation must be set:
30° + (90° - 70°) = 50°.
4-26
REG 316*4 1MRB520049-Uen / Rev. B
Fig.
Fig.
4.12) which is applica-
and X
d
q
Fig.
4.13). The centre of the circle lies on
Fig. 4.12
as a func-
4.12) to avoid tripping
at the stability limit
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