Siemens Siprotec 7UM611 Manual page 80

2 Functions
The trip characteristics of the underexcitation protection in the admittance value
diagram are composed of straight segments which are respectively defined by their
admittance 1/xd (=coordinate distance) and their inclination angle α. The straight seg-
ments (1/xd CHAR.1)/α
form the static underexcitation limit (see the following figure). (1/xd CHAR.1) corre-
sponds to the reciprocal value of the related synchronous direct reactance.
If the voltage regulator of the synchronous machine has underexcitation limiting, the
static characteristics are set in such a way that the underexcitation limiting of the
voltage regulator will intervene before characteristic 1 is reached (see figure 2-24).
Figure 2-22
Characteristic If the generator capability diagram (see the following Figure) in its preferred represen-
Curve Values tation (abscissa = positive reactive power; ordinate = positive active power) is trans-
formed to the admittance plane (division by U
matched directly to the stability characteristic of the machine. If the axis sizes are
divided by the nominal apparent power, the generator diagram is indicated per unit
(this diagram corresponds to a per unit representation of the admittance diagram).
80
(characteristic 1) and (1/xd CHAR.2)/α
1
Underexcitation Protection Characteristics in the Admittance Plane
(characteristic 2)
2
2
), the tripping characteristic can be
C53000-G1176-C127-3
7UM61 Manual