2 Functions
80
7UM61 Manual
C53000-G1176-C127-3
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
1
(characteristic 1) and (1/xd CHAR.2)/α
2
(characteristic 2)
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 Underexcitation Protection Characteristics in the Admittance Plane
Characteristic
Curve Values
If the generator capability diagram (see the following Figure) in its preferred represen-
tation (abscissa = positive reactive power; ordinate = positive active power) is trans-
formed to the admittance plane (division by U
2
), the tripping characteristic can be
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).
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