2 Functions
78
7UM61 Manual
C53000-G1176-C127-3
The underexcitation protection in the 7UM61 makes available three independent,
freely combinable characteristics. As illustrated in the following figure, it is possible for
example to model static machine stability by means of two partial characteristics with
the same time delays (T CHAR. 1 = T CHAR 2). The partial characteristics are distin-
guished by the corresponding distance from the zero point (1/xd CHAR. 1) and (1/xd
CHAR. 2) as well as the corresponding inclination angle α
1
and α
2
.
If the resulting characteristic (1/xd CHAR.1)/α
1
; (1/xd CHAR.2)/α
2
is exceeded (in the
following figure on the left), a delayed warning (e.g. by 10 s) or a trip signal is trans-
mitted. The delay is necessary to ensure that the voltage regulator is given enough
time to increase the excitation voltage.
Figure 2-20 Stator circuit criterion: Pick–Up Characteristic in Admittance Diagram
A further characteristic (1/xd CHAR.3 /α
3
can be matched to the dynamic stability
characteristic of the synchronous machine. Since stable operation is impossible if this
characteristic is exceeded, immediate tripping is then required (time stage T CHAR 3).
Excitation Voltage
Query
With a faulty voltage regulator or excitation voltage failure, it is possible to switch off
with a short delay (time stage T SHRT Uex<, e.g. 1.5 s). For this purpose, excitation
voltage failure must be communicated to the device via a binary input.
Undervoltage
Blocking
The admittance calculation requires a minimum measurement voltage. During a
severe collapse (short-circuit) or failure of stator voltages, the protection is blocked by
an integrated AC voltage monitor whose pickup threshold 3014 Umin is set on deliv-
ery to 25 V. The parameter value is based on phase-to-phase voltages.
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