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2.11 Underexcitation (Loss-of-Field) Protection (ANSI 40)
77
7UM61 Manual
C53000-G1176-C127-3
2.11 Underexcitation (Loss-of-Field) Protection (ANSI 40)
The underexcitation protection protects a synchronous machine from asynchronous
operation in the event of faulty excitation or regulation and from local overheating of
the rotor. Furthermore, it avoids endangering network stability by underexcitation of
large synchronous machines.
2.11.1 Function Description
Underexcitation De-
termination In order to detect underexcitation, the unit processes all three terminal phase currents
and all three terminal voltages to form the stator circuit criterion.
For the stator circuit criterion the admittance is calculated from the positive sequence
currents and voltages. The admittance measurement always produces the physically
appropriate stability limit, independently of voltage deviations from rated voltage.
Even in such circumstances the protection characteristic can be thus optimally
matched to the stability characteristic of the machine. By virtue of the positive se-
quence system evaluation, protection operates reliably even during asymmetrical
current or voltage conditions.
Characteristic
Curves The following figure shows the loading diagram of the synchronous machine in the ad-
mittance plane (P/U
2
; –Q/U
2
) with the statistic stability limit which crosses the reactive
axis near 1/X
d
(reciprocal value of the synchronous direct reactance).
Figure 2-19 Admittance Diagram of Turbo Generators
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