2.11 Underexcitation (Loss-of-Field) Protection (ANSI 40)
81
7UM61 Manual
C53000-G1176-C127-3
Figure 2-23 Capability Curve of a Salient-Pole Generator, Indicated per Unit
The primary setting values can be read out directly from the diagram. The related
values must be converted for the protection setting. The same conversion formula can
be used if the protection setting is performed with the predefined synchronous direct
reactance.
with
x
dsec
related synchronous direct reactance, secondary,
x
d mach
related synchronous direct reactance of the machine,
I
NMach
Nominal Current of the Machine
U
NMach
Nominal Voltage of the Machine
U
N VT. prim
Primary Nominal Voltage of the voltage transformers
I
N CT prim
Nominal primary CT current
Instead of 1/x
d mach
the approximate value I
K0
/I
N
can be used (with I
K0
= short-circuit
current at no-load excitation).
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