ABB RELION REX640 Technical Manual page 956

Section 4
Protection functions
950
mainly protects the generator bus, the low-voltage part of the step-up transformer and
a part of the stator winding against any short circuits.
The voltages should be measured from the generator terminals and the phase currents
from the neutral point of the generator.
GUID-73DE8771-721D-45D1-9EC8-15D3FB39A44D V1 EN
Figure 515: Current and voltage signals for underimpedance protection
To prevent malfunctioning of the underimpedance protection in case of nearby faults,
the Polar reach setting is set to a value equal to 70 percent of the step-up transformer
short circuit impedance.
In directly connected machines where the impedance towards the network is limited
only by the lines or bus bars, it must be secured that the function does not cause any
selectivity problems. In such cases, it is recommended to use the voltage-dependent
overcurrent protection instead of the underimpedance protection.
Function blocking guidelines
The operation of the underimpedance protection must be restrained if the voltage in
one or more phases suddenly drops close to zero without any significant change in the
current observed at the same time. This situation is considered as a fuse failure or a
miniature circuit breaker operation in the voltage transformer secondary circuit. The
voltage drop could cause an unwanted operation of the function block since the
1MRS759142 C
REX640
Technical Manual