ABB RELION REX640 Technical Manual page 1035

1MRS759142 F
Figure 576: Current and voltage signals for underimpedance protection
To prevent malfunctioning of the underimpedance protection in case of nearby
faults, the
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 calculated impedance could fall below the set operating limit even if there
is no actual fault in the primary system.
The blocking operation is provided by an external function block, the fuse failure
supervision SEQSPVC, whose output is connected to the BLOCK input of UZPDIS.
4.7.4.7 Signals
REX640
Technical Manual
Polar reach setting is set to a value equal to 70 percent of the step-up
Protection functions
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