ABB RELION REX640 Technical Manual page 1034

Protection functions
Figure 575: Typical impedance trajectory during a short circuit
Underimpedance protection for transformers
The underimpedance function is used as a short circuit protection instead of the
overcurrent protection. It is also used as a backup to the differential protection of
transformers.
The voltage and current phasors are measured with the VTs and CTs connected on
the HV side of the transformer.
The
short circuit impedance. The setting also provides a backup protection for the
busbar and feeder faults on the HV side.
Underimpedance protection for generators
The underimpedance protection is set to protect the zone between the generator
windings and the generator side windings of the step-up transformer. The function
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.
1034
The phase and current shifts due to transformer D/Y connections and
other factors complicate the settings for the faults in the secondary
winding (as seen from the protection relay), and detailed calculations are
necessary for a good coverage.
Polar reach setting is set to a value equal to 150 percent of the transformer
1MRS759142 F
REX640
Technical Manual