ABB RELION REX640 Technical Manual page 955

1MRS759142 C
REX640
Technical Manual
GUID-DBC15F04-6CBB-4616-883B-00F4B6EA9122 V1 EN
Figure 514: 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 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.
The Polar reach setting is set to a value equal to 150 percent of the transformer 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
Section 4
Protection functions
949