ABB 615 series Technical Manual page 719

1MAC059074-MB A
615 series ANSI
Technical Manual
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Figure 355: 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.
Section 4
Protection functions
713