ABB 615 series Technical Manual page 720

Section 4
Protection functions
714
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.
GUID-E1B90616-E865-49E2-9366-616B0112CA85 V1 EN
Figure 356: 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.
1MAC059074-MB A
615 series ANSI
Technical Manual