ABB RELION 620 Series Technical Manual page 726

Section 4
Protection functions
720
If the magnitude of the voltage is less than 0.05 · U
reliable and the impedance calculation is disabled. U
voltage.
The calculated impedance magnitudes and angles are available in the Monitored data
view. The impedance angles are provided between -180...180 degrees.
The calculated apparent impedance is converted to pu impedance as
the operating characteristics are defined with the pu settings.
Impedance reach check
The operating characteristic is a circular offset mho on the impedance plane. The
operating characteristics are defined with the Offset, Diameter and Displacement
settings. If the calculated impedance value enters the circle in the impedance plane, the
module sends an enabling signal to start the Timer.
X (Reactance) p.u.
-R
Offset
Diameter Displacement
GUID-A023B281-CA68-4A0C-8E90-9075DF588504 V2 EN
Figure 365: Operating region of the impedance mho circle
A fault in Automatic Voltage Regulator (AVR) or in the excitation system may cause
a total loss of excitation. A short circuit on the slip rings reduces the excitation voltage
to zero. This causes a gradual reduction of the excitation current and eventually a loss
, the calculated impedance is not
N
R (Resistance) p.u.
Operating Region
-X
1MRS757644 F
is the rated phase-to-phase
N
620 series
Technical Manual