If the polarity of the voltage signals is opposite to the normal polarity, the
correction can be done by setting
Voltage reversal to "Yes", which rotates the
impedance vector by 180 degrees.
If the magnitude of the voltage is less than 0.05 · U
N
, the calculated impedance
is not reliable and the impedance calculation is disabled. U
N
is the rated phase-to-
phase 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.
Offset
Displacement
Diameter
X (Reactance) p.u.
R (Resistance) p.u.
-R
-X
Operating Region
Figure 375: 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 of excitation. An open circuit in the field circuit also causes
a loss of excitation. These are typical examples which cause underexcitation in
synchronous machines. This module detects the underexcitation condition for the
above cases when the calculated impedance enters the operating characteristics.
Protection functions
1MRS757644 H
722
620 series
Technical Manual