ABB RELION REX640 Technical Manual page 941

1MRS759142 C
REX640
Technical Manual
X (Reactance) p.u.
-R
Offset
Diameter Displacement
GUID-F1D894E6-9661-43B5-8C4E-7E410012AEF5 V1 EN
Figure 506: 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.
External loss detection
The module checks the status information of the excitation system. It is activated
when the External Los Det Ena setting is set to "Enable". The total loss of excitation
current or a failure in the excitation system is indicated by connecting the external
binary signal to the EXT_LOS_DET input. The Timer is enabled immediately when
the EXT_LOS_DET input is activated.
Timer
Once activated, the Timer activates the START output. The time characteristic is
according to DT. When the duration of the underexcitation exceeds the set definite
Operate delay time, the OPERATE output is activated. If the impedance locus moves
out of the offset-mho operating characteristics before the module operates, the reset
R (Resistance) p.u.
Operating Region
-X
Section 4
Protection functions
935