ABB RELION 620 Series Technical Manual page 522

Section 4
Protection functions
516
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Figure 246: 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 PICKUP output. The time characteristic is
according to DT. When the duration of the underexcitation exceeds the set definite
Operate delay time, the TRIP output is activated. If the impedance locus moves out of the
1MAC504801-IB E
620 series ANSI
Technical Manual