ABB RELION REX640 Technical Manual page 1020

Protection functions
The directional restraint shown in
Rst offset X angle are set with negative signs.
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
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 timer is activated. If the reset timer reaches the value set by
the operating timer resets and the START output is deactivated.
The Timer calculates the start duration value START_DUR, which indicates the
percentage ratio of the start situation and the set operating time (DT). The value
isavailable in the Monitored data view.
Blocking logic
There are three operation modes in the blocking functionality. The operation
modes are controlled by the BLOCK input and the global setting Configuration >
System > Blocking mode which selects the blocking mode. The BLOCK input can be
controlled by a binary input, a horizontal communication input or an internal signal
of the protection relay's program. The influence of the BLOCK signal activation is
preselected with the global setting
The
mode, the operate timer is frozen to the prevailing value. In the "Block all" mode, the
whole function is blocked and the timers are reset. In the "Block OPERATE output"
mode, the function operates normally but the OPERATE output is not activated.
4.7.3.6 Application
There are limits for the underexcitation of a synchronous machine. A reduction of
the excitation current weakens the coupling between the rotor and the external
power system. The machine may lose the synchronism and start to operate like an
induction machine, which increases the consumption of the reactive power. Even if
the machine does not lose synchronism, it is not recommended to operate in this
state. The underexcitation causes excessive heating in the end region of the stator
winding. This can damage the insulation of the stator winding and even the iron
core.
1020
External Los Det Ena setting is set to "Enable". The total loss of excitation
Blocking mode setting has three blocking methods. In the "Freeze timers"
Figure 566
is obtained when both
Blocking mode .
1MRS759142 F
Rst offset X and
Reset delay time ,
REX640
Technical Manual