ABB RELION REX640, Техническое руководство, Страница: 517 / 1960

Level detector 1
The measured rotor earth-fault current (DFT value) is compared to the
Operate start
value setting. If the measured value exceeds that of the Operate start value setting,
Level detector 1 sends a signal to start the Timer 1 module.
Level detector 2
The measured rotor earth-fault current (DFT value) is compared to the set
Alarm
start value. If the measured value exceeds that of the Alarm start value setting, Level
detector 2 sends a signal to start the Timer 2 module.
For MREFPTOC, the earth-fault current is the current that flows due to
the voltage injected by the injection device in the rotor circuit when an
earth fault arises.
A considerable amount of harmonics, mainly 3rd and 6th, can occur in the
excitation current under normal no-fault conditions, especially with the
thyristor excitation and rotating diode rectifier systems. MREFPTOC uses
DFT value calculation to filter DC and harmonic components which could
otherwise give out false alarms or trips.
Timer 1
Once activated, the Timer activates the
START
output. The timer characteristic is
according to DT. When the operation timer has reached the value set by
Operate
delay time in the DT mode, the
OPERATE
output is activated. If a drop-off situation
occurs, that is, a fault suddenly disappears before the operating delay is exceeded,
the timer reset state is activated. The reset time depends on the
Reset delay time
setting.
The binary input
BLOCK
can be used to block the function. The activation of the
BLOCK
input deactivates all outputs and resets the internal timers.
Timer 2
Once activated, the Timer activates the alarm timer. The timer characteristic is
according to DT. When the alarm timer has reached the value set by Alarm delay time
in the DT mode, the
ALARM
output is activated. If a drop-off situation occurs, that is,
a fault suddenly disappears before the alarm delay is exceeded, the timer reset state
is activated. The reset time depends on the
Alm reset delay time setting.
The binary input
BLOCK
can be used to block the function. The activation of the
BLOCK
input deactivates all outputs and resets the internal timers.
4.2.5.6 Application
The rotor circuit of synchronous machines is normally isolated from the earth. The
rotor circuit can be exposed to an abnormal mechanical or thermal stress due to, for
example, vibrations, overcurrent and choked cooling medium flow. This can result
in the breakdown of the insulation between the field winding and the rotor iron
at the point exposed to excessive stress. If the isolation resistance is decreased
significantly, this can be seen as an earth fault. For generators with slip rings, the
rotor insulation resistance is sometimes reduced due to the accumulated carbon
dust layer produced by the carbon brushes. As the circuit has a high impedance to
earth, a single earth fault does not lead to any immediate damage because the fault
current is small due to a low voltage. There is, however, a risk that a second earth
1MRS759142 F Protection functions
REX640
Technical Manual
517