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.
4.2.5.5
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 fault appears, creating a rotor winding interturn fault and causing
Protection functions
1MRS757644 H
456
620 series
Technical Manual
