(Equation 81)
UB
3H
Magnitude of the third harmonic bias voltage
Beta
Setting to achieve the required degree of security under healthy conditions
Ū
3H_N
Neutral side third harmonic voltage phasor
The third harmonic bias voltage calculation shown in
Equation 81
is valid under all
operating conditions if there is no generator circuit breaker between generator and
transformer. But if the generator circuit breaker is used, it is needed to reduce the
sensitivity of the protection when it is open. The use of the generator circuit breaker
is defined by
Generator CB used
setting set to "Yes" and the open position is sensed
when the binary input GCB_CLOSED available is FALSE.
With the generator breaker in the open position, function desensitizes the
protection by multiplying the value of the
Beta
setting with the set constant
CB
open factor
setting.
UB CB open factor Beta
3H
= × × U
H N3 _
(Equation 82)
Neutral side third harmonic voltage is measured via a voltage transformer between
the generator neutral point and the earth. The magnitude of the third harmonic
biased voltage UB_3H is available in the Monitored data view.
Level detector
In the third harmonic differential method, Level detector compares the third
harmonic differential voltage with the third harmonic bias voltage. If the differential
voltage exceeds the biased voltage, the module sends an enabling signal to start
the timer.
If the terminal voltage is not available, that is,
Voltage selection
is set to "No
voltage" the module compares the neutral side third harmonic voltage U_3H_N to
the set
Voltage N 3.H Lim
.
Timer
Once activated, 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
, the OPERATE output is activated. If the fault disappears before the
module operates, the reset timer is activated. If the reset timer reaches the value
set by
Reset delay time
, the operation timer resets and the START output is
deactivated.
Timer calculates the start duration START_DUR value, which indicates the
percentage ratio of the start situation and the set operation time. The value is
available in the Monitored data view.
The binary input BLOCK can be used to block the function. The activation of the
BLOCK input deactivates all outputs and resets internal timers.
1MRS759142 F
Protection functions
REX640
Technical Manual
545
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