Bias calculation
The amplitude of the third harmonic bias voltage can be calculated using the following
equation.
GUID-8F320BC7-0F6E-4C5A-960C-3272BA65EB18 V1 EN (Equation 48)
VB
3H
Magnitude of the third harmonic bias voltage
Beta
Setting to achieve the required degree of security under healthy conditions
Neutral side third harmonic voltage phasor
The third harmonic bias voltage calculation shown in Equation 48 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.
VB Beta
H
H N
3
3
=CB open factor V⋅ ⋅
_
GUID-C9813B89-E86E-4E07-8848-648E88F92EE6 V1 EN (Equation 49)
Neutral side third harmonic voltage is measured via a voltage transformer between the
generator neutral point and the ground. 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, the 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 V_3H_N to the set Voltage N
3.H Lim.
Timer
Once activated, the Timer activates the PICKUP output. The Timer characteristic is
according to DT. When the operation timer has reached the value set by Trip delay time,
the TRIP output is activated. If the fault disappears before the module operates, the reset
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 427
Technical Manual
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