ABB RELION REX640 Technical Manual page 544

Protection functions
selection set to "No Voltage", third harmonic-based earth-fault protection is
based on third harmonic neutral side undervoltage protection.
Voltage selection setting is set to "Measured Uo" if the third harmonic voltage
•
is measured directly from an open-delta voltage connection of the voltage
transformer or the third harmonic voltage is calculated using all three phase-to-
earth voltages. If the terminal side voltage is fed from an open-delta voltage
connection of the voltage transformer, the terminal side third harmonic voltage
Û3H_T is the same as the measured open-delta voltage Û3H_RES.
U
If all three phase-to-earth voltages are available, the terminal side third harmonic
voltage is calculated as a vector average of the third harmonic voltages of all
three phases.
U
• If only one phase-to-earth voltage is available, the
set to the respective phase, that is, "Phase A" or "Phase B" or "Phase C" based
on the available phase. In this case, the magnitude of the terminal side third
harmonic voltage is assumed to be equal to the third harmonic voltage of the
phase available.
U
The function is internally blocked if the magnitude of calculated Ū
the set
Differential calculation
The amplitude of the third harmonic differential voltage can be calculated using the
following equation.
UD
3
UD
Ū
3H_T
Ū
3H_N
The magnitude of the third harmonic differential voltage UD_3H and the phase
angle difference between the terminal side and neutral side third harmonic voltage
U_3HANGL_T_N are available in the Monitored data view.
Bias calculation
The amplitude of the third harmonic bias voltage can be calculated using the
following equation.
544
U
3 H T _ 3 H RES _
=
1
U U
(
= × +
3 H T _ 3 H A _
3
U U
or
3 H T _ 3 H A _ 3 H B
=
Voltage block value . This also activates the INT_BLKD output.
U U
= +
3 H T _ 3 H N _
H
Magnitude of the third harmonic differential voltage
3H
Terminal side third harmonic voltage phasor
Neutral side third harmonic voltage phasor
U
)
+
3 H B _ 3 H C _
U
or
_ 3 H C _
1MRS759142 F
(Equation 77)
(Equation 78)
Voltage selection setting is
(Equation 79)
is lower than
3H_T
(Equation 80)
Technical Manual
REX640