ABB RELION REX640 - Page 544

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 U
H T H RES3 3_ _
=
(Equation 77)
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 U U U
H T H A H B H C3 3 3 3
1
3
_ _ _ _
(= × + + )
(Equation 78)
• If only one phase-to-earth voltage is available, the Voltage selection setting is
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 U U U
H T H A H B H C3 3 3 3_ _ _ _
= or or
(Equation 79)
The function is internally blocked if the magnitude of calculated Ū
3H_T
is lower than
the set Voltage block value . This also activates the INT_BLKD output.
Differential calculation
The amplitude of the third harmonic differential voltage can be calculated using the
following equation.
UD U U
H
H T H N
3
3 3
= +
_ _
(Equation 80)
UD
3H
Magnitude of the third harmonic differential voltage
Ū
3H_T
Terminal side third harmonic voltage phasor
Ū
3H_N
Neutral side third harmonic voltage phasor
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.
Protection functions
1MRS759142 F
544 REX640
Technical Manual