ABB RELION REX640 Technical Manual page 518

Section 4
Protection functions
512
3rd harmonic calculation (terminal side)
3rd harmonic calculation calculates the magnitude and phase angle of the third
harmonic voltage at the generator terminal Ū
voltage depends on the availability of terminal side voltage and is specified by the
Voltage selection setting.
• Voltage selection setting is set to "No voltage" if phase-to-earth voltages are not
available at the terminal side. Even in a situation where only phase-to-phase
voltages are available, Voltage selection is set to "No voltage" because phase-to-
phase voltages do not contain a third harmonic component. With Voltage
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 "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
=
3 H T _ 3 H RES _
GUID-8F4195B7-CCEA-455C-81B2-9D223F009039 V1 EN
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.
1
U U
(
H T = ⋅ H A +
3 _ 3 _
3
GUID-384EBC24-F639-4114-A46B-B01E4B248410 V1 EN
• 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 or U
=
3 H T _ 3 H A _
GUID-A1B9E7DA-31AC-4A2F-8196-FF0560024B6E V1 EN
The function is internally blocked if the magnitude of calculated Ū
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
= +
3 H T _ 3 H N _
3 H
GUID-6D6A5834-253B-4C68-9776-4A8379D81517 V1 EN
3H_T
U U )
H B + H C
3 _ 3 _
or U
3 H B _ 3 H C _
1MRS759142 C
. Calculation of the third harmonic
is lower than
3H_T
Technical Manual
(Equation 65)
(Equation 66)
(Equation 67)
(Equation 68)
REX640