ABB RER670 Applications Manual page 105

1MRK 506 375-UEN A
Railway application RER670 2.2 IEC
Application manual
healthy phase increases to phase-to-phase voltage level since the neutral
displacement is equal to the phase voltage level at a fully developed earth fault.
Increase of the healthy phase voltage together with slow tripping increases the risk
a second fault in a healthy phase and the second fault can occur at any location.
UL2'
UL2
IEC15000382 V2 EN-US
Figure 39: Phase preference logic
The phase preference logic is mainly used in systems where the single phase-to-
earth faults are not automatically cleared, only alarm is given and the fault remains
until a suitable time to send people to track down and repair the fault. When the
cross country fault occurs, the practice is to trip only one of the faulty lines.
Alternatively, the residual earth fault protection is used to trip both lines. But due to
the low fault currents, long tripping times are used.
Phase preference logic is used to decide the tripping mode for distance protection
and the following two selection modes are provided:
• Equal priority
• L1 before L2
The typical waveform of recovery voltage following arc extinction (earth faults) in
compensated earthing systems is shown in Figure 40. The resonance established
between the capacitance of system and inductance of Petersen coil delays the
recovery voltage build up after arc extinction. The resonance enables the dielectric
strength of the insulation at the point of fault to recover and prevent restriking of
the arc. Hence, special care has to be taken in clearing earth faults in compensated
earthed systems.
Impedance protection
2U0
Section 7
UL1'
UL1
IEC15000382-2-en.vsdx
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