Hitachi Relion 670 Series Applications Manual page 97

1MRK506375-UEN Rev. N
UL2'
UL2
IEC15000382 V2 EN-US
Figure 41:
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
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.
Voltage (V)
10
5
0
- 5
- 10
0
IEC15000383 V2 EN-US
Figure 42:
Three situations are possible when a cross country double earth fault occurs in the system:
• Line L1 is tripped, voltage on line L2 recovers after a while, the distance and residual current
protections reset.
• Line L1 is tripped, voltage on line L2 recovers after a while, the distance and residual current
protections reset, line L2 restrikes immediately after the reset.
• Line L1 is tripped, voltage on line L2 does not recover, the distance and/or residual current
protections does not reset.
After tripping the line with fault on phase L1, wait for a while to make sure that the voltage on L2 has
recovered and the distance and residual protection functions have reset, refer to
delay to detect cross country fault is normally (0.1 - 0.15)s.
Railway application RER670
Application manual
Phase preference logic
Figure 42. The resonance established between the capacitance of
50 100
Recovery voltage after arc extinction
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2U0
150 200 250
Section 7
Impedance protection
UL1'
UL1
IEC15000382-2-en.vsdx
Time (mS)
IEC15000383-1-en.vsdx
Figure 42. The time
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