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2.1.2.4 Directional Sensitive Earth Fault Protection
GRD150-200 and 400 series provide directional earth fault protection with more sensitive settings
for use in applications where the fault current magnitude may be very low. A 4-stage directional
sensitive earth fault function is provided, with the first and second stage programmable for inverse
time or definite time operation. The third and fourth stages provide definite time operation.
The sensitive earth fault element includes a digital filter which rejects all harmonics other than the
fundamental power system frequency.
The sensitive earth fault quantity is measured directly, using a dedicated core balance earth fault
CT.
This input can also be used in transformer restricted earth fault applications, by the use of external
metrosils and setting resistors.
The directional sensitive earth fault elements can be configured for directional operation in the
same way as the standard earth fault pole, by polarising against the residual voltage. An additional
restraint on operation can be provided by a Residual Power element RP, for use in protection of
power systems which utilise resonant (Petersen coil) earthing methods.
Scheme Logic
Figure 2.1.43 shows the scheme logic of inverse time or definite time selective two-stage
directional sensitive earth fault protection DSEF1.
In Figure 2.1.43, the definite time protection is selected by setting [MDSE1] to "DT". The definite
time element is enabled for sensitive earth fault protection and stage 1 trip signal DSEF1 TRIP is
given through the delayed pick-up timer TDSE1.
The inverse time protection is selected by setting [MDSE1] to either "IEC", "IEEE", "US" or
"CON" according to the inverse time characteristic to employ. The element SEF1I is enabled and
stage 1 trip signal DSEF1 TRIP is given.
Both protection provide stage 2 trip signal DSEF1-S2 through a delayed pick-up timer TDSE12.
When the standby earth fault protection is applied by introducing earth current from the
transformer low voltage neutral circuit, stage 1 trip signals are used to trip the transformer low
voltage circuit breaker. If the definite time or inverse time element continues operating after stage
1 has operated, the stage 2 trip signal can be used to trip the transformer high voltage circuit
breaker(s).
The forward operate zone or reverse operate zone is selectable by the scheme switch [DSE1-DIR].
ICD is the inrush current detector ICD, which detects second harmonic inrush current during
transformer energisation, and can block the DSEF1 protection by the scheme switch [DSE1-2F].
See Section 2.1.7.
SEF protection can be disabled by the scheme switch [DSE1EN] or PLC logic signal DSEF1
BLOCK. Stage 2 trip of standby earth fault protection can be disabled by the scheme switch
[DSE1S2]. Residual element RP also can be disabled by the scheme switch [RPEN]. The switch
[RPEN] is common for DSEF1 to DSEF4 protection.
Figure 2.2.44 shows DSEF2 protection. The DSEF2 provides the same logic of DSEF1 except for
the stage 2 trip.
Figure 2.1.45 and Figure 2.1.46 show the scheme logic of the definite time sensitive earth fault
protection DSEF3 and the DSEF4. The DSEF3 and DSEF4 give trip and alarm signals DSEF3
TRIP and DSEF4 ALARM through delayed pick-up timers TDSE3 and TDSE4.
The forward operate zone (DSEF3F and DSEF4F) or reverse operate zone (DSEF3R and
DSEF4R) is selectable by the scheme switches [DSE3-DIR] and [DSE4-DIR].
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