Sensitive Earth Fault Protection; Standby Earth Fault Protection; Restricted Earth Fault Protection - Toshiba GRE110 Instruction Manual

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2.3 Sensitive Earth Fault Protection

The sensitive earth fault (SEF) protection is applied for distribution systems earthed through high
impedance, where very low levels of fault current are expected in earth faults. Furthermore, the
SEF elements of GRE110 are also applicable to the "standby earth fault protection" and the "high
impedance restricted earth fault protection of transformers".
The SEF elements provide more sensitive setting ranges (1 mA to 250mA) than the regular earth
fault protection.
Since very low levels of current setting may be applied, there is a danger of mal-operation due to
harmonics of the power system frequency, which can appear as residual current. Therefore the SEF
elements operate only on the fundamental component, rejecting all higher harmonics.
The SEF protection is provided in Models 420, 421, 422, 820 and 821 which have a dedicated earth
fault input circuit.
The element SEF1 provides inverse time or definite time selective two-stage earth fault protection.
Stage 2 of the two-stage earth fault protection is used only for the standby earth fault protection.
SEF2 provides inverse time or definite time selective earth fault protection. SEF3 and SEF4
provide definite time earth fault protection.
When SEF employs IEEE, US or C (Configurable) inverse time characteristics, two reset modes
are available: definite time or dependent time resetting. If the IEC inverse time characteristic is
employed, definite time resetting is provided. For other characteristics, refer to Section 2.1.1.
In applications of SEF protection, it must be ensured that any erroneous zero-phase current is
sufficiently low compared to the fault current, so that a highly sensitive setting is available.
The erroneous current may be caused with load current due to unbalanced configuration of the
distribution lines, or mutual coupling from adjacent lines. The value of the erroneous current during
normal conditions can be acquired on the metering screen of the relay front panel.
The earth fault current for SEF may be fed from a core balance CT, but if it is derived from three
phase CTs, the erroneous current may be caused also by the CT error in phase faults. Transient
false functioning may be prevented by a relatively long time delay.

Standby earth fault protection

The SEF is energised from a CT connected in the power transformer low voltage neutral, and the
standby earth fault protection trips the transformer to backup the low voltage feeder protection, and
ensures that the neutral earthing resistor is not loaded beyond its rating. Stage 1 trips the
transformer low voltage circuit breaker, then stage 2 trips the high voltage circuit breaker(s) with a
time delay after stage 1 operates.
The time graded tripping is valid for transformers connected to a ring bus, banked transformers and
feeder transformers.

Restricted earth fault protection

The SEF elements can be applied in a high impedance restricted earth fault scheme (REF), for
protection of a star-connected transformer winding whose neutral is earthed directly or through
impedance.
As shown in Figure 2.3.1, the differential current between the residual current derived from the
three-phase feeder currents and the neutral current in the neutral conductor is introduced into the
SEF elements. Two external components, a stabilising resistor and a varistor, are connected as
shown in the figure. The former increases the overall impedance of the relay circuit and stabilises
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