Diretctional Sensitive Earth Fault Protection (At Model 820 And 821) - Toshiba GRE110 Instruction Manual

V = CT knee point voltage
k
I = maximum prospective secondary current for an internal fault
F
When a Metrosil is used for the varistor, it should be selected with the following characteristics:
β
V = CI
where:
V = instantaneous voltage
I = instantaneous current
β = constant, normally in the range 0.20 - 0.25
C = constant.
The C value defines the characteristics of the metrosil, and should be chosen according to the
following requirements:
1. The current through the metrosil at the relay voltage setting should be as low as possible,
preferably less than 30mA for a 1Amp CT and less than 100mA for a 5Amp CT.
2. The voltage at the maximum secondary current should be limited, preferably to 1500Vrms.
Restricted earth fault schemes should be applied with high accuracy CTs whose knee point voltage
V is chosen according to the equation:
k
≧ 2×V
V
k S
where V is the differential stability voltage setting for the scheme.
S

Diretctional Sensitive Earth fault protection (at model 820 and 821)

The models 820 and 821 are espetialy models for the directional sensitive earth fault protection
using earth fault current and zero phase voltage at the non- grounding system,.
Figure 2.3.5 illustrates the directional characteristic, with the forward operate zone shaded. The
reverse zone is simply a mirror image of the forward zone. The forward operate zone or reverse
operate zone is selectable by the scheme switch [SE-DIR] As shown in Figure 2.3.6, the directional
characteristic is composed of a forward directional characteristic, reverse directional characteristic
and overcurrent thresholds.
Polarising signals for directional elements are shown in Figure 2.3.7. Polarisation for directional
sensitive earth fault element SEF is derived from a residual voltage.
Figures 2.3.8 to 2.3.11 show the scheme logic for the directional sensitive earth fault protection.
The directional control characteristic can be selected to "FWD" or "REV" or "Non" by scheme
switch setting [SE∗-DIR].
At the model 820 and 821, 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 Zero phase sequence
Power element ZP, for use in protection of power systems which utilise resonant (Petersen coil)
earthing methods.
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