Detection of Earth Faults in LV Systems - Part II
In the last issue of L&T Current Trends
we had learnt about different methods of
detecting the earth fault in TPN system.
In the following issue we will discuss
the problems related to it and their
respective solutions.
III) CT on trafo neutral & earthing at trafo
end:In this scheme, earthing is done at transformer end &
CT is mounted as shown in (Fig 7).
Fault current on bus section flows back to trafo neutral
point through trafo neutral CT causing the relay to trip.
Unbalance current does not flow through the CT &
hence no nuisance tripping in unbalance.
However as shown in (Fig 8), if buscoupler is
TPN type, earth fault on Bus-A has 2 paths.
First path is through neutral CT of trafo-1
back to trafo-1 neutral. Another path is through
the neutral CT of Trafo-2 & neutral of buscoupler
Fig.7
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During the earth fault on the bus
section, current flows through trafo
end neutral CT back to trafo neutral
point,causing the relay to trip.
If system is having unbalance
current, that current will flow back
to trafo neutral point through the CT
mounted in switchboard neutral,
causing the nuisance tripping of the
relay.
If the buscoupler is TPN type, E/F
current of one bus section passes
through both the CTs of both bus
section as shown in (Fig 12). First
path is through trafo-1earth back to
trafo-1 neutral point. Another path
is through trafo#2 neutral earthing
– Buscoupler neutral link – trafo-1
neutral CT mounted in switch
board-back to trafo#1 neutral point.
Fig.8
back to neutral point of trafo-1.
Magnitude of current passing
through these paths depend on the
impedance offered by each path.
Relay on trafo-2 may start
responding to faults on BUS-A,
causing a nuisance tripping. This
problem can be resolved by
providing 4 pole bus coupler.
IV) CT on neutral & earthing
towards switchboard
side:-
tion.
Since relay trips on unbalance
which is a common phenomenon,
this method is not used.
V) 2: CT connection (One at
trafo earthing end & one
in switchboard N):Two CTs are mounted as shown in
(Fig.11). One CT at trafo neutral end
& another in switchboard neutral.
In this scheme, earthing is
done within switchboard
before the isolating link as
shown in (Fig 9).
Fault current on bus section
flows back to trafo neutral
point through the CT. Relay
mounted on the incomer
senses the fault & trips the
incomer.
If system is having unbalance
current, that current will also
flow back to neutral point
through the CT causing the
nuisance tripping of the
relay.
However even if the buscoupler is TPN type, E/F
current of one bus section
does not pass through the CT
of another bus section as
shown in (Fig 10).Thus it does
not give any nuisance
tripping under this condi-
Fig.9
Thus realy on incomer-2 may give
nuisance tripping for the fault on
bus-1 & may also cause non
tripping of the relay on faulty bus
section.This problem can be
solved by using 4pole buscoupler.
Since this scheme trips on
unbalance which is a common
phenomenon, it is not used.
VI) Restricted earth
fault protection: It is usually applied for star
winding of transformer. This is
a special type of earth fault
protection provided for a
selected zone. Special
protection class CTs i.e. Class
PS CTs are employed for
achieving the protection.
Protection works on
differential principle. As shown
in (Fig 13), if current entering
the circuit is not equal to
current leaving the circuit, the
difference in current flows
through the relay and makes
the relay to operate. However
for external earth fault as
shown in (Fig 14), associated
phase & neutral CT sees the
same fault current. Secondary
reflected current forms the
series connection & does not
operate the relay. Depending
upon the earthing position,
either 4 CTs or 5 CTs are
Fig.10
dition. During the unbalance
condition, unbalance current
flows back through neutral CT.
Since current through neutral
CT is in opposite direction as
that of phases, it avoids
nuisance tripping of the relay
on unbalance. If this CT is not
provided, then relay may trip
during the unbalance condition. During the earth fault
within the zone, current flows
back to trafo star point through
CT on trafo neutral (T5).
Thus it detects the fault & trips
the system. If fault is outside the
zone, fault current flows through
phase CT as well as CT mounted
on trafo neutral. Current through
trafo neutral flows in opposite
direction as that of phase CT. Thus
blocks the nuisance tripping.
Case-2:
If transformer earthing is as per
(Fig 16), only four (4) CTs are
required for reliable operation. In
this case, Unbalance current flows
back through CT (T4) in the
Fig.11
used.
Case-1:
If earthing of the transformer is as
shown in (Fig 15), five (5) CTs are
required for reliable operation. CT
on neutral within switchboard (T4)
is required to avoid nuisance
tripping during unbalance con-
Fig.12
Conclusion:-
LOAD
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In this article and in the previous
article, we have covered the
different methods of detecting the
earth fault in TPN system.
Problems genereally encountered
& solution for that are also
discussed. Use of correct scheme,
location of CT's and connection of
CT's is a must to improve the
reliability of operation.
Fig.13
opposite direction as that of phase,
thus avoids nuisance tripping of
the relay during unbalance. During
earth fault within the zone, current
flows back to trafo star point through
T4. It detects the earthfault
current & thus gives the protection
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Fig.14
Fig.15
against REF. For earthfault outside
the zone, fault current flows through
phase CT & T4 in opposite direction
preventing the nuisance tripping.
Thus CT (T4) performs all the three
operation.In this case, 5thCT should
not be provided within the switchboard to avoid nuisance tripping.
Fig.16
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