Detection of Earth Faults in LV Systems - Part II

In the last issue of L&T Current Trendswe 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 throughthe neutral CT of Trafo-2 & neutral of buscoupler

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Fig.7

IV) CT on neutral & earthing towards switchboard side:-

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.

In this scheme, earthing isdone 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 therelay.

However even if the bus-coupler 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 g ive any nuisance tripping under this condi-

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.

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.

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 se lected zone. Specia l protection class CTs i.e. Class PS CTs are employed for achieving the protection. P r o t e c t i o n w o r k s o n 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.8

Fig.9

used.

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-

Case-1:

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 condi-tion. 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.

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

Case-2:

Fig.10

Fig.11

Fig.12

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

Conclusion:-

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.

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

thoperation.In this case, 5 CT should not be provided within the switch- board to avoid nuisance tripping.

Fig.16

Fig.15

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Fig.14

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Fig.13