This is about the recent problem that we have faced in our project site. We

This is about the recent problem that we have faced in our
project site.
We have 6/7.5 MVA. 66/11kV three phase star - star power
transformer. The neutral was solidly connected to the
earthing mat. But because of high resistance of earthing
mat (130 ohm on LV and 180 ohm on HV), one day LV side
neutral earthing conductor melted and we had a huge damage.

Can any one plz tell me what might happen when we connect
power transformer neutral to the earthing mat which has
high resistance (>100 ohm ) ?????

Question Posted / gairhe

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This is about the recent problem that we have faced in our project site. We have 6/7.5 MVA. 66/1..

Answer / k.prakashchandra

Have you measured IR properly, as the values are very high
?The IR should not exceed 2 ohms for a 66 KV MUSS .I do not
know whether primary neutral is to be earthed in case of
star/star tr.It is stated that LV side IR is 130 ohms ,and
hence the maximum earth fault current will be 50A.For 7.5
MVA tr the rated LV line current is about 400 A and EFR is
likely to be set at 80A.It appears that the earth wire
melted due to continuous earth fault current.If there is
conductor to ground fault at a distance of 100 m from tr
earthing ,there will be a pd of 63.5 V /m along the line
between the fault point and the neutral earth. If anybody
stands barefooted with feet 1 m apart along this line, he is
likely to be electrocuted as the power line does not
trip.Further the ground around the earthing also may be at
nearly phase voltage during earth fault

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This is about the recent problem that we have faced in our project site. We have 6/7.5 MVA. 66/1..

Answer / gairhe

I think you are right.

for the LV even if the imbalance neutral current is 10 A,
the 130 ohm resistance will produce GPR ( Ground Potential
Rise ) of 10 X 130 = 1.3 kV.

Also at the site the transformer neutral earth conductor was
touching the 185 sq mm single core copper power cable and
control cable. May be because of this high GPR , sparking
occurred and heat was generated.

The neutral earthing conductor is of size 95 sq mm. so 10 A
current is not a big issue.

In my view 1.3 kV and sparking developed heating at the
group of XLPE power cable, control cable and fire started .
Then the XLPE power cable was damaged and because of the hig
earth fault from the damage of XLPE cable, the near by
neutral earthing conductor got melted.