Insulation of cable from transformer neutral terminal (Y-point)

[ingar]

We have a 115/22kV power transformer that is directly earthed to the earth grid. The same is the transformer chassi.

We normaly use a 120mm2 Cu / 1000V conductor between the neutral terminal and earth, but have been told that this insulation is not good enough for transient situations. In the IEC standard 60076-1 I only find that the neutral conductor should be dimensioned for earth-fault current. Is there any recommandations for the insulation level for this purpose?

 

[jwerthman]

If the neutral is solidly grounded (not through an impedance) then the insulation of your cable is not a problem. Many people ground these neutrals with bare cable or bare copper bus. If the neutral is grounded through an impedance, then it's a different story. For this situation, the cable should be insulated to withstand the maxium voltage across the grounding impedance during a ground fault, which is typically line-to-neutral voltage.

 

[RRaghunath]

Ingar,

IEC 76 states the transformer neutral shall be rated at 17.5kV, in case the neutral is solidly earthed. I have seen transformer neutrals being provided 33kV class bushings and the bare copper conductor supported on 33kV class insulators brought down to ground level for connecting to the earth mat.

Is this useful. Raghunath

 

[wilkin]

IN THE UK THE GRID TRANSFORMERS THAT ARE SOLIDLY EARTHED 132 KV AND ABOVE
HAVE 11 KV RATED INSULATION STAND OFF BUSHINGS THIS ENSURES THAT EARTH FAULTS FOLLOW A FIXED PATH TO EARTH VERY OFTEN THROUGH A PROTECTION CURRENT TRANSFORMER