Transformer Neutral Earthing

1. Earth fault protection
2. Sustaining unbalanced load
3. Completing returning path of transmission line

Explaining HOW?

If single line to ground fault occurs on isolated(neutral not grounded) transformer. No return path is available so, no current will flow with this line. This will result in increase in voltage of other two line which is unfaulted. This high voltage may lead to stress on insulation and may cause of insulation failure.

At the distribution end, load to each line is not equal. If they are equal then no problem at all without neutralizing. But if load are unbalanced(most common case). Neutral current will not be 0. So, we need to provide path for it, otherwise again same problem and result in insulation failure

Sometimes, it complete the circuit through earth and we don't need extra wire to carry returning current

Why Transformer rating in KVA?

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Power factor of the transformer depends on the load

Neutral Earthing ,
1. Protective devices reference ( eg, earth fault relay, elcb )
2. For protection transformer ( unbalanced load )
3. To maintain neutral voltage 0v ( unbalanced voltage regulation (eg,rb=415,by=406,yr=380,ne=some amount of voltage))
4. Vector line sync
5. Neon tester neutral path