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Nuetral Earthing Transformers

Nuetral Earthing Transformers

Nuetral Earthing Transformers

(OP)
Can anyone explain how a nuetral earthing transformer (NET) works.

We have a three phase 6.6Kv system where the generation has no NER. I would like to know & understand the basic principles of how it works. i.e from the fault on the bus-bar, how the NET detects the fault, how it limits the current and where it limits the current.

I have attached a document that is a basic drawing I have made from the information avalible.

Thanks in advance

RE: Nuetral Earthing Transformers

If you draw your zero sequence diagram, with impedances added, you will see that a ground fault will not drive much current into a fault. A grounding transformer basically adds a lower impedance path to ground for ground faults, such that it rases the zero sequence current into a fault to a detectable level.

RE: Nuetral Earthing Transformers

Purchasing a NGT allows the resitance on the secondary size to be much smaller becuase the impedance is reflected from the sedondary to the primary. You just have to size the NGT for the line to line voltage and the resistor on the secondary for a single phase fault.

4160V pri: 240V secondary stepdown transformer. When 2400V is applied to the primary, 138V
is the voltage on the secondary. This would equate to a 86.5 amps and 1.6
ohm resistor across the secondary to limit the primary to 5 amps at 2400V.
If you use this transformer you will need atleast a 20.76kva transformer.
The reason for this is because you aren't using a direct 2400V pri: 240V sec
transformer and using those voltages. Since the voltages of the transformer
and the application differ, a derating is required.

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