[WALKERP]

A 3-core cable was installed to supply a DC UPS system and it was realised they actually needed a 4-core cable , 3 phase & neutral.

To overcome this oversight” they are proposing to install a local autotransformer of vector group Zn0 to generate a neutral for the load. The “new” neutral will not be earthed at this point however the source where the 3-phase eminates from is a 415v transformer with solidly earthed star point.

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[Hamid]

As you have mentioned, they will want to make an artificial neutral point as indicated in following diagram. Theoretical this is possible, but you must check the isolation of UPS grounding system and that artificial neutral bar. Also the auxiliary transformer must be rated for withstand phase-neutral short circuit current (equal to upstream system earth fault current) additional of normal neutral loads; it seems this point can be increased the cost of mentioned transformer very much.

[R K Mohapatra]

Normally the impedence of the Zig – Zag transformer is very high . The earthing transformers normally use Zig-Zag connection to harness this advantage .

[Author]

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