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Three-winding Trfr Impedance Questions

Three-winding Trfr Impedance Questions

Three-winding Trfr Impedance Questions

(OP)
A customer has a 3-wdg trfr, 11kV/6.6kV/440V (3MVA/3MVA/100kVA) with vector group 4Dyn11yn11. Primary is delta connected, whilst secondary and tertiary are star connected. Anyone explain what the 4 stands for please?

Nameplate says:
Imp 11kV/6.6kV = 5.37
Imp 11kV/440V = 0.7
Imp 6.6kV/440V = 0.94

a) What would be the MVA base at which these impedances are specified as the nameplate does not say?

b) Can I take it that I can calculate a 3-phase fault at the sec terminals the same way I would a two wdg 11/6.6kV with Z% = 5.37%?

c) Ditto for 3-phase fault at the 440V terminals? Does this mean I have effectively a 11kV/440V trfr with Z% = 0.7? This seems awfully low.

Thanks.

RE: Three-winding Trfr Impedance Questions

My GUESS is that the P-T and S-T impedances are on the 100 kVA base. Nothing else really makes sense. the P-S impedance would be on a 3000 kVA base.

RE: Three-winding Trfr Impedance Questions

The transformer must be an old one as per British standard.In old standard,before 1970, vector groups were grouped in to tables with numbers/.4 is from that table.

RE: Three-winding Trfr Impedance Questions

(OP)
The trfr was manufactured in 1993 to Australian Standard AS 2374-1982 (wich I'm trying to get hold of). I agree, it will almost certainly be based on some British or IEC standard.

RE: Three-winding Trfr Impedance Questions

I think AS2374 is based on IEC60076.
You would need to solve the impedances as a star connection to do fault level (or voltage regulation) calculations.

Regards
Marmite

RE: Three-winding Trfr Impedance Questions

(OP)
If the P-T is on a 100kVA base with z% = 0.7% then I get a 440V bolted fault current value of 19874A or 143 x FLA! Assumed infinite source (11kV) bus. Is this possible to have a leakage reactance between the P-T being so low?

Thanks.

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