Three-phase transformers connections

[Cubrilo]

I’m not quite clear about three-phase connections that are in use. For example: in Europe is in use 63 MVA, 110/35/10 kV, YNyn0d5 three-phase transformer with both neutral points grounded. I assume that magnetic core has three legs (I didn’t actually saw it so I’m not sure). The 35 kV network is not grounded on the other end which means that there is no zero-sequence component of current (except in short-circuit fault with earth but thats is very short period of time. My question is: "Why do we need tertiary winding when there is no zero-sequence component of current and the form of magnetic flux is good (flux' third harmonic is small because of the magnetic core shape)?"
Second question is: "Are there three-phase power transformers with five legs magnetic core? If there is, please specify me they rated power, voltages and connection"
Please somebody answer! Thanks for your time.

 

[busbar]

Cubrilo—It appears you may prefer an answer in IEC-based standards, but IEEE C57.105-1978 IEEE Guide for Application of Transformer Connections in Three-Phase Distribution Systems addresses the matter also. See: §3. Magnetic Circuits, §5. Primary Faults and §6. Unbalanced Loads and Secondary Ground Faults It is usual for these standards to have a very detailed reference list for further studies. INDEX at:

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.105-1978_desc.html

Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part V Transmission Systems and Subtransmission Systems INDEX at:

http://standards.ieee.org/reading/ieee/std_public/description/surge/C62.92.5-1992_desc.html

§6. Transformer Tertiary Systems

With ∆ tertiary, it is customary to keep these external connections quite localized, as ground faults on the associated bus are somewhat more difficult to detect, but the winding is essential for maintaining zero-sequence currents in the Y windings. A single CT placed in the tertiary “loop” can provide polarizing quantities for primary- and secondary-side relaying.

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