transformer voltage ratio

[144x]

an auxiliary transformer were to be designed with the following specification:
U1/U2=3.3/0.4 KV(+2*2.5%, -2*2.5%).
V full load =3 KV
Uk=4%
after FAT it became clear that transformers have the ratio 3/0.4 KV.
manufacturer says that the problem could be solved by changing tap steps to be (+2*2.3%, -2*2.5%)
can anybody help me to understand this problem?

 

[alehman]

The propsed change to the tap designations does not seem to make sense. 3.0kV +2*2.3% = 3.138kV - not close enough to 3.3kV even at the top tap. Even if the taps were within range, this would not be an acceptable solution because this would leave no room for further adjustment up.

 

[kittiga]

144x

You just do not have the number of turns on the primary side for the required ratio; no amount of effort to find a tap changer will help.

 

[busbar]

It's hard to make a precise judgment without more details, but it seems that if the purchase order clearly specified a 3.3kV primary, the manufacturer screwed up and should simply furnish the specified device and pay for replacement labor and shipping expense. 'Outboard' or ‘aftermarket’ fixes will likely be unsuitable, particularly given the anticipated life of the equipment.

 

[jbartos]

Suggestion to original posting: Please, would you elaborate on the nominal system voltage the transformer is meant for? Also, clarify V full load 3kV. Have you followed any industry standard spec for transformers?

 

[144x]

the transfromer full load voltage is the voltage that should appear at the secondary of the power transformer which I should have mentioned it.it appears at the primary of the auxiliary transformer.power transformer secondary is 3.3 KV.the standard is IEC 76

 

[jbartos]

Suggestion: There is a difference between the transformer rated voltage ratio and the transformer rated turns ratio. It means that the no load transformer will have the higher secondary voltage. E.g. if the turns ratio is 3kV/.4kV, then the transformer voltage ratio will be 3.3kV/.4kV=(3.0 x 1.1)/.4kV for transformer impedance 10%, all at the transformer rated values, including transformer rated load. It is necessary to be specific, which transformer ratios are considered, i.e. transformer voltage ratio or transformer turns ratio.