Connection polarity in transformers.

[prc]

Transformer connections are always negative polarity except small distribution transformers in US where it is positive polarity.Why is it so?What is the historical background to it?Then why in US it is positive polarity for distribution transformers?

 

[RalphChristie]

prc

Sorry, but what do you mean by positive and negative polarity? Do you mean subtractive and additive polarity?

Thanks

Regards
Ralph

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[tomad]

prc,

Do you refer to the direction (CW or CCW) of the winding for the secondary with regard to the primary winding , in order to determinate the beginning and the end of each winding ?

 

[prc]

Yes Ralph, I meant subtractive and additive polarity.It is not exactly CC&CCW of windings.It means when primary terminals are 1-2 current flows 1 to 2 in the winding and secondary circuit current flows from terminal 1 to 2 in the external load.

When we look at the cover of trf, with subtractive polarity, H1 And X1 will be near by.With additive polarity it will be diagonally opposite. (see ANSI/IEEE C57.12.70-2000)
As per ANSI standards, for all trfs polarity shall be subtractive,except for single phase trfs of rating 200 kvA and below (with HV ratings of 8660 V&below)for which additive polarity is standard.
For CTs also similar conventions are prevalent.