Corner Grounding Transformer

[daltonian69]

I have been asked to hook up a 208V 480V step up transformer. Transformer has delta primary and delta secondary. Existing service is 120-208V. Rather than leaving the secondary ungrounded, I was thinking of corner grounding it but have never done that before and cant seem to find and good diagrams to help explain on how exactly to do this. Any help would be greatly appreciated. BTW transformer is already on site so switching to a 277-480 wye secondary is not an option at this point. All loads will only be 3P 480V motors.

 

[hillbilly1]

Corner grounding is exactly as it sounds, you ground one phase of a delta. Does not matter which one, as long as it is only one. Delta-Wye transformers usually show (at least on Square D) a dashed mark to the inside, on the winding diagram showing the optional ground.

 

[jim dungar]

Corner grounding is exactly as it sounds, you ground one phase of a delta. Does not matter which one, as long as it is only one.

Delta-Wye transformers usually show (at least on Square D) a dashed mark to the inside, on the winding diagram showing the optional ground.[/quote]
One issue with corner grounded systems, is the rating of the overcurrent protective devices.

So are you planning to use fuses or breakers for the 480V protection?

Remember if you ground a corner, that 'phase' conductor must then be colored/marked white or grey.

Delta-Wye transformers usually show (at least on Square D) a dashed mark to the inside, on the winding diagram showing the optional ground.

A dashed line (pointing from a corner towards the middle of the triangle) is used to denote the phase relationship of the primary and secondary windings. A reference to ground would be shown using a traditional symbol (an upside down tree).

 

[winnie]

'Wye' grounding is still an option; you add something known as a 'zig-zag' transformer to derive the 'wye' neutral. Additionally, if the delta secondary has a mid-point tap on any of the coils, you can ground that.

The 'zig-zag' transformer is an autotransformer configuration (meaning a single circuit, not a separate primary and secondary) that creates a neutral from 3 phase legs. It can be built up from a bank of three 1:1 single phase transformers (analogous to the way a 'buck-boost' transformer is just a normal step down transformer, wired as an autotransformer). Zig-zag transformers are often used to 'resistance ground' a delta system. See http://www.electrical-contractor.ne...?ubb=showflat&Number=148726&page=2#Post148726 for a schematic

-Jon