A common use for a zig-zag autotransformer is to establish a grounded neutral in a 3ø3w system. A low-voltage example would be to derive a grounded neutral {for 277V loads} where the building is served by a 480V 3-wire ungrounded-delta transformer—literally providing a 480Y/277V 4-wire circuit from the ungrounded 3-wire service.
Likewise, “north of the border” a zig-zag autotransformer can be used to derive a neutral conductor for 347V lighting from a 600V 3-wire ungrounded-delta transformer—literally providing a 600Y/347V 4-wire circuit from the ungrounded 3-wire service. Three 240:240V 1ø transformers of the correct kVA could be used for either 3ø3w voltage.
EC&M magazine has had a couple of articles about serving 277V equipment from ungrounded 480V. See
http://ecmweb.com/ar/electric_unconventional_transformer_connections/index.htm {Online version is without illustrations.}
