Supply Transformers

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•Supply transformers are not sources of short-circuit current. Transformers merely deliver short-circuit current from the utility generators to the fault point. In the process, transformers change the voltage and current magnitudes.

•Transformer impedance s will also limit the amount of short-circuit current from the utility generators. Standard tolerance on impedance is plus or minus 7.5 percent for two-winding transformers and plus or minus 10 percent for three-winding transformers. The minus tolerance should be used for short circuit studies and the plus tolerance for load flow and voltage regulation studies.