IEEE C57.105-1977 (R2008) pdf free.IEEE Guide for Application of Transformer Connections in Three-PhaseDistribution Systems.
This guide concerns transformer connections in 3-phase distribution systems. Distribution systems are characterizedby primary voltages up to and including 34.5 kV, usually have a preponderance of connected transformers with low-voltage windings below 1000 V and furnish electric service to consumers. The characteristics of the varioustransformer connections and possible operating problems under normal or abnormal conditions are treated.
All combinations of A and Y, grounded and ungrounded, T connected,zigzag, and certain special connections areconsidered.Only two-winding transformers are included.
Phasing procedures and loading practices are not covered.Guides for loading of oil-immersed and dry-typetransformers are available in publications of the American National Standards Institute:American National StandardC57.91-1974,Appendix to C57.12.00-1973,Guide for Loading Mineral Oil-Immersed Overhead-Type DistributionTransformers with 55°C or 65C Average Winding Rise; American National Standard C57.92-1962,Appendix toC57.12 Standards,Guide for Loading Oil-Immersed Distribution and Power Transformers; and American NationalStandard C57.96-1959，Appendix to C57.12 Standards，Guide for Loading Dry-Type Distribution and PowerTransformers, respectively.
The primary supply may be either an ungrounded, effectively grounded,or impedance-grounded source. Anungrounded supply may have a grounded shield wire or cable sheath, but such conductor should not be mistaken for aneutral conductor if it is not intended for such service.The ungrounded supply is frequently called a delta(A) sourcebecause such systems generally consist of the three phase conductors only.
The effectively grounded distribution system normally includes a solidly grounded neutral conductor derived from thesource.Usually the neutral is multigrounded along the feeder. Such systems are referred to as “grounded Y” systems.
The impedance-grounded systems are usually found in industrial plant circuits operating at distribution voltage levelsof 4160 V and higher.Such systems may be Y or A.If Y, the neutral is derived from the source, but is insulated andgrounded through a resistor or reactor. If A, the grounding is through a grounding transformer to which an externalimpedance may or may not be connected.Such systems allow the flow of sufficient ground fault current to affordpositive high-speed protective relaying, but limit the ground fault current in order to minimize damage to faultedequipment.Although the impedance-grounded Y systems are sometimes said to have a“floating neutral,””thisterminology is misleading.The neutral is actually stabilized at the source, and with fairly well-balanced loads, theneutral voltage remains near ground voltage except during fault conditions.IEEE C57.105 pdf download.