Distribution Transformers and Connections
Foundation
Understand transformer action: voltage ratio equals turns ratio, and power in approximately equals power out minus losses. Know the common distribution connections - single-phase, open wye–open delta, and three-phase wye or delta banks - and how each serves different load types. Understand the purpose of the neutral, the meaning of a grounded wye secondary, and why a delta secondary may be ungrounded or corner-grounded.
Concepts to master
Transformer polarity and additive versus subtractive marking matter when paralleling or banking units. Know how to identify the polarity of a bank from nameplate data. Understand percent impedance and how it affects fault current and voltage regulation - lower impedance means higher secondary fault current. Know the effect of load imbalance on a wye–wye bank and why a delta bank tolerates single-phase loading differently. Recognize the purpose of a tap changer and how it adjusts output voltage.
Traps
Questions often confuse turns ratio with impedance, or assume a delta bank can supply a neutral for single-phase loads without a grounding transformer.
Rule of thumb
Secondary fault current is roughly the transformer full-load current divided by its per-unit impedance - a 5% impedance unit yields about 20 times full-load current.
Worked example
A 50 kVA, 7200/240 V transformer has a full-load secondary current near 208 A; at 2% impedance the available secondary fault current is on the order of 10 kA before cable impedance is considered.