Chapter 4 Transformer Connections

1. Chapter 4Transformer Connections Voltage Transformation · Three-Phase Circuits · Delta Connections · Wye Connections · Delta-to-Wye Connections · Wye-to-Wye Connections · Wye-to-Delta Connections · Delta-to-Delta Connections · Open Delta Connections · Delta Connection with High Leg to Ground · Three-Phase Transformer Polarity

2. The low‑voltage winding of a standard distribution transformer connected to a single-phase circuit is normally made with two equal coils. These sections are arranged so that they may be connected in series or parallel.

3. Transformer polarity may be subtractive or additive.

4. The polarity of a transformer may be determined using a known transformer or with AC voltage.

5. A delta connection has the wires from the ends of each coil connected end-to-end to form a closed loop.

6. Studying the current in a delta-connected system at both 0° and 90° illustrates the current flow in a delta-connected system.

7. A wye connection has one end of each coil connected together and the other end of each coil left open for external connections.

8. Studying the current in a wye-connected system at both 0° and 90° illustrates the current flow in a wye-connected system.

9. A 3-phase delta-to-wye connection is often used for distribution where a four-wire secondary distribution circuit is required.

10. Three single-phase transformers may be connected in a wye-to-wye connection to form a 3-phase transformer.

11. A wye-to-delta connection permits single-phase and 3-phase loads to be drawn simultaneously from the delta-connected secondary at the same voltage.

12. In a wye-to-delta connection of three single-phase transformers, one unit may be disconnected from the circuit and service maintained with the secondary operating in open delta connection at 57.7% of normal bank capacity.

13. In a delta-to-delta connection, the voltage of each transformer is equal to the 3-phase line voltage and the current in each of the transformers is only 57.7% of the line current.

14. Grounding one of the midpoints of the windings provides 240 V/120 V, single-phase, 3-wire service for lighting applications.

15. An open delta connection makes 3-phase power available anywhere along the distribution line with the use of only two transformers rather than the usual three.

16. A delta connection with a high leg to ground provides 240 V, 3-phase power to equipment and 120 V power for lighting and receptacles.

17. The phase sequence on the secondary of a three-phase transformer is the same as the phase sequence on the primary.

18. Transformers are classified into connection groups according to their phase shift.