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THREE PHASE TRANSFORMER WINDING CONNECTIONS

1) DELTA DELTA CONNECTION

1. Suitable for both un-grounded and effectively grounded sources.
2. Suitable for a three-wire service or a four-wire service with a mid-tap ground.

2) DELTA WYE CONNECTION

1. Suitable for both un-grounded and effectively grounded sources.
2. Suitable for a three-wire service or a four-wire grounded service with XO grounded.
3. With XO grounded, the transformer acts as a ground source for the secondary system.
4. Fundamental and harmonic frequency zero-sequence currents in the secondary lines supplied by the transformer do not flow in the primary lines. Instead the zero sequence currents circulate in the closed delta primary windings.
5. When supplied from an effectively grounded primary system does not see load unbalances and ground faults in the secondary system.

3) WYE DELTA CONNECTION

1. Suitable for both un-grounded and effectively grounded sources.
2. Suitable for a three-wire service or a four-wire delta service with a mid-tap ground.
3. Grounding the primary neutral of this connection would create a ground source for the primary system. This could subject the transformer to severe overloading during a primary system disturbance or load unbalance.
4. Frequently installed with mid-tap ground on one leg when supplying combination three-phase and single-phase load where the three-phase load is much larger than single-phase load.
5. When used in 25 kV and 35 kV three-phase four-wire primary systems, Ferro resonance can occur when energizing or de-energizing the transformer using single-pole switches located at the primary terminals. With smaller kVA transformers the probability of Ferro resonance is higher.

4) WYE WYE CONNECTION

1. Suitable for both un-grounded and effectively grounded sources.
2. Suitable for a three-wire service only, even if XO is grounded.
3. This connection is incapable of furnishing a stabilized neutral and its use may result in phase-to-neutral over voltage (neutral shift) as a result of unbalanced phase-to-neutral load.
4. If a three-phase unit is built on a three-legged core, the neutral point of the primary windings is practically locked at ground potential.

5) GROUNDED WYE WYE CONNECTION

1. Suitable for four-wire effectively grounded source only.
2. Suitable for a three-wire service or for four-wire grounded service with XO grounded.
3. Three-phase transformers with this connection may experience stray flux tank heating during certain external system unbalances unless the core configuration (four or five legged) used provides a return path for the flux.
4. Fundamental and harmonic frequency zero-sequence currents in the secondary lines supplied by the transformer also flow in the primary lines (and primary neutral conductor).
5. Ground relay for the primary system may see load unbalances and ground faults in the secondary system. This must be considered when coordinating overcurrent protective devices.
6. Three-phase transformers with the neutral points of the high voltage and low voltage windings connected together internally and brought out through an HOXO bushing should not be operated with the HOXO bushing un-grounded (floating). To do so can result in very high voltages in the secondary systems.

6) DELTA DELTA CONNECTION WITH TAP

1. Suitable for both un-grounded and effectively grounded sources.
2. Suitable for a three-wire service or a four-wire service with a mid-tap ground.
3. When using the tap for single-phase circuits, the single-phase load kVA should not exceed 5% of the three-phase kVA rating of the transformer. The three-phase rating of the transformer is also substantially reduced.
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