Three-Phase Transformer Connections:

Any three-phase transformer or three single-phase transformers that constitute a three-phase transformer can be connected in different fashions known as Three-Phase Transformer Connections.

The Primary and Secondary three-phase windings can be connected in different ways. Each model has a certain application.

Types of Three-Phase Transformer Connections:

1. Primary Y – secondary Y  *(Y-Y connection)
2. Primary Δ – secondary Δ  *(Δ-Δ connection)
3. Primary Y – secondary Δ *(Y-Δ connection)
4. Primary Δ – secondary Y *(Δ-Y connection)
5. Primary and secondary open Δ  *(V-V connection)
6. Primary T – secondary T *(Scott connection)
7. Zig-Zag connection (Z or z Connection)

Let us discuss these Three-Phase Transformer Connections in detail along with the applications.

1. Primary Y – secondary Y (Y-Y connection):

This type of connection has the following characteristics:

1. Line voltage is √3 times the phase voltage.
2. Neutral is unstable because of the third harmonic component in the exciting current. So this connection can be used only if the connected load is balanced.
3. No phase shift between primary and secondary voltages.

Application:

• Y-Y connection is economical for small H.V. transformers.

2. Primary Δ – secondary Δ (Δ-Δ connection)

This type of connection has the following characteristics:

1. These connections can tolerate large load unbalance.
2. The ratio of line voltages on the primary and secondary sides is equal to the transformation ratio of the transformers.

Application:

• Δ-Δ connection is economical for large H.V. transformers.
• Even if one transformer is disabled, the system can continue to operate in an open delta connection but with reduced available capacity.

3. Primary Y – secondary Δ (Y-Δ connection)

This type of connection has the following characteristics:

1. The ratio of secondary to primary line voltage is 1/√3 times the transformation ratio.
2. There is a 30° shift between the primary and secondary line voltages.

Application: star-delta connection is used to step down the voltage at the substation end of the transmission line.

4. Primary Δ – secondary Y (Δ-Y connection)

This type of connection has the following characteristics:

1. It provides 3 phase 4 wire service with secondary winding connected in star with grounded neutral.
2. It cannot be operated parallel with Star-Star or delta-delta bank.
3. The ratio of secondary to primary line voltage is √3 times the transformation ratio
4. There is a 30° shift between the primary and secondary line voltages.

Application:

• Delta-star connection is used to step up the voltage at the beginning of the transmission line. Frequently used for low voltage distribution systems.

5. Open Δ (V-V connection)

This type of connection has the following characteristics:

1. Two transformers are used in this connection.
2. The total load-carrying capacity of an open delta connection is 57.7% and that would be for Δ-Δ connection.
3. The increase in capacity is 73.2 % when converting from a V – V system to a ∆-∆ system.

Application:

1. When it is anticipated that in the future the load increase, then it requires the closing of the open delta. In such cases, an open delta connection is preferred.
2. When one of the transformers in a Δ-Δ bank fails, service may be continued until the faulty transformer is repaired or a good one is substituted.

Other Three-Phase Transformer Connections:

Scott connection and Zig-Zag connections are different from the above connections.

6. Scott Connection (T-T connection):

This type of connection has the following characteristics:

1. Two single-phase transformers are used in this type of connection. One of the transformers has center taps on both primary and secondary windings (which is called the main transformer). The other transformer is called a teaser transformer.
2. The main transformer has a center or 50 percent tap and is connected between the two lines of the three-phase wires. The teaser transformer has 86.6 percent tap and is connected between the third phase wire and 50 percent tap of the main transformer.
3. The secondary voltages in the two transformers will be equal in magnitude if both transformers are wound for an equal number of turns on the secondary. And produced voltages are 90 degrees out of phase with each other.
4. The secondary winding of each transformer provides the phases of two-phase systems.

Application:

• Employed for 3-phase to 2-phase conversion or vice versa.
• Scott connection is also used to supply power to the two-phase motor.

7. Zig-Zag Three-Phase Transformer Connection: (Z or z connection)

Each phase of winding is split into two equal halves. The first half of the winding is on a given core, and the second half is on a separate core. This pattern is repeated for each of the other phases and their respective cores. There will therefore be parts of two phases on each limb, with one winding on each limb being connected together at the endpoints.

With the neutral point connected to the ground and when balanced voltages are applied to the phases, the system remains passive.

The windings are configured so that induced voltages cancel each other where the phase current is essentially zero establishing the transformer as a very high impedance to positive and negative sequence voltages.

However, when an unbalanced state occurs, such as for a ground fault, the windings offer a very low impedance path for a return to the source of zero sequence current. The current entering the neutral point is split evenly into three currents and returns to the respective phases.

Additionally, the impedance of the unit can be adjusted to set the maximum value of the ground-fault current or the transformer can be used with a ground resistor to fix a value throughout a medium-voltage system.

Application:

• A Zig-Zag transformer is mainly used where the load unbalances can occur between the phases and neutral, most commonly used as a grounding (earthing) transformer means to establish an enhanced protection scheme like Stator Earth Fault Protection for High Impedance Earthing.