Three Phase Transformer
Three phase transformers are used throughout industry to change values of three-phase voltage and current. Since three-phase power is the most common way in which power is produced, transmitted, and used, an understanding of how three phase transformer connections are made is essential.
1. Delta-Delta (Δ-Δ) Connection:
The delta-delta connection of three identical single-phase transformer is shown in the figure below. The secondary winding a1a2 is corresponding to the primary winding A1A2, and they have the same polarity. The polarity of the terminal a connecting a1 and c2 is same as that connecting A1 and C2. The figure below shows the phasor diagram for lagging power factor cosφ.
The magnetizing current and voltage drops in impedances have been neglected. Under the balanced condition, the line current is √3 times the phase winding current. In this configuration, the corresponding line and phase voltage are identical in magnitude on both primary and secondary sides. The secondary line-to-line voltage is in phase with the primary line-to-line voltage with a voltage ratio equal to the turn’s ratio. If the connection of the phase windings is reversed on either side, the phase difference of 180° is obtained between the primary and the secondary system. Such a connection is known as an 180º connection.
2. Star-Star (Υ-Υ) Connection of Transformer:
The star-star connection of three identical single-phase transformer on each of the primary and secondary of the transformer is shown in the figure below. The phasor diagram is similar as in delta-delta connection.
The phase current is equal to the line current, and they are in phase. The line voltage is three times the phase voltage. There is a phase separation of 30º between the line and phase voltage. The 180º phase shift between the primary and secondary of the transformer is shown in the figure above.
3. Delta-Star (Δ-Υ) Connection:
The ∆-Y connection of the three winding transformer is shown in the figure below. The primary line voltage is equal to the secondary phase voltage. The relation between the secondary voltages is VLS= √3 VPS.
Secondary phase voltage Van leads the primary phase voltage VAN by 30°. Similarly, Vbn leads VBN by 30º and Vcn leads VCN by 30º.This connection is also called +30º connection.
4. Star-Delta (Υ-Δ) Connection:
The star-delta connection of three phase transformer is shown in the figure above. The primary line voltage is √3 times the primary phase voltage. The secondary line voltage is equal to the secondary phase voltage.
There is a phase shift of 30 lead exists between respective phase voltage. Similarly, 30° leads exist between respective phase voltage. Thus, the connection is called +30º connection
5. Open delta or V-V Connection:
If one transformer of delta-delta connection is damaged or accidentally opened, then the defective transformer is removed, and the remaining transformer continues to work as a three-phase bank. The rating of the transformer bank is reduced to 57.7% of that of the actual bank. This is known as the open delta or V-V delta. Thus, in open winding transformer, two transformers are used instead of three for the 3-phase operation.
If the three transformers are connected in delta-delta configuration and are supplying rated load and if the connection becomes V-V transformer, the current in each phase winding is increased by √3 times. The full line current flows in each of the two-phase windings of the transformer. Thus, each transformer in the V-V system is overloaded by 73.2%. It should be noticed that the load should be reduced by √3 times in case of an open delta connected transformer. Otherwise, serious overheating and breakdown of the two transformers may take place.
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