Transformer Delta-Delta Connection

Fig: Delta-Delta or ∆-∆ Connection

Delta-Delta Connection Procedure of Three-Phase Transformer in Power Transmission Network 

This connection is economical for large, low-voltage transformers in which the insulation problem is not so urgent, because it increases the number of turns/phases. 

The transformer connections and voltage triangles are shown in the above figure. The ratio of transformation between primary and secondary line voltage is exactly the same as that of each transformer. 

Further, the secondary voltage triangle ABC occupies the same relative position as the primary voltage triangle ABC i.e., there is no angular displacement between the two. 

Moreover, there is no internal phase shift between phase and line voltages on either side as was the case in Y − Y connection. 

Why is Delta-Delta Connection necessary for Three-Phase Transformer?

The major application of the Delta-Delta three-phase transformer connection in the power transmission network is as below:

Suitable for large, low-voltage transformers of the distribution system.

Delta-Delta type of connection is normally uncommon but used in some industrial facilities to reduce the impact of SLG faults on the Primary System.

Delta-Delta connection is generally used in systems where it needs to carry large currents on low voltages and especially when continuity of service is to be maintained even though one of the phases develops a fault.

Advantages of delta-Delta three-phase transformer connection

 Delta-delta connection of a three-phase transformer has the following advantages:

1. As explained above, in order that the output voltage to be sinusoidal, it is necessary that the magnetizing current of the transformer must contain a third harmonic component. In this case, the third harmonic component of the magnetizing current can flow in the ∆-connected transformer primaries without flowing in the line wires. The three phases are 120° apart which is 3 × 120 = 360° with respect to the third harmonic, hence it merely circulates in the ∆. Therefore, the flux is sinusoidal which results in sinusoidal voltages.

2. No difficulty is experienced from unbalanced loading as was the case in Y − Y connection. The three-phase voltages remain practically constant regardless of load imbalance.

3. An added advantage of this connection is that if one transformer becomes disabled, the system can continue to operate in open-delta or in V − V although with reduced available capacity. The reduced capacity is 58% and not 66.7% of the normal value.

Disadvantages of delta-delta transformer connection

The main disadvantage of the delta-delta transformer is that there is no star-point or neutral terminal available. Therefore, the delta-delta connected transformer is used when neither primary nor secondary requires a neutral terminal and the voltages are low and moderate.

You may know the details about the electrical transformer from the following articles:

 

1. Working Principle of Transformer;
2. Transformer Construction;
3. Core-type Transformers;
4. Shell-type Transformers;
5. Elementary Theory of an Ideal Transformer;
6. E.M.F. Equation of Transformer;
7. Voltage Transformation Ratio;
8. Transformer with losses but no Magnetic Leakage;
9. Transformer on No-load;
10. Transformer on Load;
11. Transformer with Winding Resistance but no Magnetic Leakage;
12. Equivalent Resistance;
13. Magnetic Leakage;
14. Transformer with Resistance and Leakage Reactance;
15. Simplified Diagram;
16. Total Approximate Voltage Drop in Transformer;
17. Exact Voltage Drop;
18. Equivalent Circuit Transformer Tests;
19. Open-circuit or No-load Test;
20. Separation of Core Losses;
21. Short-Circuit or Impedance Test;
22. Why Transformer Rating in KVA?;
23. Regulation of a Transformer;
24. Percentage Resistance, Reactance, and Impedance;
25. Kapp Regulation Diagram;
26. Sumpner or Back-to-back-Test;
    
27. The efficiency of a Transformer;
28. Condition for Maximum Efficiency;
29. Variation of Efficiency with Power Factor;
30. All-day Efficiency;
31. Auto-transformer;
32. Conversion of 2-Winding Transformer into Auto-transformer;
33. Parallel Operation of Single-phase Transformers;
34. Questions and Answers on Transformers;
35. Three-phase Transformers;
36. Three-phase Transformer Connections;
37. Star/Star or Y/Y Connection;
38. Delta-Delta or ∆/∆ Connection;
39. Wye/Delta or Y/ Connection;
40. Delta/Wye or ∆/Y Connection;
41. Open-Delta or V-V Connection;
42. Power Supplied by V-V Bank;
43. Scott Connection or T-T Connection;
44. Three-phase to Two-Phase Conversion and vice-versa;
45. Parallel Operation of 3-phase Transformers;
46. Instrument Transformers;
47. Current Transformers;
48. Potential or Voltage Transformers.