Why Parallel Operation of Transformer is Required ?

Fig- Parallel Operation connection diagram of  transformer A & B 

What is Parallel Operation of Transformer and Why Does it Required?

Transformer Parallel Operation is means a multiple transformers with same characteristics operates using same voltage from a unique voltage source to supply the same level of voltage for a common load. In short the transformers in parallel operation are connected as primary windings are in a common voltage supply, and the secondary windings are in a common load. The simple parallel operation connection diagram of two transformers is shown in the figure above.

For supplying a larger load of the rating of an existing transformer A, a second or additional transformer B may be connected in parallel with existing transformer that is shown in above figure. 

You will find here that primary windings are connected to the supply bus bars and secondary windings are connected to the load bus-bars. In connecting two or multiple transformers in parallel, it is essential that their terminals of similar polarities are joined to the same bus-bars as in figure above. 

Important issues for transformer parallel operation is if the above condtions are not fulfilled, the two e.m.fs. induced in the secondaries which are paralleled with incorrect polarities, will act together in the local secondary circuit even when supplying no load and will hence produce the equivalent of a dead short-circuit.

Read more about:

1. How do you know if a transformer is step up or step down?
2. What is the Function of Earthing or Grounding Transformer?

How Oparallel Operation of Transformer Works

The Reason of Transformer Parallel Operation in Practical

The most important reasons of Parallel operation for electrical transformer is summarized as below:

1. Physically it is impractical and uneconomical to have a single large transformer for heavy and large loads. So, the better and wise decision to connect a number of transformers in parallel;
2. Parallel Operation of transformer reduces the spare capacity of the electrical substation. It is possible to operate the appropriate number of transformer as per requirement of demanded load of the substation;
3. Transformer parellel opetration provide the phasility to increase the substation capacity just adding the additional transformer of a substation to supply a load beyond the capacity of the transformer already installed;
4. Parallel Operation provide a great oppurtunity to replace a faulty transformer without interruption, just replace faulty transformer by a newone;
5. In Parallel Operation, if any of the transformer from the system is taken out of service due to maintenance or inspection, the power supply will continue by remaning transformers andwill not get disturbed.

Necessary Conditions for Parallel Operation of Multiple Transformers

Two main conditions are necessary for satisfactory parallel operation of the transformer without any incident.

1. Polarities of the transformers must be same;
2. Turn Ratio of the transformer should be equal.

We may summarize the necessary and followed certain definite conditions which must be satisfied in order to avoid any local circulating currents that causes unexpected incident. To share the common load as per proportion of transformer KVA rating, the necessary conditions are  as below:

1. Primary windings of the transformers should be suitable for the supply system voltage and frequency;
2. The transformers should be properly connected with regard to polarity;
3. The voltage ratings of both primaries and secondaries should be identical;
4. Avoid the circulating current and operation at different power factor the percentage impedance should be equal in magnitude;  
5. With transformers having different kVA ratings, the equivalent impedances should be inversely proportional to the individual kVA rating.

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Parallel operational Conditions for Three Phase Transformer

Figure in top explained the connection diagram for parallel operation of single phase transformer. Now, we would like to know the three phase transformer parallel operation connection diagram as below:

Fig- Parallel Operation Connection Diagram for Three Phase Transformer

As per connection, three phase transformer may two types-

1. Delta connection; & 
2. Star or Y connection.

Combination of three phase transfoemer for Operative Parallel Operation and Inoperative parallel operation:

Three Phase transformer Parallel Operation combination that may operative and inoperative list is as below:

Following four vector group is possible to operate parallel operation for three phase transformer.

Operative Parallel Operation
Sr. No.	Transformer-A	Transformer-B
1	∆∆	∆∆ or YY
2	YY	YY or ∆∆
3	∆y	∆y or Y∆
4	Y∆	Y∆ or ∆y

Following four vector group is not possible to operate parallel operation for three phase transformer.

Inoperative Parallel Operation
Sr.No	Transformer-A	Transformer-B
1	∆∆	∆Y
2	∆y	∆∆
3	Y∆	YY
4	YY	Y∆

Parallel Operation between three phase Transformers ideally occurs when there are no circulating currents on open circuit, and the load division between the Transformers is proportional to their kVA ratings.

There are necessary conditions for three phase transformer in parallel operation between- two or more: 

1. The no load ratio of transformation is same;
2. The percentage impedance of transformer is same;
3. The resistance to reactance ratio of transformer is same;
4. The polarity of transformer is same;
5. The phase rotation of transformer is same;
6. The inherent phase-angle displacement between primary and secondary terminals of transformer is same;
7. The power ratio between the corresponding windings of transformer is same.

The above conditions from sirial number one to five are common for three phase Transformers whether two winding or three winding and condition number six and  seven are additional for three winding Transformers to perform in suitable parallel operation.

Disadvantages of Transformer Parallel Operation

Though there are many advantages in transformer parallel operation, but we cannot avoid some disadvantages of parallel operations. The major disadvantages of transformer parallel operation is as below:

1. Circuit Breaker capacity need to increase due to increase short-circuit currents;
2. The risk of circulating currents increases that increases loss and diminish load capability;
3. The bus ratings could be too high;
4. Paralleling transformers reduces the transformer impedance significantly;
5. The control and protection of three units in parallel is more complex.

Finally we can say two or more transformers are connected in parallel means that the multiple primary windings are connected to supply bus and the same number of secondary windings are connected to load bus-bars on transformer parallel operation network.

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.