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Home / Technical Articles / Delta-Delta Transformer Connection Overview
Delta-Delta Transformer Connection Overview
Delta-Delta Transformer Connection Overview (on photo: SQUARE D transformer nameplate)

Delta-Delta Connection

In this type of connection, both the three phase primary and secondary windings are connected in delta as shown in the Figure 1:

Delta-delta transformer connection
Figure 1 – Delta-delta transformer connection

The voltages on primary and secondary sides can be shown on the phasor diagram (Figure 2):

Phasor diagram - Delta delta transformer connection
Figure 2 – Phasor diagram of delta delta transformer connection

This connection proves to be economical for large low voltage transformers as it increases number of turns per phase.

Key points

  1. Primary side Line Voltage = Secondary Side Line Voltage.
  2. Primary side Phase Voltage= Secondary Side Phase Voltage.
  3. No phase shift between primary and secondary voltages.

Advantages of Delta-Delta Transformer Connection

Sinusoidal Voltage at Secondary

In order to get secondary voltage as sinusoidal, the magnetizing current of transformer must contain a third harmonic component. The delta connection provides a closed path for circulation of third harmonic component of current. The flux remains sinusoidal which results in sinusoidal voltages.

Carry 58% Load if One Transfer is Faulty in Transformer Bank

If there is bank of single phase transformers connected in delta-delta fashion and if one of the transformers is disabled then the supply can be continued with remaining tow transformers of course with reduced efficiency.

Economical for Low Voltage

Due to delta connection, phase voltage is same as line voltage hence winding have more number of turns. But phase current is (1/√3) times the line current. Hence the cross-section of the windings is very less. This makes the connection economical for low voltages transformers.

Absent of Third Harmonic Voltage

Due to closed delta, third harmonic voltages are absent. The absence of star or neutral point proves to be advantageous in some cases.

Disadvantages of Delta-Delta Transformer Connection

Due to the absence of neutral point it is not suitable for three phase four wire system. More insulation is required and the voltage appearing between windings and core will be equal to full line voltage in case of earth fault on one phase.


  1. Suitable for large, low voltage transformers.
  2. This type of connection is normally uncommon but used in some industrial facilities to reduce impact of SLG faults on the primary system.
  3. It is generally used in systems where it need to be carry large currents on low voltages and especially when continuity of service is to be maintained even though one of the phases develops fault.

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More Information

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. Annie
    Apr 07, 2023

    Respected Sir,
    I have been visiting your website to gather knowledge. It is very helpful. I appreciate your thoughts in sharing your knowledge. I have a question. What is delta feedback? How will this impact surge arrestor calculations. Please let me know. Appreciate your help on this question.

    Thank you


  2. Suyudi Karsohutomo
    Dec 13, 2022

    dear Mr. Parmar,
    would you please explain that in Japan delta-delta transformer they put S line secondary side direct grounded, the voltage between phase keep the same. I still cannot find this theory.

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