Delta-Delta Transformer Connection Overview

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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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About Author


Jignesh Parmar

jiguparmar - Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.