Delta-Delta Connection of Transformer


(2)  Delta-Delta Connection:    

  • In this type of connection, both the three phase primary and secondary windings are connected in delta as shown in the Fig.

  •  The voltages on primary and secondary sides can be shown on the phasor diagram.


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

Key point:

  • Primary Side Line Voltage = Secondary Side Line Voltage.
  • Primary Side Phase Voltage= Secondary Side Phase Voltage.
  • No phase shift between primary and secondary voltages

Advantage of Delta-Delta 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.
  • Suitable for Unbalanced Load: Even if the load is unbalanced the three phase voltages remains constant. Thus it suitable for unbalanced loading also.
  • 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.
  • No Distortion in Secondary Voltage: there is no any phase displacement between primary and secondary voltages. There is no distortion of flux as the third harmonic component of magnetizing current can flow in the delta connected primary windings without flowing in the line wires .there is no distortion in the secondary voltages.
  • 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. 
  • Reduce Cross section of Conductor: The conductor is required of smaller Cross section as the phase current is 1/√3 times of the line current. It increases number of turns per phase and reduces the necessary cross sectional area of conductors thus insulation problem is not present.
  • 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.

Disadvantage of Delta-Delta 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.


  • Suitable for large, low voltage transformers.
  • This Type of Connection is normally uncommon but used in some industrial facilities to reduce impact of SLG faults on the primary system
  • 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.