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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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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 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


    Dec 10, 2018

    For DC Drives Siemens 6RA22 Synchronisation supply used is through Synchronisation PT 33KV/110V, Delta/Delta connection.
    My question is can we use PT 33KV/110V star/star connection? Or can we use 415V/110 V LT Delta/delta PT ?

  2. Royce Day
    Dec 06, 2018

    On the secondary side of a delta/delta transformer with 240 volt between phases, what will the voltage be should phase A become grounded?
    When onboard a ship during my Navy years, I remember we were using this type of connection and wiring.

  3. B.D.Shah
    Feb 15, 2018

    Transformer rating 2.5MVA,6.6KV/0.69KV, Dd0 (Delta/Delta) vector Group Transformer.
    6.6KV HV Side CT Ratio=250/1 Relay Type-REF615 ABB make
    KV LV Side CT Ratio= 3200A ACB with static release Schneider make
    Pl advise the Protection Philosophy in Overcurrent and Earth fault. Especially earth fault considering the delta winding on HV & LV side.
    LV side being unearthed, Current operated protection will not sense.
    HV side source being earthed, Earth fault current will be dependent on source earthing type & NGR. Hence, current operated earth fault protection will be effective.
    Should it have any protection against harmonics [due to delta-delta transformer and APFC load

  4. bhavesh
    Jan 21, 2017

    delta connection in lv wdg what is the formula for vt

  5. Amey Venikar
    Nov 22, 2016

    If we connect a load on secondary of delta delta transformer, which demands harmonic what will happen ?

  6. felix
    Aug 03, 2016

    What is the difference between a Starr-star Transformer connection and a Delta-delta transformer connection?

    • Rt
      Feb 01, 2017

      A Delta Delta is a 3-phase connection without a neutral you’re going the same voltage on the primary to the same voltage of the secondary but it’s used for high current applications like stated in this article. Your star star would be the same type of system except now you’re driving a neutral at the center point of the three phases. It would most likely be used to block fault current and voltage is from the primary or secondary side to not have a substantial impact on your load.

  7. Raman
    Jun 25, 2015

    Power supply transformer connection is preferred

  8. LI Xinglong
    Apr 09, 2015

    Jayesh, we can use separate transformer to change voltage for single phase load

  9. jayesh
    Dec 03, 2014

    what will be voltage in secondary of delta-delta transformer if single phase supply is applied?

    • Yuoi
      Feb 01, 2017

      Depending on the voltage you have present it probably be just 208 or 480 that’s what we have in the US anyway. You would just be missing voltage on that third leg if you didn’t have a voltage present On the third leg of your primary. Or there’s a possibility of also seeing an induced voltage on your third leg of your secondary but it may not be as large as the other two legs.

  10. Esmail afshari
    May 28, 2014

    Limit the extend of damage by providing a credible path to ground and using this to interrupt and take a Transformer out of service during a fault

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