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Home / Technical Articles / Delta-Star Transformer Connection Overview
GE Transformer delta-star nameplate
GE Transformer delta-star nameplate

Delta-Star Connection of Transformer

In this type of connection, the primary connected in delta fashion while the secondary current is connected in star.

Delta-Star Connection of Transformer
Delta-Star Connection of Transformer

The main use of this connection is to step up the voltage i.e. at the begining of high tension transmission system. It can be noted that there is a phase shift of 30° between primary line voltage and secondary line voltage as leading.

Phase shift of 30° between primary line voltage and secondary line voltage
Phase shift of 30° between primary line voltage and secondary line voltage

Key points

  1. As primary in delta connected:
  2. Line voltage on primary side = Phase voltage on Primary side.
  3. Now Transformation Ration (K) = Secondary Phase Voltage / Primary Phase Voltage
  4. Secondary Phase Voltage = K X Primary Phase Voltage.
  5. As Secondary in Star connected
  6. Line voltage on Secondary side = √3 X Phase voltage on Secondary side. So,
  7. Line voltage on Secondary side = √3 X K X Primary Phase Voltage.
  8. Line voltage on Secondary side = √3 X K X Primary Line Voltage.
  9. There is s +30 Degree or -30 Degree Phase Shift between Secondary Phase Voltage to Primary Phase Voltage

Advantages of Delta-Star Connection

Cross section area of winding is less at Primary side:
On primary side due to delta connection winding cross-section required is less.

Used at Three phase four wire System:
On secondary side, neutral is available, due to which it can be used for 3-phase, 4 wire supply system.

No distortion of Secondary Voltage:
No distortion due to third harmonic components.

Handled large unbalanced Load:
Large unbalanced loads can be handled without any difficulty.

Grounding Isolation between Primary and Secondary:
Assuming that the neutral of the Y-connected secondary circuit is grounded, a load connected phase-to-neutral or a phase-to-ground fault produces two equal and opposite currents in two phases in the primary circuit without any neutral ground current in the primary circuit.

Therefore, in contrast with the Y-Y connection, phase-to-ground faults or current unbalance in the secondary circuit will not affect ground protective relaying applied to the primary circuit. This feature enables proper coordination of protective devices and is a very important design consideration.

The neutral of the Y grounded is sometimes referred to as a grounding bank, because it provides a local source of ground current at the secondary that is isolated from the primary circuit.

Harmonic Suppression:
The magnetizing current must contain odd harmonics for the induced voltages to be sinusoidal and the third harmonic is the dominant harmonic component. In a three-phase system the third harmonic currents of all three phases are in phase with each other because they are zero-sequence currents. In the Y-Y transformer connection, the only path for third harmonic current is through the neutral.

In the ∆ -Y connection, however, the third harmonic currents, being equal in amplitude and in phase with each other, are able to circulate around the path formed by the ∆ connected winding. The same thing is true for the other zero-sequence harmonics.

Grounding Bank:
It provides a local source of ground current at the secondary that is isolated from the primary circuit. For suppose an ungrounded generator supplies a simple radial system through ∆-Y transformer with grounded Neutral at secondary as shown Figure. The generator can supply a single-phase-to-neutral load through the -grounded Y transformer.

Let us refer to the low-voltage generator side of the transformer as the secondary and the high-voltage load side of the transformer as the primary. Note that each primary winding is magnetically coupled to a secondary winding.

The magnetically coupled windings are drawn in parallel to each other:

Magnetically coupled windings
Magnetically coupled windings

Through the second transformer law, the phase-to-ground load current in the primary circuit is reflected as a current in the A-C secondary winding. No other currents are required to flow in the A-C or B-C windings on the generator side of the transformer in order to balance ampere-turns.

Easy Relaying of Ground Protection:
Protective relaying is MUCH easier on a delta-wye transformer because ground faults on the secondary side are isolated from the primary, making coordination much easier.  If there is upstream relaying on a delta-wye transformer, any zero-sequence current can be assumed to be from a primary ground fault, allowing very sensitive ground fault protection.

On a wye-wye, a low-side ground fault causes primary ground fault current, making coordination more difficult.  Actually, ground fault protection is one of the primary advantages of delta-wye units.

Disadvantages of Delta-Star Connection

In this type of connection, the secondary voltage is not in phase with the primary. Hence it is not possible to operate this connection in parallel with star-star or delta-delta connected transformer.

One problem associated with this connection is that the secondary voltage is shifted by 300 with respect to the primary voltage. This can cause problems when paralleling 3-phase transformers since transformers secondary voltages must be in-phase to be paralleled. Therefore, we must pay attention to these shifts.

If secondary of this transformer should be paralleled with secondary of another transformer without phase shift, there would be a problem.


Commonly used in a step-up transformer

As for example, at the beginning of a HT transmission line. In this case neutral point is stable and will not float in case of unbalanced loading. There is no distortion of flux because existence of a Δ -connection allows a path for the third-harmonic components.

The line voltage ratio is √3 times of transformer turn-ratio and the secondary voltage leads the primary one by 30°. In recent years, this arrangement has become very popular for distribution system as it provides 3- Ø, 4-wire system.

Commonly used in commercial, industrial, and high-density residential locations

To supply three-phase distribution systems.

An example would be a distribution transformer with a delta primary, running on three 11kV phases with no neutral or earth required, and a star (or wye) secondary providing a 3-phase supply at 400 V, with the domestic voltage of 230 available between each phase and an earthed neutral point.

Used as Generator Transformer

The ∆-Y transformer connection is used universally for connecting generators to transmission systems because of two very important reasons.

First of all, generators are usually equipped with sensitive ground fault relay protection. The ∆-Y transformer is a source of ground currents for loads and faults on the transmission system, yet the generator ground fault protection is completely isolated from ground currents on the primary side of the transformer.

Second, rotating machines can literally be.

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


  1. Harold LeRay
    Jul 29, 2020

    ideal delta-wye xfrm & no load on transformer (480-208/120), for a L-G fault on the secondary side, what would be the line fault current flow on the primary which only involves the A-B primary winding?

    Nov 24, 2019

    Sir, its my heartiest honour to you. Sir I’m a electrical engineer n doing a govt. Job. I have Two Dyn11 11000/433 Volt Transformers with same X/R ratio. One already in service. Just recenty, I connected the 2nd transformer to 11kv side indoor metalclad switchboard for parallel operation. Phase sequence in secondary 433 Volt side of both transformer is ok. After testing the voltage on 433 Volt side of both x’mer thro. Test lamp, it’s showing voltage between same phases of two xmer. Hence polarity of both are NOT correct. Both transformers are connected thro.sealed pack cable box. Shutdown of transformer already in service is NOT permitted. I have only option to change the 11kv side cable connection of 2nd transformer. How to change the 3 leads of delta side of 2nd xmer to match the polarity with 1st xmer. Regards.

  3. Tirawolf
    Nov 18, 2019

    Hi! I can changes the connection of factory of the secondary side in dry trafo. Of star to delta. What happens?

  4. Ahmad Niazi
    Aug 31, 2019


  5. Kazi Anwar Hossain
    Jan 01, 2019

    Dear Mr.Jignesh

    Happy new year.

    I have seen your name and articles in the website and got interest to receive some right suggestions fro you.

    We are a engineering company a do electrical business in Bangladesh.Recently we are talking a customer for their
    Textile Spinning project.The criteria are as follows:

    Primary source 6 nos 1500 KW Gas Generator( 415 V,3 phase 4 wire system).Customer need transfer this 8 MW
    Load from Generator house to main factory building.Distance between Generator Hose to main factory is 800 meters. So we planned to transfer 8MW Generator power from Generator house to factory building by 8 nos 2500KVA Cast Resin Dry type Transformer(2500 KVA,11/0.415V,50Hz).We planned as follows:

    a) 6 nos 1500KW Generator power will come to Main panel,where 6 nos Generator will be synchronised.

    b) From mail LT panel we planned to take power by 4 nos 5000Amp Air circuit Breaker(415 V,50Hz)

    c) we planned to use 4x2500KVA Step up Transformer through 4 nos 11KV HT Switchgear as incoming and
    1 nos 11KV Switchgear as out going by 12KV HT cable to factory side.

    d) we planned 1 no 11KV HT panel as incoming and 4 nos 11KV HT panel for 4 nos 2500 KVA Step down
    Transformer to Factory LT panel.

    Now we please confirm us that we are using
    a) 4 nos 2500 KVA step up Transformer, Yd1( primary side 415 V, Delta connection),Secondary side 11KV star connection with neutral grounded)

    b) 4 nos 2500KVA step down transformer Dyn11(Primary side 11KV and Secondary side 415v with neutral grounded).Power will come factory LT panel and from there power will go through BBT(Bus Bar Trunking System)

    Please confirm us our selection of transformer is ok.Because 4 nos step up transformer will wok as parallel operation.On the other side 4 nos step down transformer will also work as parallel.

    Considering vector group and other factor of transformer parallel operation,above Transformer selection is ok
    or if you have any recommendations pls let us know.

    Best Regards
    Engr.Kazi Anwar Hossain
    Managing Director
    KDH Engineering Ltd

    • Warez Ali
      Feb 07, 2021

      Pls give the reply

  6. Gulnawaz Arshad
    Dec 26, 2018

    Hello Sir,
    Can you please give the solution of this?
    How do I get 230V and 15 amps three stable AC outputs from three-phase 440V/415VAC input?

  7. Justin Low Thion
    Dec 17, 2018

    Hi Sir
    Just want to know if a 400V supply from a generator is connected to a step up transformer400v/11000V. What would be the vector type of the transformer and do you connect the neutral cable?

    Justin Low Thion

  8. Veeramani
    Jun 19, 2018

    I have a question what is the reason for fault occur in load side fuse in transformer

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