Star-Delta Connection of Transformer

(4)  Star-Delta Connection:    

  • In this type of connection, then primary is connected in star fashion while the secondary is connected in delta fashion as shown in the Fig.


  • The voltages on primary and secondary sides can be represented on the phasor diagram as shown in the Fig.

Key point:                

  • As Primary in Star connected
  • Line voltage on Primary side = √3 X Phase voltage on Primary side. So
  • Phase voltage on Primary side = Line voltage on Primary side / √3
  • Now Transformation Ration (K) = Secondary Phase Voltage / Primary Phase Voltage
  • Secondary Phase Voltage = K X Primary Phase Voltage.
  • As Secondary  in delta connected:
  • Line voltage on Secondary side = Phase voltage on Secondary side.
  • Secondary Phase Voltage = K X Primary Phase Voltage. =K X (Line voltage on Primary side / √3)
  • Secondary Phase Voltage = (K/√3 ) X Line voltage on Primary side.
  • There is s +30 Degree or -30 Degree Phase Shift between Secondary Phase Voltage to Primary Phase Voltage

Advantages of Star Delta Connection:

  • The primary side is star connected. Hence fewer numbers of turns are required. This makes the connection economical for large high voltage step down power transformers.
  • The neutral available on the primary can be earthed to avoid distortion.
  • The neutral point allows both types of loads (single phase or three phases) to be met.
  • Large unbalanced loads can be handled satisfactory. 
  • The Y-D connection has no problem with third harmonic components due to circulating currents inD. It is also more stable to unbalanced loads since the D partially redistributes any imbalance that occurs.
  • The delta connected winding carries third harmonic current due to which potential of neutral point is stabilized. Some saving in cost of insulation is achieved if HV side is star connected. But in practice the HV side is normally connected in delta so that the three phase loads like motors and single phase loads like lighting loads can be supplied by LV side using three phase four wire system.
  • As Grounding Transformer: In Power System Mostly grounded Y- ∆ transformer is used for no other purpose than to provide a good ground source in ungrounded Delta system. Take, for example, a distribution system supplied by connected (i.e., un-grounded) power source. If it is required to connect phase-to-ground loads to this system a grounding bank is connected to the system, as shown in Figure


  • This system a grounding bank is connected to the system, as shown in Figure. Note that the connected winding is not connected to any external circuit in Figure.
  • With a load current equal to 3 times i, each phase of the grounded Y winding provides the same current i, with the  -connected secondary winding of the grounding bank providing the ampere-turns required to cancel the ampere-turns of the primary winding. Note that the grounding bank does not supply any real power to the load; it is there merely to provide a ground path. All the power required by the load is supplied by two phases of the ungrounded supply

Disadvantages of Star-Delta 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


  • It is commonly employed for power supply transformers.
  • This type of connection is commonly employed at the substation end of the transmission line. The main use with this connection is to step down the voltage. The neutral available on the primary side is grounded. It can be seen that there is phase difference of 30° between primary and secondary line voltages.
  • Commonly used in a step-down transformer, Y connection on the HV side reduces insulation costs the neutral point on the HV side can be grounded, stable with respect to unbalanced loads. As for example, at the end of a transmission line. The neutral of the primary winding is earthed. In this system, line voltage ratio is 1/√3 Times of transformer turn-ratio and secondary voltage lags behind primary voltage by 30°. Also third harmonic currents flow in the to give a sinusoidal flux.

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 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”,”Smart Energy”, “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.

26 Responses to Star-Delta Connection of Transformer

  1. uday says:

    sir rely Important for improve our technical points hats off u sir

  2. praveen agrawal says:

    great info in a very concise manner.

  3. sankar says:

    good notes for understanding. TanQ.

  4. hemantha says:

    thank you sir it is realy help full .

  5. BEng says:

    really benefitn from this,very happy.

  6. amit gupta says:

    dear sir, as per your thery about star, in step down transformer in primary side how can we reduces turns then delta, i think for step down we have to put higher turn in primary side then secondary.

  7. sachin says:

    best….. evry point is clear …. thnks a lot sir

  8. manivannan says:

    good thanks sir

  9. Arvind kumar Pandey says:

    A lot of thanks for proper concept.

  10. simi says:

    Hi, I want to know what are the drawbacks if we ground Y – phase of a transformer

  11. Andrew Mithi says:

    Help me with more details of electrical and electronics engineering.

  12. Reza says:

    Hi dear
    Thank you for your helpful website. I have a question. Is there any phase difference (30 degree) between primary and secondary voltage in star-delta connection? I know there is 30 degree difference between line voltage but about phase voltage I don’t know and it’s better that to say I can not prove it for phase voltage. Could you please help me to find it? Thank you again

  13. satishG says:

    if we wish to use this transformer for distribution purpose and as distribution system require neutral,How we will get neutral from delta connection??

  14. Mageshwaran says:

    thanks sir

  15. Ganesha K says:

    Hi, I have a 400V / 650V Star-Delta Transformer with Delta connected to unbalance load. (RY, YB, BR sequence). How to calculate primary current?

  16. Albertus Haeseb says:

    Sir give me a reason why the transformers under distributions are normally connected in star on the secondary side.?

  17. These is very good website

  18. Kuldeep Singh says:

    thanks sir………… your articles are very helpgf
    ul and informative..

  19. navin singh says:

    this is very helpful for us
    plz upload power system studty material also power plant

  20. aman ullah al sajid says:

    thank you very much

  21. Abdul Hameed says:

    Nice article about Star Delta Connection of Transformer.Great Work.Power Engineers Manufactures Step Down Transformers In India

  22. Olukay says:

    Thank you. You just save me from carrying over EEE303 ELECTRICAL POWER

  23. Sujit Kindo says:

    what is Star And Delta connection transformer

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