Delta-Star Connection of Transformer

(3) Delta-Star Connection of Transformer

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

  • 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.

Key point:                

  • As primary in delta connected:
  • Line voltage on primary side = Phase voltage on Primary side.
  • Now Transformation Ration (K) = Secondary Phase Voltage / Primary Phase Voltage
  • Secondary Phase Voltage = K X Primary Phase Voltage.
  • As Secondary in Star connected
  • Line voltage on Secondary side = √3 X Phase voltage on Secondary side. So,
  • Line voltage on Secondary side = √3 X K X Primary Phase Voltage.
  • Line voltage on Secondary side = √3 X K X Primary Line Voltage.
  • 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 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.

  • 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


About Jignesh.Parmar (B.E,Mtech,MIE,FIE,CEng)
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.

39 Responses to Delta-Star Connection of Transformer

  1. Raju says:

    Sir If I want to connect a 1MVA, 600v PV source to the grid by stepping up 11kv, what will be the number of primary and secondary turn of a delta-star 3 phase transformer ?

  2. Sir please clear, that when one of the phase incomming on the main side of the transformer is cut and grounded what voltage and current conditions wil it reflect on the secondary of 11KV/433V, 100KVA Transformer

  3. enggsanjeev says:

    transformer name plate show ( DY11) ITS MEANING explained . and transformer how many type of test, explained. transformer rating is 33kv/415 volt ,3.15 MVA ,

  4. Saransh Sharma says:

    Delta- star Confirguration is also useful in 300V / 33KV ….

  5. Jeff says:


    Why would communication messengers only be bonded to a primary grounding conductor in a delta Y system ? Would it best not to ground to it where the messenger is bonded to the secondary neutral at other intervals ?


  6. VENKATESH says:

    Hello sir,
    name plate details shows a LV amperes is 84 A in Dyn11 63 KVA 11KV/433V 3 phase transformer, yes In star side line current = phase current.. my question is
    can i connect R phase to load A which draws an amount of 52 A and Y phase to load B which draws 58 A and B phase to load C which draws 43 A ? (i am assuming that transformer gives 84 A in each phase)

    • Amritpal Singh says:

      Yes you can attache the load as you have mentioned, as in the 63 KVA TF the 84A current is per phase full load current. As per knowledge. If any doubt we can discuss at 998902530.

  7. muslim hashim says:

    hello sir,
    i’m a new tech. how i connect my machine to output transformer 3 phase star with neutral connect to ground. is it i need to combined the ground and neutral shorted?

  8. riyaz says:

    Mr. Jignesh… i got a doubt…
    can we use a delta / star transformer in ship?
    in above T/F, is star point need to be grounded?
    in ships, cant point cant be achieved? so can i use a delta / star T/F in ships???

  9. ahmed says:

    very good website thanks alot

  10. Karan says:

    Kuchh naya likho

  11. faisal says:

    Where use star delta

  12. Jude says:

    hi i got some questions for you

  13. sreedhar says:

    Dear Sir , is it possible to design a transformer as per the below spec …
    We feel that primery delta is complicated , pls suggest ..

    PRY VOLTAGE:200-208-215-223V-DELTA ;
    FRQ-400 HZ,

  14. Portable generator 6000 watts says:

    Asking questions are really pleasant thing if you are not understanding
    something totally, except this post gives nice understanding even.

  15. Amir says:

    I have a query here. Is it necessary to provide the grounding transformer to the delta side of generator transformer where the generator neutral is impedance earthed.

  16. anirbanmra says:

    Sir what is the second reason, its incomplete as rotating machines can generally be………….. ????

  17. sumeet mehta. says:

    can you give us connection of 3 phase delta \ star connection in which primary 415 volt +\-5% delta secondary 220 volt star

  18. sravani says:

    sir why we giving 415 volts to star wdg and 230 volts for delta winding in 3 phase armature wndg or induction motor.

  19. sanjeev says:

    Sir, here why the DY trf is used as GT the second reason is not clear. could you please elaborate the second reason.

  20. hari says:

    Sir, I had a doubt on
    Selection of transformer configuration

    Generator supply from 400V to 33KV, a transformer of a different configuration can be selected. Generator is 3phase 4 wire. But most commonly the stepup transformer configuration selected is Dyn11 ??

    Some experts say that this configuration will enhance the alternator life by limiting the harmonics effects on alternator. Is this so?

  21. swati chadha says:

    Kindly explain how to coordinate earth fault in Delta-star transformer (HT/LT) by an example.

  22. sai says:

    sir if a transformer with delta star,ungrounded.if line to ground fault occurs at secondary side. Is there any zero sequence components in transformer.if any on which side?

  23. Ramesh.N says:

    Sir ,

    I have used 3 different 230v ac load.
    Here I need to Use isolation transformer.
    3 PH 415, delta to star transformer.
    Transformer secodary side i will use phase voltage only.

    My load is 1.230 V ,0.5 A
    2.230 V, 1.0 A
    3.230 V,1.5 A

    So,avg Current is 1.75 A (I have included my Factor of safety also).
    Then i have calculated below method.

    =1.732 x 230 x 1.75
    =697 VA
    So ,Finally I got 0.7 KVA

    I need to know about this calculation is right or wrong.

  24. Susanta Mondal says:

    what type of transformer connection generally we use from electrical power generation to secondary distribution ????
    sir please give me a list…. please , it is very urgent . I am waiting for your reply.

  25. Thabelo says:

    I like to know how star-delta connected or delta-star connected transformers

  26. M.B.Parthasarathy says:

    10kva/delta-star/Dyn11/440-230V lighting transformer. Delta current is 7.57A and the star current is 25A. Pls examine whether it is correct or not?

  27. shubham kumar Raut says:

    Can Dyn0 vector group be possible ?

  28. Amna Ali Butt says:

    At what condition star delta and delta star connection have same fault current?

  29. Sir, Under ground cable 11 KV, When change substation in root 3 given will mean 6.3 kv. Then the cable joint kit is failed .. why? I Want to take your opinion?

  30. Nikhil Hari says:


    Could u please help me to calculate the HV current during an unbalance in LV side of the transformer.

    Ex: If The transformer rating is 1500kvA, 11k/0.415kVA, 78/2086A. What will be the HV current if the LV side currents in 3 phases are 1800A, 1900A and 2000A.

    Thanks and Regards,

  31. Dr Sanjay R Joshi says:

    Dr Sanjay R Joshi
    Please do not write that the third harmonic currents are in phase as they are zero sequence currents.
    U need to write here more responsibly, as someone may believe it is right.
    The frequency of zero sequence currents is fundamental only and not 150 Hz.

  32. Ronak Bera says:

    Hello sir,

    I want Dy3 & Dy9 Transformer connection so please send me Details about this transformer connection,

  33. Er. Amritpal Singh Sidhu says:

    Mr. Jignesh, I wan to discuss about termination of HT XLPE cables & AB Cables. I have experience that both of them cables burst at termination point so I want to discuss in detailed with you. Please give me your contact no.
    Er. Amritpal Singh Sidhu

  34. Deepak Agrawal says:

    Mr Jignesh, Kindly let me know in a Delta-Star Transformer, If there is load in only one phase on the secondary side, what is the effect of current drawn in each of the phase of the primary.


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