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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.


Applications

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

46 Comments


  1. Sumanta
    May 23, 2018

    For a Y-D Transformer , if the D side gets grounded what will happen.?
    if there is a second grounding what will happen.?


  2. narendra ganvir
    Apr 24, 2018

    sir,
    I have 500 kva outdoor transformer, I want to change tapping for the same,
    please let me know how to calculate output voltage of the LT side.


  3. Omer
    Apr 11, 2018

    Hi Jignesh,
    I have a delta/zigzag+N (DZno) isolation transformer (415v/440V) connected to any IT system (floating / unearthed). My question is, do I need to terminate the neutral to ground, even though the transformer is connected to an IT system? or do I not connect the neutral?
    The transformer enclosure is grounded.


  4. VINAYAK G
    Mar 12, 2018

    Hi , I have a doubt , for 1000 KVA transformer, the cable to main panel we are using is 1Core 630 mm2 of two runs for each phase and one run for neutral, ie total 7 runs , the distance between panel and transformer is 45meter , Can you suggest if any problem in doing the same? What will be maximum current capacity of mentioned above cable?


  5. Benwari
    Mar 05, 2018

    Hi Parmar, i have a medium voltage system comprising of four 2188kVA, 11kV gas generator with star point synchronized together on a HT bus with two 2250KVA, 415V diesel sets connected to two 2.5MVA (0.4/11KV) step up delta/star transformer. the two diesel sets will be synchronized at LV side and stepped up to 11kV to be synchronized on the 11 KV bus. do i need to connect the neutral cables of the 11KV gas generator and 415V of the diesel generators, because if the two diesels are started and synchronized at 11KV bus, the 11KV gas generator can sync with them, but if the gas is started first, the diesel set sees an inbalance bus reference. please what can cause this inblance?


  6. Harish Kumar
    Jan 13, 2018

    Is there any problem if we connect YNd1 instead YNd11 in generator step up Tranformer , and UAT WITH Dyn11 instead Dyn1.
    what will be problem if secondary side (LOAd side winding leading) is it mandotory to design always secondary winding load side to be lagging ? please confirm.


  7. Shrey
    Jan 11, 2018

    Sir what is instantaneous current in 3 wire 2 phase transmission system and how instantaneous current differ from line current. Plz reply


  8. Johan Borman
    Dec 17, 2017

    I have a 100kva trf and want to use it for step up from 400v to 3300 volt, but input is delta and ht side is star, with a neutral.
    What do I do with neutral on input side and what do I connect to neutral terminal on output side.


  9. sk shakeel
    Nov 27, 2017

    Sir above we said delta(primary) star(secondary) connection T/f is a step up transformer but 11kv/440v is a step down voltage so how can said star delta connected T/f is a step up transformer


  10. Irfan
    Oct 25, 2015

    I have a standby DG 3ph-380V, 1250kVA, 1800rpm, 60 Hz with a Star connection, while its Neutral is connected to the Ground. This Generator is supposed to provide 33kV via Step up Transformer to an assembly of 33kV switchgears containing Vacuum type circuit breaker.
    Please tell me, what is the best configuration for the Transformer in this case. Note that the Step up transformer is rated to be 1250kVA.
    Which side is to be Star and which to be Delta for this step up transformer?
    And How the earthing connections should be done?
    Please provide comprehensive response.
    Thanks
    Irfan


  11. arvind
    Sep 20, 2015

    sir,
    star connection 3phase supply of 400V r y b conductor. Then r in phase Voltage ,r & y in phase voltage,
    who to calculate When r y connection in 400V and r y b is also in 400V why.


  12. Rakesh Dewangan
    Aug 01, 2015

    what effect in transformer if we are incoming supply 2 phase fed in a 3phase 25KVA delta -star 6.6/110V transformer.


  13. Nizam
    Jul 26, 2015

    Sir;
    how Can we detect theft in any type of line


    • DEBASISH KHUNTIA
      Mar 28, 2016

      sir what is the effect of third harmonic in step up and step down transformer


  14. Aamir Hashmi
    Jun 29, 2015

    Sir.
    In Delta-star connection of a transformer if a phase is faulty or switched off,then what will happen??


  15. Nitin Joshi
    May 30, 2015

    We are having Biogas genrtor connected in star feeding to delta connected step up transformer.Protection system frequently trips on earth fault.On checking we could not find any problem.Kindly give probable reason/suggestion.
    Nitin

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