Zig-zag Connection of Transformer

(5) The Zigzag Connection:

• The zigzag connection is also called the interconnected star connection.  This connection has some of the features of the Y and the ∆ connections, combining the advantages of both.
• The zigzag transformer contains six coils on three cores. The first coil on each core is connected contrariwise to the second coil on the next core. The second coils are then all tied together to form the neutral and the phases are connected to the primary coils. Each phase, therefore, couples with each other phase and the voltages cancel out. As such, there would be negligible current through the neutral pole and it can be connected to ground
• One coil is the outer coil and the other is the inner coil. Each coil has the same number of windings turns (Turns ratio=1:1) but they are wound in opposite directions. The coils are connected as follows:
• The outer coil of phase a1-a is connected to the inner coil of phase c2-N.
• The outer coil of phase b1-b is connected to the inner coil of phase a2-N.
• The outer coil of phase c1-c is connected to the inner coil of phase b2-N.
• The inner coils are connected together to form the neutral and our tied to ground
• The outer coils are connected to phases a1,b1,c1 of the existing delta system.
• If three currents, equal in magnitude and phase, are applied to the three terminals, the ampere-turns of the a2-N winding cancel the ampere-turns of the b1-b winding, the ampere-turns of the b2-N winding cancel the ampere turns of the c1-c winding, and the ampere-turns of the c2-N winding cancel the ampere turns of the a1-a winding. Therefore, the transformer allows the three in-phase currents to easily flow to neutral.
• If three currents, equal in magnitude but 120° out of phase with each other, are applied to the three terminals, the ampere-turns in the windings cannot cancel and the transformer restricts the current flow to the negligible level of magnetizing current. Therefore, the zigzag winding provides an easy path for in-phase currents but does not allow the flow of currents that are 120°out of phase with each other.
• Under normal system operation the outer and inner coil winding’s magnetic flux will cancel each other and only negligible current will flow in the in the neutral of the zig –zag transformer.
• During a phase to ground fault the zig-zag transformer’s coils magnetic flux are no longer equal in the faulted line. This allows zero sequence.
• If one phase, or more, faults to earth, the voltage applied to each phase of the transformer is no longer in balance; fluxes in the windings no longer oppose. (Using symmetrical components, this is Ia0 = Ib0 = Ic0.) Zero sequence (earth fault) current exists between the transformers’ neutral to the faulting phase. Hence, the purpose of a zigzag transformer is to provide a return path for earth faults on delta connected systems. With negligible current in the neutral under normal conditions, engineers typically elect to under size the transformer; a short time rating is applied. Ensure the impedance is not too low for the desired fault limiting. Impedance can be added after the secondary’s are summed (the 3Io path)
• The neutral formed by the zigzag connection is very stable. Therefore, this type of transformer, or in some cases an auto transformer, lends itself very well for establishing a neutral for an ungrounded 3 phase system.
• Many times this type of transformer or auto transformer will carry a fairly large rating, yet physically be relatively small. This particularly applies in connection with grounding applications. The reason for this small size in relation to the nameplate KVA rating is due to the fact that many types of grounding auto transformers are rated for 2 seconds. This is based on the time to operate an over current protection device such as a breaker. Zigzag transformers used to be employed to enable size reductions in drive motor systems due to the stable wave form they present. Other means are now more common, such as 6 phase star.

• The ∆ -zigzag connection provides the same advantages as the ∆-Y connection.
• Less Costly for grounding Purpose: It is typically the least costly than Y-D and Scott Transformer.
• Third harmonic suppression: The zigzag connection in power systems to trap triple harmonic (3rd, 9th, 15th, etc.) currents. Here, We install zigzag units near loads that produce large triple harmonic currents. The windings trap the harmonic currents and prevent them from traveling upstream, where they can produce undesirable effects.
• Ground current isolation: If we need a neutral for grounding or for supplying single-phase line to neutral loads when working with a 3-wire, ungrounded power system, a zigzag connection may be the better solution. Due to its composition, a zigzag transformer is more effective for grounding purposes because it has less internal winding impedance going to the ground than when using a Star type transformer.
• No Phase Displacement: There is no phase angle displacement between the primary and the secondary circuits with this connection; therefore, the ∆-zigzag connection can be used in the same manner as Y-Y and ∆- ∆ transformers without introducing any phase shifts in the circuits.

Application:

• An Earthing Reference: Occasionally engineers use a combination of YD and zigzag windings to achieve a vector phase shift. For example, an electrical network may have a transmission network of 220 kV/66 kV star/star transformers, with 66 kV/11 kV delta/star for the high voltage distribution network. If a transformation is required directly between the 220 kV/11 kV network the most obvious option is to use 220 kV/11 kV star/delta. The problem is that the 11 kV delta no longer has an earth reference point. Installing a zigzag transformer near the secondary side of the 220 kV/11 kV transformer provides the required earth reference point.
• As a Grounding Transformer:The ability to provide a path for in-phase currents enables us to use the zigzag connection as a grounding bank, which is one of the main applications for this connection.
• We rarely use zigzag configurations for typical industrial or commercial use, because they are more expensive to construct than conventional Star connected transformers. But zigzag connections are useful in special applications where conventional transformer connections aren’t effective.
• D or Y / Zig-zag are used in unbalanced low voltage system – mostly with single phase appliances

Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 14 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. 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.

17 Responses to Zig-zag Connection of Transformer

1. Robert Clark says:

Hi Jignesh, Nice Article, Thanx. IMy Transformer skills are a bit rusty at present, and I am trying to get my head around the So-Called Voltage optimizer like POWERSTAR that they claim will reduce energy consumption in buildings. Are these not simply ZigZag Connected with some taps that one can reduce the voltage a bit? Supplying a facility with reduced Voltage and 3rd Harmonics will probably reduce the energy use of some equipment a bit. devices that they simply branded so that they sound a bit more like something out of Hollywood. I can send you a electric diagram from their brochure if you need that. Maybe you can write an article on how these things work, and if there is anything special about them.

2. yahya el meraabi says:

how we can protect the electrical network against the unbalancing?

3. Philip Leech says:

Dear Jignesh

I only have an HNC in electrical enginerring but understand your explanation of the application of ZIG ZAG transformers perfectly. We are are using such transformers on a large remote project here in Northern Alberta Canada. This was a great article to read.

Thank you

Philip Leech

4. Manish Gharat says:

how zigzag transformer reduce the phase angle?

Dear Jignesh, We have 6.6kV/433V star/433VDelta 3 windingTransformer. Delta side is grounded with zigzag transformer with 400A MCCB in line. during Earth Fault MCCB in Zigzag circuit will tripped and give alarm. In this condition, first Earth Fault would be hanging and will be rectified after taking transformer Shutdown as per convenience. Is it ok or we should connect 400Amp MCCB output to trip the 6.6kV Incomer? What should be the protection Philosophy for earth fault in delta system earthed through zigzag transformer? any Standard/ guideline on this?

6. laxmi says:

it is easy to understand

7. krishna rao says:

The drawing shown by you has 9 windings.In some other drawings only six windings are considered.Why and how the difference.

8. LiLi says:

Could you tell me why do we use zigzag transformer(Dz0n) in bypass isolation transfromer in UPS ?

9. Minju says:

Dear Sir,
I am having delta connected 11kV generators (2nos) connected to paralleling switchgear to power four step down transformers [delta(11kV) / star (415V)].

Since there is no neutral for the generator, is it a must to have for neutral grounding?

If yes, how can it be done? neutral grounding transformer? or do we call it zig-zag transformers with Neutral ground resistor?

How is the connection done? Whether i should be providing NGT for each genset or can I provide one NGT at the paralleling switchgear busbar?

Thanks & Kind regards,

10. anurag jain says:

11. Er.K.K.Murty says:

Dear Er.Jignesh Parmar,Kindly apprise me, as to how to size ZIG-ZAG transformer for grounding a Star/Delta Transformer,Delta side to be loaded.Say the Transformer is 10 MVA,33/11kV Star/Delta Transformer?

• Er Parmar, I am waiting for your guidance regardingf the Zig -Zag transfomer sizing for a 40MVA,132/11 kV, YNd11 Transformer to be connected to delta system. Regards. Er.K.K.Murty.

12. Santhosh kumar says:

In Star to zig zag transformer is vector group possible if it’s yes how if it’s no then how

• Er.K.K.Murty says:

Dear Mr.Santosh Kumar,

Ans: Yes, there can be either YZn11(+ 30 degrees). or YZn1(─ 30 degrees) Zig-Zag Transformers as the case may be.However, either of the 2-vector groups shown above can be used for Grounding applications of the ungrounded Power systems.
From.Er.K.K.Murty.