What is an Open Delta Transformer 

By on

In three phase systems, the use of transformers with three windings (or legs) per side is common.  These three windings are often connected in delta or star, resulting in common transformer configurations such as delta-delta or delta-star.  An open delta transformer is a special arrangement which uses only two windings.

If your new to transformers, have a look at the Power Transformers - An Introduction note.

Transformer Configuration

The illustration shows how an open delta transformer is connected.  On both the primary and secondary there are only two windings.  Even in this configuration, it is still possible to transform a three phase voltage.

Open Delta Transformer
Open Delta Transformer

Open delta transforms are not the commonly used.  Typically they would be used for small loads where cost is important.  Alternatively, they could be used as an emergency measure, should one winding only of a transformer fail. 

Sometimes you may hear an open delta transformer referred to as a V-connection transformer.

Power Delivered

Sometimes the power delivered by an open delta transformer is compared to that of an equivalent three winding transformer.  Typically figures like having 57.7% of the capacity of an equivalent three winding transformer or 87% of two transformers (same winding size) are quoted. While you can think of the transformer in this manner, it is more fruitful not to consider comparisons but to the necessary calculations on the open delta transformer.

Open Delta Transformer Current

Consider the illustration, showing the output from both a close delta and open delta transformer.  Note, that in the delta connection to line current is √3 times the phase current, whereas in the open delta, they are the same.

The transformer output power (in VA) is for a balanced transformer system for the closed delta connection (using phase current), this give:

  VA=3 V L I ph

And for the open delta connection:

  VA= 3 V L I ph

Taking the ratio of open delta to closed delta power, gives:

  3 V L I ph 3 V L I ph =0.577 (or 57.7%)

Summary

Open delta transformers are three phase devices, with only two windings on each of the primary and secondary sides.  While cheaper than a conventional three winding transformer, the open delta will only deliver 57.7%  of the power of a conventional transformer (not two thirds, 66.7% as may be expected).  There is limited adoption of open delta transformers, although they can be useful in certain situations.



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



ANSI (IEEE) Protective Device Numbering

The widely used United Sates standard ANSI/IEEE C37.2 'Electrical Power System Device Function Numbers, Acronyms, and Contact Designations' deals with...

Arc Flash Calculations

Working in the vicinity of electrical equipment poses an hazard. In addition to electric shock hazard, fault currents passing through air causes Arc Flash...

Load Flow Study – how they work

A load flow study is the analysis of an electrical network carried out by an electrical engineer. The purpose is to understand how power flows around...

Cable Insulation Properties

Cable insulation is used to provide electrical separation between conductors of  a cable.  During the historical development of cables, numerous types...

Laplace Transform

Laplace transforms and their inverse are a mathematical technique which allows us to solve differential equations, by primarily using algebraic methods...

Electrical Engineering

Electrical engineering is a field that covers a wide variety of sub-fields, including electricity and electronics. It is a field that goes back to the...

Calculating Cable Fault Ratings

When selecting a cable, the performance of the cable under fault conditions is an important consideration. It is important that calculations be carried...

Closed Doors

"I can live with doubt and uncertainty and not knowing. I think it is much more interesting to live not knowing than to have answers that might be wrong...

Photovoltaic (PV) Panel - Performance Modelling

In an earlier note on the site [Photovoltaic (PV) - Electrical Calculations], the theory of solar (PV) cell calculations was introduced.  In particular...

How to measure power supply quality

If your are ever called out to troubleshoot something on your electrical system, one of the first things consider is the supply voltage. You want to ensure...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note