The forgotten properties of Isolation Transformers

Tuesday, Jan 14, 2020

Designed specifically for applications where harmonic currents are the rule, the Admagnetics Isolation Transformer has a number of fundamental features that are not always apparent. Over time these features appear to become overlooked.

The first of these features is the Dzn0 connection. The primary winding being a conventional delta connection whilst the secondary connection is an inter-connected star, more commonly referred to as a zig-zag connection. The importance of this zig-zag connection lies in its ability to cancel the balanced portion of triplen harmonics within the secondary winding. It’s important to cancel these triplen harmonics because if not, they will travel through the distribution system until they find a delta connected transformer. This will then cause a circulating current to set up in that winding. This transformer may well be the main step down, 11kV to 400V system distribution transformer, a unit not generally not designed with harmonic handling in mind.

So Admagnetics range of bespoke Dzn0 transformers work as a blocking mechanism to harmful triplen harmonics. This can be demonstrated by a zero sequence test on an isolation transformer. In a demonstration at our Accrington facility, we had 500 Amps flowing in the secondary winding while approximately 1 Amp flowed in the primary, a level totally harmless to the transformer. Zero sequence currents and triplen harmonics are of the same form. It should be noted that this would not be the case for a delta star connected transformer which would have full load current flowing in both windings. This illustrates the inability of a delta star connected transformer to block such harmonics other than trapping them within its primary winding, which can cause it to overheat. This demonstrates the importance of the use of the right type of isolation transformer.

The cancellation of harmful triplen harmonics by the secondary windings also brings another benefit. The zig-zag transformer connection produces a low zero sequence impedance and as such this means that triplen harmonics do not act on the normal or positive sequence impedance of the transformer rather only at the lower level. Therefore there is a reduction in voltage distortion that one would expect from a transformer having specific impedance.

Another benefit of the Dzn0 transformer is that there is no phase shift between the input and the output. With the input and output windings having their characteristic like for like voltage, the transformer can simply be by passed using a circuit breaker to connect input and output together. Traditionally isolating input and output circuit breakers complete the arrangement.

Finally the inter-connected star secondary provides a neutral connection that depending on the dictates of the power system design can provide a permanently connected neutral and earth if required, irrespective of the requirements of switching arrangements on the primary.

Besides acting as a blocker of harmful harmonics this type of Admagnetics transformer is equipped in standard configuration with a special arrangement of earthed screens that provide virtually equal attenuation of transients whether considered from output to input or input to output. This level of attenuation cannot be achieved by the use of single earthed screen arrangements as supplied by many other manufacturers.

A keystone of Admagnetics design philosophy has been to reduce the level of eddy and stray loss occurring within designs across this range of transformers. The reason that this is important is that in the presence of harmonic load currents, the heating and hence losses due to eddy and stray losses are increased. It is these losses that are basically multiplied by the K factor. In a typical distribution transformer the eddy and stray losses may account for 5% of the total load losses. If subjected to a load with harmonic currents equivalent to K factor 20, then the loss due to eddy and stray losses would be in the same order as the load losses. For this reason Admagnetics take a number of steps within the design process to achieve a reduction in eddy and stray losses compared to those in a distribution transformer.

Although Admagnetics PD range of transformers (typically 150 to 450kVA, K13) are custom made to suit the specific requirements of each contract, efficiencies in the region of 97 to 97.8% are regularly achieved under working conditions.

The Admagnetics PD Isolation Transformers stand as sentinels within the supply system always active, suppressing harmonics and electrical noise on a fit and forget basis.

Why Admagnetics?

At every step of the process from enquiry to delivery, we give a service second to none. From a complex design to a simple winding, Admagnetics produce equipment that you can rely on. Admagnetics work with you to design and produce a transformer or choke optimised to your requirement. Manufacturing directly from raw materials we control every step of the design and manufacturing process. Our products are used in a vast variety of industries. From some of the most prestigious construction projects to intricate OEM machinery, our transformers and chokes are chosen again and again.

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