THREE WINDING TRANSFORMER BASIC INFORMATION
What Is Three Winding Transformer?


The three-winding transformer is a subset of multiwinding transformers. In addition to the usual primary and secondary windings, a third tertiary winding is added to each phase. Having three winding can serve several purposes:

• Three windings allow connecting three systems together where each system has a different operating voltage.

• Three windings provide electrical isolation between dual input circuits or dual output circuits having the same operating voltage.

• If the third winding is Δ-connected, this can stabilize voltages, supply third harmonic currents to magnetize the transformer core, filter third harmonics from the system, and provide grounding bank action when the primary and secondary windings are both Y-connected.

Sometimes a tertiary winding may serve more than one function at the same time. For example, a 13.8 kV Δ-connected tertiary winding on a 230 kV– 69 kV Grd.Y-Grd.Y transformer helps to stabilize the primary and secondary voltages, provides grounding bank action to partially shield the primary circuit from secondary ground currents, and provides 13.8 kV supply voltage to a station-service auxiliary transformer.

(Note: When a group of windings are connected in parallel to increase the current capability of a secondary winding, the parallel group is considered one winding and not several separate windings. Using multiple sets of low-voltage windings in parallel is common in large generator step-up transformers; however, these are still considered two winding transformers.)

Sometimes a tertiary winding is intended only to magnetically interact with the primary and secondary windings so it may not have any external terminal connections. In these cases, the tertiary winding is said to be an imbedded tertiary.

Imbedded tertiary windings are found only in three-phase transformers and are always Δ-connected. One corner of the Δ-connected imbedded tertiary winding is sometimes grounded internally to limit capacitively coupled voltages.

For single-phase transformers, the standard labels for the tertiary bushings are Y1 and Y2. For three-phase transformers, the standard labels for the tertiary bushings are (Y0), Y1, Y2, Y3.

ADVANTAGES AND DISADVANTAGES OF THE AUTOTRANSFORMER CONNECTION

AUTOTRANSFORMERS CONNECTION ADVANTAGES AND DISADVANTAGES
What Are The Advantages And Disadvantages Of Autotransformer Connection?


Summarizing the advantages of the autotransformer connection:

 • There are considerable savings in size and weight.

• There are decreased losses for a given KVA capacity.


• Using an autotransformer connection provides an opportunity for achieving lower series impedances and better regulation.

Summarizing the disadvantages of the autotransformer connection:

• The autotransformer connection is not available with certain threephase connections.

• Higher (and possibly more damaging) short-circuit currents can result from a lower series impedance.

• Short circuits can impress voltages significantly higher than operating voltages across the windings of an autotransformer.

• For the same voltage surge at the line terminals, the impressed and induced voltages are greater for an autotransformer than for a two winding transformer.

In many instances, the advantages of the autotransformer connection outweigh its disadvantages.

For example, when very large MVA capability is required and where a Grd.Y-Grd.Y connection is suitable, an autotransformer is usually the design of choice.

Because an autotransformer cannot provide a Δ-Y connection, autotransformers are not suitable for use as generator step-up transformers.