Autotransformer

Autotransformer• An autotransformer (sometimes called autostep down

transformer)[1] is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting on itself and not to any kind of automatic mechanism.

• Autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation.

• Other advantages of autotransformers include lower leakage reactance, lower losses, lower excitation current, and increased KVA rating.[2]

Autotransformer

In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer. The winding has at least three taps where electrical connections are made.

Autotransformer

1 1inS V I

2 2outS V I

Problem

• The primary and secondary voltages of an autotransformer are 500 V and 400 V respectively. Show with the aid of a diagram, the current distribution in the winding when the secondary current is 100 A. Calculate the output power for unity power factor.

V1

V2

N1

N2

a

I1

I2

1

a

Auto-TransformerTurns link the same flux

V1

V2

N1

N2

a

I1

I2

1

a

Variable-voltage autotransformers

Three-Phase Transformers

Three-phase transformer (Courtesy of Waukesha Electric Systems).

Three Phase Transformers1. Constructing a three-phase transformer on a common

magnetic structure

2. Connecting a bank of three single-phase transformers

Y - ∆: Step down high voltage to lower voltage

∆ - Y: Step Up voltage

∆ - ∆: One transformer can be removed

Y - Y: Rarely used due to problem with exciting current and induced voltages

Three Phase Transformers

3-phase transformer connections1. Y-Y connection:

The primary voltage on each phase of the transformer is

3LP

P

VV

The secondary phase voltage is

3LS SV V

The overall voltage ratio is

3

3PLP

LS S

VVa

V V

The Y-Y connection has two very serious problems:

1. If loads on one of the transformer circuits are unbalanced, the voltages on the phases of the transformer can become severely unbalanced.

2. The third harmonic issue. The voltages in any phase of an Y-Y transformer are 1200 apart from the voltages in any other phase. However, the third-harmonic components of each phase will be in phase with each other. Nonlinearities in the transformer core always lead to generation of third harmonic! These components will add up resulting in large (can be even larger than the fundamental component) third harmonic component.

3-phase transformer connections

Wye-wye connection

• When transformers are connected in wye-wye, specialprecautions have to be taken to prevent severe distortion of the line-to-neutral voltages. (1) connect the neutral of the primary to the neutral of the source, usually by way of the ground

Fig.6 Wye-wye connection with neutral of the primary connected to the neutral of the source.

Wye-wye connection (ต่�อ)

(2) provide each transformer with a third winding,called tertiary winding.

Fig.7 Wye-wye connection using a tertiary winding.

Wye-wye connection

• Note that there is no phase shift between the incomingand outgoing transmission line voltages ofa wye-wye connected transformer.

2. Y- connection:

The primary voltage on each phase of the transformer is

3LP

P

VV

The secondary phase voltage is

LS SV V

The overall voltage ratio is

33PLP

LS S

VVa

V V

3-phase transformer connections

The Y- connection has no problem with third harmonic components due to circulating currents in . It is also more stable to unbalanced loads since the partially redistributes any imbalance that occurs.

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.

In the U.S., it is common to make the secondary voltage to lag the primary voltage. The connection shown in the previous slide will do it.

3-phase transformer connections

3. -Y connection:The primary voltage on each phase of the transformer is

P LPV V

The secondary phase voltage is

3LS SV V

The overall voltage ratio is

3 3PLP

LS S

VV a

V V

advantages and the same phase shift as the Y-

connection.

3-phase transformer connections

4. - connection:The primary voltage on each phase of the transformer is

P LPV V

The secondary phase voltage is

LS SV V

The overall voltage ratio is

PLP

LS S

VVa

V V

No phase shift, no problems with unbalanced loads or

harmonics.

3-phase transformer connections

Open-delta connection

• It is possible to transform the voltage of a 3-phase system by using only 2 transformers, connected in open-delta.

• The open-delta arrangement is identical to a delta-delta connection, except that one transformer is absent.

• The open-delta connection is seldom used because the load capacity of the transformer bank is only 86.6 percentof the installed transformer capacity.

Open-delta connection (ต่�อ)

• The open-delta connection is mainly used in emergency situations. Thus, if three transformers are connected in delta-delta and one of them becomes defective and has to be removed, it is possible to feed the load on a temporary basis with the two remaining transformers.

Fig.8a Open-delta connection.

Three-Phase Transformers

One-line Diagram

One-line diagrams are simplified schematics used to show the main power distribution systems for commercial or industrial settings.

Three-Phase Transformers

Industrial one-line power distribution schematic.

Parallel coupling of transformers? What are the conditions?

Delta-delta connection

• Example 1

Delta-delta connection

Delta-delta connection

Delta-delta connection

Delta-wye connection

Fig.4 Schematic diagram of a delta- wye connection and associated phasor diagram.

Delta-wye connection

• Example 2

Delta-wye connection

Fig.5

Delta-wye connection