Transformers 19-9-2004

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8/6/2019 Transformers 19-9-2004


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8/6/2019 Transformers 19-9-2004


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If a coil is wound around an iron core and a current passes through the

coil, a magnetic field is induced in the core. The force of the magnetic

field will depend on the KVA present in the winding.

IV1

V2

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If another coil is wound around the same core, the magnetic field in the

core will induce a current and voltage in the secondary winding.

VP1

VP2

∆V = V - VP1 P2P

IP

IS

VS1

VS2

∆V = V - VS1 S2S

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The winding connected to the voltage source and induces the magnetic

field is called the Primary,

while the winding that receives its voltage by induction is called the

Secondary.

VP1

VP2

VS1

VS2∆V = V - VP1 P2P

∆V = V - VS1 S2S

IP

IS

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It is possible to predict the voltage (and the current) induced in the

secondary side if we know the relation of the "turns" in each winding.

VP1

VP2

IP

IS

VS1

∆V = V - VP1 P2P VS2

∆V = V - VS1 S2S

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This type of transformer is called a Potential Transformer because it can

be used to change one voltage to another.

VP1

VP2

VS1

VS2

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If the number of turns in both windings is not the same, the voltage in the

secondary side will be different than the voltage in the primary side.

IP

IS

VP1 VS1

VP2

VS2∆V = V - VP1 P2P

∆V = V - VS1 S2S

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The transformer shown has more turns in the primary side than in the

secondary. Specifically it has 8 primary turns and 4 secondary turns.2:1

VP1

VP2

VS1

VS2

IP

ISV = V - VP1 P2P

∆V = V - VS1 S2S

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A potential transformer which changes one voltage to a higher voltage is

called a " step-up " transformer.

V <P

VS

VP1

VP2

VS1

VS2

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When the transformer ratio is not very high, it is possible to use one

winding for both the primary and secondary sides. This type of transformer is called an Autotransformer .

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Transformers can be either single phase or three phase. A single

phase transformer generally appears similar to the figure shown.

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Normally when using single phase transformers for three phase power,

three individual transformers are connected together.

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It is possible to connect them in various configurations.

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Delta connection "Wye" or Star connection

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Depending on the area (or the country) the high line voltage could be in

the range of 7200V, 12470V, 24960V, etc. This is the "line to line"voltage. Depending on how the primary winding of the transformer is

connected, this will determine the effective line-to-line voltage entering

the primary side of the transformer.As an example, we will consider a case where the line to line voltage is

12470V.

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If the transformers are connected in "Delta", the phase to phase voltage will

be the same as the line-to-line voltage. The phase to phase voltage will be12470V.

12470V

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If the line voltage is 12470V and the amperage is 6A, the KVA will be:

12470 x 6 x 1.732KVA = = 129.6 KVA1000

12470V

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On the other hand, if the transformers are connected in Wye on the primary

side, there will be an electrical angle of 60° between each phase.

φ =60°

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To calculate the value of line voltage applied to the transformer windings, it

is necessary to divide the line voltage by the square root of three.

φ = 60°

Phase to PhaseVoltage

Line Voltage1.732Phase Voltage =

Phase to NeutralVoltage

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If the line voltage is 12470V and the amperage is 6A, the KVA will be:

Phase Voltage = 12470 / 3 = 7200V

7200 x 6 x 3KVA = = 74.8 KVA

1000

φ = 60°

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This relationship can be very useful. For example, if a transformer has a

primary winding which is only rated for 7200 volts but the overhead power is 12470V, the transformer can still be used as long as it is connected in

Wye on the primary side.

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This arrangement shows three transformers connected "Delta-Delta".

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In this example, the transformers are connected "Wye-Wye".

Many time the field engineer will select a slightly higher voltage than

that calculated. This is because, in many power systems, thesecondary voltage will be slightly lower under load than it is at no-load.

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• Secondary winding can be split, then reconnected either series or

parallel to obtain different winding voltages

• H1H2 - High Voltage winding has many turns

• X1X2 - Low Voltage winding has fewer turns

• Primary is always connected to the power supply

• Secondary is always connected to the load.

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• Primary Side

– If Y connected the winding voltage is equal to the phase-phase voltage divided by 1.73

– If delta connected the winding voltage is equal to the phase-

phase voltage

• Secondary Side

– If Y connected the output voltage is equal to the phase-phase

voltage x 1.73

– If delta connected the output voltage is equal to the winding

voltage.

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• KVA Rating

– When a transformer is fully loaded according to the KVA rating thedifference between No-load voltage and loaded voltage can be quite

significant. (Due to impedance losses)

• Correct Voltage

– Ensure the primary voltage does not exceed the winding voltage rating.

Make sure output voltage is compatible with required surface voltage.

• Mount

– Exposed or Covered bushing covers

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• VSD Rated Transformers

– Special purposed transformers are used on VSD applications. Theycontain more iron required for developing more flux at lower frequencies.

• Frequency

– 50 cycle rated transformers can be used on 60 cycle systems but a 60

cycle transformer operating on a 50 cycle system needs to be derated.

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Weatherford Transformer

Transformers 19-9-2004

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