Transformer Voltage Regulation

Fact: As the load current is increased, the voltage (usually) drops.

Transformer voltage regulation is defined as:

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flsP

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VaVVR

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Other useful formulas

Note: If there were only resistive loads, the VR would not be much large in real transformers, but because loadsnormally have different power factors, VR may be considerably large in real transformers.

Prefer small VRFor an ideal transformer : VR=0

VR in real transformers

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flsP

V

VaVVR

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At unity power factorResistive load

At lagging power factorInductive load

At leading power factorCapacitive load

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The voltage in a distribution line is not constant.It may be 1.05 p.u. at generator terminal and 0.95 at the load side. Depending on the place the transformer is used, we may need to adjust the transformer ratio to get similar load voltage. That’s why we need Tap

In Tap Changing Under Load (TCUL) transformer, the tap changes automatically to keep the output voltage constant

Transformer Taps

Current Inrush

If a transformer is switched to a system, there may be some current with large magnitude (Current Inrush), in the first few milliseconds after the switch is closed.

The reason behind the current inrush is transient

TransientsTransient in electrical systems is the severe changes in current and voltage after a switch is closed or opened

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cos.

Special Transformers

-Instrument Transformers-Autotransformers

In instrument transformers, accuracy is very important. In their design they do their best to make it as close as possible to ideal transformers. Accuracy is important in magnitude and phase.They make it shell type to reduce the flux leakage

Instrument Transformers

Power Transformers (PT) Current Transformers (CT)

We need voltage and current samples for:-measurement (voltmeters, ammeters, wattmeters, scopes)-Protection-Control

Power Transformers (PT)

Specially wound transformer with a high-voltage primary and low-voltage secondary (0-120 V), very low power rating, almost no load. Insulation on high voltage is very important. Thousands of turns of very thin conductors.

Current Transformers (CT)

Reduces the line current to a safe (measurable) level (0-5 A)

The secondary should be connected to an instrument or short-circuited

Autotransformers

This form of transformers are more economical if the voltage level of the primary and the secondary is almost the same. For example if we want to change the voltage from 110 to 120 or 13.2 kV to 13.8 kV.

Conventional Transformer

Step-up autotransformer Step-down autotransformer

New terminology in autotransformers:

Common coil : The coil common in the primary and secondarySeries coil : The coil in series with the common coil

We write the equation based on the voltages and current in the common coil and series coil. These relations can be used for both cases (step-up and step-down)

It is easy to prove (assuming ideal transformer)

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SEC

H

L

SEC

C

H

L

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V

V

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An autotransformer to get a adjustable magnitude sinusoid voltage from zero to nominal value (the one in the lab)

Apparent Power Advantage of Autotransformers

Remember, we emphasized that a transformer designed for a particular voltage and apparent rating can not be easily used for other ratings

Now we show that if a transformer designed to work for a particular ratings, is reconnected to work as an autotransformer, it can handle much more apparent power

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Let’s defineWinding apparent powerInput-output apparent power

In conventional transformers WIO SS

But in autotransformers:

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CSE

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CSE

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CSE

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The autotransformer can handle more apparent power specially when the series windings is small. The closer the two voltages are the greater the autotransformer power advantage

The disadvantage of increased apparent power is that the parameters of transformer is decreased in p.u. values. This will cause higher currents during faults in power systems

Example: