Transformer Voltage Regulation

1. Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:

2. Other useful formulas Prefer small VR For an ideal transformer : VR=0 Note: If there were only resistive loads, the VR would not be much large in real transformers, but because loads normally have different power factors, VR may be considerably large in real transformers.

3. VR in real transformers

4. At unity power factor Resistive load At lagging power factor Inductive load At leading power factor Capacitive load

6. Transformer Taps 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

7. 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

8. Transients Transient in electrical systems is the severe changes in current and voltage after a switch is closed or opened

10. Special Transformers -Instrument Transformers -Autotransformers

11. Instrument Transformers Power Transformers (PT) Current Transformers (CT) • We need voltage and current samples for: • measurement (voltmeters, ammeters, wattmeters, scopes) • Protection • Control 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

12. 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.

13. 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

14. 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.

15. Conventional Transformer Step-down autotransformer Step-up autotransformer

16. New terminology in autotransformers: Common coil : The coil common in the primary and secondary Series 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)

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

18. 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 Let’s define Winding apparent power Input-output apparent power In conventional transformers

19. But in autotransformers: 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

20. 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: