transformer voltage regulation fact: as the load current is increased, the voltage (usually) drops....
Post on 21-Dec-2015
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![Page 1: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/1.jpg)
Transformer Voltage Regulation
Fact: As the load current is increased, the voltage (usually) drops.
Transformer voltage regulation is defined as:
fls
flsnls
V
VVVR
,
,,
![Page 2: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/2.jpg)
fls
flsnls
V
VVVR
,
,,
fls
flsP
V
VaVVR
,
,/
fls
flsP
V
VVVR
,
,
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
![Page 3: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/3.jpg)
VR in real transformers
fls
flsP
V
VaVVR
,
,/
seqseqsP IjXIRVaV ../
![Page 4: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/4.jpg)
At unity power factorResistive load
At lagging power factorInductive load
At leading power factorCapacitive load
seqseqsP IjXIRVaV ../
![Page 5: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/5.jpg)
![Page 6: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/6.jpg)
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
![Page 7: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/7.jpg)
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
![Page 8: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/8.jpg)
TransientsTransient in electrical systems is the severe changes in current and voltage after a switch is closed or opened
![Page 9: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/9.jpg)
tN
VtVe m
m
sin..
cos.
![Page 10: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/10.jpg)
Special Transformers
-Instrument Transformers-Autotransformers
![Page 11: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/11.jpg)
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
![Page 12: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/12.jpg)
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.
![Page 13: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/13.jpg)
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
![Page 14: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/14.jpg)
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.
![Page 15: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/15.jpg)
Conventional Transformer
Step-up autotransformer Step-down autotransformer
![Page 16: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/16.jpg)
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)
C
SEC
H
L
SEC
C
H
L
N
NN
I
I
NN
N
V
V
,
![Page 17: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/17.jpg)
An autotransformer to get a adjustable magnitude sinusoid voltage from zero to nominal value (the one in the lab)
![Page 18: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/18.jpg)
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
ssPPIO
WWW
IVIVS
VIS
..
.
Let’s defineWinding apparent powerInput-output apparent power
In conventional transformers WIO SS
![Page 19: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/19.jpg)
But in autotransformers:
SE
CSE
W
IO
CSE
SEIO
CSE
SELL
CSE
CLLLL
HLLLHLLCCW
HHLLIO
SESECCW
N
NN
S
S
NN
NS
NN
NIV
NN
NIVIV
IVIVIIVVIS
IVIVS
VIVIS
.
...
.).(.
..
..
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
![Page 20: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/20.jpg)
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:
![Page 21: Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops. Transformer voltage regulation is defined as:](https://reader035.vdocuments.mx/reader035/viewer/2022062313/56649d635503460f94a45789/html5/thumbnails/21.jpg)