Transformer taps are preferred on High Voltage Side why?
Transformer taps are used to changing the transformer voltage level when it is connected to the load circuits. 99% of the power transformers are designed to operate using these tap changers only. At the same time these tap changers are always preferred to operate in high voltage side. Two types of tap changer are used which are on load tap changer and off load tap changer. Let see the true reason behind it.
On load tap changer is preferred on High Voltage Side:
Mostly the power transformer is used for both step up and step down action. In that, the power transformer’s turns are in the high voltage side is higher than the low voltage side. Therefor placing on load tap changer circuit on high voltage side which give fine voltage regulation.
The current in the low voltage winding side is always higher than the current in the high voltage side, therefore the size of the carbon brushes get reduced. We can place small size of carbon brushes on high voltage side.
The LV winding of the transformer is placed inside of the high voltage winding, so on the account of the practical consideration, it is simpler to provide tappings on the high voltage.
When you increase the number of turns on high voltage side which decrease the flux and flux density of the transformer. Reducing the flux density reduces the core losses of the transformer (eddy current losses and hysteresis losses). It increases the efficiency of the transformer.
Providing tapping on low voltage side may not get exact voltage regulation. i.e transformer tap changer is designed to get ± 2.5 % voltage regulation in a three phase 6.6 kV/440 Star/Delta transformer designed for 11.55 Voltage per turn on secondary. In output of the low voltage side single phase to ground is 230 V. The number of turns are 20 (230/11.55). It is possible to tap a whole number of turns. Here, The minimum number of turns possible is one. Therefore, we will get the minimum voltage regulation of from the transformer tap changers is ± 5% (440V /20 =11.55 V => regulation in % = 11.55 / 440 Volts= 5 %). So that we cannot get our desired voltage regulation of ±2.5% if we provide the transformer taps on low voltage winding.
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