A step-down transformer converts the high voltage (HV) and low current from the primary side to the low voltage (LV) and high current value on the secondary side. This transformer type has a wide application in electronic devices and electrical systems. When it comes to the operation voltage, the step-up transformer application can be roughly divided
The first LV application refers to the transformers in electronic devices. Supplying the electronic circuits requires a low voltage value (e.g. 5V, even lower values nowadays).
A step down transformer is used to provide this low voltage value which is suitable for electronics supplying. It transforms home voltage (230/120 V) from primary to a low voltage on the secondary side which is used for the electronic supplying. If electronic devices are designed to have higher nominal power, transformers with high operating frequency are used (kHz-s). The transformers with higher nominal power value and 50/60 Hz nominal frequency would be too large and heavy. Also, the daily used battery chargers use the step-down transformer in its design.
The step-down transformers have a very important function in power system. They lower the voltage level and adapt it for energy consumers. It is performed in several steps described below:
- A long distance energy transmission system should have voltage level as high as possible. With high voltage and low current, the transmission power loss will be significantly decreased. A power grid is designed that has to be connected with the transmission system with the different voltage levels. Step-down transformers are used in interconnection of transmission systems with different voltage level. They decrease voltage level from high to lower value (e.g. 765/220 kV, 410/220 kV, 220/ 110 kV). These transformers are huge and have very high nominal power (even 1000 MVA). In this case, when the transformer turns ratio is not high the auto transformers are usually installed.
- The next voltage level transformation step is adapting transmission voltage to the distribution level. The characteristic voltage ratios in this case are 220/20 kV, 110/20 kV (also the LV secondary voltages 35 kV and 10 kV can be found). The nominal power of those transformers is up to 60 MVA (usually 20 MVA). The on-load tap changer is almost always installed in these transformers. A voltage regulation is the main function of tap changer. In USA the tap changer is based on LV side, and in rest of the world mostly on the HV transformer side.
- The final voltage transformation step is adapting the voltage to the home voltage level
These transformers are known as small distribution transformers with nominal power up to 5 MVA (mostly below of 1 MVA) and with nominal voltage values 35, 20, 10 kV on HV side and 400/200 V on LV side. It is noticeable that those transformers have high turns ratio. They usually have de-energized tap changer with 5 tap position (+/- 2 tap position) and do not have on load tap changer.