Where are high voltage transformers use?

To answer this correctly, we must first ask what is "High Voltage". This can vary quite a bit in electrical engineering. If you are working on a circuit board, then "High Voltage" might be above 50 volts. These would be Transformers used in circuit board power supplies. If you are working in Industrial Environment, then you might consider "High Voltage" to be 230 Volts and above. If you are working on power distribution in a facility, then it might be above 15,000 volts. If you are dealing with power transmission withing towns, or between cities, then it might be several hundred thousand volts.

Now, to the answer. If you read the above correctly, you will find the answer for each level.

The electrical regulations in your country will define what is meant by 'high voltage'; for example, in the UK, 'high voltage' is clearly defined as being 'voltages exceeding 600 V a.c.' So a high-voltage transformer is one whose either winding is rated above 600 V.

So, high-voltage transformers are either 'power transformers' (transformers used in the electricity transmission system) or 'distribution transformers' (transformers used in the electricity distribution system).

Power tranformers may be 'step-up' or 'step-down', and are essential components in the electricity system, which can only work at very high voltages (in the UK, 400 or 275 kV). Distribution transformers are 'step-down' transformers -in the UK, for example, 11 kV is stepped down to 400/230 V for distribution to residences or commercial premises.