Power Transformers are used for a variety of reasons, but they all have one thing in common; namely that the power being received must be tailored for the specific use.
  One of the main applications for Power Transformers is to convert the power being transmitted to a very high voltage as it is much more efficient to transmit power at exceedingly higher voltages. In this way less power is lost due to resistance. Step Up Transformers and 3 Phase Transformers are used at this stage.
  Once the power reaches a region, area or facility where power is to be used, a Power Transformer or series of Power Transformers must bring the received power to a specific range that will fit the local needs.
  The Power Transformers would specifically include Buck Boost Transformers (which adjust to low voltage applications, motor applications and adjustments to chronic high or low voltage conditions), Step Up Transformers (to raise any consistently low voltage condition as in lines that repeatedly test out at only 108 volts where 120 is desired), and Step Down transformers (where the voltage is being reduced from a chronically high voltage condition).
  Often, once the power is channeled through the line, it can acquire unwanted characteristics. Due to fluctuations in available power, increased or decreased usage at certain times of the day, and other factors, the voltage may be intermittently a bit higher or lower than optimum or voltage may be right but there may be instances of feedback, sudden spikes or drops, induced currents or ghost currents, harmonic currents and instances of static charges that could affect sensitive equipment or even bleed into people exposed to that equipment.
  These are mainly addressed through the use of a variety of Power transformer types; AC Line Reactors (designed to handle harmonics, surges, fleeting currents and anything that affects optimum performance of motors and electronics), Isolation Transformers (which can include shielding so as to eliminate local induced currents and static charges by draining them off to ground before the current reaches the final stage) and Medical Isolation Transformers (which specifically bring all equipment within closely regulated legal and safety tolerances designed to protect electronics, personnel and patients exposed to the equipment.)
  Some Power Transformers apply to converting the power from one phase type to another. A major transmission line is usually 3 phase and, as mentioned above, a 3 phase Transformer will be used at the power end and sends the power on three individual lines which will have to be converted later at the receiving or "load" end.