A power transformer can be used to increase or decrease voltage, primarily in power and distribution transmission. Instrument transformers are devices used to isolate or transform voltage or current levels. 

Power transformers are used in the step-up and step-down operation in high transmission systems. It is primarily used in conjunction with electricity coupled from a power line to system circuit. There is a method to transfer energy at high voltage because the electricity cannot be directly transmitted to the consumers; the output is decreased to the desired level through the step-down transformer.

Power transformers are different from other types of transformers because they are designed to meet regulatory requirements for power interface, work on high voltages and relatively high currents. The most critical feature of a transformer is the galvanic isolation from the primary to secondary transformers, typically in kV. This is a vital aspect of protection when it comes to protecting people from potentially harmful earth defects.

In the transmission network, the load fluctuation will be very low if the power transformer is connected. These are not connected directly to the consumer end, but there are variations in load if aligned with the distribution network. The cost of a power transformer is effective when power is produced at low voltage levels, and if the level of voltage is raised at that point the current of the power transformer is decreased, bringing about I2R losses and so the voltage regulation is also increased.

The transformer is used primarily to link electricity from a power supply line to a circuit system or at least one part of the framework. The power transformer is used with strong state circuits known as the transformer rectifier. The rating of a power transformer is given for the peak voltage and the current supply capability of the secondary transformer.

On the other hand, the Instrument transformers are electrical instruments used for transforming or isolating high precision current or voltage levels. They’re applied to electrical parameters, such as current, voltage, energy, power factor and frequency in alternative currents (AC) systems.

To protect the power system, instrument transformers are also used with protection relays. Instrumental transformer involves current and potential transformers used by traditional instruments to minimise high voltages and current to lower value. 

Current Transformer (CT) 

• A current transformer is a device used to switch the current from higher and lower values to the balanced current. Current transformers minimise high voltage currents to a considerably smaller value and provide the simplest way to safely track down the actual current flow in an AC transmission line using a standard ammeter.
• The current transformer is used with the AC instrument, meters or control when the measuring current is of such a scale that a meter or instrument coil cannot be adequately formed of sufficient current-carrying capacity.
• The current transformers are equal to each other in their primary and secondary current. 
• The current is used to measure the high voltage current due to the problem that the meter itself lacks security. The current transformer is used in meters to calculate up to 100 amperes in the current. 

Potential Transformer (PT)

• A potential transformer is also called voltage transformers (VT),  and are a parallel-connected type of instrument transformer.
• A potential transformer might be characterised as an instrument transformer due to the change of voltage from a higher voltage to the lower. 
• A potential transformer is a voltage step-down transformer which reduces the voltage of a high voltage circuit to a lower level for the purpose of measurement. 
• They can be measured with a standard measuring instrument like a voltmeter, wattmeter and watt-hour meters, etc. 
• These transformers have a large number of primary turns and a smaller number of secondary turns. It is normally used with a standard 120-volt voltmeter.