As you hear the word transformer, we assume that they are meant for providing substantial power voltage. We immediately tune up to seek for the step-up transformers and step-down transformers to supply increased or decreased voltages. But, there are certain significant types of transformers called current transformers (CT) and potential transformers (PT) which are functional for special purposes.

CT and PT are called Instrument Transformers.

Well, as you know, instruments are devices that are meant to measure something.

Instrument transformers are the transformers that are used for measuring electrical quantities in a transformer. They measure voltage, current, energy, power, power factor, and frequency engaged in any kind of transformer. This is necessary to check and observe the levels of current or voltage changes happening with respect to the transformer function.

One more thing is we cannot measure larger electrical quantities engaged in transformers. It is required to design and standardize huge equipment for measuring such enormous entities. If we have measurable small rating equipment decreased by relative magnitude, then accordingly we can measure the concerned electrical quantities.

So, generally, instrument transformers are designed for measuring minimum ranges of 5A and 110V.

Now, you might have got an idea of what is the use of CT and PT.

· CT – Current Transformers measure current.

· PT – Potential Transformers measure voltage.

Let’s first know about CT and its uses.

Current Transformer

Like any transformer, it has primary and secondary coils. But, the primary coils have a few turns when compared to the secondary coils. Sometimes, the primary side of the current transformer has no coils and simply has a bar. Always, its primary side is in series connection with the power circuit. So, it is often termed as a series transformer.

Now, we come to the secondary end of the CT. This end is connected to the Ammeter (The device which measures current).

Uses of a current transformer

The CT decreases or steps-down the current to the lower level. Such levels will be easily measured by simple electrical ammeters.

· CT finds the current in a circuit. So, it is used to determine and control the high-voltage lines in a power grid.

· It is used more commonly in electric meters to measure electric usage. As you know, you have electric meters everywhere to measure the usage of current and you will be charged according to the per unit rate. Such meters are coupled with CT – the reading from the CT is used to calculate the current usage and power bills simultaneously.

· It also protects sensitive electrical equipment – whether industrial or residential. It isolates the equipment in case of higher levels of power and avoids consequential damage.

· Often, it is used along with relay controls to ensure safe and effective measurement of the current.

Potential Transformer

Potential transformers are also called voltage transformers. They measure voltage levels of power. Contrast to the CT, PT has a larger number of turns in its primary side.

This primary end is connected generally across the line between on line and the earth. So, as the connection is parallel, it is termed as ‘parallel transformer’. The secondary end is connected to a voltmeter to measure the voltage.

Uses of a potential transformer

The PT decreases or steps-down the voltage of the power system to such a basic level (110V – 120V) that is measured by a voltmeter.

· PT is used for a metering purpose – it is used to determine the voltage levels of a transformer or any other electrical equipment. In case of any deviated levels of voltage, appropriate measures are taken to avoid unnecessary losses.

· It is used to enable the protection of the feeders.

· It is also meant to synchronize the generators and feeders.

· In order to protect the impedance of the generators also, PT is useful.

· It insulates the protective equipment from the primary circuit and, therefore, used in relay schemes.

Therefore, CT and PT are important instrument transformers having outstanding uses. They have got many feasible and significant applications across various sectors. They are used for simple and complex power-based applications.

Hope this information is helpful to you to understand what is a CT and PT and what their uses are.