Explaining the Difference Between Current Transformers and Voltage Transformers

One of the most common questions in the world of electrical engineering is the difference between current transformers (CT) and voltage transformers (VT). Since, both belong to the family of instrument transformers, a lot of people from non-engineering or technical backgrounds tend to consider both of them to be identical in their functions.

Although their basic premise is similar to a certain extent, which is to step down high current and voltages in power systems, their construction, functioning and usages differ significantly and it is important to understand these differences for anyone dealing with them. One of the prime reasons for this confusion is that both of them are collectively termed as instrument transformers and in normal parlance, people tend to use the terms current and voltage transformers interchangeably. In this article, we will try to explain the similarities and differences that exist between the two, their basic functioning and usage in the electrical grid to our readers.

Both current and voltage transformers belong to the category of instrument transformers. Their primary function is to lower or step-down high current and voltage in electrical power systems to lower current and voltages to facilitate measurement of electricity by measuring instruments. In short, they separate the high voltage or high current circuits and the measuring equipment. This safeguards the metering instruments from getting burnt from the high temperatures in high system voltages.

The standard rating used in most metering instrument transformers are 5A & 110V, which means that the primary winding of an instrument transformer will be connected to the high system voltages or high current circuits like 11Kilovolts (kv in which 1 kv equals 1000 volts), 25 (kv) 33 kv, and likewise till 840 kv and even higher while the metering instrument is connected to the secondary circuit that discharges 5 ampere (in case of current transformers) and 110 Voltage (in case of voltage transformers).

Basics of Current Transformers

Current Transformers are instrument transformers that transform a proportional amount of current in its secondary terminal compared to the primary. The proportion is calculated using various models and given possibility with the copper windings that form the core of the transformer. Current transformers when used in live power systems, step-down high voltage currents to lower value currents so that these can be measured by measuring equipment. The most common such instrument is the ammeter.

The other big difference between a CT and a VT is in the construction and make of its core and windings. In CT, the primary winding consists of one or a sometimes a few more turns in its primary winding. However this is not the case in VT.

Basics of Voltage Transformers

Voltage Transformers are types of instrument transformers that are built to step down the voltage in power systems. Voltage is different from current and if you are not aware of these differences, please read up the internet. You will find several articles on the same in the internet. (We will deal with that in another future article)

It is built in such a manner so that makes it possible to measure voltages by simple measuring instruments. The standard instrument used for measuring voltage in power systems is a voltmeter. The universally standard measure of a voltmeter is 110-120 Volts. There is a difference in their construction as well. A PT has large number of the primary windings and the secondary has lesser turns which is then connected to a measuring voltmeter

Both current and voltage transformers are important elements of power generation and electricity distribution grids. Without them, it will be impossible to measure electricity in an efficient and cost effective manner. Instrument transformers help in reducing the costs incurred in deployment of measurement of electricity.