Current Transformer Working Principle: Understand the Concept

Energy efficiency minimized costs and high system availability represent now three central aspects of plant management. To achieve them it is necessary to know when, where, and how the energy is consumed. This is why measuring and monitoring the principal electrical parameters of the network is becoming more important. The current transformer is one of the essential parts of electric power systems. All types of protection and control devices need current transformers. For this reason, all professionals should know the “Current Transformer Working Principle”. After reading this article you will have a basic knowledge of the current transformer.

The current transformer is a special type of electrical equipment that steps down from high primary currents to low secondary currents. The primary winding is connected with the current being measured and the secondary winding is to measuring instruments.

The current transformer can be used in the following applications:

• Ammeters
• Wattmeters
• Varmeters
• Kilowatt-hour meters
• Power factor meters
• Control relays
• Measuring transducers

The primary winding of the current transformer consists of a few turns and is connected in series with the line carrying the current. The secondary winding has a larger number of turns and is connected with the instruments.

The current transformer is used for measuring and protecting applications. By using a current transformer we can measure high currents easily. It is recommended to apply current transformers for currents of 40 A or higher.

Current transformers have two basic functions:

• Limiting and minimizing the current for metering and protection devices.
• Isolating power circuits from the metering and/or protection circuit.

The current transformer consists of a primary winding, a secondary winding, a magnetic core and an insulated body. The high-grade silicon steel core is annealed, varnished then insulated with polycarbonate core caps. The secondary winding is toroidally wound by high-precision semi-automatic machinery. For the tape wound ring-type current transformer, the PEW-coated windings are then covered with elephantine paper, varnished and double-tapped with PVS tapes. The windings are enclosed in a compact and heat-resistant split cap for the encapsulated current transformer.

Current Transformer Working Principle

The current transformer works to transform or change the magnitude of AC (50…400 Hz) in a system, typically from a higher current value to a lower current value. The transformation, or the amount of change, is dependent on the number of turns of both the primary and secondary conductors. CT consists of three main components: a primary winding, a core and a secondary winding.

The relationship or ratio, between the number of turns in the primary and secondary windings, is responsible for reducing or stepping down the current in a system to a value that is usable to the current monitoring device such as an overload relay or power monitoring product. The following formula demonstrates how the ratio between the windings can lower the current:

Current Transformer Ratio

The CT ratio is the ratio of primary current input to secondary current output at full load. For example, a current transformer with a ratio of 300:5 is rated for 300 primary amps at full load and will produce 5 amps of secondary current when 300 amps flow through the primary.

If the primary current changes, the secondary current output will change proportionally. For example, if 150 amps flow through the 300 amp rated primary the secondary current output will be 2.5 amps (150:300 = 2.5:5)

Current Transformer Accuracy

The current transformer accuracy is determined by its certified accuracy class that is stamped on the nameplate. For example, a CT accuracy class of 0.3 means that the CT is certified by the manufacturer to be accurate to within 0.3 percent of its rated ratio value for a primary current of 100 percent of rated the ratio.

A current transformer with a rated ratio of 200/ 5 with an accuracy class of 0.3 would operate within 0.45 percent of its rated ratio value for a primary current of 100 amps. To be more explicit, a primary current of 100A is certified to produce a secondary current between 2.489 amps and 2.511 amps.

Current Transformer Polarity

The polarity of a current transformer is determined by the direction in which the coils are wound around the core of the CT (clockwise or counterclockwise) and by the way the leads, if any, are brought out of the transformer case.

All current transformers are subtractive polarity and will have the following designations to guide proper installation:

(H1) primary current, line facing direction; (H2) primary current, load facing direction; and (X1) secondary current.

Taking care to observe proper polarity is important when installing and connecting current transformers to power metering and protective relays.

Models

There are several different models of current transformers each facilitating the step-down and metering of current but how that is accomplished can be different. The following explains the characteristics of the three main models of current transformers.

Wound-type

The wound current transformer has a primary winding of more than one full turn wounded on the core. The primary and secondary winding of the wound current transformer is insulated from each other consisting of one or more turns encircling the core. Constructed as multi-ratio CTs by the use of taps on the secondary winding. The wound type provides excellent performance under a wide operating range.

Toroidal-type

The toroidal current transformer does not contain a primary winding. Instead, the line that carries the current flowing in the circuit passes through a window or hole in the toroidal transformer. Some current transformers have a “split core”, which allows them to be opened, installed and closed without disconnecting the circuit to which they are attached.

Bar-type

The bar-type current transformer uses the actual cable or busbar of the main circuit as the primary winding, equivalent to one turn. Bar types are available with higher insulation levels and are usually bolted to the current caring device.

Connection

Single ratio CT

Multi ratio CT

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