What is Current Transformer (CT)? Construction, Working Principle & Types

What is Current Transformer (CT)?

Current transformer is an electrical device which is used for the measurement of electric current and power of transmission and distribution lines. CT’s are mostly used on grid stations, small power houses and substations for the measurement of power and current. Current transformer is also called as Series Transformer because it is connected in series connection with any circuit for the purpose of measurement of various parameters of electric power. the primary winding of the current transformer is connected in series with the main line which carries main current. The secondary winding is stepped down many times as compared to the primary current of CT. The secondary winding is connected across ammeter for the measurement of current or with wattmeter for the measurement of power in the circuit or in line. The circuit diagrams of the applications of the Current Transformer (CT) is shown in figure below:

Construction of Current Transformers (CT)

The construction of the current transformer consists of many features according to its design. The design features are described below:

Number of Primary ampere-turns

The numbers of the primary ampere turns are in the range of 5000 to 10000. The number of primary ampere turns are determined by the primary current.

Core

To attain the low magnetizing ampere turns. The core material should have low reluctance and low iron losses. Core materials such as an alloy of iron and nickel containing copper has properties of high permeability, low loss and low retentivity is used in current transformers.

Windings

 Primary and secondary windings of current transformer are placed close to each other to reduce the leakage reactance. The SWG wires are used for secondary windings and copper strips are used for primary winding. The windings are designed for proper robustness and tight bracing without any damage.

Insulation

The windings of the current transformer are insulated with tape and varnish. Higher voltage applications require oil immersed insulation arrangements for the windings.

Theory of Current Transformer (CT)

The equivalent circuit and the phase diagram of the current transformer is shown in figure below during its operation:

VP =    Primary Supply Voltage

EP =    Primary Winding Induced Voltage

VS =    Secondary Terminal Voltage

ES =    Secondary Winding Induced Voltage

IP =     Primary Current

IS =     Secondary Current

I0 =     No-Load Current

IC =     Core Loss Component of Current

I_M =     Magnetizing  Component of Current

rP =     Resistance of Primary Winding

xP =    Reactance of Primary Winding

rS =     Resistance of Secondary Winding

xS =    Reactance of Secondary Winding

RC =   Imaginary Resistance Representing Core Losses

XM =  Magnetizing Reactance

Re =    Resistance of External load including resistance of meters, current coils, etc.

xE =    Reactance of External load including reactance of meters, current coils, etc.

 NP =  Primary Winding Number of Turns

NS =   Secondary Winding Number of Turns

N =      Turns Ratio   = NS / NP

Φ =      Working Flux of the CT

Θ =      Phase angle of the CT

δ =       Phase angle between secondary winding induced voltage and secondary winding current

β =      Phase angle of Secondary Load circuit

α =      Phase angle between no-load current I0 and flux φ

Magnetizing component of the current is in phase with the flux and the flux is along the positive x-axis. The core loss component leads by the magnetizing component by 90^o. The sum of the core loss component and magnetizing component produces the no load current which is the phase angle of flux.

The induced voltage of secondary winding is 180^o out of phase with the primary winding induced voltage. The secondary lags with the secondary winding induced voltage with the δ angle. The secondary output voltage are obtained by subtracting the secondary winding resistive and reactive voltage drops I_s r_s and I_s X_s from the secondary induced voltage E_s.

The phase angle difference between the primary current and the secondary current is β. It is the phase angle of the load. The secondary current when goes back to the primary side then the shifted phasor is represented by 180^o and it is indicated by nIs. The phase angle difference between the primary current and the secondary back current is called the phase angle of the CT.

Types of Current Transformer (CT)

Indoor Type Current Transformers (CT)

Indoor type current transformers are mounted on control panels and control tables. These are of two types wound type CT’s and bar type CT’s.

Wound type indoor current transformers have few turns on its primary winding. The primary conductor is bolted and the secondary winding of current transformer consists of a large number of turns. More than one turn is used to attain exciting current and high accuracy. The ratings of these transformer are 800 Amperes.

Bar type current transformers consist of a single bar as a primary winding. This bar is connected in series with the circuit conductor. These CT’s include the laminated core and a secondary winding. These CT’s are named as single turn primary type current transformers. Figure shows the cross section of bar type current transformer as follows:

Clamp on Type CT / Portable Type CT

It is possible to measure the electric current in a current conductor without breaking the current circuit. The core of the current transformer with the secondary winding is clamped around the main conductor which acts as the primary winding of the current transformer.

Bushing Type Current Transformer

Bushing type current transformers are similar to bar type current transformer. The core and secondary winding are mounted on the single primary conductor. It consists of a circular core that carries the secondary winding wounded on it. This secondary winding forms a unit. The primary winding is the single conductor between the bushings.

Causes of Errors in Current Transformer (CT)                    

1. The following are the errors caused in current transformer:

2. The primary winding of the current transformer needs magnetive motive force (MMF) 

   to produce flux and this force draws magnetizing current.

3. The no load current of current transformer has a part of core loss component and it occurs eddy current losses and hysteresis losses.

4. When the core of a current transformer is saturated then the fluxdensity finishes the linear function of the magnetizing force and other losses are occurred.

5. Primary and secondary flux linkages vary due to leakages of flux.

Reducing Errors in Current Transformers (CT)

Usually magnetizing current component causes major errors and losses in current transformers. So, the following methods / schemes are used to reduce the value of magnetizing component as small as possible:

Low Flux Density

The magnetizing component of current is reduced by using low values of flux density. Low flux density is attained by using large cross section of core. That is why current transformers are designed with low flux densities as compared to power transformers.  

High Permeability Core Material

The magnetizing component may be reduced by using high permeability core materials such as Perm alloy, Hipernik has high permeability at low flux densities. These materials are frequently used for the manufacturing of Current transformers.

Modifications of Turns Ratio

The current transformers are improved by improving the accurate number of turns instead of using the higher turns. This change in turns is made in secondary side of current transformers because the primary turns are so less and if these are reduced then it results a wide variation in the turn ratio.

Wound Core

The wound core construction is used for current transformer to improve the magnetization characteristics. This type of construction is used in distribution transformers. The silicon steel is used as core material and is used to carry flux in it. This construction is used in current transformers to reduce the ratio and phase angle errors.

Applications of Current Transformer (CT)  

Current transformers are usually used as a measurement of electric current, electric power in grid stations, power houses, industrial power houses, industrial control rooms for metering and analyzing of the circuit current and for protection purposes etc.