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.
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.
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.
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
IM = 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 90o. 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 180o 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 Is rs and Is Xs from the secondary induced voltage Es.
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 180o 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)
The following are the errors caused in current transformer:
The primary winding of the current transformer needs magnetive motive force (MMF)
to produce flux and this force draws magnetizing current.
The no load current of current transformer has a part of core loss component and it occurs eddy current losses and hysteresis losses.
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.
Primary and secondary flux linkages vary due to leakages of flux.