For measuring of Electrical current, Ammeter been used. From analog to digital, Ammeters are available in multiple models.
An GIF shows working of Ammeter connected across line
When current level is low, Measurement is simply easy because it can be measured directly. But measuring the heavy current; is quite difficult. Measuring the heavy current, make the size of measuring instrument bulky & heavy. That’s why high Voltage & high current been measured indirectly.
There are two types of power supplies AC & DC. Both the powers have their own working mechanism that’s why for measuring these powers, There are different devices working on different mechanism. For Alternating current, ‘Current transformers’ are used. While for the DC current, ‘hall effect based CT coils’ or ‘DC shunts’ are used.
In this post, We will talk about the Current transformers which are utilized to measure AC current.
CURRENT TRANSFORMER
Current transformer with Highest system voltage <600Volts| ratio 200/5 |
Current transformer is a type of instrumental transformer which measure the heavy current, indirectly. Current transformer works on the working principle of Transformer, On the method of faraday law of Electromagnetic Induction. Current transformer is a type of step down transformer. In which measuring quantity act like a primary side of transformer while current transformers act like secondary.
To know better about current transformer, We should know about transformer first.
Transformer– An Electrical machine which transfer power from one circuit to another. Generally, transformer has two windings primary & secondary. Both the windings are winded on common silicon steel stamping. These windings are not directly connected rather they are magnetically coupled to each other.
Transformer without any movement changes the voltage level on the secondary side while maintaining the constant frequency on both the sides. Both primary & secondary windings have number of turns. The variation of voltage & current on secondary side depend upon the ratio of number of turns.
Secondary side’s voltage increases & current decreases when secondary side’s number of turns are greater than primary side’s number of turns.
Secondary side’s voltage decreases & current increases when secondary side’s number of turns are lower than primary side’s number of turns.
An schematic diagram of an ideal Transformer
Transformer actually follows the equation, Vp/Vs=Np/Ns=Is/Ip
Where,
Vp= Primary Voltage
Vs= Secondary Voltage
Np= Primary number of turns
Ns= Secondary number of turns
Ip= Primary side current
Is= Secondary side current
“The phenomenon Ns>Np makes Vs>Vp which further makes Is<Ip; That phenomenon is used in Current transformer. “
Current Transformers placed on line conductors
Currenttransformer in general seems like a ring type bulk coil which is having a hollow internal diameter as shown in an image above.
Current transformer itself act as a secondary side of Transformer while the measuring conductor act as a primary side which crosses in between through the Current transformer. When a heavy current flows through the measuring conductor, A changing magnetic field link with the conductors of current transformer which makes an EMF get generated across the terminals of CT coil.
A generated EMF in current transformer is directly proportional to the current flowing through the measuring conductor. Terminals of CT coil are connected to the measuring device(an ammeter) which makes the circuit of current transformer; A closed loop. Current flow through the current transformer is ultimately proportional to current flowing through the measuring conductor.
Since this full arrangement, Current transformer placed on measuring conductor forms a type of step up transformer where primary has single turn while secondary has multiple turns which further makes the current be lower at secondary side or at a line of current transformer.
GIF shows current measurement by current transformer
There are some of the clamp meters or power meters which contains a clamp; A Clamp is just a type of current transformer for current measurement. An image of energy meter3510 by MECO Is shown below which is having a clamp.
Jaw of clamp meter which act as CT coil Energy meter3510 by MECO
TECHNICAL DETAILS OF CURRENT TRANSFORMER
1. CT Ratio
An important parameter of current transformer is CT ratio. (CT)Current ratio; primary current to secondary current ratio which is been published on the name plate of current transformer. Ratio is basically, the maximum rated current on primary to the maximum rated current on secondary. Where measuring conductor act as an primary while the current transformer itself act as a secondary.
An images of current transformers are shown below which areHaving a CT ratio’s of 200/5 & 100/5.
200/5 represents primary max rated current 200Amps while secondary max rated of 5Amps. Which means 200Amps current on measuring conductor can produce 5 Amps of current on current transformer side.
Current Transformer with CT ratio’s (200/5)(above) & (100/5 (down)
The CT ratio has been chosen according to the maximum rated current of the system as per according to the rated current of the circuit breaker connected on that line. So that permissible level of current flows across the CT coil, that is less than 5amps.
Modern Electronic energy meters having a provision that we can select the CT coil’s ratio.
An images of Elite 440 energy meter by SECURE been shown below. The images shows the selection of CT’s & PT’s primary & secondary.
Elite 440 Energy meter by SECURE
Elite 440 Energy meter by SECURE
2. Voltage
There are two voltage levels for current transformer. One is the measuring system’s voltage & another is CT coil’s insulation level voltage which is much higher than the system’s voltage.
image below of the coil, shows the system voltage should be lower than 600Volts while insulation level voltage is 3000Volts.
Current transformer with ratio 200/5 Having system voltage<600Volts, Insulation voltage 3000Volts
3. Power Rating
Power rating represents the apparent power of current transformer Which is rated in VA.
4. Accuracy
Energy meters are available in different accuracy classes. Greater the accuracy class; lower the error; Costlier the energy meter. Similarly, CT coils are also available in accuracy classes. The CT coils are available in 2, 1, 0.5, 0.5s, 0.2, 0.2s classes.
Two classes 0.5 & 0.2 contains s class which are 0.5s & 0.2s respectively. The term s containing class is more accurate.
Tabular format shows the accuracy classes of Energy meter
5. Frequency
Frequency represents the system’s frequency & Frequency rating should always remains the same on CT coil.
SUMMARY
Current transformers (CTs) are designed to measure AC current indirectly, relying on electromagnetic induction principles. They consist of primary and secondary windings to transfer power without direct electrical connection. These transformers generate an EMF proportional to the current in the measuring conductor.
Technical details of current transformer include CT ratio, voltage specifications, apparent power ratings, accuracy classes, and frequency ratings, essential for effective current measurement and monitoring.
Hope, You like the post. If you have any doubt, suggestion or query please do comments !
