A current transformer is an instrument transformer. It is similar to the voltage transformer but instead of transforming voltage it is used to transform current.
“It’s the main purpose is to step-up/step-down the current from higher power transmission line. The current in the secondary winding is proportional to that of primary winding but the magnitude of the current is reduced”.
It’s main purpose is to ac measure current.
Why we use the current transformer?
The question arises in your mind that why we use current transformers. Since we have the potential transformer to step-up/step-down the voltages….
- Since a standard ammeter (i.e. measuring device) is not capable of handling high value of current and not be connected to the high power line (i.e. transmission line). In order to measure the actual current in the transmission line using a standard ammeter. We use a current transformer because the current in secondary of winding is proportional to that of primary winding but the magnitude is reduced.
- It is used to isolate the measuring device from high power lines.
- It is used to provide current input to relay for the protective purpose (i.e. from high transmission line), if we want to operate relay then we use CT which produce current in secondary proportional to primary but reduce the magnitude of current).
The basic working of CT is similar to that of PT, but the primary winding of the current transformer is connected in series with high power lines. Since the current in high magnitude lines is very large if we use a small diameter of primary wire then it will be damaged. So, the primary coil is a heavy duty wire. Since the secondary current of CT is proportional to primary but is reduced to a small magnitude. Now, what is the turn ratio of primary and secondary windings? Let’s see the basic equation of transformer current and number of turns.
According to this relation, we see that the turn ratio of secondary is greater and primary is lesser turns in order to step-down the current.
Since the primary coil is in series with the power line it is also called “series transformer”.
The secondary winding is wounded on the laminated core which has low-loss magnetic material of large cross-sectional area.
In the current transformer, the load connected to the secondary winding is very small to protect to secondary from high voltage breakdown. At this point, the above-mentioned point is to be understood in order to use CT.
This is a step-down transformer because secondary winding is greater than the primary winding.
Suppose that the turn ratio is 10:1 which means ( NS/NP=10/1=IP/IS ) that primary current is ten times greater than the secondary current, but the primary voltage is ten times less than that of secondary voltage.
“The impedance (i.e. the burden) connected to the secondary is very small”.
To understand this, let’s suppose
- If the RL=1Ω, then the voltage drop at RL is VO=IR =.>(1A)(1Ω) => VO=1V
- If the RL=10Ω, then the voltage drop at RL is VO=IR =.>(1A)(10Ω) => VO=10V
- If the RL=100Ω, then the voltage drop at RL is VO=IR =.>(1A)(100Ω) => VO=100V
- If the RL=1KΩ, then the voltage drop at RL is VO=IR =.>(1A)(1KΩ) => VO=1KV.
Now by the above calculation, we observe that if the RL is increasing the VO across RL is increasing. There is a particular limit of RL to be connected to the secondary winding. If the RL exceeds this limit then the flashover will occur and CT will burn. We cannot remove RL because is RL is removed then the resistance will become maximum (i.e. ∞) and flashover will occur. If RL is exceeding above the minimum value the CT will burn.
We cannot disconnect the secondary winding from the rated burden;
“The burden is defined as the load connected across secondary winding”.
Therefore CT is always operated in a short circuit. The load (i.e. the burden) is defined as the product of current & voltages (i.e. VA) of secondary. CT is capable of handling low power.
Current Transformation Ratio:
“ the ratio of primary current to the secondary current is called the current transformation ratio.”
The primary current from the transmission line is above 10-3000A or greater, whereas the secondary current is around 0.1A – 5A