In the power system, the level of current and voltage is very very high and that’s why it is not possible to measure it with conventional instruments without damaging anything. For such cases, an instrument transformer is used to step down the voltage and the current within the range of the existing measuring instruments.
Instrument transformers are current transformer ( CT ) and voltage or potential transformer ( PT ). CT is used to extend the current ranges of instruments and PT is used for increasing the voltages ranges.
What is the current transformer?
It is an instrument transformer that is used to measure a large current where the current in the secondary winding is proportional to the primary winding current.
In the current transformer, the primary winding is connected in series with the main line, and the secondary winding of CT is connected across respective apparatus used for different purposes like ammeter is connected for measuring current, etc.
The current in the secondary winding of CT is proportional to the primary current or maybe you can say it is many times stepped down. The number of turns in the primary winding of CT is kept very such that there is no voltage drop across it.
Working of CT
When alternating current is applied to the primary winding, an alternating magnetic flux induces which then induces an alternating current in the secondary winding. As the burden of the current transformer is very small, this results in the operation of the current transformer under short-circuit conditions.
Phasor diagram of CT
A phasor diagram of the current transformer is given below:
IP and Is = primary and secondary current
Ep and Es = primary and secondary induced voltage
I0 = no-load current
IC = core loss component of current
IM = magnetising component of current
Vs = secondary terminal voltage
n = turns ratio
Construction of CT
- Number of primary turns
It is suggested for CTs that the number of ampere-turns may vary from 5000 to 10000. Current transformers have a single bar as their primary winding. In CT the number of primary ampere-turns depends upon the primary current.
Mumetal (an alloy of iron and nickel) is used to make the core of CT. mumetal has properties like high permeability, low loss, etc. which make it an appropriate material to use as a CT.
Generally, copper material is used for primary winding and SWG wire for secondary windings. Dimensions of primary windings depend upon the primary current. Both the primary and secondary winding should be placed close together to reduce leakage reactance.
Insulation for lower voltage and higher voltage application require different insulation methods. In lower voltage applications windings are insulated with tape and varnish and higher voltage applications require oil-based insulation for windings.
Types of current transformer
Three types of CT are listed below:
- Wound type
In this type of CT, the primary winding consists of few turns of a heavy conductor.
- Bar type
This includes the laminated core and secondary winding but no primary winding.
- Window type
This CT doesn’t consist of any primary winding. To measure any current, the line carrying current must be inserted through a hole or a window.
Errors in CT
The current transformer has two types of errors
- Ratio error
[( Nominal ratio – actual ratio) / actual ratio ] * 100
- Phase angle error
Practical current transformer always has a phase difference between primary and secondary. This is because the primary current is supplied with the existing current. But for an ideal CT, the vector angle between primary and secondary is always zero.
Causes of error
In an ideal CT, the transformation ratio should have been equal to the turn ratio and the phase angle should have been zero. But due to the physical limitation of the CT, errors are introduced in the CT.
How to reduce error in CT?
- Low flux density
CTs are designed with lower flux densities as compared to a normal power transformer.
- Use of shunts
The shunt is used when the current in secondary is found to be too high. By placing shunt across the primary or secondary winding will reduce the current. But this only applies to specific values and types of burden.
- High permeability core material
Manufacturing of core material of CT is mostly with the materials which have high permeability.
- Modification of turns ratio
By modifying the number of turns the accuracy of a current transformer may improve.
Applications of CT
A current transformer is mainly used for measuring the current and protection.