What is a current transformer?

The current transformer would step down or reduce the high alternating current. So in order to measure the usage of current in a high voltage system, it is not possible because the measuring meters can’t handle it due to the insulation problems. It would also create a lot of problems to use the current directly from the system for certain purposes so to solve these problems we can use a current transformer. The current which is in the power system has KA ratings and because of this the current to the transformer is really high and the required accuracy is low, so the secondary resistance is low.

So this transformer would convert the high current to low current and in order to do the measurement, there is no need for anything else. The measurement can be done by using a small current, so in the secondary winding, there will be only a small current which will be proportional to the high current in the primary. 

How to choose the current transformer for our application?

  • The current transformer must be chosen by considering the nominal frequency
  • It should be selected by considering the networks highest rating voltage
  • It should be able to withstand the rise in voltage like the lightning voltage
  • While choosing the current transformer we must consider the primary and the secondary CT ratio
  • We should choose the CT according to the environmental conditions to which it will be exposed, like the temperature, altitude, pollution, seismic conditions
  • Required performance for the application
  • Rated burden
  • Accuracy
  • It must be selected according to the number of cores
  • Knee point voltage

Why current transformer is very important in a power system?

It is not possible to use the high voltage from the power system directly to the equipment if we do so then it will damage the equipment. The equipment may be able to handle this amount of power by improving its size and also the insulation but it will be really costly so in order to use this power we can use a current transformer.

  • Reduction of the line current to a value that will be suitable for the instruments
  • The measuring instruments can be isolated from the high voltage
  • Instruments can be protected from short circuit current
  • It would also find out if there are any abnormalities in the current line sudden variation in the current level etc

Why the secondary should never be open-circuited in a current transformer?

If there is current in the primary then the secondary should not be open circuited because it would damage the transformer. So the secondary of the CT should be connected to a burden in case if there is any primary current. The open-circuited secondary could create a great amount of secondary voltage and this would create insulation failure and due to this, the winding will be short-circuited. The CT which is energized and is not in use should be short-circuited. In case if CT is going to be shorted then we must do it safely.

What are the factors that would determine the ability of the current transformer?

  • Insulation level
  • Rated primary current
  • Burden and accuracy

How to avoid CT saturation?

  • In order to avoid saturation, we need to use higher ratio CT
  • For high fault current tripping, we need to use a separate set of high ratio CT
  • Use step down auxiliary CT
  • The secondary burden should be reduced in order to do that use low burden relays and meters, the single-phase burden should be distributed among phases, increase the size of the secondary leads and reduce the length of secondary leads.

How current transformers are constructed?

The core of the current transformer is constructed by a wound strip of magnetic material. The major parts of the current transformer are a laminated steel core, secondary winding around the core, and also an insulating material.

The core could be of rectangular, shell type, or ring form. The windings of the CT should be enclosed together so that the leakage resistance can be reduced and thus the ratio error can be reduced too. The windings will be separately wound and insulated by tape.

If the CT is measuring type then the core material must have high permeability with low saturation levels so nickel will be a suitable core material. If the CT is a protective type then it needs to have a high saturation level, so steel core can be used.

How is a current transformer connected?

The primary windings and the secondary windings would be wound on the magnetic core. The secondary will be connected to the metering or the protective devices. The primary winding which is connected to the power system would convert the line current in a way that, will be suitable for the metering or the protective devices which are connected to the secondary. The primary winding of this transformer will be connected in series to the power system and it would have a high impedance which would determine the primary winding current magnitude. This magnitude is not dependent on the load of the secondary winding.

How to do the mounting and installation of the current transformer?

Mostly the current transformer is mounted in the main breaker panel or in the branch distribution panel. The current transformer doesn’t need to be installed ninety degrees to the conductor run, because the conductors would go through the CT at any angle. The window type CT should be installed in a way that the H1 side of the window must be in the direction of the power source. The CT should not be installed in a hot service, during the CT installation the power must be disconnected, sometimes it won’t be possible due to certain loads such as computers, and other devices which are not possible to turn off. The split-core CT must not be installed, on hot uninsulated bus bars in any condition.

How does a current transformer work?

If the current flows through the primary of the current transformer, then the iron core inside the CT will be magnetized. The primary of the current transformer has a single turn while the secondary has many numbers of turns. The secondary of the current transformer would have an induced voltage due to the magnetic field in the core. So if the secondary circuit is closed then the current would flow through it. This transformer needs a slight amount of energy in order to magnetize the iron core. The AC current can be stepped down by using a current transformer and the secondary current will be proportional to the primary current.

How to do the CT testing?

Before testing certain checks should be done

  • Check the nameplate rating
  • Check for any physical damage or for any defects
  • Check if the transformer is connected to the system properly
  • Check if there is proper clearance between the primary and the secondary winding
  • Electrical connections must be proper
  • Check if the grounding and shorting connections are proper
  • Check if the grounding of each core is properly done

Insulation resistance test

Before doing this test the temperature must be noted. This test is done with the help of megger and the testing should be done between the primary to earth, primary to secondary cores, secondary cores to earth, and also to the secondary core to the core.

Testing of the secondary winding

This test is done to determine the resistance there shouldn’t be any discontinuity in the winding. The resistance must be equal to the specified value in the nameplate. This resistance testing is done to determine if the load is properly connected or not.

Polarity test

This test is done to determine the polarity of the current transformers’ primary and the secondary. We can use a battery to do this test, open and close the battery switch which is connected to the primary and check the movement of the pointer to know the polarity. By determining the polarity we would get an idea about how to connect the secondary in order to do the protection and metering function properly.

Type test and routine test

Type test

  • Checking the terminal markings and polarity
  • Short time current test
  • Temperature test
  • Impulse voltage test
  • Power frequency voltage withstand test
  • Error measurement

Routine test

  • Overvoltage test
  • Error determination and accuracy class
  • High voltage withstand test on the secondary

How current transformers are classified?

Classification according to the location of the installation

  • Indoor
  • Outdoor

Classification based on application

  • Protection CT
  • Metering CT

Classification based on the construction

  • Bar
  • Ring
  • Wound
  • Split-core
  • Linear
  • Cascade

Classification based on the type of insulation

  • Dry-type
  • Epoxy
  • Oil impregnated paper
  • SF6

What are the applications of the current transformer?

  • These transformers are used in panelboards of substation so that the bus bar current can be measured which is really high
  • It is used in power measuring circuits
  • These transformers are widely used in powerhouses
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