Current Transformer: Basics

A current transformer is a type of instrument transformer used to step down large values of current on the primary side to a manageable level of secondary current proportional to the amount of current flowing through the primary.

When you see a CT with 800:5 ratio, it means the circuit is operating in the range of 0 to 800 Amps on the primary side and it would step the current down proportionally to the range 0 to 5 Amps for the devices on the secondary side.

The output from the secondary side of a current transformer is usually used for operating measuring devices like ammeters and protective devices like relays. The current transformer provides electrical isolation for the measurement and protective devices on the secondary side as these devices are insulated away from the high-voltage and current on the primary side.

Working Principle

The working principle of the current transformer is the same as that of the voltage transformer it contains a primary and a secondary winding. Whenever an alternating current flows through the primary winding, an alternating magnetic flux is produced, which then induces an alternating current in the secondary winding but unlike the voltage or power transformer, the current transformer consists of only one or very few turns as its primary winding. The primary winding could be just a conductor placed through a hole on the current transformer and the secondary winding consists of copper coils wrapped around an iron core

The current transformer in its most basic form is just an inductor placed around a current-carrying conductor. The inductor is used to sense the magnetic field caused by the changing current flowing through the conductor. Just like in the image below.  

Although in actual construction, an iron core is used to confine the magnetic flux generated by the current-carrying conductor so that much of it gets to the secondary coil and induce current in the secondary coil. A more accurate representation of the current transformer would be the image below.

Most current transformers have a standard secondary rating of 5 amps or 1 amp.

Current flows on the secondary side of a CT only if the secondary circuit is closed or connected to a load. In fact the secondary side of a current transformer should never be left open because as we know:

In essence the voltage ratio is opposite to the current ratio. As the CT steps down current,  also steps up voltage and for this reason, extremely high voltages would be induced at the terminals of the CT which could pose a risk of electrocution to human and a risk of damaging the CT due to insulation failure caused by high voltages stresses.

An in-depth explanation of why the secondary side of a CT should never be left open is that the current flowing in the primary side produces a magnetic flux on the core of the current transformer and the current flowing on the secondary side produces a counter magnetic flux. When the secondary is open, there will be no secondary current flowing, and no counter magnetic flux will be produced. Due to the absence of a counter-magnetic flux, a very high flux is set up in the core of the transformer. The high flux leads to core overheating due to excessive core losses. It also causes very high voltages to be induced across the terminals on the secondary side.

For this reason the secondary of a current transformer should always be shorted if it is not connected to a load. When it is shorted voltage becomes zero and the maximum current flows but we all and we know that the current is low in most cases, 5A or 1A.

Types Of Current Transformer

Window or Ring Type Current Transformer.

These are the most commonly used type of current transformers. They do not contain a primary winding. Instead, the current-carrying conductor is threaded through the window or hole in the current transformer and thus conductor becomes the primary winding in the setup. Some of them have a “split core” which allows them to be opened, installed, and closed, without disconnecting the circuit to which they are attached to.

Bar Type Current Transformer

These type of current transformers are usually bolted to the current-carrying conductor. In most cases, the conductor is a bus bar and the transformer is installed on the bus bar to measure the amount of current the bus bar is carrying.

Wound Type Current Transformer.

This current transformer is referred to as wound type because of its construction. The primary winding is wound on the same core as the secondary winding.  The primary winding is wound on the top of the secondary winding and a suitable insulating material is used to separate the two windings. Typically, they are used in applications that require small current transformation ratios. Also, they are more accurate and they have a higher burden capacity.

Modifying the turns ratio.

The CT ratio is the ratio of primary current input to secondary current output at full load. Most current transformers have a standard secondary rating of 5 amps with the primary and secondary currents being expressed as a ratio such as 75:5, 100:5, 150:5, 300:5 and so on and so forth.

 Let’s take 300:5 as an example. This current transformer will produce 5 amps at the secondary when 300 amps flow through the primary. In other words, the primary current is scaled down by 60.

This is so because, in a current transformer, rated primary amperage divided by the rated secondary amperage is proportional to the turns ratio of the current transformer. We can see that 300 ÷ 5 equals 60 or  60:1. Meaning 60 turns in the secondary to 1 turn in the primary.

So if we double the turn in the primary it becomes 60:2 or, similarly if we triple the turn in the primary it becomes 60:3.

The rated secondary amperage (Is) of the current transformer is fixed at 5A and cannot be modified so when we increase the primary turns we have to figure out what the new rated primary amperage (Ip) would be. This can be achieved using the following formula.

Where:        Ip = Rated primary amperage of CT

                    Is = Rated Secondary Amperage of CT

                    Ns = Number of secondary turn

                    Np = Number of Primary turn

Calculating our new Rated Primary amperage (Ip) for 2 turns we can say

Ip x Np = Is x Ns

Ip x 2 = 5 x 60

Ip x 2 = 300

Ip = 300/2

Ip = 150A

Similarly to find the new Rated Primary amperage (Ip) for 3 turns

Ip x Np = Is x Ns

Ip x 3 = 5 x 60

Ip x 3 = 300

Ip = 300/3

Ip = 100A