Current Transformer

Current Transformer produces an output in proportion to the current flowing from the primary winding as a result of the constant potential in the primary winding.

The Current Transformer (CT) is a type of "measuring transformer" that is designed to produce an alternating current secondaryly of wrapping, which is proportional to the primary measured current.Current transformers reduce high voltage currents to a much lower value and provide a convenient way to safely monitor the actual electric current flowing in an AC transmission line using a standard amperemeter.The principle of operation of the basic current transformer is slightly different from that of an ordinary voltage transformer.

current transformer
Current Transformer

Unlike the voltage or power transformer we looked at last time, the current transformer consists of only one or very few rotations as its primary winding.This primary winding can be a single straight turn, a heavy-duty wire coil wrapped around the core, or a conductor or bar placed through a central hole, as shown.

However, the secondary winding can have a large number of coil rotations wrapped over a laminated core from low-loss magnetic material.This core has a large cross-sectional area, so the magnetic flux density generated is reduced using much smaller cross-sectional wire, depending on how much the current should be reduced when trying to extract a constant current regardless of the one connected.

Secondary winding will feed a current into the short circuit in the form of an amperometer or to a resistant load until the voltage induced in the secondary is large enough to saturate the core or cause an overvoltage failure.

Unlike a voltage transformer, the primary current of a current transformer is not connected to the secondary load current, but is instead controlled by an external load.Secondary current is usually rated as standard 1 Amp or 5 Amps for larger primary current ratings.

There are three basic types of current transformers: Wound, toroidal and bar.

  • Wound Current Transformer – The primary winding of transformers is physically serially connected by the conductor carrying the measured current flowing in the circuit.The size of the secondary current depends on the rotational rate of the transformer.
  • Toroidal Current Transformer – These do not contain primary winding.Instead, the line carrying the current flowing through the network is passed through a window or hole in the toroidal transformer.In some current transformers, it has a "split core" that allows the circuit to be opened, installed and closed without disconnecting.
  • Bar Type Current Transformer – This type of current transformer uses the actual cable or bar of the main circuit as the primary winding, which is equivalent to a single turn.They are completely ins isolated from the high operating voltage of the system and are usually bolted to the current-bearing device.

The current transformercan reduce or "reduce" current levels from thousands of amps to 5 Amps or 1 Ampere to a standard output at a known rate for normal operation.Therefore, small and precise instruments and controllers can be used in con con conceived with CT's, as they are ins isolated away from any high-voltage power line.Wattmeters, power factor meters, watt-hour meters, protective relays or magnetic circuit breakers have various measurement applications and uses for current transformers such as opening coils or MCBs.

current transformer
Current Transformer

Usually current transformers and amperemeters are used together as a harmonious pair, where the design of the current transformer is to provide a maximum secondary current corresponding to a full-scale deviation in the amperemeter.In most current transformers, there is an approximate inversion rate between the two currents in the primary and secondary windings.Therefore, the calibration of the CT is usually for a specific type of amperemeter.

Most current transformers have a standard secondary 5 amps, in which primary and secondary currents are expressed as a ratio of 100/5.This means that the primary current is 20 times larger than the secondary current, so when flowing 100 amps in the primary conductor, it causes 5 amps to flow in the secondary winding.For example, a current transformer of 500/5 will produce 5 amps in the secondary and 100 times more for 500 amps in the primary conductor.

By increasing the number of secondary windings by increasing Ns, secondary current can be made much smaller than the current in the measured primary circuit because as the Ns increases, is reduced by a proportional amount.In other words, the number of windings in the primary and secondary windings and the current are inversely proportional.

A current transformer, like any other transformer, must provide the amperage rotation equation, and we know from our tutorial on double-winded voltage transformers that this rotation rate is equal to this:

current transformer

where do we get it:

current transformer

The current rate will determine the rotation rate, and since the primary usually consists of one or two turns, the secondary can have several hundred turns, the ratio between primary and secondary can be quite large.For example, suppose the current value of the primary winding is 100A.The standard value of secondary winding is 5A.Then the ratio between primary and secondary currents is 100A to 5A or 20:1.In other words, the primary current is 20 times greater than the secondary current.

However, it should be noted that a current transformer rated 100/5 is not the same as subsections of 20/1 or 100/5.This is because the ratio of 100/5 refers to the "degree of input/output current" of the primary current, not the actual ratio of the primary current to secondary currents.Also note that the number of turns and current in the primary and secondary windings are inversely proportional.

However, relatively large changes in the rate of rotation of a current transformer can be achieved by changing the primary rotations from the window of the CT, where a primary rotation is equal to a pass, and multiple passes through the window result in a change in the electric ratio.

For example, a current transformer with a 300/5A relationship can be converted to another 150/5A or even 100/5A by passing the main primary conductor through its internal window two or three times, as shown.This allows for a higher value current transformer to provide maximum output current for the amperemeter when used in smaller primary current lines.

Current Transformer Primary Rotation Rate

current transformer

Current Transformer Question Sample 1

A rod-type current transformer with 1 lap in the primer and 160 laps in the secondary will be used with a standard amperemeter range with an internal resistance of 0.2Ω.When the primary current is 800 Amps, the amperemeter must give a full-scale deviation.Calculate the maximum secondary current and secondary voltage along the amperemeter.

Secondary Current:

current transformer

Voltage along the amperemeter:

current transformer

Above, since the secondary of the current transformer is connected to the amperemeter, which has a very small resistance, we can see that the voltage drop in the secondary winding is only 1.0 volts at the full primary current.

But if the amperemeter is removed, the secondary winding will effectively be on circuit, and therefore the transformer acts as an amplifier transformer.This is partly due to the huge increase in magnetization flux in the secondary nucleus, since the secondary leakage reaccount affects the secondary induced voltage, since there is no opposing current in the secondary winding to prevent this.

The results are a very high voltage induced in the secondary winding, equal to the Vp (Ns/Np) ratio in the secondary winding.For example, suppose our current transformer above is used in a grounding three-phase power line from 480 volts.Then:

current transformer

The reason for this high voltage is that the voltage rate per turn is almost constant in primary and secondary windings, and vs = Ns*Vp values are high values, so vs is extremely high.

Therefore, a current transformer should never be left open or operated without load when passing through the main primary current, just as a voltage transformer never short-circuits.If the amperemeter (or load) is to be removed, a short circuit must first be placed between the secondary terminals to eliminate the risk of shock.

The reason for this high voltage is that when the secondary is open circuit, the transformer's iron core runs at a high saturation and there is nothing to stop it, producing an abnormally large secondary voltage, and this is calculated in our simple example above. At 76.8kV!…This high secondary voltage can damage insulation or cause electric shock if the terminals of the CT are accidentally touched.

HandHeld Current Transformers

There are many special types of current transformers currently available.The type, which is a popular and portable "multimeter" that can be used to measure circuit load, is called "clamp meter", as shown.

current transformer

It turns the current-carrying conductor on and off and measures its current by determining the magnetic field around it, usually providing a quick measurement reading without interrupting or turning on the circuit on a digital display.

In addition to the handheld clamp type CT, discrete core current transformers are available with one end removable, so that the load conductor or bar does not need to be disconnected to load.These are available to measure currents from 100 to 5000 amps with square window sizes from 1" to 12" (25 to 300mm).

To summarize later, the Current Transformer (CT) is a type of instrument transformer used to convert primary current into secondary current through a magnetic environment.The secondary winding then provides a much lower current, which can be used to detect overcurrent, low current, peak current or average current conditions.

The primary coil of a current transformer is always serially connected with the main conductor, which is also called a serial transformer.The nominal secondary current is rated 1A or 5A for ease of measurement.As with wound, toroidal, donut or bar types, there can be a single primary turn or usually several winding primary turns for low current rates.

Current transformers are designed to be used as proportional current devices.Therefore, the secondary winding of a current transformer should never be operated on an open circuit, just as a voltage transformer is never operated on a short circuit.

The open circuit of the secondary circuit of an energized current transformer will cause very high voltages, so if the amperemeter is to be removed or if a CT is not in use before the system is turned on, its terminals should be short-circuited.

In the next tutorial on transformers, we will look at what happens when we connect three separate transformers in a star or delta configuration to produce a larger power transformer called the Three Phase Transformer, which is used to provide 3-phase resources.