Current Transformer: 19 Useful Answers For Beginners

When I was an electrical engineering student, current transformer was a confusing topic for me.

Back then, it didn’t make any sense for me to transform the current. I knew how a voltage transformer works.

A question about current transformer was asked from the instructor, I can remember how this question was confusing to me!

Now, after more than 15 years, I’m answering important questions about CT to help you and to help the beginners. Let’s get started.

Table of Contents

What happens to the voltage in a current transformer?

With a current transformer, high voltage is reduced to a much lower value, and a standard ammeter can be used to safely monitor the actual electrical current flowing in an AC line, thereby providing a convenient and safe way to monitor the actual flow of electricity.

A basic current transformer operates on a somewhat different premise than a typical voltage transformer.

The output voltage of a current transformer (CT) may be checked in the field using a digital multimeter with a millivolt AC (mVac) range.

This test is helpful in determining whether the CT is operating correctly and whether current is flowing through the conductor it is mounted on.

The CT outputs’ AC millivolts between the white and black wires are measured. When the CT’s full-scale current rating is flowing through the conductor, its output voltage should be 333 mVac.

What Is The Current Transformer Burden?

The burden of the current transformer stands the terminating impedance of the measuring instrument or the CT load.

The instrument that is being used to measure can be an energy meter, either digital or analog, and a data logger or recorder.

Any instrument that uses an electric CT to gauge current in the line must terminate the CT by resistance (impedance in the same way that there is an inductance).

There are some essential points to understand the CT burden mentioned below:

  • The most common burden rating for CT: 2.5, 5, 10, 15, and 30VA.
  • The external load that is applied on the secondary of a current transformer is referred to as “burden”
  • The burden can be described as the highest amount (in VA) applied on CT secondary side.
  • It is possible to have the burden expressed in two different ways:

The burden could be defined by the sum of impedance in ohms of the circuit or as the total voltage-amperes (VA)

power factor at a certain amount of voltage or current and frequency.

  • It is common to describe the CT burden using the volt-amperes (VA) or power factor (PF), the volt-amperes consumed in the burden’s impedance at secondary rating current. So, a burden of 0.5 O impedance could be described by using “12.5 VA at 5 amperes” when we consider the standard 5-ampere secondary rating.

Current Transformer Burden Calculation, Formula And Example?

Formula to calculate the CT Burden is mentioned below:

The total burden of measuring CT = the total burden of meters in VA (Ammeter and Wattmeter), Transducer, etc. that are connected to CT secondary circuit + the Secondary Circuit Burden of cable in VA.

Cable burden =I2 x R x 2 L. In this equation, I is the CT second current, and R is the resistance of the cable per length, 2L represents distance of the cable length L from CT to meters. If the correct length and size of wire are utilized, the cable burden is left out.

The CT Secondary circuit load must not exceed the CT VA ratings. When the CT burden is smaller than the CT load, the other meters attached to the CT should give the correct reading.

In a Measuring Current transformer, the burden is determined by the connected meters. Amount of meters that are connected to the secondary side of the cable, i.e., Ammeters, Kvar meters, KWh meters the Kwh Meters transducers as well as a load of connecting cables(I 2 x R x 2 L) to metering should be considered.

Note: Meter burdens values can be taken from the manufacturer’s catalog.

The selected CT burden should be greater than the burden calculated.

CT burden Calculation Example

 If the resistance for the relay equals 0.2 Ohm, the resistance of the wires that are connected is 0.2 Ohm, the secondary winding resistance of the is CT 0.2 Ohm.

Total resistance in the Secondary side circuit = 0.2+0.2+0.2=0.6 Ohm.

The total burden on CT is 0.6 Ohm.

If the rated current of the secondary side of CT is 5 amps.

Secondary voltage = 0.6*5 = 3 Volts.

The main burden for an existing transformer within VA is;

CT (VA) = CT secondary current rated by CT * Secondary Voltage of CT

=5*3 = 15 VA.

Current Transformer Ratio Meaning And Formula

The ratio of the primary current input to the secondary current output at the full load is called Current Transformer (CT) Ratio.

At full load, the primary current input to secondary current output ratio is known as the CT ratio.

For instance, a CT with a ratio of 100/5 is designed to create 5 amps of secondary current when 100 amps pass through the primary, even if it is rated for 100 main amps at maximum load.

The secondary current output will adjust according to variations in the primary current. The secondary current output, for instance, will be 2.5 amps if 50 amps pass through the 100-amp main.

To simplify calculations, the CT secondary current will always be 1 or 5, and we can design secondary circuits with low ratings that can handle up to 5 Amps.

The 1 Amps secondary current is always included with the sensitive metering current transformer.

Current Transformer ratio formula

CT ratio = I(P) / I(s) 

Where, I(P) = Primary current in Amps and I(s) = Secondary current in Amps

I have written a detailed article about Electrical Current, AC & DC. You can find it here.


An CT with a ratio of 300:5 is assessed to be 300 primary amps when fully loaded. This will result in 5 amps of the secondary voltage if 300 amps flow across the primary.

When the current in the main fluctuates or changes, the output of the secondary current changes according to that change.

If 150 amps flow through a primary with a 300-amp capacity secondary current, the secondary current will be 2.5 amps.

Let’s use one more easy example of a current transformer with a CT ratio of 2000:1 and compute the secondary current in relation to the primary current, assuming the CT primary has 1500 Amps.

Assuming x is the secondary current in Amps,

Apply our CT ratio formula,

CT ratio = I(P) / I(s) 


Secondary Current= x =1500/2000=0.75A

How To Select Current Transformer Ratio?

To select the current transformer ratio, it is vital to understand the lowest and maximum operating current for each of the loads to be monitored for your application, as each CT has a suitable operating current range.

As long as the current transformer does not operate over its maximum usable operating current, it can be used securely.

The lowest useful working current range should be kept in mind since this is the level at which the current transformer will no longer be able to monitor the current, causing the Power Meter to “Snap to Zero.”This indicates that the Power Meter will display “0” current use even if only a little quantity of current is flowing.

Due to the CT’s higher resolution, smaller size, and overall cost savings, a typical split/solid-core type CT is advised for applications where the maximum operating current is less than 600 Amps.

Rogowski Coils should be used when the wire size or buss bar is too large to use the standard solid core or split-core CT’s aperture or window, as well as in applications with multiple conductors and panels rated above 600 Amps.

The smaller, solid, and split-core CTs have a useful operating current range of 1 to 200 Amps (100A Midi CT) or 1 to 300 Amps (200A Midi CT), respectively.

What Is The Error In Current Transformer And Its Ratio?

An error in a current transformer can be classified into two types. Specifically, they are known as ratio errors and phase angle errors.

Ratio Error In CT

A current transformer’s ratio error results from a deviation between the turn ratio and the actual current ratio.

We know that a current transformer’s current ratio must match the turn ratio, or I1/I2 = N2/N1. However, because of the power factor of the secondary winding and the magnetizing and cross loss components of the main winding current. The turn ratio N2/N1and the current ratio I1/I2 will differ.

Because of the power factor and load current of the load or burden linked to the secondary, the real current ratio will not be constant.

The current in the primary cannot be accurately estimated as a result of this change in the real current ratio, which results in a mistake known as a ratio error.

The percentage ratio error formula is as follows:

% Ratio Error= (Nominal Ratio – Actual Ratio/ Actual Ratio) * 100

% Ratio Error= (Kn– R / R) * 100

Phase Angle Error in CT

The CT’s secondary current must be precisely in phase opposition to the primary current or by a 180-phase difference.

There is a discrepancy in the phase angle between the primary and secondary currents at the moment of power measurement.

This is because the CT’s core loss and the CT’s magnetizing components, for which the primary current loses some phase angle, must be supplied by the primary current.

The secondary current would not be in precise phase opposition as a result. A change in the phase angle causes it, leading to an error called the ‘Phase Angle Error.’ This angle denotes θ the phase angle error.

The phase angle error formula is as follows:

Phase Angle = 180/π [Im cos δ – IC sin δ / nIS] degrees

Relays, instruments, and pilot lights are the most common types of loads linked over secondary in practise. is positive and relatively tiny for inductive. As a result, sine is 0 and cos is 1.

In the equation above, substituting, we obtain.

Phase Angle = 180/π [Im/nIs] degrees

What Is The ALF And ISF Of Current Transformer?

 ALF (Accuracy Limit Factor):

CT’s Accuracy limit value (ALF) means the ratio of the rated accuracy primary current limitation to the rated primary current.

ALF has been utilized in the protection class Current Transformers (CTs). The purpose of the Protection Class CT is to detect current during fault conditions and then feed the result to the protective relay/circuit breaker, which will then cut off power.

The class of protection for the CT is identified with 5P10 and 5P20. In this case, P represents the protected class. 20 represents the accuracy limit of the primary current, and 5 represents the Composite error for the CT when the accuracy limits current flows through the primary transformer.

ISF (Instrument Safety Factor):

Instrument Safety Factor (ISF) is the ratio between saturation current and highest-rated current at primary section.

Metering CTs is devised with ISF, having typically value less than 5 or less than 10. CT will be saturated if the amount of current generated is greater than 5 or 10 times the maximum output.

If the CT is saturated at a flow of 10 kA current via its main and the CT ratio is 2000/1, the instrument safety factor is given as:

ISF = saturating Current of CT / Rated current of CT

ISF= 2000×5 / 2000 = 5

Only the metering current transformer is where ISF is defined (CT). The CT used for metering is known as a “metering CT.”

In most cases, a CT has four cores or more. Core-1, Core-2, and Core-4 are intended for protective purposes, while Core-3 is intended for metering. Different cores are developed for various purposes.

As a result, ISF will be established for core-3 of CT, the metering core. Since meters are only intended to handle modest current levels, it is crucial to safeguard them against excessive current levels.

Because meters are directly linked to the terminals of the CT metering core, it is possible that, in the event of a malfunction, the secondary current of the CT may be high, causing it to flow through the attached meter. This might result in meter coil damage.

Therefore, steps must be made to safeguard meters against such an incident. For metering CT, we establish Instrument Safety Factor ISF.

What Causes A Current Transformer To Burn?

All electrical devices and equipment can get damaged or burnout. Each device has factors that cause damage. Current transformers are not an exception. Here under some common causes of the current transformer burning and breaking:

  • Secondary open circuits of the current transformer generate a high voltage; the secondary voltage instantly jumps into thousands of volts, which reduces the secondary insulation. It produces arcs internally, which generates lots of heat that burns out or breaks the transformer.
  • Long-term use of the transformer causes the insulation to overage due to poor insulation resulting in an overvoltage due to this local discharge, or break down occurs, and the current transformer burns out or breaks.
  • The contact surface of the aluminum of the primary connections of the current transformer is severely oxidized, resulting in an enormous resistance to contact. The contact resistance releases heat and burns out the current transformer.
  • The long-term overloading process causes the current transformer to emit heat and burn out.
  • High Voltage fluctuation from sub-station can be cause of burn or break current transformer.

Read also my articles: Transformer moisture, causes and protection, and the other one, Transformer burnout, causes and solutions for more information about what can cause an ordinary transformer to burn.


What Happens When Current Transformer Secondary Opens On Live?

If you open the secondary of a current transformer while energized, there will be a very high voltage across the secondary side of a current transformer. This high voltage leads to a buildup of a large magnetizing current on the secondary side, which in turn generates a large flux and saturates the core.

The CT secondary cannot be left open for a number of reasons, including:

  • Back emf is 0 when there is no secondary current. Because of the low primary resistance, primary current increases significantly. The winding and core risk burning from the high heat created.
  • A step-up transformer is essentially what a CT is. Induced high voltage may therefore saturate the core, causing irreparable damage to the core. The operating point moves into a nonlinear zone as a result, which causes the transformer to operate incorrectly (the secondary current won’t copy the primary according to the turn ratio).
  • The lives of workers on the secondary side may be in jeopardy due to high voltage spikes (on the order of kV) created there.
  • The transformer insulation can be destroyed by high voltage spikes on the secondary side (order of kV).

The prospect of open circuiting the CT secondary can be hazardous for the personnel working on the second side.

The secondary part of a transformer’s current should not be left in an open state, because if it left in the open position, a high voltage is present on this secondary portion due to this action, this high voltage is able to make a short circuit and cause serious injuries.

This high voltage triggers an intense magnetizing current to accumulate on the secondary end, which creates high flux and causes the intense flux could saturate the core and cause enormous residual magnetism within the core, increasing the magnetization current and creating an error in the conversion ratio.

Due to exceptionally high secondary voltage across of CT can cause flashovers and damage the insulation due to the rupture of the dielectric layer between two points of CT. So, Its secondary requires to be connected to a low-resistance device like an ammeter at all times.

Is Current Transformer A Step Up Or A Step Down?

High currents exist on power lines and within high-power circuits. It is often difficult to measure these currents directly.

Current transformers are used to reduce the line current to make it easier to determine. The current Transformer is usually a step-down transformer. It will lower the current’s level to that level measured with an ammeter.

As we all know, ammeters can measure a range of current that can be approximately 500 to 750 A. However, the current levels of AC circuits are high enough that they can’t be measured with an ammeter. Thus, the CT is installed in series, with the primary winding connected to the circuit and the secondary winding attached to a current measurement instrument.

The power of the transformer is the same for both the primary side and the secondary side. The only way to reduce the current is to step-up the voltage.

For more information about Step up and Step Down Transformers, read my detailed article here.

What is Difference Between 0.2 and 0.2S Class Current Transformer?

It is worth noting that 0.2 and 0.2S are both indicators of current transformer accuracy.

A CT of 0.2S class, on the other hand, boasts a higher level of accuracy than a CT of 0.2 class. CTs with an S class of 0.2 delivers an error of +/- 0.2 percentage when the loading on the CT is between 100 and 120 percent.

The CT measurement error is not guaranteed to be +/-0.2% at lower loading; it is substantially higher at 5,20,50 and 80% loading. Any mistake might occur if the CT is run with less than 5% loading.

0.2S class CT is employed for revenue metering to increase measurement precision. Utility companies now measure revenue using 0.2S class current transformers.

At lower loading, the 0.2S class CT is significantly more accurate than the 0.2 class CT. The 0.2 S class CT’s guaranteed error ranges from 20 to 100 percent loading and is +/- 0.2 percent. CT inaccuracy is +/-0.75% with a 5 percent loading.

With slightly additional inaccuracy, the CT may read loading up to 1 percent. So, from 1 to 120 percent loading, 0.2S class CT may be employed for an energy assessment.

Why Is Current Transformer Knee Point Voltage Important?

The knee point voltage parameter is extremely important when dealing with protective current transformers, particularly for Class-PS, which is a special purpose current transformer.

Knee point voltage is only a measurement of the current transformer’s saturation limit. Each CT with a protection class need must be used in its non-saturation zone.

The highest permissible voltage limit that can be applied to the secondary winding of the current transformer must thus be calculated.

According to IEC, a current transformer’s knee point voltage is the point at which a 10% increase in the secondary voltage causes a 50% increase in the secondary current.

Take transformer voltage transformation ratio into account. Es = 4.44 × flux x current x conductor count. According to this equation, the current is exactly proportional to the mmf, and the flux transfer to the core is directly proportional to the applied voltage (ampere-turns.).

As a result, the applied voltage in the transformer increases the flux in the core. However, the core has a limit for flux transition, and once that limit is reached, the core opposes the flux.

In order to maintain the transformer’s minimal voltage, the winding needs to draw additional current, which raises the core temperature. We refer to this as core saturation. The knee point voltage is the limit mentioned.

Knee Point Voltage Testing Procedure

The current transformer’s input voltage will be gradually raised by 10% increments up to 120 per cent for knee point voltage testing.

There will be a notation of the secondary voltage and current. Within a limited range, adding a modest voltage causes the transformer to draw a greater current.

For example, while applying 10% of the secondary voltage, the transformer draws 50% of its secondary current. Knee point refers to certain voltage limitations.

Knee Point Voltage Calculation:

By using this formula, you can calculate CT Knee Point Voltage;

Vkp = K * If/CTR * (RCT + RL + RR)


Vkp = The minimum Knee Point Voltage

K = Constant, conventionally taken as 2.0

If = Max fault current

CTR = CT Ratio

RCT = Resistance of the CT secondary winding, in Ohms

RL = 2-way Lead Resistance, in Ohms

RR = Relay Burden, in Ohms

Let’s take Some to calculate the Knee point Voltage

System fault current If= 30 KA

CTR= 600/5

RCT+RL+RR=0.5 Ohm

Assuming that the current transformer has a knee point voltage of;

Vkp = K * If/CTR * (RCT + RL + RR)

Vkp = 2*30000/120*0.5 =250 Volt

Check the rated knee point voltage of the CT based on the aforementioned computation. There is a potential for CT saturation during a power network fault if the knee point voltage determined using the power network is higher than the set knee point voltage of current transformer, and the protection relay will likely not activate.

Difference between current transformers and ordinary power transformer?

In the case of a current transformer, it is simply an inductor wrapped around a wire to sense the magnetic field produced by the changing current.

In other words, measuring a current CT reduces the current signals. In the case of a power transformer, the ratio between the number of windings on the two coils determines the voltage level.

An ordinary power transformer is used for step-up or step-down purposes. It reduces the high voltage to lower voltage and lower voltage to high voltage.

Let’s take a deep look at the differences between current transformer and power transformer:

The difference in structure

The main winding of the current transformer is wound with a thick wire frame, often only one turn or a few turns, and is linked to the load of the measured current.

The power transformer is a step-down or step-up transformer, which has a large number of turns at once.

Different in working principles

  • The power transformer can be opened a second time but cannot be short-circuited, whereas the current transformer can be short-circuited a second time but cannot be opened in either direction.
  • The power transformer’s primary internal impedance is comparatively low for the load on the secondary side and may even be disregarded. The power transformer may be considered a power source. The current transformer’s principal internal resistance, which is very high, can be considered an internal resistance, a source of high resistance to current.
  • The magnetic flux density of the power transformer is nearly at saturation state levels during normal operation. The magnetic flux density drops when a fault develops; the current transformer’s magnetic flux density is extremely low during all normal operations, and short-circuit failures frequently result in problems such as the magnetic flux density will also grow significantly, sometimes even exceeding the saturation state value, as a result of the short-circuit defect on the main side increasing the amount of current.

Difference in function

The distinction between them is that one measures current, and the other measures voltage. The power circuit consists of connections between current transformers.

The main winding is better than the secondary winding because it has fewer turns and can signal direction while the secondary winding cannot.

The power transformers are linked to one another in the power circuit. A second short circuit is impossible since the winding coil has a lot of turns.

Current transformer is connected in series and potential transformer in parallel

Like an ammeter, the CT measures current, so it is connected in series with the conductor that carries current. A CT’s secondary is linked to measuring instruments, while its primary is connected in series to the wire carrying the current.

A CT secondary must not be left open-circuited since this might result in an unusually high voltage appearing on the secondary and harming the core and windings.

On the other hand, the purpose of PT is to measure the voltage across an element (like a voltmeter), so in contrast to a CT, which is connected in series with a circuit, a potential transformer is connected in parallel with a circuit.

A direct connection is made between the primary of the PT and the power line whose voltage is being measured. During the course of the measurement of voltage, the secondary leads are connected to a voltage measuring device such as a voltmeter, wattmeter, etc.

What if current transformer gets saturated?

The CT is considered to be in saturation when the primary current is so high that the core can no longer take extra flux. When the main current varies in saturation, there is no flux change (as the core already carries maximum flux).

There is no secondary current flow since there is no flux change. Because of this, none of the ratio currents passes into the load connected to the CT during saturation and all of it is utilized as magnetizing current.

The knee point, also known as the point at which CT saturates, is defined by the IEC as the voltage at which a 10% increase in the secondary voltage of CT causes a 50% increase in secondary current.

Even for slight changes in voltage across the secondary terminals over the knee point voltage, the magnetizing current increases noticeably.Only loads that generate neutral current necessitate a neutral CT.

When a 4-wire system is utilised with an electronic-trip circuit breaker that offers a ground-fault option (alarm or trip), a neutral CT is necessary. For precise sensing, neutral current must be taken into account along with phase current.

What is current transformer blast?

CT is a device that has oil in it. There is a possibility that the oil in CT will boil and begin to evaporate due to overheating. The vaporization of the CT oil is going to create pressure in the CT housing, which will cause it to blast due to the pressure that is created.

We know that a current transformer (CT) monitors a significant amount of current by scaling down the current value. If the secondary winding circuit of the CT is opened while the primary winding is carrying current, the transformer oil temperature will rise.

Due to the primary winding’s handling of the high current, the primary winding generates a lot of heat.

The secondary current will drop to zero if the secondary winding is opened, raising the voltage across the secondary open terminal and increasing the primary current.

Therefore, we utilize insulating oil, which helps to offer insulation as well as cooling. As the main current increases, the temperature of the oil in CT also rises, and the oil begins to burn, which causes a blast in CT.

Reasons Of The Current Transformer Blast?

CT is oil-filled, so if it overheats, the oil will boil and vaporize. It will result in the CT housing becoming pressurized and blasting as a consequence of the vaporization of CT oil.

This is one of the major reasons to CT blast lets see other reasons that can be cause to the CT blast:

  • The current transformer has an excessively long service life, the insulation is deteriorating, and partial breakdowns or discharges take place. This causes overvoltage, which causes the current transformer to heat up and burn out or blast.
  • The user overload operating period is prolonged, causing the current transformer to overheat and burn out or blast.
  • The current transformer is opened twice, producing tremendous voltage and causing the current transformer to heat-up, leading to burnout or blast.
  • The current transformer can burn out or blast because the contact surface of the primary connection aluminum bar is overoxidized, and the contact resistance is too high.

What Does 5p 10 Mean In a Current Transformer?

A CT that is classified as a protection class CT can be recognized from its specifications or name plate by the designation 5P10.

Considering that this CT is categorized as a protection class CT, we can interpret it as having an accuracy of 5% over a range of currents that are 10 times the normal primary current rating.


P stands for protection class CT.

5 stands for the error percentage is 5 when the CT-rated current is flowing.

10 stands for to stay accurate, the CT must be within the allowed error margin by a factor of 10.

The error for a CT rated at 1000/5A, 5P10 will thus be 5 percent or less when a fault current of 10 x 1000A flows, i.e., 10kA, and a 5 percent error indicates that a 500A would be the maximum error (5 percent).

How Does Current Transformer Work In DC Circuit?

As a general rule, CTs are used to measure AC currents in AC circuits. The CT is technically sensitive to DC as well. When we use it with dc circuits, the core will become saturated quickly, and the indication will stop working.

The issue with this method of sensing DC current is that the CT’s windings eventually get saturated when the current flows in just one direction, making it impossible for the CT to continue taking precise measurements.

Hall effect sensors with a gap in the core for the sensor are frequently used for DC, but when used in permanent installations, they frequently exhibit sensitivity drift.

The usage of orthogonal fluxgates, which employ two orthogonal coils, one of which alternatively enters saturation, is another choice for a permanent installation.

It is the second coil from which the DC current is measured. To find an AC component, if it’s there, a Rogowski coil is inserted.

What is the creepage distance of current transformer?

Whether it be a current transformer or another electrical device, the creepage distance is the distance traveled over the insulator’s surface in the least amount of time.

The machine’s live components must be kept apart from the ground and the operator. By ensuring a safe distance between the live portion and the ground, this is accomplished.

The tank in a current transformer must be kept separate from the secondary box and the CT’s base. This is accomplished with the porcelain insulator’s assistance.

If a 420 kV Current Transformer is being insulated, then the distance between the CT tank and base needs to be extended in order for it to withstand a 1250 kVp lightning impulse.

Due to construction and economic constraints, we cannot expand the CT height over a certain point; therefore, we have instead increased the distance along the porcelain’s surface to safeguard the CT against surge and flashover. In a typical situation, the Creepage distance for 420 kV CT is 10500mm.

How A Current Transformer Is Connected With The Load?

There are two windings of a current transformer: the primary winding is connected to the load in series, which carries the actual current flowing to the load, while the secondary winding of the transformer is connected to a measuring device or a relay on the secondary winding of the transformer.

The measuring, sensing, and protection equipment are attached to the transformer’s secondary, which receives an alternating current as its primary.

The current-carrying conductor is the sole turn made by the primary of a CT, which normally has just one window. Its main never has more than a very small number of turns.

Depending on the size of the current that has to be stepped down, the secondary of the transformer has a number of turns.

The measuring equipment can be attached to the ends of the secondary coil, which is twisted around a laminated core made of ferromagnetic material.

Is Polarity Important When Connecting To Current Transformers?

Yes, it is very important Correct polarity must be observed when installing current transformers and connecting them to power metering and protection relays.

A CT’s polarity may occasionally be represented by an arrow; in this case, the arrow should point in the direction of the current flow when the CT is placed.

The polarity of a current transformer is defined by the clockwise or counterclockwise direction in which the coils are twisted around the CT’s core and by the direction in which the secondary leads are brought out of the transformer casing.

With the following designations to help with appropriate installation, all current transformers are subtractive polarity devices:

H1  = Primary current, the direction of the line

H2 = Primary current, forward-facing load

X1  = Secondary current

The H1 main lead and the X1 secondary lead are located on the same side of subtractive polarity transformers.

When a CT’s polarity is represented by an arrow, the arrow shall point in the direction of current flow when the CT is placed.

Can I Connect Current Transformer In Reverse Direction?

If the current transformer is switched to reverse direction, several devices begin to malfunction, including the energy meter, which displays negative values, and the relay, which may sense negative current and so trigger the trip circuit.

Current transformers are constructed somewhat differently from ordinary transformers; they resemble sensors more than transformers, and sensors are frequently one-way devices.

It doesn’t help that current sensing transformers often have extremely high resistance secondaries, they don’t need to handle much power, and their intended applications don’t care too much about voltage loss.

Why Is The Secondary Of current Transformer Grounded?

An extremely high voltage can be produced across the CT winding if the load or burden is either eliminated or not there. If secondary end of CT is grounded, a circuit board terminal that should be close to ground potential won’t accidentally float at a high voltage.

In terms of safety and the proper operation of the protective relays, the grounding of the current transformer is crucial.

The secondary circuit of the current transformer should only have one point connected to the station ground in accordance with the current transformer’s grounding requirement. No matter how many secondary windings of current transformers are connected to the circuit, this is true.

If a current transformer contains three sets of secondary winding, none of the individual windings should be connected to the ground.

Instead, the circuit created by the CT’s secondary winding sets must be connected just once, at the neutral creation point of the CTs. Problems with the voltage generated at several current transformer ground points are eliminated by the use of a single ground for all current transformers.

The typical current does not cause any issues if the current transformers are grounded at various ground locations.

The size of the potential rise at the various ground points of the current transformer will vary during a fault state, though.

Even though there is no defect in the protection zone, the relay may trip because the increase in ground potential may not be a precise reflection of the primary current.

One of the most frequent annoyance trips is when differential relay trips because of a fault that is not inside the zone of protection.

For more about Electrical Grounding Read My Article Here.

Does A Current Transformer Has Oil?

Yes, some kinds of current transformers have oil. High transmission line currents (up to 8000 A) are converted using oil-insulated current transformers to low, standardized values that are simple to measure and may be utilised for high voltage system metering, protection, and control.

As a result, precise and trustworthy current transformation is crucial. Numerous measuring, revenue metering, and protection applications can make use of oil-type current transformers. Peak Demand OCTs are created and produced using client requirements and IEEE, IEC, or GOST standards.

Every OCT required routinely tested 100 per cent in line with the required standard.

In order to monitor primary currents of up to 8000A, oil-type CTs can be submerged. Since there are no standards for OCTs, each unit is unique. You must provide details about the apparatus, such as the current ratio, degree of precision, and frequency of the power to manufacture to design Oil Type CT for you.

It is important to point out that our oil-immersed current transformers are compliant with international safety standards, including explosion proof, multiple chopped insulating ageing, seismic qualification, and temperature extremes.

Reason of Using Oil In CT

Heat Transfer: Heat is produced during a current transformer’s operation from both the load on the current transformer and the surrounding environment (which has a lot to do with how hot the current transformer operates). This heat from the core and coils is dispersed by the oil.

Dielectric Strength: Inside a current transformer, both the oil and the paper have dielectric strength. The oil and paper work together to boost dielectric strength by 23%. This combination is well-liked as an insulating team because of the improvement in dielectric strength.

A Testing Medium: Oil in current transformers can be a good testing medium for the transformer’s health.

A representative oil sample can be pulled and sent off to a lab for testing. Then, the results are analysed, and decisions can be made about what needs to be done to prolong the health of CT. This is our goal when testing current transformers – to extend their life span.

Protects the Solid Insulation: Solid insulation is shielded by transformer oil. The role of the oil in this is by far the most crucial.

There are only two ways to restore the current transformer to a decent, dependable piece of machinery if the paper’s integrity has been compromised: replace it or rewind it.

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