Understanding Current Transformers and Their Applications

Current transformers (CTs) are essential components in modern electrical systems. They allow for the safe measurement, monitoring, and control of high currents by producing a scaled‐down replica of the current flowing in a conductor. This article delves into how current transformers work, their various types, and the diverse applications in which they are used.

What Are Current Transformers?

Current transformers are devices designed to measure high electrical currents by generating a lower, proportional current that can be easily and safely measured by standard instruments. They serve as a bridge between high-power circuits and sensitive electronic equipment, ensuring that monitoring and control systems can operate without being exposed to dangerous high currents.

How Do Current Transformers Work?

At their core, current transformers work on the principle of electromagnetic induction. When a current flows through the primary conductor (which can be as simple as a single wire passing through the CT’s core), it produces a magnetic field. This magnetic field induces a current in the CT’s secondary winding. The ratio of the number of turns in the primary winding (often just the conductor itself) to the number of turns in the secondary winding determines the scale factor, which in turn sets the output current relative to the primary current.

For example, a CT designed to provide a 1 A secondary output might have a ratio of 3000:1, meaning that when 3000 A flows in the primary circuit, the CT outputs 1 A. In many cases, the secondary output is standardized (such as 1 A, 5 A, or even a voltage output like 0.333 V) to simplify instrumentation and protection system designs.

Types of Current Transformers

There are several types of current transformers available, each suited to different applications:

Solid-Core (or Wound-Core) CTs:
These are built with a fixed, solid magnetic core. They are typically used in new installations where the CT can be designed and manufactured as an integral part of the system.
Split-Core (or Open-Core) CTs:
These CTs are designed with a hinge or split mechanism, allowing them to be clamped around an existing conductor without the need to disconnect it. This makes them ideal for retrofitting into operational systems and for applications where safety and ease of installation are paramount.
Bar-Type CTs:
In these models, the CT is integrated directly into busbars or conductors, offering robust performance in high-current scenarios.
Rogowski Coils:
Although technically not transformers in the traditional sense, Rogowski coils are used to measure alternating currents. They offer the advantage of being flexible and non-intrusive but require integrator circuits to produce a voltage output proportional to the current.

Key Applications of Current Transformers

Current transformers have a wide range of applications across various sectors:

1. Energy Metering

CTs are fundamental in electricity metering, allowing utilities and consumers to accurately measure energy consumption. By stepping down the high currents found in power distribution systems to measurable levels, CTs enable precise billing and energy monitoring.

2. Protection Systems

In protection relays and circuit breaker control, CTs provide crucial information about the current flowing through a circuit. This data helps in detecting abnormal conditions, such as overloads or short circuits, and initiates protective measures to isolate and protect equipment.

3. Control and Monitoring

Industrial automation and control systems use CTs to monitor current levels in motors, transformers, and other equipment. This monitoring helps in managing system performance and efficiency while ensuring that components operate within safe parameters.

4. Renewable Energy Systems

In renewable energy installations, such as wind farms and solar power plants, CTs are used to monitor the flow of electricity from generation to the grid. They ensure that systems operate efficiently and safely while providing data for performance optimization.

5. Safety and Instrumentation

CTs help in ensuring personnel safety by enabling the use of isolated measurement instruments. By providing a low-level secondary current, CTs allow for the safe operation of meters and sensors that would otherwise be at risk if connected directly to high-current circuits.

Used to characterize dynamic behavior of a transducer, step response time is the delay between the primary current reaching 90% of its final value and the transducer’s output reaching 90% of its final amplitude. The primary current shall behave as a current step, with a given di/dt slope (usually 100A/µs) and with the amplitude close to the nominal current value I_PN .

To avoid magnetization of a current transducer, the following measures can be taken:

1. Prevent Overload Currents:

Avoid High Overload Currents: Magnetization occurs when the transducer’s core is exposed to excessive currents that can saturate the magnetic circuit. To minimize this, ensure that the current transducer is not subjected to currents exceeding its rated capacity. Proper protective devices, such as fuses or circuit breakers, should be in place to limit high overload currents.

2. Ensure Continuous Power Supply:

Maintain Power Supply: Interruption or a lack of power supply to the transducer can result in an undesirable magnetic offset. To prevent this, ensure a reliable power supply and avoid unnecessary interruptions.

3. Use a Transducer with Lower Magnetic Remanence:

Select Materials with Lower Residual Flux: Some magnetic materials used in transducers have higher residual flux (magnetic remanence), leading to higher magnetic offsets. Choosing transducers made from materials with low remanence can reduce the likelihood of magnetization.

4. Demagnetization (Degaussing) Process:

Perform Periodic Demagnetization: If magnetization occurs, a degauss cycle can be used to restore the transducer’s core to its initial state. This involves applying a low-frequency AC current through the core, gradually decreasing the amplitude to remove residual magnetization. This process should be performed after any overload or interruption event.
Reversal of Primary Current: Another method of demagnetization is to apply a reverse current to the primary winding, effectively neutralizing the magnetic offset.

5. Regular Monitoring:

Monitor for Magnetic Saturation: Regularly checking the transducer’s performance can help in detecting any offsets early. If a magnetic offset is detected, performing demagnetization can restore accuracy and prevent long-term performance degradation.

By taking these precautions, magnetization of the current transducer can be minimized or avoided, ensuring accurate and stable performance over time.

Process of Demagnetization:

To demagnetize a transducer’s magnetic core, the following steps are typically used:

1.AC Excitation: A low-frequency AC source drives the magnetic core through its entire B-H hysteresis loop (the magnetic field vs. magnetization curve), applying alternating current to cycle the core’s magnetization from positive to negative saturation.

2.Gradual Decrease of Excitation: After cycling the core through the B-H loop for at least 5 full cycles at maximum amplitude, the amplitude of the AC excitation is reduced gradually. The rate of reduction should be no faster than 4% per cycle to ensure a smooth return of the core’s magnetization to zero (the origin of the B-H curve).

3.Time Requirements: Typically, a demagnetization cycle will require around 30 cycles at a low frequency, such as 500 milliseconds at 60 Hz.

Special Considerations for Closed-Loop Devices:

For closed-loop transducers (devices that have a compensation coil), additional care is needed because the compensation coil can interfere with the demagnetization process by negating its effects. This interference can prevent the core from reaching its neutral state. Therefore, extra steps may be required to ensure proper demagnetization.

Partial Demagnetization:

Alternatively, partial demagnetization can be achieved by applying a specific signal of opposite polarity to the core’s magnetization. However, this method is more complex because it requires precisely determining the correct amplitude and duration of the signal to effectively reduce the magnetization without overshooting or undercompensating.

In practice, if a specific application is well understood, it is possible to determine the necessary correction signal empirically and apply this whenever demagnetization is needed.

Demagnetization is essential for maintaining the accuracy and reliability of a transducer’s measurements, especially after strong magnetic influences or long-term use.

Let’s take a product with a supply voltage of +5V and an output of 2.5V ± 0.625V with a precision of 1% as an example.
Our Vref pin serves two functions: one is to act as a reference point for output, commonly used in differential input acquisition devices; the other function is for single-ended input acquisition devices, where it is used to control the sensor output, making the static output (2.5V) more accurate. It only affects the static output (2.5V when there is auxiliary power and no input signal) and does not affect the amplification (0.625V).
Due to the discreteness of product parameters, sensors cannot be perfectly identical and ideal, and there will always be some degree of error. For most of our products, the static reference point error is within ±15mV. This means the static output of 2.5V (with 0V or GND as the reference point) will be between 2.485V and 2.515V, which is considered within the acceptable range.
For example, for a product with a power supply of +5V and an output of 2.5V ± 0.625V with a precision of 1%:

If the output is referenced to 0V (GND), we calibrate it as follows:
First, check if the Vref voltage meets the requirements (standard is 2.5V ± 1%, generally no more than 0.5%).
The second step is to adjust the static output to between 2.485V and 2.515V (i.e., 2.5V ± 15mV).
The third step is to apply the input signal to the rated input and adjust the output to within 2.5V + (0.625V ± 1%).
Finally, check if the output at 10% of the rated input is accurate. If there is significant error, adjust the zero-point component for correction. Then, check if the zero-point and full-scale values meet the requirements. If everything is correct, the product moves on to the next process step.

If the output is referenced to Vref, we adjust it as follows:

First, check if the Vref voltage meets the requirements (standard is 2.5V ± 1%, generally no more than 0.5%).
The second step is to adjust the static output to between 2.485V and 2.515V (i.e., 2.5V ± 15mV).
The third step is to connect the black probe of the multimeter to the Vref point, apply the input signal to the rated input, and adjust the output to within 0.625V ± 1%.
Finally, check if the output at 10% of the rated input is accurate. If there is significant error, adjust the zero-point component for correction. Then, check if the zero-point and full-scale values meet the requirements. If everything is correct, the product moves on to the next process step.

If a product adjusted using the first method is used in a differential input acquisition device, the acquired signal will be inaccurate and may even produce negative output errors.
If a product adjusted using the second method is used in a single-ended input acquisition device, the acquired signal will also be incorrect and may result in overflow errors.

By default, our products are calibrated using the first method (referenced to 0V/GND), as most customers request this.

Example 1: A product calibrated using the first method, with Vref = 2.510V and Vout = 2.505V in the static state. This product is qualified, and the precision is acceptable. However, when used in a differential input acquisition device, since Vout is lower than Vref, the static output will be negative, which is incorrect.
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Example 2: A product calibrated using the second method, with Vref = 2.510V and Vout = 2.520V in the static state. This product is also qualified, as the zero-point error does not exceed 15mV. However, when used in a single-ended input acquisition device, the output of 2.520V exceeds the calibrated 15mV error, and the full-scale value will also be higher, which could reach up to 2% in extreme cases, affecting the customer’s use.
Now that some customers have reported this type of error, we are asking them to provide calibration requirements to correct the issue. Sales representatives are also being asked to label products based on different calibration methods to ensure correct production for customers.
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To calculate the input current of a transformer, you can use the following formula based on the transformer’s power rating, input voltage, and efficiency. The input current depends on the power demand on the secondary side (load), and this can be calculated for both single-phase and three-phase transformers.

1. Single-Phase Transformer Input Current Calculation

For a single-phase transformer, the input current $I_{\text{input}}$ can be calculated using the formula:

$I_{\text{input}} = \frac{P_{\text{out}}}{V_{\text{primary}} \times \eta}$

Where:

$I_{\text{input}}$ = Input Current (in Amperes)
$P_{\text{out}}$ = Output Power (in Watts or VA)
$V_{\text{primary}}$ = Primary Voltage (in Volts)
$\eta$ = Efficiency of the transformer (expressed as a decimal)

If the efficiency is not given, it can be assumed based on typical transformer efficiency (for instance, 0.98 for a high-efficiency transformer).

2. Three-Phase Transformer Input Current Calculation

For a three-phase transformer, the input current $I_{\text{input}}$ is calculated with a similar formula but with an additional factor for three-phase power:

$I_{\text{input}} = \frac{P_{\text{out}}}{\sqrt{3} \times V_{\text{primary}} \times \eta}$

Where:

$I_{\text{input}}$ = Input Current (in Amperes)
$P_{\text{out}}$ = Output Power (in Watts or VA)
$V_{\text{primary}}$ = Primary Line-to-Line Voltage (in Volts)
$\eta$
$\sqrt{3}$ = 1.732, a factor used for three-phase power

3. Example Calculation (Three-Phase Transformer)

Suppose you have a 500 kVA transformer, with a primary voltage of 11 kV (11,000 V) and an efficiency of 98%.

Step 1: Determine Output Power:

For a 500 kVA transformer, the power $P_{\text{out}}$ is 500,000 VA (since 1 kVA = 1,000 VA).

Step 2: Apply the Formula:

$I_{\text{input}} = \frac{500,000}{\sqrt{3} \times 11,000 \times 0.98}$

$I_{\text{input}} = \frac{500,000}{1.732 \times 11,000 \times 0.98}$

$I_{\text{input}} = \frac{500,000}{18,577.176} \approx 26.92 \, \text{A}$

So, the input current for the primary side of the transformer is approximately 26.92 A.

Important Notes:

The output power $P_{\text{out}}$ is usually given in kVA or VA. To convert from kW to VA, use the apparent power formula $P_{\text{out}} = \frac{\text{kW}}{\text{power factor}}$ if necessary.
Efficiency $\eta$
For three-phase systems, make sure you use the line-to-line primary voltage.

This calculation helps determine how much current is drawn on the primary side of the transformer based on the output load.

In a current transformer (CT), the burden refers to the total impedance (resistance and reactance) presented by the connected measuring devices, such as ammeters, relays, or meters, to the secondary winding of the CT. It is usually expressed in volt-amperes (VA) at a specific secondary current (typically 1A or 5A) and can include the impedance of the connecting wires and instruments.

Key Points:

Burden Components: The burden consists of the impedance of the connected devices (like meters, protection relays, and wires) which draw power from the CT’s secondary winding.
Burden Rating:
- CTs are rated for a maximum burden in VA. For example, a CT rated for 10 VA means that the connected load should not exceed 10 VA.
- If the burden exceeds the CT’s rated value, it can cause errors in current measurement or protection malfunctions because the CT may not be able to maintain its accuracy or properly step down the current.
Importance of Proper Burden:
- The burden affects the accuracy of the CT. If the burden is too high (i.e., the impedance of the connected devices is too large), the CT will not provide an accurate current transformation, leading to incorrect measurements or protection relay malfunctions.
- On the other hand, if the burden is too low, the CT may operate closer to saturation, which also leads to inaccuracies.
Practical Consideration:
- When installing a CT, it is crucial to calculate the burden of all connected devices and ensure it falls within the CT’s rated burden for proper performance.

In summary, the burden is the load imposed on the secondary winding of the current transformer, and ensuring it is within the CT’s rated capacity is essential for accurate current measurement and safe operation.

1. Single-Phase Circuit:

Connection Mode: A single current transformer is connected in series with the live wire or neutral wire of the circuit.
Application: This is used for measuring single-phase current, power, and providing protection against single-phase faults, such as ground faults or short circuits.
Diagram:

2. Incomplete Star (V Connection) for Two Current Transformers (Three-Phase Circuit):

Connection Mode: Two current transformers are connected in a “V” configuration (incomplete star), where the primary windings are connected to two of the three-phase conductors. The secondary windings are then connected together to form a measuring point.
Application: This connection is used to measure three-phase current, power, and to protect the system from phase-to-phase short-circuit faults. Although only two transformers are used, it provides sufficient information about the three-phase system for protection and measurement in a balanced system.
Diagram:

3. Two-Phase Current Difference Between Two Current Transformers (Phase Difference Protection):

Connection Mode: Two current transformers are placed in two different phases (e.g., Phase A and Phase B), and their secondary windings are connected to each other to detect any current difference between these phases.
Application: This configuration is used for measuring one-phase current and for protecting against phase-to-phase short-circuit faults. It is based on detecting differences in the phase currents, which indicates a fault between phases.
Diagram:

4. Star (Wye) Connection Mode for Three Current Transformers (Three-Phase Four-Wire System):

Connection Mode: Three current transformers are connected in a star (wye) configuration. Each CT is placed on one phase of a three-phase system, and the secondary windings are connected to a common point (neutral).
Application: This setup is suitable for measuring three-phase current, power, and protecting the system in a three-phase four-wire system. It can detect any form of fault, including ground faults, phase-to-phase faults, and phase-to-neutral faults.
Diagram:

A Residual Current Sensor (RCS) is an electrical safety device designed to detect residual current (also known as leakage current) in a circuit. Residual current occurs when there is an imbalance between the current flowing into a system (through the live wire) and the current returning from the system (through the neutral wire). This imbalance indicates that some current is leaking to the ground, often due to insulation failure, faulty wiring, or a person coming into contact with the electrical system.

How Does a Residual Current Sensor Work?

Current Imbalance Detection: The RCS measures the difference between the current in the live wire and the neutral wire. Under normal operating conditions, the current flowing through these two wires should be equal. If there is a discrepancy, it means current is leaking somewhere.
Detection Threshold: If the difference between the live and neutral current exceeds a certain threshold (usually set to a few milliamps, such as 10 mA or 30 mA, depending on the application), the sensor detects it as residual current.
Activation of Safety Mechanism: When residual current is detected, the sensor typically signals a protective device, such as a Residual Current Device (RCD) or Ground Fault Circuit Interrupter (GFCI). These devices will quickly disconnect the power to prevent hazards like electric shock or fire.

Key Features of a Residual Current Sensor:

High Sensitivity: RCS devices can detect very small current differences, often in the range of milliamps (mA).
Non-Intrusive: The sensor can monitor current flow without interrupting the circuit.
Rapid Response: It triggers safety mechanisms immediately upon detecting leakage, minimizing risks.
Accuracy: Modern RCS units provide precise detection and monitoring of residual current to avoid nuisance tripping.

Applications:

Household Electrical Safety: RCS is widely used in household RCDs and GFCIs to protect users from electric shocks, particularly in wet areas like kitchens and bathrooms.
Industrial Systems: In industrial environments, RCS devices monitor equipment to detect insulation failures or potential fault conditions that could lead to electrical hazards.
Renewable Energy: RCS is also employed in renewable energy systems, such as solar power installations, to ensure safe operation and prevent electrical leakage to ground.

Benefits:

Prevention of Electric Shock: RCS can detect when current flows through a person or faulty wiring to the ground, preventing serious injury.
Fire Prevention: By detecting leakage currents caused by insulation failure, the RCS can help prevent electrical fires.
Compliance with Safety Standards: Residual current sensors are essential for meeting electrical safety regulations in homes, offices, and industrial settings.

In summary, a Residual Current Sensor is a critical safety device that continuously monitors circuits for leakage current, helping to protect people and property from electrical hazards by detecting imbalances in current flow and triggering protective mechanisms.

Rogowski coil also called differential current sensor, it is a “hollow” circular coil, arranged around the conductor, so that an alternating magnetic field generated by the current induces a voltage in the coil. The coil is actually a ransformer coupled to the conductor under test, and the voltage directly output from the coil is proportional to the rate of change of current, e.g. @50Hz/1kA Vout=85mV, @60Hz/1kA Vout=85*60/50=102mV. If you want to obtain the current waveform or current value that doesn’t matter to frequency also need to add the integral circuit to achieve 90° phase shift compensation and frequency equalization.

RF series is a flexible current sensor based on Rogowski coil principle, which is in small size, light weight and easy to install and offers a choice of different sizes, which can also be customized design of the customer requires a special order. It has no magnetic saturation and a shielding layer to resist the influence of external magnetic field, so stable measurements can be achieved in the range of low current to hundreds of kA. The flexible rogowski coil is an extremely comfortable solution for current measurement,
particularly suited to current monitoring and electrical retrofitting, can be used in many cases where a traditional current transformer (CT) is not available, or can replace it.Systems that use an ADC chips(ADS131M04) or a power metering ICs(ADE7753) that support the Rogowski coil principle are even more advantageous.

Leakage Current Sensors

Current Sensors

Voltage Sensors

Current Transformers

Rogowski Coil And Integrator

Voltage Transformers

High Frequency Transformers

Power Transformers

Common Mode Chokes