Rogowski coils are specialized devices for measuring alternating current (AC) or high-speed current pulses, offering unique advantages for various applications. They are particularly useful in scenarios where flexibility, accuracy, and safety are crucial.
Rogowski coils are flexible and can be wrapped around live conductors without disconnection, making them perfect for tight spaces or crowded electrical panels. This non-invasive installation reduces downtime and simplifies setup, especially in industrial or commercial settings.
Unlike traditional current transformers, Rogowski coils use an air core, preventing magnetic saturation. This allows them to accurately measure very high currents, such as those in power transmission or welding, and handle rapid changes, like transients or pulses, making them suitable for high-frequency applications.
They are safer because they don’t produce high voltages when open-circuited, reducing electric shock risks. Additionally, their lightweight and simple design lowers construction and installation costs, offering a cost-effective solution for current measurement.
A Rogowski coil is a specialized electrical device designed for measuring alternating current (AC). It consists of a helical coil of wire wound uniformly around a non-magnetic, air-filled core, typically arranged in a toroidal (doughnut-like) shape. The coil is placed around a current-carrying conductor, and as the AC flows through the conductor, it generates a changing magnetic field. This changing magnetic field induces a voltage in the Rogowski coil, which is proportional to the rate of change of the current (the time derivative, di/dt).
To obtain a direct measurement of the current, the induced voltage is typically processed through an integrator circuit, which converts the voltage signal into one proportional to the current itself. This makes the Rogowski coil particularly useful for applications requiring the measurement of high currents or fast transient currents, such as in power systems, industrial equipment, or research settings.
Air Core Design: Unlike traditional current transformers with iron cores, the Rogowski coil’s air core prevents magnetic saturation, allowing it to measure very high currents without distortion.
Flexibility: Many Rogowski coils are designed to be flexible and can be opened to wrap around a conductor without needing to disconnect it, making installation straightforward and non-invasive.
Wide Bandwidth: The coil can accurately measure currents with high-frequency components or rapid changes, such as those in lightning strikes or switching transients.
Power system monitoring and protection.
Measuring high currents in welding or industrial processes.
Research applications, such as analyzing transient phenomena.
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.
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.
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.
There are several types of current transformers available, each suited to different applications:
Current transformers have a wide range of applications across various sectors:
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.
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.
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.
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.
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 IPN .
To avoid magnetization of a current transducer, the following measures can be taken:
By taking these precautions, magnetization of the current transducer can be minimized or avoided, ensuring accurate and stable performance over time.
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.
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.
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.
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.
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.
For a single-phase transformer, the input current can be calculated using the formula:
Where:
If the efficiency is not given, it can be assumed based on typical transformer efficiency (for instance, 0.98 for a high-efficiency transformer).
For a three-phase transformer, the input current is calculated with a similar formula but with an additional factor for three-phase power:
Where:
Suppose you have a 500 kVA transformer, with a primary voltage of 11 kV (11,000 V) and an efficiency of 98%.
For a 500 kVA transformer, the power is 500,000 VA (since 1 kVA = 1,000 VA).
So, the input current for the primary side of the transformer is approximately 26.92 A.
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.
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.
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.
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.
In essence, CTs are used for current measurement and VTs for voltage measurement, both essential for protection, control, and monitoring in electrical power systems.
The secondary of a current transformer (CT) is always short-circuited (or connected to a low-impedance load like a meter or relay) for several important reasons related to safety and proper functioning:
To connect a CT sensor to an Arduino, the output signal from the CT sensor needs to be conditioned so it meets the input requirements of the Arduino analog inputs, i.e. a positive voltage between 0V and the ADC reference voltage.
This give the example of an Arduino board working at 5 V and of the EmonTx working at 3.3 V. Make sure you use the right supply voltage and bias voltage in your calculations that correspond to your setup.
The circuit consists of two main parts, their functions are to change the c.t’s current into a voltage of the correct amplitude, and position this voltage in the centre of the ADC’s input range.
The voltages and currents shown are for a 5 V Arduino, with a 0 – 5 V range for the analogue input, about 1.6 V rms for a sine wave. For the emonTx V2 & V3 and the emonPi, the analogue input range is 0 – 3.3 V, so the midpoint voltage is 1.65 V and the analogue input voltage swings between 0 and 3.3 V (approximately 1 V rms for a sine wave). For the emonTx4 and emonPi2, the analogue input range is 0 – 1 V and is intended for use with 0.333 V rms output current transformers, which do not need a burden. so this resistor is omitted.
The secondary of a current transformer (CT) should never be left open when current is flowing through the primary because doing so can create dangerous high voltages. Here’s why:
Take the HSTS016L model as an example, it is capable of measuring current values from 10A to 200A. Using the split core current sensor type, we can turn the CT on and off without changing the existing system, and just use a clip to put the measuring cable inside the CT. The output voltage of this sensor is 2.5V +/- 0.625V, which is a good accuracy. Although the voltage output range of this sensor is between 1.875V and 3.125V, it is also suitable for 3.3V analog sensors, especially Arduino Nano and NodeMCU microcontrollers.
The sensor has 4 output pins: RED (5V input), BLACK (0V Gnd), YELLOW (Analog Output), and WHITE (Analog for Calibration). Sometimes the supply voltage may not be exact 5V thus we will need additional Analog Pin to measure the exact middle point from the White Pin. For Arduino UNO, there are 6 analog input pins (A0-A5) where we can use one of the pins to measure AC current and 1 more to use for calibration purpose. The analog input pins will map input voltages between 0 and 5V into integer values between 0 and 1023 with resolution of 4.9mV per unit (5.00V / 1023 units).
We connect RED to 5V input, BLACK to ground, YELLOW to analog pin A1, and WHITE to analog pin A2. It is highly recommended to use a 12V power adapter to power the Arduino Uno and the sensor. Using a 5V power supply over USB will have an initial offset value, you may need to manually add the offset value when uploading the code.
A transformer does not work with direct current (DC) because its operation relies on changing magnetic fields, which DC does not provide. Here’s a detailed explanation of why transformers are ineffective with DC:
A current transformer (CT) is a vital electrical device used in high-power systems for the accurate measurement of alternating current (AC). It operates on the principle of electromagnetic induction, enabling the safe reduction of high current levels to a manageable value for measurement or protection. Below is a detailed explanation of its construction and working principles:
A current transformer consists of two main windings: the primary winding and the secondary winding.
The turns ratio between the primary and secondary windings is a crucial parameter and is usually very high, often in the range of 100:1 to 5000:1. This means the secondary winding has many more turns compared to the primary winding. This ratio determines the magnitude of the current transformation and helps scale down the high current to a level suitable for standard measuring devices.
When an alternating current flows through the primary winding, it generates a corresponding alternating magnetic field around the conductor. According to Faraday’s Law of Electromagnetic Induction, this changing magnetic flux induces a proportional voltage in the secondary winding. The magnitude of the induced voltage and the resulting current in the secondary is dependent on the turns ratio.
The CT effectively steps down the high current from the primary side to a much lower current on the secondary side. While the turns ratio governs the reduction, the CT ensures that the secondary current is a scaled, proportional version of the primary current. For instance, a current transformer with a turns ratio of 1000:1 will produce 1 ampere in the secondary circuit when 1000 amperes flow in the primary conductor. This step-down effect enables standard measuring instruments, which are designed for low current ranges, to monitor the current accurately.
The burden refers to the load connected to the CT’s secondary winding, which can be an ammeter, protective relay, or another measuring device. The impedance of this burden significantly affects the CT’s accuracy and performance. For accurate measurement, the burden impedance must match the CT’s specified range. If the burden is too high, the accuracy may be compromised, and in extreme cases, it may even damage the CT.
The current ratio of a CT is defined as the ratio of the primary current to the secondary current and is determined by the turns ratio. It is a fixed value for any given CT. For example, in a CT with a 1000:1 turns ratio, 1000 amperes in the primary circuit will produce 1 ampere in the secondary circuit. This consistent current ratio allows for the easy conversion of high primary currents into lower secondary currents for safe monitoring and protection.
Current transformers are categorized into various accuracy classes based on their precision under specified conditions. The accuracy class defines the maximum permissible error in the measurement. The common accuracy classes are 0.1, 0.2, 0.5, 1, and 3, with class 0.1 offering the highest accuracy. CTs with higher accuracy are typically used in metering applications, whereas those with lower accuracy may be sufficient for protection purposes.
The current transformer operates by stepping down high AC currents in a power system to a safer and manageable level, using the principle of electromagnetic induction and a high turns ratio. The CT’s secondary current is proportional to the primary current, which enables accurate measurement and protection of electrical systems. Proper matching of the CT’s burden and adherence to its accuracy class ensures reliable performance in various applications.
By following these steps, you’ll be able to accurately measure DC current with the help of a Hall effect sensor.
By following these steps and ensuring proper calibration, you will be able to accurately measure AC current using a Hall effect sensor.
Connecting a current sensor with a NodeMCU involves a few steps. Here’s a general overview:
1. Choose a current sensor: There are many types of current sensors available, such as Hall effect sensors or shunt resistors. Select a sensor based on your requirements and available resources.
2. Identify the sensor pins: Look for the pinout diagram or datasheet of your selected current sensor and identify the pins for power supply, ground, and signal output.
3. Connect power supply and ground: Connect the power supply pin of the sensor to the 3.3V pin of the NodeMCU, and connect the ground pin of the sensor to the GND pin of the NodeMCU.
4. Connect signal output: Connect the signal output pin of the sensor to any available analog input pin of the NodeMCU, such as A0.
5. Code the NodeMCU: Write code to read the analog input from the current sensor and process the data. For example, you can use the analogRead() function to read the voltage at the analog input pin and convert it to current using the sensor’s sensitivity. Then, you can use the WiFi capabilities of the NodeMCU to send the data to a cloud service or to a local server.
Here’s an example code snippet that reads the voltage at pin A0 and converts it to current using a Hall effect sensor with a sensitivity of 100 mV/A:
Note: This is just an example code snippet and may need to be modified based on your specific requirements and sensor characteristics.
A current transformer (CT) operates on the principle of electromagnetic induction, similar to a standard transformer. Its primary purpose is to step down high currents from a power system to a lower, manageable value for metering, protection, and control devices. Here’s a breakdown of the working principle of a current transformer:
The CT works based on Faraday’s Law of Electromagnetic Induction, which states that a changing magnetic field within a conductor induces a voltage in another conductor near it.
CTs are designed to operate with high accuracy, so the secondary current is directly proportional to the primary current within a specified accuracy class (e.g., 1% or 0.5% error). Accuracy is critical in both metering and protection applications.
This allows the safe and accurate measurement of high currents in power systems, protecting both personnel and equipment.
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