In power systems, precise energy metering is the cornerstone for fair transactions, energy conservation, and stable system operation. As a critical component in energy monitoring systems, the accuracy of current transformers (CTs) directly determines the precision of final readings. However, beyond the well-known “ratio error” (amplitude error), a frequently overlooked yet crucial parameter—phase angle error—can have a critical impact on energy measurement accuracy under specific conditions.
To understand phase angle error, we must first recognize the gap between ideal theory and practical reality.
Ideal scenario: An ideal current transformer should output a secondary current signal that is exactly opposite in phase to the primary (main circuit) current signal, maintaining a perfect 180° phase difference.
Reality: In actual transformers, the secondary current phase does not precisely oppose the primary current by 180°, but instead exhibits a slight angular deviation. The angle between this actual secondary current vector and the ideal opposing primary current vector is defined as the phase angle error.
According to international standards, when the secondary current vector leads the ideal position in phase, the phase angle error is positive (+); conversely, when it lags, the phase angle error is negative (-). This seemingly insignificant angle is crucial for high-precision energy metering.
The root cause of phase angle error lies in the inherent characteristics of the transformer’s physical structure and electromagnetic principles, specifically the presence of the excitation current.
Current transformers operate similarly to transformers: primary current flowing through the core generates a magnetic field, which in turn induces a secondary current. To establish this necessary magnetic field in the core, the transformer itself consumes a portion of energy. The current corresponding to this energy consumption is the exciting current.
Magnetizing Component: Used to establish the magnetic field in the core, its phase lags behind the induced electromotive force by 90°.
Iron loss component: Compensates for energy losses in the core due to hysteresis and eddy currents under alternating magnetic fields. Its phase is in phase with the induced electromotive force.
Since the primary current I₁ equals the vector sum of the secondary current I₂ (converted to the primary side) and the excitation current I₀ (I₁ = I₂ + I₀), the presence of the non-coincident excitation current I₀ inevitably creates a phase difference between I₁ and I₂. This phase difference ultimately manifests as the angle difference in the mutual inductor.
Simply put, the angular difference is essentially a “disturbance” in the phase relationship of currents caused by the excitation current. All factors affecting the magnitude and phase of the excitation current—such as the magnetic properties of the core material, the load (burden) connected to the secondary side, and the magnitude of the primary current itself—directly influence the angular difference.
3. How Does the Angular Difference Affect the Accuracy of Electrical Energy Monitoring?
The formula for calculating electrical energy (active power) is:
P = U × I × cos(φ), where φ is the phase angle between voltage and current, and cos(φ) represents the power factor.
Phase angle error in the transformer directly alters the phase angle φ measured by the energy meter, leading to errors in power calculation.
Error propagation relationship: The total error in energy measurement is approximately equal to the sum of the voltage ratio error, current ratio error, and phase angle error. A simplified formula can be expressed as:
Power measurement error ≈ Voltage ratio error + Current ratio error + tan(φ) × Δφ
Where Δφ represents the phase angle error introduced by the instrument transformers.
Key Influencing Factor: Power Factor
High power factor (cos(φ) ≈ 1, φ ≈ 0°): tan(φ) is very small, and the angular error Δφ contributes minimally to the total error. Measurement error is primarily determined by the “ratio error” (amplitude error).
Low power factor (cos(φ) → 0, φ → ±90°): tan(φ) becomes extremely large. Even a small phase angle error Δφ, amplified by tan(φ), can cause significant power measurement errors.
For example: In a circuit with an extremely low power factor, even if the ratio error (amplitude error) of the instrument transformer is very small, the final calculated energy reading may still deviate significantly from the true value if any phase angle error exists.
4. Manifestations of Phase Angle Error Impact
Inductive loads (e.g., motors, transformers with lagging power factor): A positive phase error typically causes the meter reading to exceed actual energy consumption, resulting in unnecessary electricity charges for the user.
Capacitive loads (e.g., capacitor banks, excessively long cables, leading power factor): A positive phase angle error causes the meter reading to understate actual energy consumption, resulting in losses for the power supplier.
In modern industrial and commercial settings, the widespread use of variable frequency drives, motors, LED lighting, and switching power supplies has made grid power factor increasingly complex and often suboptimal. Against this backdrop, the phase angle difference of current transformers has evolved from a secondary parameter into a critical factor determining whether an energy monitoring system can operate with “accuracy and fairness.”
Electromagnetic compatibility (EMC) is a critical design consideration in modern electronic equipment. Electromagnetic interference (EMI) not only affects the normal operation of equipment, but can also violate regulatory requirements. To effectively suppress these disturbances, engineers use a wide range of filtering components, with inductors playing a central role. Among the many types of inductors, Common Mode Chokes and Differential Mode Inductors are two of the most commonly used and have very different functions.
Although both are used for EMI filtering, the types of disturbances they target, their structures and operating principles are very different. In this article, we will discuss the essential differences between common mode choke and differential mode inductors, their application scenarios, and provide practical selection points to help engineers make the right choice in circuit design.
A common mode inductor, also known as a common mode choke, is a filtering element primarily used to suppress common mode interference.
Common Mode Noise (CMN) is a noise signal with the same direction and size on two signal lines (e.g., power lines L and N, or data lines D+ and D-) relative to the ground or reference plane. This interference is usually caused by external EMF coupling or unbalanced circuitry within the device and is radiated outward through the cable or introduced into the device internally through the cable.
The typical construction of a common mode inductor is that two or more sets of coils are wound on the same high permeability ferrite core with the same number of turns and orientation.
The operating principle is based on magnetic field cancellation and high impedance effects:
For differential mode signals (useful signals): When normal differential mode currents (i.e., signals of opposite directions and equal sizes on two lines) flow through two windings, the magnetic fluxes they generate in the cores are in opposite directions and cancel each other out. Therefore, the impedance of a common mode inductor to a differential mode signal is extremely low and hardly affects the transmission of useful signals.
For common-mode signals (noise): When common-mode currents (in the same direction) flow through two windings, they generate magnetic flux in the core in the same direction, superimposed on each other. This causes the core to exhibit high inductance, which creates an extremely high impedance to common-mode noise and achieves the purpose of suppressing and attenuating common-mode interference.
1.2 Structural Characteristics
Common mode choke usually have four pins (two inlets and two outlets) and are composed of two windings with the homonymous ends of the windings located on the same side of the magnetic ring.
Differential Mode Inductor
2.1 Definition and Working Principle
Differential Mode Inductor is a traditional inductive component mainly used to suppress differential mode interference.
Differential Mode Noise (DMN) is a noise signal with opposite direction and unequal size between two signal lines. This kind of interference is directly superimposed on the useful signal and is usually caused by internal factors such as the switching action of the switching power supply, load changes or signal line impedance mismatch.
The structure of a differential mode inductor is similar to that of an ordinary inductor, which usually has only one winding, wound on a magnetic core.
Its working principle is based on the self-inductive effect of the inductor:
For differential mode signals (useful signals and noise): A differential mode inductor impedes the change of current flowing through itself. Whether it is a normal differential mode signal or differential mode noise, as long as the current flows through the inductor, it will be affected by its inductance.
Filtering effect: differential mode inductor through its high frequency impedance characteristics, the high frequency of the differential mode noise attenuation, while the low frequency of the useful signals (such as power supply DC or low-frequency AC component) to maintain a low impedance, so as to achieve filtering.
2.2 Structural Characteristics
Differential mode inductors usually have only two pins and only one winding. In EMI filtering circuits, it is usually connected in series on the power or signal line.
Core Differences Comparison
The core distinction between common-mode chokes and differential-mode chokes lies in the types of noise they address, their structural design, and their impact on useful signals. The table below summarizes the key differences between the two:
Common-mode interference (noise in the same direction relative to ground)
Common-mode interference (noise between signal lines)
Working Principle
Flux superposition produces high impedance (for common mode), while flux cancellation produces low impedance (for differential mode).
Self-inductance creates high impedance (to all high-frequency currents)
Structure
Two windings, wound in the same direction on the same magnetic core
A winding, wound around the magnetic core
Number of pins
4 (two in, two out)
2 (one in, one out)
For useful signals
Extremely low impedance with virtually no attenuation
There is a certain amount of impedance, which may cause slight attenuation of the useful signal.
Application Location
Typically used at the input/output ends of power cables or high-speed signal lines to filter out radiated interference.
Typically used in series with power cables or signal lines to filter out conducted interference.
Application Scenarios
4.1 Application Scenarios for Common-Mode Chokes
Due to their excellent suppression of common-mode noise without affecting differential-mode signal transmission, common-mode chokes are widely used in the following scenarios:
Switching Power Supplies (SMPS): At the input stage of AC/DC or DC/DC converters, they suppress common-mode noise generated by switching operations, preventing its conduction and radiation through power lines.
Data Communication Interfaces: High-speed differential signal lines such as USB, HDMI, and Ethernet (RJ45 connectors). Common-mode chokes effectively filter externally coupled common-mode noise while preserving differential signal integrity.
Automotive Electronics: Employed in vehicle networks (e.g., CAN, LIN) and power lines to withstand complex electromagnetic environments.
4.2 Differential Mode Choke Applications
Differential mode chokes primarily suppress differential mode noise on power lines, often paired with capacitors to form π-type or L-type filters:
Power Line Filtering: At power inputs, combined with X capacitors to create differential mode filter circuits that attenuate internally generated differential mode conducted interference.
Low-frequency applications: Suitable for low-frequency power filtering or signal processing circuits where high-frequency signal attenuation is not critical.
DC/DC converter outputs: Used to smooth output current and suppress the differential mode component in switching ripple.
Selection Criteria
Proper selection is critical to ensuring effective EMI filtering. When choosing common-mode and differential-mode inductors, the following core parameters must be considered:
Rated Current (I_rated): The choke must withstand the circuit’s maximum continuous operating current.
Common Mode Impedance (Z_CM): This is the most critical parameter. Select chokes exhibiting maximum impedance at the target noise frequency (typically tens to hundreds of MHz). Higher impedance yields better suppression.
Differential Mode Impedance (Z_DM): Select inductors with low differential mode impedance to minimize attenuation of useful signals.
DC Resistance (DCR): Lower DCR is preferable to reduce power loss and temperature rise.
Operating Frequency Range: Select appropriate core material and winding structure based on the circuit’s operating frequency and noise frequency.
5.2 Key Considerations for Differential Mode Inductor Selection
Inductance (L): Determine the inductance value based on the required cutoff frequency and filtering characteristics. Higher inductance provides better attenuation of low-frequency noise.
Rated Current (I_rated): Similar to common-mode inductors, it must meet the circuit’s maximum operating current requirement.
Saturation Current (I_sat): When current increases beyond a certain threshold, the core saturates, causing inductance to drop sharply. During selection, the saturation current must exceed the peak current in the circuit to ensure filtering performance.
DC Resistance (DCR): Similarly, DCR should be as low as possible.
Conclusion
Common-mode ckoke and differential-mode inductors are the two indispensable forces in EMI filter circuits. Common-mode inductors, with their unique dual-winding structure, efficiently suppress common-mode interference while protecting the integrity of useful signals. They are the preferred choice for high-speed signal and power line radiation suppression. The differential mode inductor, through the self-inductance effect, directly attenuates the differential mode interference and is the cornerstone of power conduction filtering.
Understanding the fundamental differences among them: different suppression objects, different structures, and different working principles, is the basis for correct circuit design and component selection. Only by reasonably combining the specific types of interference and circuit requirements can an electronic system that meets strict EMC standards be constructed.
In power systems, measuring devices such as electricity meters typically need to be connected to high-voltage or high-current circuits via current transformers (CTs). A common standard is that the interface parameters of electricity meters and the rated secondary current of their matching CTs are almost always 5A (or 1A). This article will delve into the reasons behind this standard and, in conjunction with common parameter markings on electricity meters, explain how to correctly select the matching current transformer.
The current transformer parameters supported by the meter are usually marked on the meter, such as 0.25-5(6)A, 0.015-1.5(6)A, 0.25-5(80)A, 0.25-5(100)A, etc. These parameters identify the operating range and technical requirements when connecting to an external CT. These parameters identify the operating range and technical requirements of the meter when connected to an external CT: Take 0.015-1.5(6)A as an example: * Starting current (0.015): the minimum current of 15mA that the meter can accurately measure. * Basic current Ib(1.5):The rated current value of the energy meter is 1.5A, and the accuracy level of the energy meter can be guaranteed when it works under this current. * Maximum current Imax(6): the maximum current at which the meter can be carried safely and continuously without damage, 6 A. This is covered in the IEC/EN 62053 specification for electricity meters.
Two core principles must be followed in selecting a matching current transformer: secondary current matching and primary current sizing. a. Matching of rated secondary current (key) The rated secondary current of the CT must match the basic current Ib of the energy meter. – For meters with parameters 0.25-5(6)A, 0.25-5(80)A, 0.25-5(100)A, you need a current transformer with a 5A rated secondary current. – For energy meters with parameters 0.015-1.5(6)A (used in specific scenarios although the base current is not 5A/1A standard), you need to equip a CT with a rated secondary current of 1.5A (these are less common and usually follow the 5A or 1A standard). b. Selection of Rated Primary Current and Variable Ratio The rated primary current of the CT (which determines the variable ratio, e.g., 100/5A, 500/5A, 100/1A) should be selected in accordance with the load current of the actual primary circuit. The selection principle is to make the actual load current of normal operation close to the rated primary current of the CT. According to IEC/EN 61869 standard, 1.2 times of the rated primary current is also in line with the nominal accuracy of the product, so the best measurement accuracy can be ensured. Therefore, for energy meters with parameters of 0.015-1.5(6)A, the best match is a current transformer (CT) with a secondary rated output of 1A. If you are interested in the measurement of small currents and do not have high requirements for measurement accuracy and range, you can also choose a current transformer (CT) with a secondary rated output of 100mA.
3. Why has 5A (or 1A) become the industry standard?
Returning to the question at the heart of the article, why has the industry generally adopted 5A or 1A as the standard secondary current for CTs? The reason is a combination of performance, standardization, and safety: Performance Requirements and Historical Reasons Early electromechanical energy meters and protective relays required a certain amount of physical energy (power) to actuate the mechanical components. 5A current provided a strong enough signal and power to ensure reliable and accurate operation of these devices. Current signals in the milliamp range are too weak, susceptible to interference and unable to drive older devices. Standardization and Global Interchangeability 5A is established as the global standard for the power industry, ensuring that devices from different manufacturers and countries are easily compatible and interchangeable, greatly simplifying system design, manufacturing and maintenance. Balancing Safety and Economy The core safety objective of the CT is to isolate the high voltage high current circuits from the low voltage measurement circuits. 5A’s relatively low secondary current reduces operational risk. At the same time, the 5A strikes a good balance between cabling costs, wire cross-section requirements and power loss. Where long wiring distances are required, the 1A standard is used to further reduce line losses (1A losses are only 4% of 5A). In summary, the selection of 5A (or 1A) as the standard secondary current for CTs is the result of engineering practices that optimize performance, cost, historical compatibility and operational reliability.
The Rogowski Coils (RC) and Current Transformers (CTs) are two core technologies for measuring alternating current, both of which operate based on the principle of electromagnetic induction. However, there are fundamental structural differences between the two the design with or without a magnetic core leads to significant differences in their performance, application scenarios and safety. CT adopts a core structure and performs exceptionally well in high-precision, low-frequency and steady-state measurements. RC adopts a coreless design, featuring greater flexibility, a wider dynamic range and anti-saturation characteristics, making it an ideal choice for high-current, high-frequency and transient applications.
I. Fundamental Differences: Principle and Structure
The core design is the most critical differentiator between the two technologies.
Requires an external integrator circuit to convert the derivative voltage signal back into a signal proportional to the current.
Output current is typically measured directly or converted to voltage via a burden resistor.
II. Performance and Safety Comparison
The coreless design of the Rogowski coil provides distinct advantages in dynamic range and safety, while the iron core of the CT offers inherent simplicity and high accuracy at rated conditions.
– DC current measurement (RCs only measure AC/rate of change).
– High-Frequency and Transient analysis (e.g., power electronics, motor inrush, fault currents).
– Applications where the cost/complexity of the integrator is prohibitive.
– Retrofitting or installations with irregular conductors or limited space.
– Standard utility metering where high-accuracy, low-frequency measurement is the sole requirement.
Current Transformer
– Standard Utility Metering and revenue applications (high accuracy at 50/60 Hz).
– High-current measurement where saturation is a risk.
– Relay Protection and industrial monitoring at power-line frequencies.
– High-frequency or transient current analysis.
– Applications where simplicity and low cost are prioritized.
– Installations with irregular busbars or tight space constraints.
Conclusion
Rogowski coils and Current Transformers are complementary technologies. The Current Transformer remains the workhorse for traditional, high-accuracy, steady-state power monitoring and protection at power-line frequencies. The Rogowski Coil, with its coreless design, offers a modern, flexible, and safer solution that excels in demanding environments involving high currents, wide dynamic ranges, and high-frequency transients, particularly in power quality analysis and power electronics testing.
In industrial measurement and control systems, current transmitters and voltage transmitters convert analog signals into standardized electrical signals for long-distance transmission to control rooms for monitoring, recording, and control. The uniformity of these standard signals is critical for ensuring interoperability between different devices and system stability. The International Electrotechnical Commission (IEC) has established 4-20mA DC and 1-5V DC (via resistive conversion) as the analog signal standards for process control systems.
1. Advantages of Current Signals
Current signals offer significant advantages over voltage signals during long-distance transmission. Industrial environments often experience severe electromagnetic interference, which easily distorts voltage signals. Additionally, voltage signals suffer voltage drops due to wire resistance during extended transmission, leading to signal attenuation and measurement errors.
The advantages of current signals are primarily reflected in the following aspects:
• Strong Interference Resistance: Noise voltages in industrial environments can reach several volts, but noise power is weak, typically generating noise currents below the nA level. Current signals are less affected by line resistance, resist attenuation, and ensure reliable data transmission in harsh electrical noise environments. • Long transmission distance: Current sources exhibit infinite internal resistance, meaning wire resistance in series within the loop does not affect accuracy. As long as the transmission loop remains unbranched, the current through the loop remains constant regardless of wire length. This allows transmission over standard twisted-pair cables for hundreds of meters or more, ensuring transmission precision. • Reduced cabling: Two-wire 4-20mA signal transmission simultaneously provides both power and signal, reducing cable usage and thereby lowering wiring costs and complexity.
2. Why the 4-20mA DC Signal
The 4-20mA DC signal is the most widely adopted standard analog signal in industrial applications. Its selection is not coincidental but based on multifaceted technical and safety considerations.
2.1 Reason for Selecting 4mA as the Starting Point (“Live Zero”)
The choice of 4mA as the starting point for the 4-20mA signal, rather than 0mA, is one of its most critical features, known as the “Live Zero.” • Fault Diagnosis: When the signal reads 0mA, it clearly indicates a line break (open circuit) or device failure, rather than a zero measurement value. This makes fault diagnosis straightforward and intuitive, avoiding confusion between a zero signal and a broken wire fault. • Sensor Powering: For two-wire transmitters, the 4mA current provides the minimum operating current required for sensors, ensuring proper function. This eliminates the need for additional power wires, simplifying wiring.
2.2 Reasons for Selecting 20mA as the Endpoint
The choice of 20mA as the full-scale signal value is also based on safety and practicality considerations.
• Safety: The spark energy generated by the 20mA current flow is insufficient to ignite gas, meeting intrinsic safety explosion-proof requirements—critical in flammable and explosive industrial environments. • Power Consumption and Cost: While ensuring signal transmission quality and response speed, 20mA represents a relatively low current value, helping reduce power consumption and equipment costs.
Conclusion
In summary, the 4-20mA DC signal has become the most widely adopted standard in industrial automation and process control due to its exceptional interference resistance, long-distance transmission capability, unique “live zero” fault diagnosis function, and inherent safety. It reliably transmits measurement data while effectively reducing wiring costs and enhancing system reliability.
The HAS series current sensors indicate the magnitude and direction of the measured current through their voltage output signals. Its core feature is that the output signal takes 2.5V as the center point and is offset according to the direction and magnitude of the current on this basis. This design enables the sensor to measure bidirectional current.
For the HAS14Z current sensor, its output voltage range is 2.5V ± 0.625V. This means:
•Zero current (0A): When no current passes through the sensor, the output voltage is 2.5V. • Forward current: When the current is forward, the output voltage will increase from 2.5V to a maximum of 2.5V + 0.625V = 3.125V. • Reverse current: When the current is in reverse, the output voltage will decrease from 2.5V to 1.875V, with a minimum of 2.5V – 0.625V = 1.875V.
This range of 2.5V±0.625V usually corresponds to the rated measurement current range of the sensor. For example, for HAS2009, its rated current is ±20A; For HAS4009, its rated current is ±100A.
3. Current calculation method
Through the nominal sensitivity (SN) of the sensor, we can convert the collected voltage signal into the actual current value. The calculation formula is as follows: IP = (Vout – 2.5V)/SN
Among them:
•IP is the measured current (unit: A) • Vout is the voltage output by the sensor (unit: V) •2.5V is the reference voltage at zero current •SN is the nominal sensitivity of the sensor (unit: V/A or mV/A).
Example:
Suppose the HAS14Z current sensor (SN = 104.2 mV/A or 0.1042 V/A) is used.
•If Vout is measured to be 2.8V: IP = (2.8V – 2.5V) / 0.1042V /A ≈ 2.88A (forward current) •If Vout is measured to be 2.2V: IP = (2.2V – 2.5V) / 0.1042V /A ≈ -2.88A (reverse current)
• VCC (+5V): Positive input of sensor power supply, connected to a stable 5V DC power supply. • GND: The power ground and signal ground of the sensor should be shared with the ground wire of the control system. • OUT (Vout): Sensor voltage output pin, used for collecting current signals.
Typical wiring diagram (taking microcontrollers as an example) :
Wiring precautions
1). Power supply: Ensure a stable 5V power supply is provided for the sensor. The quality of the power supply directly affects the measurement accuracy. 2). Common ground: The sensor and the microcontroller must share the same ground to avoid measurement errors caused by ground potential differences. 3). Wiring: The wire of the current to be measured should be correctly passed through the measurement hole of the sensor and centered as much as possible to ensure the best measurement effect.
5. Signal acquisition steps
The 2.5V±0.625V voltage output signal of the current sensor is mainly collected through an analog-to-digital converter (ADC). The following are the detailed steps: 1). ADC configuration:
•Select the ADC pin: Connect the OUT pin of the sensor to one of the ADC input pins of the microcontroller. •Set the reference voltage: Set the reference voltage of the ADC to 5V (if the sensor output range is 0.25V to 4.75V), or select an appropriate reference voltage based on the actual situation. Ensure that the ADC can cover the entire output range of the sensor. •Set the resolution and sampling rate: Select the resolution (such as 10-bit or 12-bit) and sampling rate of the ADC according to the application requirements. Higher resolution provides more accurate measurements, and a higher sampling rate is suitable for rapidly changing currents.
2). Read the ADC value:
•The digital values of the ADC input pins are periodically read through microcontroller programming.
3). Convert the ADC value to voltage:
•Convert the read digital value to the actual voltage value Vout. For example, for a 10-bit ADC (0-1023) with a 5V reference voltage, the conversion formula is: Vout = (ADC_Value / 1024.0) * 5.0 (unit: V)
4). Calculate the actual current
•Use the following formula to convert the voltage Vout to the actual current IP: IP = (Vout – 2.5V)/SN, where SN is the nominal sensitivity of the sensor (unit: V/A). For instance, the SN of HAS14Z is 0.1042 V/A.
Sample calculation Suppose the HAS14Z current sensor (SN = 0.1042 V/A) is used.
•If the output voltage Vout measured by the ADC is 2.8V: IP = (2.8V – 2.5V) / 0.1042V /A ≈ 2.88A (forward current) •If the output voltage Vout measured by the ADC is 2.2V: IP = (2.2V – 2.5V) / 0.1042V /A ≈ -2.88A (reverse current)
By following the above steps, you can accurately collect and calculate the magnitude and direction of the current being measured.
6. Application scenarios
The HAS series closed-loop principle current sensors are widely used in the following fields due to their bidirectional current measurement capability and voltage output characteristics:
• Motor control: Achieve precise speed and torque control for electric vehicles, industrial robots and other equipment. • Battery Management System (BMS): Monitors the charging and discharging status of the battery to optimize battery health and efficiency. • Power management: Used in UPS, SMPS, solar inverters, etc., it precisely controls the power flow. Renewable energy systems: Monitor the current at the connection points between energy storage systems and the power grid to optimize energy conversion. • Industrial automation: Provides real-time current data for process control, fault diagnosis and safety protection.
What is the difference between current transformer, voltage transformer and Control/Safety Isolation Transformer?
Current transformers (CTs), voltage transformers (VTs, also known as potential transformers (PTs), and power transformers all operate based on the principle of electromagnetic induction, but they differ significantly in their design, purpose, and operating characteristics. Simply put, CTs and VTs are instrument transformers, primarily used to measure and protect current or voltage in power systems, while power transformers are power transformers, primarily used for voltage conversion in power transmission and distribution. Below, I will compare them from multiple perspectives.
1. Basic Definition and Principle
Current Transformer (CT): Used to convert high-voltage, high-current to low-voltage, low-current (typically 1A or 5A) for instrument measurement or relay protection. Its operating principle is that the primary winding is connected in series in the circuit, and the secondary winding outputs a signal proportional to the primary current.
Voltage Transformer (VT): Used to convert high-voltage, high-current to low-voltage (typically 100V or 110V) for voltage measurement and protection. The primary winding is connected in parallel in the circuit, and the secondary output is proportional to the primary voltage.
Control/Safety Isolation Transformer: Used to change voltage levels (e.g., step up or down) in power systems to achieve efficient power transmission and distribution. It is not used for measurement but directly supplies power to the load.
2. Key Differences Comparison
The following table summarizes the key differences between the three types of transformers:
Current Transformer (CT)
Voltage Transformer (VT)
Control/Safety Isolation Transformer
Applications
Measure and monitor current and protect equipment (such as relays).
Measure and monitor voltage and protect equipment.
Provide electrical isolation and voltage conversion for control circuits and safe power supply (such as preventing electric shock and powering low-voltage equipment).
Working Mode
When the primary is connected in series with the load circuit, the secondary must be short-circuited (or connected to a low-impedance load) to avoid dangerous high voltages.
When the primary is connected in parallel, the secondary is open-circuited or connected to a high-impedance load (such as a meter).
The primary and secondary terminals are connected to the power supply and load as required for power transmission and have no measurement function.
Transformation ratio characteristics
Current step-down ratio (high current → low current, e.g., 1000A:5A), low secondary voltage.
Voltage step-down ratio (high voltage → low voltage, e.g., 1kV:10V), high secondary impedance.
Voltage step-down ratio (e.g. 220V:24V), with emphasis on isolation and power transfer.
Design Features
Fewer primary turns, more secondary turns; high precision, low saturation; small capacity (several VA).
Similar to a small transformer, high precision; small capacity (several VA to tens of VA).
Dual-winding isolation design with shielding for enhanced safety; low-voltage PCB or chassis-mountable; high efficiency, suitable for intermittent or continuous loads; large capacity (several VA to several kilovolts).
Load requirements
The secondary must be short-circuited or low-impedance to avoid dangerously high voltages.
The secondary can be left open for high-impedance loads (such as instruments).
When connecting the secondary to an actual load (such as a control device), it can be left open, but this is not recommended.
Accuracy and Error
Emphasizes current measurement accuracy (error <1%).
Emphasizes voltage measurement accuracy (error <0.5%).
Emphasis is placed on isolation and efficiency (>65%), not measurement accuracy; voltage regulation is low.
Application Scenario
Power metering, protection systems, ammeters.
Voltmeters, power meters, protection relays.
Industrial automation, machine tool controls, medical equipment, security lighting, PCB power modules.
Connection between current transformer and Atmel M90E32AS chip
Introduction
In modern energy metering systems, accurate current measurement is crucial. The Atmel M90E32AS is a high-performance, wide-dynamic-range multiphase energy metering IC widely used in various energy meters and power monitoring equipment. Current transformers (CTs) play an indispensable role in safely and accurately transmitting high-voltage, high-current signals to the M90E32AS for processing. This article will delve into the operating principles of current transformers and how to properly connect them to the M90E32AS for reliable current measurement.
Atmel M90E32AS Chip Overview
The Atmel M90E32AS is a high-performance integrated circuit designed for multiphase energy metering. It integrates six independent second-order Sigma-Delta ADCs capable of simultaneously processing three-phase voltage and current signals. The chip’s internal digital signal processor (DSP) performs complex calculations on the data collected by the ADCs to derive key energy parameters such as active power, reactive power, apparent power, power factor, frequency, and RMS values of voltage and current. The M90E32AS supports current sampling through a current transformer (CT) or a Rogowski coil, and voltage sampling through a resistor divider network.
Current input channel
The M90E32AS chip provides three sets of differential current input pins: I1P/I1N, I2P/I2N, and I3P/I3N, corresponding to the measurement of three-phase current. These pins are the input terminals of the analog ADC channels, allowing flexible channel mapping to adapt to different three-phase system configurations (such as three-phase four-wire or three-phase three-wire).
Internal ADC and sampling
The ADC inside the chip is responsible for converting the analog current signal into a digital signal. The M90E32AS has a high dynamic range and high accuracy, which can ensure accurate measurement over a wide current range. The data sheet states that current sampling is performed through a current transformer (CT) or a Rogowski coil (di/dt coil) [1]. This means that the external sensor converts the current signal into a voltage signal acceptable to the M90E32AS.
Signal Processing and Calibration
The M90E32AS’s DSP not only calculates energy parameters but also supports gain and phase angle compensation to correct for errors introduced by external sensors and circuits. This is crucial for ensuring the accuracy of the entire metering system. The chip also features programmable startup and no-load power thresholds, as well as current and voltage transient signal monitoring. These features enable the M90E32AS to operate stably and reliably in complex power environments.
Current Transformer (CT) Operating Principle
A current transformer (CT) is a special type of transformer used to proportionally convert large currents into smaller ones for measurement, protection, and control. Its primary functions are current isolation and current scaling. The operating principle of a CT is based on the law of electromagnetic induction: when current flows through the primary winding (usually the wire running through the center of the CT), it generates magnetic flux in the core. This flux induces an electromotive force in the secondary winding, generating a secondary current.
Key Features
Ratio:
A CT’s ratio is the ratio of its primary-side rated current to its secondary-side rated current, for example, 100A/5A or 100A/0.1A. This means that when 100A flows through the primary, the secondary output will be 5A or 0.1A. In energy metering applications, CTs with a secondary output of milliamperes or less are typically selected to allow direct connection to the current input of a metering chip.
Accuracy Class:
A CT’s accuracy class indicates its measurement error within its rated current range. For example, a 0.2S-class CT indicates an error within a specific current range of ±0.2%, which is critical for high-precision energy metering.
Burden:
A CT’s burden refers to the impedance of the measuring instrument or device connected to its secondary side. The secondary side of a CT must always be closed; otherwise, an open circuit will cause high voltage to develop on the secondary side, potentially damaging the CT or posing a danger to personnel. Isolation: CTs provide electrical isolation between the primary-side high-voltage circuit and the secondary-side low-voltage measurement circuit, thereby improving system safety and reliability.
In energy metering, CTs are often used to measure high currents in the main circuit. CTs can reduce the voltage and current of the high-current signal in the main circuit, converting it into a low-current or low-voltage signal that the metering chip can process. This not only protects the metering chip from direct impact from high voltage and current, but also makes metering system design more flexible and secure.
Connecting Current Transformers to the M90E32AS Chip
Connecting current transformers to the Atmel M90E32AS chip requires careful consideration of the signal conditioning circuit to ensure measurement accuracy and chip safety. The M90E32AS chip’s current input pins (I1P/I1N, I2P/I2N, and I3P/I3N) are differential inputs, meaning they measure the voltage difference between the two pins. Therefore, the current transformer output typically needs to be converted to a voltage signal through a sampling resistor before being fed into the chip’s differential inputs.
Typical Connection Circuit
We can see how a current transformer (CT) is connected. Typically, the secondary side of the CT is connected to a small-value shunt resistor. When current flows through the shunt resistor, a voltage drop proportional to the current is generated across it. This voltage drop serves as the input signal for the M90E32AS chip’s ADC.
Using single-phase current measurement as an example, the CT’s secondary output is connected to the shunt resistor, which is then connected to the I_P and I_N pins of the M90E32AS chip. To improve measurement accuracy and suppress common-mode noise, a capacitor is typically connected in parallel across the shunt resistor to form an RC filtering network. Furthermore, a voltage divider network or bias circuit may be required to provide an appropriate bias voltage to ensure the ADC input is within the operating range of the M90E32AS.
Potential for cavitation, leading to partial discharge, hot spots, and thermal cycling cracks Service life may be reduced by 40–60%
Advantages of vacuum potting:
Evacuates air at ≤5 mbar Injects UL94V-0 epoxy resin and cures in a moisture-free environment Significantly improves insulation performance and thermal stability
Actual Results and Test Data
A vacuum-potted medical device transformer passed a 50,000-hour accelerated aging test. A non-vacuum-potted device failed after 18,000 hours.
Ability to cope with extreme environments
Stressor
Consequence
Vacuum Molded Benefit
Vibration (5–2000Hz)
Wire fatigue
Dampens resonance 300% better
Thermal shock (-40~125°C)
Solder joint cracks
CTE-matched resin absorption
Humidity (85% RH)
Corrosion tracking
Zero permeability barrier
Recommendations for verifying potting quality
–Don’t rely solely on the datasheet, request: –X-ray scan showing void distribution –Cross-section report measuring bubble size –Thermal shock test (-55°C to +150°C, 500 cycles) –Partial discharge test <5 pC (at 2x the operating voltage)
A split-core current transformer makes it easy to measure current without cutting wires. The core of a split-type current transformer can be opened, allowing wires to be threaded through the center and secured. This transformer is suitable for applications where cutting wires is not possible. The video demonstrates how to connect and test a current transformer, monitor output current and voltage, and use a rectifier to convert AC to DC. Split-core current transformers provide excellent isolation and ease of use for power measurement.
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