Current Transformer

The Working Principle of Current Transformer
Jul 22, 2026

In the power supply lines, there is a huge difference in current and voltage, ranging from a few amperes to tens of thousands of amperes. For the convenience of secondary instrument measurement, it is necessary to convert the current into a relatively uniform current. In addition, the voltage on the circuit is relatively high, making direct measurement very dangerous. The current transformer plays a role in current conversion and electrical isolation. Previously, most display instruments were pointer type current and voltage meters, so the secondary current of current transformers was mostly in ampere level. Nowadays, most electricity measurement is digitized, and the sampling signal of computers is generally in the milliampere range (e.g. 0-5V, 4-20mA). The secondary current of micro current transformers is in the milliampere range, mainly serving as a bridge between large transformers and sampling. Miniature current transformers are also known as "instrument current transformers". The term "instrument current transformer" refers to a multi current ratio precision current transformer used in laboratories, generally used to expand the instrument range Principle circuit diagram of current transformer Miniature current transformers work similarly to transformers based on the principle of electromagnetic induction. Transformers convert voltage, while miniature current transformers convert current. As shown in the diagram, winding N1 is connected to the measured current, which is called a set of windings (or primary winding, primary winding); Connect winding N2 to the measuring instrument and become the secondary winding (or secondary winding, secondary winding). The current ratio between the primary winding power I1 and the secondary winding I2 of a miniature current transformer is called the actual current ratio K. The current ratio of a miniature current transformer when operating at the rated working current is called the rated current ratio of the current transformer, expressed in Kn. Kn=I1n/I2n Micro current transformers can be roughly divided into measuring current transformers and protective current transformers. A. Measurement current transformer The measuring current transformer is mainly used in conjunction with measuring instruments to measure current, voltage, power, etc. under normal operating conditions of the circuit. The main requirements for measuring micro current transformers are: 1. Reliable insulation; 2. High enough measurement accuracy; When a high current occurs due to a fault in the side line, the transformer should saturate within an appropriate 20% (such as 500% of the rated current) to protect the measuring instrument. B. Protective current transformer The protective current transformer is mainly used in conjunction with the relay device to provide a signal to the relay device to cut off the faulty circuit in case of short circuit overload or other faults, in order to protect the safety of the power supply system. The working conditions of protective miniature current transformers are completely different from those of measuring transformers. Protective transformers only start working effectively at currents several times or tens of times higher than normal. The main requirements for protective transformers are: 1. Reliable insulation; 2. A sufficiently large accurate limit coefficient, 3. Adequate thermal and dynamic stability. The maximum primary current that the protective transformer can meet the accuracy level requirements under rated load is called the rated accuracy limit primary current. The accurate limit coefficient is the ratio of the rated accurate limit current to the rated primary current. When the current is large enough, the iron core will saturate and cannot reflect the current. The accurate limit coefficient represents this characteristic. The accuracy level of the protective transformer is 5P and 10P, indicating that the allowable error at the rated accuracy limit for one current is 5% and 10%. When a fault occurs in the circuit, the surge current generates heat and electromagnetic force, and the protective current transformer must withstand it. The effective value of the primary current that a current transformer can withstand without damage within one second in the event of a short circuit in the secondary winding is called the rated short-time current. The peak value of the primary current that the current transformer can withstand without damage in the event of a short circuit in the secondary winding is called the rated dynamic stability current. Protective current transformers are divided into: Overload protection current transformer Differential protection current transformer Grounding protection current transformer (zero sequence current transformer) C. Micro voltage transformer Micro voltage transformers, due to size and manufacturing reasons, usually use current type voltage transformers. In fact, it is a current transformer with a rated current ratio of 1 and both primary and secondary currents in milliampere level (e.g. 2mA/2mA): During operation, the primary winding of the transformer is connected in series with the current limiting resistor R to measure the voltage, and the secondary output is connected to the operational amplifier for I/V conversion (or direct resistance sampling). At this point, the primary current is I1=U (R+r), and the secondary current I2=I1/Kn, where r is the internal resistance of the primary winding and Kn is the rated current ratio.
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Current Transformer Error
Jul 29, 2026

Reasons for errors In fact, there must be energy loss during the operation of the current transformer, which causes errors in the current transformer. To generate current in the secondary winding, an excitation current I0 is required to excite and generate induced potential and current. The excitation current is provided by the primary winding. The product I0W1 of the excitation current I0 and the number of turns W1 of the primary winding is called the excitation ampere turn or excitation electromotive force. It means that the first winding turns need to be deducted from the example turns before they are transmitted to the first winding, resulting in errors. The error is caused by providing excitation turns. The error of current transformers consists of two parts: non ratio difference and phase difference. It should be noted that the transformer ratio is the percentage of the secondary current error value to the actual primary current, not the percentage of the rated current. This is different from the method of expressing the error of other general measuring instruments as a percentage of fullness (% F.S). There is no concept of full capacity for current transformers, only the measurement range and rated current. For example, a micro transformer with an accuracy level of 0.1 has an allowable ratio difference of+/-0.1% at 100% rated current and+/-0.4% at 5% rated current. Assuming the rated current is used as the full scale, the allowable ratio difference at 100% rated current is expressed as a percentage of full scale, which is+/-0.1% F.S. The allowable ratio difference at 5% rated current is expressed as a percentage of full scale, which is+/- (0.4x5%/100%)%=+/-0.02% F.S Error compensation of current transformer Current transformers without compensation have negative specific differences and positive angular differences. The allowable range of error for current transformers at all levels is positive and negative deviation. Therefore, the surplus range of positive and negative deviations can be utilized to improve the accuracy of the transformer. In order to improve the accuracy of transformers, various compensation methods are generally used. In general, due to the small compensation value, it can be considered that the magnetic field of the iron core should not be abstracted. This can be calculated using error superposition. The compensation methods for current transformers include turn compensation, auxiliary iron core compensation, capacitor compensation, etc. Turn compensation The compensation method for the number of turns of micro current transformers is the simplest, as long as the secondary winding is wound Nx fewer than the rated number of turns. The ratio difference before compensation of the current transformer is replicated, and increasing the current of the secondary winding by fewer turns serves as compensation. The compensation amount is as follows: △f=Nx/(N2-Nx)x100% The ratio difference of the turns compensation team plays a compensating role, and the compensation amount is independent of the secondary load and current size. The compensation turns are generally only a few turns, and the turn compensation should calculate the error between the maximum current low-end secondary impedance and the minimum current high-end secondary impedance. Du Yu's high-precision micro current transformer can compensate for excessive turns even if it only compensates for one turn. At this point, half turn or fractional turn compensation can be used. However, the number of turns of a current transformer is calculated based on the closed circuit passing through the iron core window, and the number of turns of a current transformer is calculated by smashing one by one, without the situation of half a turn. The use of half turn or fractional turn compensation requires the use of auxiliary terminals, such as dual windings, dual iron cores, etc. Auxiliary iron core compensation The auxiliary iron core compensation has a compensating effect on the contrast difference and angle difference, but the manufacturing process of the auxiliary iron core compensation method is relatively complex. Capacitor compensation Capacitor compensation can be achieved by directly connecting capacitors in parallel at both ends of the secondary winding of the current transformer. Its contrast difference plays a positive compensation role, and the compensation size is proportional to the X component in the secondary load Z=R+iX, and proportional to the size of the compensation capacitor; It has a negative compensation effect on the cross, and the compensation size is proportional to the R component in the secondary load Z=R+iX, and proportional to the size of the compensation capacitor. Capacitor compensation is an ideal compensation method. In micro precision current transformers, the secondary winding is generally directly connected to the current/voltage conversion of the operation and discharge, and the secondary impedance is basically 0. At this time, the role of capacitor compensation is relatively small. Generally, adding a phase shift circuit in the interpretation of current/voltage conversion can solve the angle difference problem. Users can adjust and calculate the phase shift circuit based on the error data of the voltage reduction in the inspection report of the current transformer that comes with it when it leaves the factory. There are many compensation methods for current transformers. The compensation of current transformers is an important way to improve the accuracy level of current transformers, and one of the important tasks in designing high-precision current transformers.
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How to Determine the Burden of a Current Transformer
Sep 02, 2026

Introduction Current transformers (CT) are core components for current measurement, energy metering, and relay protection in power systems. “Burden” is one of the most critical parameters for a current transformer. Incorrect burden matching will cause waveform distortion, large measurement errors, inaccurate energy-meter reading, or even failure of protection relays. Understanding how to calculate and select a proper CT burden is essential for stable power‑system operation. Here we explain CT burden fundamentals, calculation steps, common pitfalls and practical selection guidelines for engineers. 1. What is Current Transformer Burden? CT burden refers to the total apparent power (unit: VA, Volt‑Ampere) consumed by all secondary‑side connected components at rated secondary current. It represents the total load on the CT secondary winding. The secondary‑side load includes: Internal resistance of CT secondary winding Resistance of connecting wires / cables Impedance of connected devices: ammeters, energy meters, protection relays, sampling circuit boards Formula: Burden can also be expressed in ohms(Ω). Conversion between VA and ohms is frequently used in engineering practice. 2. Why Burden Matching Matters Too small actual burden The CT works under light load. For measuring‑class CTs, accuracy can stay within specification. However, for protection‑class CTs, excessively low burden may alter saturation characteristics under fault conditions. Too large actual burden (over‑burden) This is the most‑common field issue: 1. Secondary voltage rises, magnetic core saturates 2. Output current waveform distorts 3. Measurement error increases, metering data deviates 4. Protection CT fails to correctly reflect fault current, leading to protection mal‑operation or refusal‑to‑operate Rule: The calculated actual circuit burden must be less than or equal to the rated burden marked on the CT nameplate. 3. Step‑by‑Step: Calculate Actual Secondary Burden Step 1: Confirm CT rated secondary current Two mainstream standards: 5A secondary output (widely used in traditional power cabinets) 1A secondary output (long‑distance wiring scenarios, reduces wire power consumption) Step 2: Sum impedance of all secondary‑side devices Check datasheet for each connected instrument / PCB sampling unit for its power consumption or impedance value. Example: Energy meter burden consumption: 1.5 VA Protection relay: 2.0 VA Sum all individual device VA values. Step3: Calculate wire burden Copper connecting cables contribute significant burden, especially for long wiring distance. Wire resistance formula: - ρ: Resistivity of copper conductor - L: One‑way cable length between CT and load device - A: Cross‑sectional area of copper wire Note: CT secondary forms a closed loop, so round‑trip length (2L) shall be adopted. Wire burden: Step4: Get total actual burden Total Actual Burden = Sum of instrument burden + Wire burden Step5: Compare with CT rated burden Requirement: Actual total burden ≤ CT rated burden on nameplate. Keep reasonable margin; it is recommended actual burden ≤75% of rated burden for reliability margin. 4. Practical Engineering Examples Case parameters CT rated secondary current: 5A Rated burden on nameplate: 10 VA Connected loads: Energy meter 2 VA, current indicator 1 VA Copper wire: 1.5 mm², one‑way length L=20 m 1. Sum instrument burden: 2 VA +1 VA = 3 VA 2. Compute round‑trip wire resistance and wire burden 3. Total actual burden = instrument burden + wire burden 4. Compare against CT rated 10 VA, verify margin. If calculated actual burden exceeds rated burden, feasible solutions: 1. Increase copper wire cross‑section, reduce wire resistance 2. Select CT with higher rated burden 3. Change to 1A secondary‑current CT to cut wire loss for long‑distance transmission 5. Common Misunderstandings 1. Ignore cable burden: Engineers only consider instrument VA, neglect long‑cable impedance, resulting in hidden over‑burden. 2. Confuse burden class and accuracy class: 0.5, 0.2S represent accuracy; burden(VA) represents load capacity. A high‑accuracy CT still fails when over‑burdened. 3. Multiple devices series connection: Each additional device adds burden; do not cascade unlimited instruments on one CT secondary loop. 4. Secondary open‑circuit risk: While calculating burden, remember CT secondary circuit must never be open‑circuited. Open circuit produces extremely high dangerous voltage and damages the magnetic core. 6. Quick Selection Checklist for CT Burden Record rated secondary current (1A /5A) Collect power consumption of every secondary‑connected device Measure actual wiring length, calculate copper wire burden Sum to get total actual burden Actual burden ≤ CT rated burden, retain 20‑30% design margin Re‑check when modifying cabinet wiring or adding secondary devices Conclusion Determining CT burden is not merely reading nameplate parameters; it requires full calculation of instruments plus wiring impedance. Reasonable burden matching guarantees measurement precision and reliable protection action for your power‑monitoring hardware. When designing current‑sampling hardware including zero‑sequence CT and ordinary current transformers, always run burden calculation at the design phase, rather than adjusting after on‑site faults For more information discuss, please contact us freely.
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How to Select the Right Current Transformer for Energy Meters
Aug 05, 2026

Energy meter accuracy depends heavily on the performance of the current transformer (CT). Even a high-precision smart meter will produce inaccurate readings, data drift, or metering errors if matched with an unsuitable current transformer. For smart grid projects, residential metering, commercial power monitoring, and solar energy measurement, selecting the right CT is critical for long-term stable and accurate energy data collection. This guide explains the key factors engineers and buyers must consider when choosing a current transformer for energy meter applications. 1.Priority Accuracy Class for Metering Applications Energy metering requires higher precision than general industrial current monitoring. For standard electricity meters, 0.2 class and 0.5 class CTs are common. For high-precision smart meters, laboratory metering, and grid-grade measurement, 0.1 class or even 0.05 class nanocrystalline current transformers are necessary. Unlike industrial monitoring CTs, metering-grade CTs must maintain low ratio error and low phase error under light load conditions, which is the biggest advantage of nanocrystalline core transformers. 2. Correct Current Ratio Matching The CT primary current must match the actual load current range of the energy meter circuit. If the CT current rating is too large, low current signals will cause severe inaccuracy; if the rating is too small, saturation and waveform distortion will occur under overload conditions. Common metering current ratios include 50A/5A, 100A/5A, 200A/5A, and custom ratios for special smart meter projects. 3. Core Material Determines Long-Term Stability Traditional silicon steel CTs are cheap but perform poorly under tiny current and low-load scenarios. For smart energy meters that require 24/7 long-term operation, nanocrystalline core current transformers are the best solution. Nanocrystalline cores feature ultra-low magnetic leakage, excellent temperature stability, and minimal phase shift, effectively solving light-load inaccuracy and temperature drift problems that plague ordinary CTs. 4. Installation Structure Selection Solid core CT is widely used for factory pre-installed energy meters, offering higher stability and better shielding performance. Split core CT is more suitable for retrofit metering projects, allowing installation without cutting off power, which improves construction efficiency for grid renovation. 5. Compliance with International Standards All meter-level current transformers must comply with IEC 61869 and local grid metering standards to ensure data validity and certification pass rate. Conclusion To achieve accurate energy metering, you must match the correct accuracy class, current ratio, core material, and structure according to your meter type and application environment. High-quality nanocrystalline metering CTs greatly improve the reliability of smart meter data. Contact us to get professional CT selection guidance, free samples, and customized current transformer solutions for your energy meter projects.
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