Why Current Transformers Struggle with High Accuracy at Small Currents

Electrical engineers, power system operators and current transformer (CT) professionals often face a common industry pain point: CTs that perform accurately at rated or high currents usually suffer a sharp accuracy drop and obvious error rise under small current or light load conditions, failing to meet the requirements of high-precision metering and online monitoring.

This is not a manufacturer-specific product defect, but a universal engineering challenge for all electromagnetic CTs, including closed-core and split-core types. The core causes focus on five aspects: magnetic circuit characteristics, winding parameters, signal strength, manufacturing process and environmental interference. Below we will analyze the core reasons and sort out the corresponding optimization ideas.

What is a “Small Current” for CTs?

In the power engineering field, small current conditions for CTs are mainly divided into two categories:

• Light load operation where the current is below 10%–20% of the CT’s rated current;
• Weak current measurement scenarios close to system stray current or equipment standby no-load current.

Under these conditions, the CT secondary output signal is inherently extremely weak, and minor interference factors that are negligible in the system will be amplified exponentially, directly converted into visible measurement errors — this is the core cause of out-of-control accuracy at small currents.

Core Reasons for Poor Small-Current Accuracy

1.Non-Linear Magnetic Core Limitations

The magnetic core is the key component of a CT, and its magnetic performance changes dynamically with the current level, with prominent performance defects at small currents: the core flux density is extremely low at small currents, falling in the initial section of the magnetization curve, leading to violent fluctuations in effective permeability, which directly breaks the fixed proportional relationship between primary and secondary current and damages measurement linearity. Meanwhile, in the low-flux region, the proportion of core remanence increases significantly, and the hysteresis effect is amplified, resulting in inconsistent secondary output for the same current change and extremely poor measurement repeatability.

2.Weak Signals Amplify Errors and Sharply Reduce SNR

Small currents correspond to weak voltage or current output on the secondary side. On the one hand, the signal-to-noise ratio (SNR) drops sharply, and electromagnetic interference, circuit noise and temperature drift, which are negligible at high currents, directly become dominant error sources in small-current scenarios, interfering with normal signal acquisition. On the other hand, the same absolute error is negligible at high currents, but the relative error surges and exceeds the standard at small currents due to the extremely small signal base — this is why most CTs only guarantee accuracy within the rated current range instead of full scale.

3.Winding and Load Mismatch Exacerbates Errors

At small currents, the resistance of the secondary winding and external connecting cables accounts for a significantly increased proportion of the total load, easily leading to insufficient secondary induced voltage and distorted output signals. In addition, problems such as long-distance wiring and mismatched input impedance of acquisition devices in practical applications will shift the CT off its ideal operating point, and this impact will be further amplified at small currents, which is difficult to completely eliminate with conventional compensation methods.

4.Visible Minor Manufacturing Defects

Tiny process flaws such as small core air gaps, split-core CT closure deviations and minor winding errors barely affect overall performance at high currents. However, under small current and low flux conditions, these tiny defects will directly damage the magnetic circuit uniformity and winding symmetry, becoming key factors affecting measurement linearity and consistency.

Solutions to Improve Small-Current Measurement Accuracy

A systematic optimization approach is required to overcome the challenge of high-precision small-current measurement: adopt special core materials with high linearity and low remanence; optimize winding structure design to reduce winding resistance and stray parameters; reasonably match the secondary load to improve the SNR of the signal acquisition circuit; equip high-precision compensation technology to offset hysteresis and nonlinear errors; and strictly control the production process to eliminate hidden defects such as core air gaps and winding deviations.

Necessity of High-Linearity CT Design

The core advantage of high-linearity current transformers is to maintain a stable proportional relationship between current and output across the full scale. They can operate stably not only at 20% and 50% rated load, but also achieve reliable and repeatable high-precision measurement under extremely weak current conditions.

For power, industrial control, new energy and other industries relying on accurate energy consumption monitoring and power quality analysis, high-linearity small-current measurement capability is not an additional advantage, but a core rigid demand to ensure system operation efficiency, meet industry compliance requirements and realize refined cost control.