Abstract: This paper analyzes the principle of zero-flux current transformers and proposes a new method to achieve "dynamic zero flux" for small current sensors by using a microcrystalline alloy core and adding an adaptive dynamic electronic circuit. Keywords: Small current sensor; Zero flux; Error Introduction: Using current sensors as sampling units in online insulation detection systems for electrical equipment has become a consensus in the industry. Currently, there are various types of current sensors, such as Hall sensors, coreless current sensors, high-permeability amorphous alloy multivibrator current sensors, and electron spin resonance current sensors. Due to the special nature of the power system operating environment, many sensors have their own limitations. Currently, most current sensors used in power systems operate on the basic principle of electromagnetic coupling. From the perspective of sampling methods, these sensors mainly include direct serial insertion, clamp-on, and closed-loop through-core types. To ensure sampling accuracy and minimize the ratio and phase angle difference between the output and input signals, researchers have adopted error compensation methods such as: short-circuit active compensation, pure resistance error compensation, secondary impedance complete compensation, and self-balancing electronic compensation. Numerous studies and experiments have shown that small current transformers based on the "zero flux principle" are more suitable for the requirements of online insulation detection in power systems. The small current sensor described in this article is based on this principle, and with the application of adaptive dynamic tracking electronic circuitry, it possesses advantages such as high precision, high stability, and strong anti-interference capability. Basic Requirements for Current Sensors in Online Insulation Detection of Power Systems: Online insulation detection systems in power systems operate for extended periods in strong electromagnetic field environments, often outdoors. As its sampling input, the small current sensor must be able to complete sampling with high precision and high stability. However, due to the small size of the sampled signal, it is highly susceptible to interference from electromagnetic fields, temperature, humidity, and other factors. To accurately sample in the strong noise interference environment of the power system, the small current sensor used for online detection should meet the following conditions: Sampling range from several hundred μA to several mA. High sensitivity, with the output capable of sensitively responding to minute changes in the input quantity; the output signal should be as large as possible. Good linearity within the measurement range, with no distortion in the output waveform, and small ratio and angle differences between the output signal and the measured signal, and these differences should be stable and not change with factors such as temperature. It has strong anti-interference ability and good electromagnetic compatibility. The principle of the through-core type small current transformer is as follows: Let I1 be the primary current of the small current transformer, I2 be the secondary current, and I0 be the excitation current. N1 and N2 are the number of turns of the primary and secondary windings, respectively. Therefore, the magnetomotive force balance equation of this small current transformer is: I1N1 + I2N2 = -I0N1. When the excitation ampere-turns I0N1 are zero, I1N1 = -I2N2, that is, the change in secondary ampere-turns can completely reflect the change in primary ampere-turns, and the error is zero. Generally, I0N1 is called the absolute error, and I0N1/I1N1 is the relative error. The error of the current transformer is a complex error, which can be expressed by the ratio difference f and the angle difference δ. ε = -I0N1/I1N1 = f + jδ where f = (I2N2/I1N1)/I1 × 100%, and δ is the angle between I2 and I1 after I2 is rotated 180° counterclockwise. Therefore, due to the existence of I0N1, there is an angle difference δ and a ratio difference f between I2N2 and I1N1. If I0=0, the excitation magnetomotive force is 0, and the error is zero. At this time, the iron core is in a "zero flux" state, operating in the initial segment (linear segment) of the magnetization curve. In this case, the current transformer output waveform will not be distorted, maintaining good linearity. This is the "zero flux principle." Therefore, if the transformer core can always be in a zero flux state, the error of the current transformer can be fundamentally eliminated. However, as we know from the working principle of the transformer, it is impossible to achieve zero flux by the transformer itself; external compensation or adjustment is necessary. Therefore, a dynamic balancing electronic circuit is used to dynamically adjust it, ensuring the iron core is always in a "dynamic zero flux state." The principle of the small current sensor is as follows: ND is the detection winding, D is the dynamic detection unit, and G is the active network that generates the secondary current. The magnetomotive force balance equation of this circuit is: I1N1 + I2N2 + IDND = —I0N1. The excitation flux generated by I1 induces an electromotive force at the two ends of ND, which is applied to the input of the dynamic detection unit D. This induces a secondary current I2 through G, which is supplied to the secondary winding. The magnetic flux generated by I2 demagnetizes the iron core, bringing it into magnetomotive force balance. Therefore, ideally, the secondary winding current I2 of this sensor is entirely supplied by the active network G, without drawing current from the induced electromotive force. D dynamically detects the potential difference across ND at high speed. When the potential difference is sufficiently small (approximately zero), the magnetic flux in the iron core is approximately zero. If the detected value deviates from the allowable value, G automatically adjusts at high speed. This high-speed tracking adjustment ensures that the iron core remains close to zero magnetic flux, achieving high sensor accuracy. Error Analysis The error of the current sensor includes three parts: capacitive error, magnetic error, and sensitivity error of the detection and adjustment electronic circuit. Capacitive error refers to the measurement error caused by the capacitive leakage current between the coils themselves. For power frequency signals, when N2 < 1000, this error can be controlled within 10⁻⁵. Since the number of turns in both the primary and secondary windings of this device is very small, the capacitive error is negligible. Although the detection winding has a relatively large number of turns, its potential difference dynamically approaches zero, so its capacitive error is still negligible. After the aforementioned high-speed dynamic adjustment, I0→0, and the iron core approaches zero magnetic flux, resulting in a very small magnetic error. However, in reality, a completely zero magnetic flux state cannot be achieved; a slight magnetic flux must exist in the iron core for G to output I2, which means that a magnetic error still exists. As can be seen from the magnetomotive force balance equation of this sensor, the magnetic error mainly consists of two parts: one is the error caused by the residual magnetomotive force brought by I0, and the other is the error caused by the additional magnetomotive force brought by the detection winding ID, that is: ε=（I0N1/I2N2）+（IDND/I2N2）=（108 EDl/222μ0NDSI2N2）+（EDND/RiI2N2） where: ED is the induced electromotive force of ND, l is the magnetic circuit length, S is the cross-sectional area of ​​the iron core, μ0 is the initial permeability of the iron core, and Ri is the input impedance of the detection unit. Therefore, to reduce the magnetic error, firstly, a core with a high μ0 value and a suitable number of turns in the detection winding should be selected. This sensor selected a microcrystalline iron core with a μ0 of 6 x 104 and an ND of 100-500 turns; secondly, a large input impedance of the detection unit is required. ED and I2 can be controlled within the required range through an active dynamic balance network. In addition, highly conductive and highly permeable materials should be used for shielding to eliminate electromagnetic interference. Microcrystalline alloys can be used as magnetic shielding materials. [b]Conclusion[/b] When using microcrystalline cores and directly connecting an active electronic circuit network to the secondary winding to form an adaptive dynamic adjustment circuit, the measurement accuracy of small current sensors can be significantly improved while maintaining high stability.