Abstract: This paper briefly discusses the current situation regarding the impact of power grid harmonics on the accuracy of low-voltage circuit breaker asymmetrical grounding (neutral grounding) fault protection, and proposes detailed solutions from both hardware and software design perspectives. This not only enables low-voltage circuit breakers to display the impact of power grid harmonics, but also improves the protection accuracy and operational reliability of low-voltage circuit breakers. Keywords: KST45 intelligent trip unit, power grid harmonics, asymmetrical grounding (neutral grounding) fault protection. I. Overview With the rapid development of my country's industrial technology, new types of electrical equipment are constantly emerging, and "non-sinusoidal wave electrical equipment" (such as silicon rectifier equipment, medium-frequency equipment, and frequency converter equipment) is increasing. The widespread use of these "non-sinusoidal wave electrical equipment" has led to a significant increase in harmonic components in the power grid, and power grid pollution is becoming increasingly serious; the increase in harmonic components in the power grid undoubtedly poses a new challenge to the operating accuracy and operational reliability of low-voltage circuit breakers. Compared with the corresponding section of international standard GB14048.2-94, the EMC section of GB14048.2-2001 adds a composite harmonic test for low-voltage circuit breakers, further illustrating the importance of low-voltage circuit breakers' resistance to power grid harmonic interference. Currently, most DW45 type low-voltage circuit breakers use three-phase current vector sum values ​​for asymmetrical grounding (neutral grounding) fault protection. This three-phase current vector sum is obtained by calculating the analog quantity from the vector sum of the A, B, and C phase current signals and then performing analog-to-digital conversion (referred to as the three-phase analog current vector sum). It is well known that the three-phase vector sum of a symmetrical 50Hz three-phase sine wave is zero; however, the three-phase vector sum of a third harmonic is the superposition of the absolute values ​​of each phase. In other words, in a power grid with high harmonic content, using the three-phase analog current vector sum as the actual current value for asymmetrical grounding (neutral grounding) fault protection is inaccurate and can sometimes even cause malfunctions. The KST45 (upgraded) intelligent trip unit, now配套 with the DW45 low-voltage circuit breaker, employs a 16-bit high-performance microcontroller (MCU). Its main frequency is doubled compared to the original, significantly reducing analog-to-digital conversion time and greatly improving processing speed. It can accurately calculate the true effective values ​​of current and voltage for each phase, quickly and effectively performing three-phase digital current vector sum calculations. This provides a physical platform for addressing the impact of grid harmonics on the three-phase simulated current vector sum. II. Harmonic Influence Coefficient K The KST45 (upgraded) intelligent trip unit adds the function of processing the impact of grid harmonics on the three-phase simulated current vector sum, introducing a comprehensive coefficient (referred to as the harmonic influence coefficient K) for the impact of grid harmonics on the three-phase simulated current vector sum. The measured value of the harmonic influence coefficient K is calculated as follows: K = (IAG - IDG) / Iz × 100% Where: IAG is the three-phase simulated current vector sum; IDG is the three-phase digital current vector sum; Iz is the comprehensive current value of asymmetrical grounding (neutral grounding). The three-phase analog current vector sum in the above formula is obtained by calculating the vector sum of the three-phase analog currents using a vector sum circuit, followed by analog-to-digital conversion. Its value is affected by power grid harmonics. The three-phase digital current vector sum is calculated by the microcontroller based on the correct true RMS value of each phase current. Its value is not affected by power grid harmonics. The asymmetrical grounding (neutral grounding) comprehensive current value is obtained by weighting the average value of the three-phase current and the three-phase digital current vector sum. Its purpose is to make the measured value of the harmonic influence coefficient K more reasonable. III. The setting of the harmonic influence coefficient K is the same as other protection parameters of the intelligent trip unit. The harmonic influence coefficient K value of the KST45 (upgraded type) intelligent trip unit can also be set by the user (the factory setting is "OFF"). The setting range of the harmonic influence coefficient K is: ON + 10% ~ 100% + OFF (with a step difference of 1%). The correspondence between the setting value of the harmonic influence coefficient K and the characteristics of the unbalanced grounding (neutral) protection is as follows: (1) When K = OFF, the actual current value of the unbalanced grounding (neutral) fault protection adopts the three-phase analog current vector sum (the same as the original KST45 intelligent trip unit); at this time, although the intelligent trip unit still displays the measured value of the grid harmonic influence coefficient K, the unbalanced grounding (neutral) fault protection is accurately affected by all grid harmonics; this setting state can be used in areas with small grid harmonics. This setting value is retained to be compatible with the original KST45 intelligent trip unit. (2) When K = ON, the actual current value of the unbalanced grounding (neutral) fault protection adopts the three-phase digital current vector sum; at this time, the unbalanced grounding (neutral) fault protection fully accommodates the influence of grid harmonics. Under this setting state, the intelligent trip unit can accurately implement unbalanced grounding (neutral) fault protection in situations where grid harmonics are very severe. (3) When the K value is set between 10% and 100%, the actual current value of the asymmetrical grounding (neutral) fault protection is taken in two cases: when the measured value of the harmonic influence coefficient K is less than or equal to the setting value, the sum of the three-phase simulated current vectors is taken as the actual current value of the protection; when the measured value of the harmonic influence coefficient K is greater than the setting value, the sum of the three-phase digital current vectors is taken as the base, and the harmonic influence component of the K value setting percentage is added as the actual current value of the protection. In this way, the degree of influence of power grid harmonics on the asymmetrical grounding (neutral) fault protection can be limited. IV. Display of Harmonic Influence Coefficient K When the KST45 (upgraded type) intelligent trip unit is working, it monitors the degree of influence of power grid harmonics on the sum of the three-phase simulated current vectors in real time, and can display the measured value of the harmonic influence coefficient K in the ammeter window. The specific operation method is as follows: Press the "Select" key to query, and the ammeter window will cycle through the following: L1 phase current (L1 light is always on), L2 phase current (L2 light is always on), L3 phase current (L3 light is always on), asymmetrical grounding (neutral) current (G light is always on), measured value of harmonic influence coefficient K (G light flashes), and maximum phase current. The asymmetrical grounding (neutral) current value displayed in the KST45 (upgraded) intelligent trip unit ammeter window is always the actual current value of the asymmetrical grounding (neutral) fault protection after processing with harmonic influence coefficient K; the displayed harmonic influence coefficient K is always the measured value of the three-phase simulated current vector and comprehensive coefficient of the grid harmonic influence. V. The KST45 (upgraded) intelligent trip unit with harmonic influence coefficient K for asymmetrical grounding (neutral) fault protection calculates the actual current value of the asymmetrical grounding (neutral) fault protection in real time based on the setting value of the harmonic influence coefficient K, and displays this actual current value in the ammeter window (see display section). When the actual current value is greater than or equal to the setting current value of the asymmetrical grounding (neutral) fault protection, the intelligent trip unit provides time-limited protection based on the setting time value of the asymmetrical grounding (neutral) fault protection. While implementing asymmetrical grounding (neutral) fault protection, the intelligent trip unit also records the actual current value and delay time of the asymmetrical grounding (neutral) fault when the asymmetrical grounding (neutral) fault trip command is issued, as well as the current value of each phase and the measured value of the harmonic influence coefficient K at that time. These fault record data do not disappear after the intelligent trip unit is powered off, for future reference, and remain stored until updated by new fault record data. The KST45 (upgraded) intelligent trip unit can solve the impact of power grid harmonics on the accuracy of low-voltage transformer asymmetrical grounding (neutral) fault protection. It was developed in response to the increasingly serious power grid harmonics. It not only improves the accuracy and reliability of protection, but also provides users with more protection options, fully embodying the concept of "continuous improvement and customer satisfaction".