Abstract: This paper introduces an example and analysis of a short-circuit fault in the control power supply caused by series resonance between the power supply filter and the high-order harmonics of the frequency converter in an AC variable frequency speed control system. Keywords: Variable frequency speed control system, power supply filter, series resonance. With the continuous improvement of the functions of frequency converters, AC speed control technology has made significant progress. Under different operating conditions, AC drive systems composed of AC asynchronous motors and frequency converters are increasingly replacing DC drive systems. Many application examples of using frequency converters to modify AC drive systems for different operating conditions have been developed, and correspondingly, some new problems are constantly being discovered during use. This paper analyzes the situation where, during normal operation of the frequency converter, the control circuit experiences series resonance due to the influence of the frequency converter's high-order harmonics, causing a system power supply fault, and the corresponding solutions. A variable frequency speed-regulating constant pressure water supply system is composed of a frequency converter, PLC, PID controller, supporting low-voltage electrical appliances, and pressure sensor. The pressure sensor detects the system's pipeline pressure, converts the pressure signal into a resistance signal, and uses this as feedback input to the PID controller. After comparing the resistance signal with the given signal, the deviation is used to output a control signal via an optimized PI algorithm to control the frequency converter's output frequency, ensuring constant pipeline pressure. The PLC, supporting low-voltage electrical appliances, and PID controller all operate on AC220V. In this system, the user requires a clear display of the water level in the storage tank; therefore, a level sensor is used in conjunction with a digital level indicator to measure and display the water level. In practice, it was found that when the frequency converter is working normally, the digital level indicator frequently displays false readings and garbled characters; the system fully returns to normal when the frequency converter stops working. Clearly, this is due to interference from the frequency converter's high-order harmonic components on the power supply. Typically, the most effective solution is to add a power filter to the power supply of the control circuit. After installing a commercially available general-purpose power filter, the liquid level display system returned to normal. However, a new problem arose: fuse FU2 in the control circuit frequently blew. An inspection of the circuit after a power outage revealed no short circuits. Detailed on-site observation revealed that during the system's gradual speed increase, the inverter's output frequently experienced short circuits within a certain frequency band. Furthermore, this fault persisted even after disconnecting the inverter's load (motor), but disappeared after removing the power filter. Therefore, the filter was inspected first. Disassembly revealed it to be a common π-type filter. Inspection showed no faults in the power filter itself. Further analysis of the inverter's operating principle revealed that in an AC-DC-AC inverter, the power grid supplies power to the inverter through a three-phase rectifier bridge. The supply current can be decomposed using Fourier series into a series of harmonic components, including the fundamental frequency and 6K±1 harmonics (K=1, 2, 3…). The harmonic content decreases as the inductance of the input line reactance and the DC filter reactance increases. According to relevant information, under normal circumstances, after adding a reactor, the fifth, seventh, eleventh, and thirteenth harmonics still account for approximately 40%, 35%, 25%, and 20%, respectively. Due to the influence of the frequency characteristics of the circuit parameters, under the influence of the nth harmonic, the inductive reactance of the inductor is , the capacitive reactance of the capacitor is , and the equivalent complex impedance of the entire power filter is ; where is the equivalent inductive reactance of the filter, and is the equivalent capacitive reactance of the filter. If, under the influence of the kth harmonic, the equivalent inductive reactance and equivalent capacitive reactance of the filter are equal, it can be concluded that the equivalent complex impedance of the circuit at this frequency is , i.e., the circuit is in a resonant state. Since R is only the line resistance at this time, its value is very small, and it can be approximated that the circuit is in a short-circuit state under the influence of this harmonic. Based on the above analysis, it can be concluded that the power supply short-circuit fault is caused by the series resonance of the power filter due to the high-order harmonic components of the inverter output. Normally, if the harmonic components causing resonance are not large enough, they should not cause short-circuit faults. However, when the effective value of the short-circuit current exceeds the protection range of the system fuse, a short-circuit fault will occur, making this type of power supply filtering method unsuitable. After analyzing the cause of the fault, further anti-interference measures were taken for the control and display system, targeting the main sources of interference in the industrial control system, namely power supply interference, process channel interference, and spatial electromagnetic interference. First, for the aforementioned system fault states, a large capacitor was used to replace the original power supply filter, effectively solving the interference problem of the control system's power supply system. However, it must be emphasized that capacitors are absolutely prohibited from being used to absorb high-order harmonics on the output side of the inverter. This is to prevent damage to the inverter tube caused by a large peak charging or discharging current at the moment the inverter tube is turned on. Secondly, since the user requires that the relative position of the level indicator and the inverter cannot be changed, and the level indicator cannot be moved away from the inverter, it was strictly metal-shielded. Furthermore, the signal shielding wire and the metal shielding layer were strictly and separately grounded from the system's working and safety grounds. The signal line and power line were spatially perpendicular to each other, effectively preventing the intrusion of electromagnetic interference from the surrounding space. After taking these measures, the entire system returned to normal operation, demonstrating that the measures were entirely feasible. Further experiments revealed that, in order to weaken interference signals propagating through the lines, a small inductor can be introduced into the control circuit when capacitor filtering is not used in the control circuit power supply. This inductor has a very low impedance at power frequency, but presents a very high impedance to high-frequency harmonic currents, effectively suppressing them. Its performance is quite good. In summary, while the anti-interference design of the system itself is very important in the design of variable frequency speed control systems, it is also necessary to fully consider the interference of the frequency converter itself to other electrical display control systems, especially the interference of high-order harmonics on the power supply system of the control circuit. When designing the power supply filter, the resonance and other problems that may be caused by the influence of high-order harmonics should be taken into account. References (1) Semiconductor converter technology, Xi'an Jiaotong University Press, Huang Jun (2) Design and maintenance of variable frequency speed control systems, Shandong Science and Technology Press, Zeng Yi et al.