Introduction: Frequency converters involve high-power diode rectification and high-power transistor inversion, resulting in the generation of high-order harmonics in the input and output circuits, interfering with the power supply system, loads, and other nearby electrical equipment. In practical applications, frequency converter harmonic interference is frequently encountered. The following is a brief introduction to the mechanism of harmonic generation, propagation paths, and effective methods for suppressing interference. I. Mechanism of Harmonic Generation in Frequency Converters The main circuit of a frequency converter is generally composed of AC-DC-AC. An external 380V/50Hz power supply is uncontrolled rectified into a DC voltage signal by a three-phase bridge circuit, then filtered by a filter capacitor and inverted into a variable-frequency AC signal by high-power transistor switching elements. In the rectifier circuit, the waveform of the input current is an irregular rectangular wave. The waveform is decomposed into a fundamental wave and various harmonics according to Fourier series, with the higher-order harmonics interfering with the input power supply system. In the inverter output circuit, the output current signal is a pulse waveform modulated by the PWM carrier signal. For GTR high-power inverter components, the PWM carrier frequency is 2-3kHz, while the PWM carrier frequency of IGBT high-power inverter components can reach 15kHz. Similarly, the output circuit current signal can also be decomposed into a fundamental wave containing only a sine wave and other harmonics, and the high-order harmonic current directly interferes with the load. In addition, the high-order harmonic current also radiates into space through the cable, interfering with nearby electrical equipment. II. Commonly used methods to suppress harmonic interference The propagation paths of harmonics are conduction and radiation. Solving conduction interference mainly involves filtering out or isolating the high-frequency current conducted in the circuit; solving radiation interference involves shielding the radiation source or the line being interfered with. Commonly used methods include: (1) The power supply of the frequency converter system is independent of the power supply of other equipment, or an isolation transformer is installed on the input side of the frequency converter and other electrical equipment to cut off the harmonic current. (2) Connect suitable reactors in series between the input and output sides of the frequency converter, or install harmonic filters. The filters must be of LC type to absorb harmonics and increase the impedance of the power supply or load to suppress harmonics. (3) The cable between the motor and the frequency converter should be laid in steel pipe or armored cable, and should be laid separately from other weak current signals in different cable trenches to avoid radiation interference. (4) Shielded cables should be used for signal lines, and the wiring should be staggered from the main circuit control lines of the frequency converter by a certain distance (at least 20cm) to cut off radiation interference. (5) The frequency converter should use a dedicated grounding wire, and a thick, short wire should be used for grounding. The grounding wires of other electrical equipment nearby must be separated from the frequency converter wiring and short wires should be used. This can effectively suppress the radiation interference of current harmonics to nearby equipment. III. Examples of Harmonic Interference Suppression Example 1 : In a variable frequency drive (VFD) switching control system, the VFD started and ran normally, but the reading of a nearby level gauge was too high. When the primary input was 4mA, the level reading was not at the lower limit; when the level had not reached the set upper limit, the level gauge displayed the upper limit, causing the VFD to receive a shutdown command and forcing it to stop. This was clearly high-order harmonic interference from the VFD to the level gauge, and the interference propagation path was the power supply circuit or signal line of the level gauge. Solution: The power supply to the level gauge was taken from a different power transformer, reducing harmonic interference. The signal line was then run through a steel conduit and separated from the main return line of the VFD by a certain distance. After these measures, the harmonic interference was basically suppressed, and the level gauge returned to normal operation. Example 2 : In a variable frequency control liquid level display system, the liquid level gauge and the frequency converter are installed in the same cabinet. The frequency converter works normally, but the liquid level gauge display is inaccurate and unstable. Initially, we suspected problems with the primary and secondary instruments, signal lines, and the fluid medium. We replaced all these instruments and signal cables and improved the fluid characteristics, but the fault persisted. The fault was caused by the high-order harmonic current of the frequency converter radiating outward through the output circuit cable and transmitting to the signal cable, causing interference. Solution: Separate the liquid level gauge signal line and its control line from the frequency converter's control line and main circuit line by a certain distance. Also, run the signal lines outside the cabinet through steel conduits, and ensure the outer casing is properly grounded. The fault was resolved. Example 3 : In a variable frequency control system, there are two frequency converters installed in the same cabinet. Both frequency converters are manually adjusted using potentiometers. When one frequency converter is running, it works normally. However, when both are running simultaneously, their frequencies interfere with each other. That is, adjusting the potentiometer of one frequency converter affects the frequency of the other, and vice versa. Initially, we thought it was a potentiometer and control line fault. After ruling out this possibility, we determined it was caused by harmonic interference. Solution: We moved one of the potentiometers to another cabinet and fixed it in place, using shielded signal cables for the leads. This reduced the interference. To completely suppress the interference, we fabricated a new control cabinet and placed it at a certain distance from the original cabinet. We moved one of the frequency converters to this new cabinet, making necessary modifications to the wiring and leads. After this treatment, the interference was essentially eliminated, and the fault was resolved. Example 4 : In a variable frequency control system, two sets of pumps were being switched. Previously, the pumps operated normally using autotransformer reduced voltage starting at power frequency. Now, they were switched to variable frequency operation. Although the frequency adjustment and deceleration functions were achieved, the output lines between the frequency converter and the motor overheated severely, increasing the temperature rise of the motor casing and frequently causing protection trips. This was because the frequency converter's output voltage and current signals contained high-order harmonics of the PWM circuit, and the harmonic current created additional power losses in the output conductors and motor windings. Solution: Separate the inverter's input and output lines, running them through their respective cable trenches. Replace the original cable with a larger cross-section cable, and keep the cable length between the output end and the motor as short as possible. This resolved the overheating problem. The above methods can generally suppress most high-order harmonic interference from inverters encountered in the field. However, for equipment with very strict requirements on harmonic composition and amplitude, completely suppressing high-order harmonic interference is very difficult and requires further research and development.