Any circuit with a rectifier on the power supply side will generate high-order harmonics due to its nonlinearity. The main circuit of a frequency converter is generally composed of AC-DC-AC. An external 380V/50Hz power supply is rectified into DC by a thyristor three-phase bridge circuit, filtered by capacitors, and then inverted into AC with a variable frequency. In the rectifier circuit, the waveform of the input current is an irregular rectangular wave. The waveform is decomposed into a fundamental wave and higher harmonics according to Fourier series, with the harmonic order typically being 6N±1 (N being the natural constant). If the power supply-side reactance is sufficiently small and the commutation overlap μ is negligible, then the effective value of the Kth harmonic current is 1/K of the fundamental current. II. Hazards of Higher Harmonics Harmonic problems have existed for a long time, and in recent years, this problem has become more serious due to the combined effect of two factors. These two factors are: the industry's widespread use of power electronic devices such as frequency converters to improve production efficiency and reliability has led to a rapid increase in the use of thyristor-related equipment, accompanied by a synchronous increase and amplification of harmonic sources; and power users' large-scale use of capacitor banks to improve power factor, with parallel capacitors exacerbating the harmonic hazards through resonance. Harmonic currents generated by nonlinear loads are injected into the power grid, increasing the harmonic voltage on the low-voltage side of the transformer. The low-voltage side load is affected by harmonic interference, impacting normal operation. Furthermore, the harmonic voltage is transmitted to the high-voltage side through the power supply transformer, interfering with other users. In a three-phase circuit, harmonic currents that are integer multiples of the third order are zero-sequence currents, which are superimposed in the neutral line. Zero-sequence harmonic currents are mainly generated by three-phase four-wire nonlinear equipment, resulting in a large neutral line current in the power supply system. When there is a large harmonic current on the neutral line, the impedance of the neutral conductor can generate a large neutral line voltage drop under harmonic conditions. This neutral line voltage drop interferes with the normal operation of computers and various microelectronic systems in the form of common-mode interference, making control equipment and precision instruments unreliable and causing a high failure rate. The hazards of high-order harmonics are specifically manifested in the following aspects: Transformers Harmonic currents and harmonic voltages will increase the copper and iron losses of transformers, resulting in increased transformer temperature, affecting insulation capacity, and reducing capacity margin. Harmonics can also generate resonance and noise. Induction Motors Harmonics also increase the copper and iron losses of motors and raise their temperature. At the same time, harmonic currents will change the electromagnetic torque, generating vibration torque, causing periodic speed fluctuations in the motor, affecting output efficiency, and generating noise. Switching Equipment Because harmonic currents cause switching equipment to generate a very high current change rate at startup, the transient recovery peak voltage increases, damaging insulation, and can also cause switch tripping and malfunctions. Harmonics in the current of protective electrical appliances can generate additional torque, altering their operating characteristics, causing malfunctions, and even burning out coils. Metering Instruments Metering instruments can experience additional torque on the induction plate due to harmonics, leading to errors, reduced accuracy, and even coil burnout. Power Electronic Equipment Power electronic equipment typically relies on the principle of precise power supply zero-crossing or the shape of the voltage waveform for control and operation. If the voltage contains harmonic components, zero-crossing shifts, waveform changes, and numerous malfunctions can occur. Computers and some other electronic devices typically require a total harmonic distortion (THD) rate of less than 5%, and individual harmonic distortion rates of less than 3%. Higher distortion rates can cause control equipment malfunctions, leading to production or operational interruptions and significant economic losses. Power Cables High-frequency harmonic currents can induce the skin effect in conductors, generating additional temperature rise and increasing copper losses. In particular, the zero-sequence third harmonic current is superimposed in the neutral line, resulting in a very large neutral line current in the power supply system. In some cases, the neutral line current may even exceed the phase current, causing the neutral line to heat up, accelerating insulation aging, and even causing fires. Furthermore, when there is a large harmonic current on the neutral line, the impedance of the conductor can generate a large neutral line voltage drop, interfering with the normal operation of various microelectronic systems. Power Capacitors Due to the increased frequency of higher harmonics, the impedance of capacitors to higher harmonics decreases, leading to overheating due to overcurrent, and even damage to the capacitors. Parallel or series circuits formed by capacitors and inductive loads in the system may also experience harmonic resonance, amplifying harmonic currents or voltages and exacerbating the harm. Parallel resonant circuits formed by capacitor banks and grid inductance can be amplified by 10-15 times. III. Solutions to High-Order Harmonic Pollution from Frequency Converters High-order harmonics mainly cause harmonic pollution to the power supply and nearby electrical equipment through two methods: conduction and inductive coupling. Conduction refers to the shunting of higher harmonics according to their respective impedances to the power system and parallel loads, interfering with parallel electrical equipment. Inductive coupling refers to the electromagnetic coupling that occurs when harmonics are conducted along conductors laid parallel to the power line, creating induced interference. In actual industrial production, to eliminate the interference of high-order harmonics from frequency converters on electrical equipment, the main solutions are to suppress the interference source and cut off the coupling path of interference to the system, as well as to avoid resonance between the power compensation capacitor and the system. Solving conducted interference mainly involves filtering out or isolating the conducted high-frequency harmonic current in the circuit; rationally arranging the distance and direction between the interference source and the interfered line can avoid or reduce coupling. IV. Application of Anti-interference Measures in Practical Engineering With the gradual improvement of industrial production technology and the increasing application range of frequency converters, the electromagnetic interference and pollution problems caused by high-order harmonics from frequency converters are becoming increasingly prominent. How to properly handle the harmonic interference and pollution problems of frequency converter systems has become increasingly important, especially in places with high requirements for harmonic pollution control. Isolation Measures Isolation technology is one of the important technologies in electromagnetic compatibility. Interference isolation refers to isolating the interference source and the susceptible parts from the circuit, preventing them from being electrically connected. (1) Install an AC reactor on the AC input side of the frequency converter to increase the rectifier impedance and increase the rectifier overlap angle, thereby reducing the high-order harmonic current. (2) Ensure that all signal lines are well insulated to prevent leakage, thus preventing interference introduced by contact. (3) Separate different types of signal lines (in different cable trays, separated by partitions). According to the different types of signals, they can be divided into several grades according to their noise interference resistance, and run separately by cable or cable tray. Grounding measures Grounding has two functions: one is to protect people and equipment from damage (protective grounding); the other is to suppress interference (working grounding). Proper grounding can effectively suppress external interference and reduce the interference of the equipment itself to the outside world. In order to ensure that the frequency converter control system and the connected instruments can operate reliably and ensure measurement and control accuracy, a reliable working ground must be established for the frequency converter. It is divided into power ground, signal ground, analog ground (AG shielding ground), and intrinsically safe ground in petrochemical and other explosion-proof systems. Before the various grounds of the frequency converter are connected to the grounding busbar, they should be kept insulated from each other to avoid grounding interference. Anti-resonance measures Harmonics are very dangerous to capacitors connected in power factor correction circuits. The capacitance of the capacitor and the inductance of the power grid form a resonant circuit. The self-resonant frequency of this circuit is typically between 250 and 500 Hz, within the 5th and 7th harmonic range. When the harmonic frequency in the power grid is close to the self-resonant frequency, it can amplify the harmonic current to about 20 times the normal value. Power grids affected by harmonics cannot use conventional capacitors for reactive power compensation. When harmonics are present in the system, using tuned filter capacitor banks is one of the best methods for power factor compensation. Untuned filter capacitor banks, composed of capacitors and reactors connected in series, can compensate reactive power in the fundamental frequency range while demodulating the self-resonant frequency of the resonant circuit. Tuned filter capacitor banks consist of several capacitor segments and tuned reactors, each segment forming a series resonant circuit, making the resonant frequency lower than the lowest harmonic frequency. For systems containing harmonics of the 5th order or higher, tuned capacitor banks with 6% reactors are used; for systems containing harmonics of the 3rd order or higher, tuned capacitor banks with 14% reactors are used. At the fundamental frequency (50Hz), the tuned filter capacitor bank exhibits capacitive behavior to provide reactive power; while at harmonic frequencies, it exhibits inductive behavior, thus avoiding the formation of a parallel resonant circuit with the network and preventing harmonic amplification. Therefore, the tuned filter capacitor bank can safely compensate for reactive power and eliminate approximately 30% of low-order harmonic currents. Filtering Technology Filters can effectively suppress conducted harmonic interference. In low-voltage power grids, when the harmonic current distortion rate (THD_I) > 10% or the harmonic voltage distortion rate (THD_V) > 3%, harmonic filters can be considered. Appropriate filtering equipment can be considered for different harmonic sources and electrical equipment. When the inverter in the system primarily uses three-phase six-pulse full-wave rectification, according to the formula K=6N±1, the harmonics are mainly 5th and 7th orders. Parallel 5th and 7th order single-tuned filters are typically used. When the inverter in the system is mainly used in single-phase circuits within a three-phase four-wire system, the harmonics are mainly 3rd order harmonics with zero phase sequence. A parallel 3rd order harmonic filter should be installed. When the system requires high anti-interference capability or has complex harmonic content, an active power filter can be connected in parallel at the power input to reduce pollution from high-order harmonics of the inverter. Active power filters can effectively filter out harmonics from the 2nd to 50th orders in the power grid, with a response time of less than 1 millisecond, making it one of the most effective filtering technologies currently available.