Variable frequency drives (VFDs) are widely used in industrial speed control and transmission. Due to the switching characteristics of the VFD inverter circuit, its power supply forms a typical nonlinear load. VFDs often operate simultaneously with other equipment in the field, such as computers and sensors, which are frequently installed close together, potentially causing mutual interference. Therefore, power electronic devices, represented by VFDs, are one of the most significant sources of harmonics in the public power grid, and harmonic pollution generated by these devices has become a major obstacle to the development of power electronics technology itself. 1.1 What are Harmonics? The fundamental cause of harmonic generation is nonlinear loads. When current flows through a load, its relationship with the applied voltage is not linear, forming a non-sinusoidal current, thus generating harmonics. Harmonic frequencies are integer multiples of the fundamental frequency. According to the analysis principle of the French mathematician Fourier, any repeating waveform can be decomposed into sinusoidal components containing the fundamental frequency and a series of harmonics that are multiples of the fundamental frequency. Harmonics are sinusoidal waves, and each harmonic has a different frequency, amplitude, and phase angle. Harmonics can be classified into even and odd orders. The 3rd, 5th, and 7th harmonics are odd harmonics, while the 2nd, 14th, 6th, and 8th harmonics are even harmonics. For example, if the fundamental frequency is 50Hz, the 2nd harmonic is 100Hz, and the 3rd harmonic is 150Hz. Generally speaking, odd harmonics cause more and greater damage than even harmonics. In a balanced three-phase system, due to symmetry, even harmonics are eliminated, and only odd harmonics remain. For a three-phase rectified load, the harmonic currents are 6n±1 harmonics, such as 5th, 7th, 11th, 13th, 17th, and 19th harmonics. Frequency converters mainly generate the 5th and 7th harmonics. A schematic diagram of the harmonic definition is shown in Figure 1. [align=center]Figure 1. Schematic diagram of harmonic definition[/align] 1.2 Relevant standards for harmonic mitigation The following standards should be emphasized for harmonic mitigation of frequency converters: Isolation standards: EN50082-1, -2, EN61800-3; Radiation standards: EN50081-1, -2, EN61800-3. Especially important are IEC10003, IEC1800-3 (EN61800-3), IEC555 (EN60555), and IEEE519-1992. Common isolation standards EN50081 and EN50082, as well as the standard for frequency converters EN61800 (IEC1800-3), define the radiation and immunity levels of equipment operating in different environments. The above standards define acceptable radiation levels under different environmental conditions: Level L, no radiation limit. Applicable to users using frequency converters in interference-free environments and users who handle radiation limits themselves. Class H, according to the limitations defined in EN61800-3, first environment: limited distribution, and second environment. As an optional RFI filter, configuring an RFI filter can bring the inverter to commercial grade, typically used in non-industrial environments. 2. Harmonic Mitigation Measures To mitigate harmonic problems and suppress radiated interference and power supply system interference, shielding, isolation, grounding, and filtering techniques can be adopted. ① Use passive or active filters; ② Increase transformer capacity, reduce circuit impedance, and disconnect transmission lines; ③ Use green inverters without harmonic pollution. 2.1 Using Passive or Active Filters Passive filters mainly change the impedance of the power supply at extraordinary frequencies, suitable for stable, unchanging systems. Active filters are mainly used to compensate for nonlinear loads. Traditionally, passive filters are often chosen. Passive filters appeared earliest and remain the main means of harmonic suppression due to their simple structure, low investment, high reliability, and low operating costs. LC filters are traditional passive harmonic suppression devices. They consist of a suitable combination of filter capacitors, reactors, and resistors, connected in parallel with the harmonic source. Besides filtering, they also provide reactive power compensation. However, these devices have some significant drawbacks. Firstly, they are easily overloaded and can burn out under overload conditions, potentially leading to excessive power factor and penalties. Secondly, passive filters are uncontrollable; therefore, over time, aging components or changes in grid load can alter the resonant frequency, reducing filtering effectiveness. More importantly, passive filters can only filter one harmonic component (e.g., some filters only filter the third harmonic). If different harmonic frequencies need to be filtered, different filters must be used, increasing equipment investment. In contrast, various active filters are available both domestically and internationally. These filters can track and compensate for harmonics with varying frequencies and amplitudes, and their compensation characteristics are unaffected by grid impedance. The theory of Active Power Filters (APFs) was established in the 1960s. Later, with the maturity of large and medium power fully controllable semiconductor devices, advancements in pulse width modulation (PWM) control technology, and the proposal of instantaneous harmonic current detection methods based on instantaneous reactive power theory, APFs developed rapidly. Their basic principle is to detect harmonic currents from the object being compensated, and then generate a compensation current spectrum equal in magnitude but opposite in polarity to the harmonic current to cancel the harmonics generated by the original line harmonic source, thus ensuring that the grid current contains only the fundamental component. The core components are the harmonic current generator and control system, which operates using digital signal processing (DSP) technology to control fast-insulating bipolar transistors (IGBTs). Currently, in specific harmonic mitigation, a complementary hybrid approach using passive filters (LC filters) and active filters has emerged. This leverages the advantages of LC filters—simple structure, ease of implementation, and low cost—while overcoming the disadvantages of active power filters—large capacity and high cost. The combined use of both results in a system with excellent performance. 2.2 Reducing Circuit Impedance and Disconnecting Transmission Lines: The root cause of harmonic generation is the use of nonlinear loads. Therefore, the fundamental solution is to separate the power supply lines of the loads generating harmonics from those of loads sensitive to harmonics. The distorted current caused by nonlinear loads generates a distorted voltage drop across the cable impedance. This synthesized distorted voltage waveform is applied to other loads connected to the same line, causing harmonic currents to flow through them. Therefore, measures to reduce harmonic hazards can also be achieved by increasing the cable cross-sectional area and reducing the circuit impedance. Currently, many domestic methods involve increasing transformer capacity, increasing cable cross-sectional area, especially the neutral cable cross-section, and selecting circuit breakers and fuses with larger settings. However, these methods cannot fundamentally eliminate harmonics; instead, they reduce protection characteristics and functions, increase investment, and increase potential hazards in the power supply system. Linear and nonlinear loads can be powered separately from the same power interface point (PCC). This prevents the distorted voltage generated by the nonlinear load from being conducted to the linear load. This is currently a more ideal solution for mitigating harmonic problems. 2.3 Using Green Frequency Converters with No Harmonic Pollution The quality standards for green frequency converters are: both input and output currents are sinusoidal, the input power factor is controllable, the power factor can be 1 under any load, and the output frequency can be arbitrarily controlled above and below the power frequency. The built-in AC reactor in the frequency converter effectively suppresses harmonics and protects the rectifier bridge from instantaneous voltage spikes. Practice shows that the harmonic current without a reactor is significantly higher than that with a reactor. To reduce interference caused by harmonic pollution, a noise filter is installed in the output circuit of the frequency converter. Furthermore, the carrier frequency of the frequency converter is reduced to a level permissible by the frequency converter. In addition, high-power frequency converters typically use 12-pulse or 18-pulse rectification, thus reducing harmonic content in the power supply by eliminating the lowest order harmonics. For example, with a 12-pulse converter, the lowest harmonics are the 11th, 13th, 23rd, and 25th harmonics. Similarly, for an 18-pulse converter, the lowest harmonics are the 17th and 19th harmonics. Low-harmonic technologies used in frequency converters can be summarized as follows: ① Parallel multiplexing of inverter units: using two or more inverter units in parallel to cancel harmonic components through waveform superposition. ② Multiplexing of rectifier circuits: using 121-pulse, 18-pulse, or 24-pulse rectification in PWM frequency converters to reduce harmonics. ③ Series multiplexing of inverter units: using 30-pulse series inverter unit multiplexing circuits, which can reduce harmonics to a very small level. ④ Using new frequency conversion modulation methods, such as diamond modulation of voltage vectors. Currently, many frequency converter manufacturers have attached great importance to the harmonic problem and have ensured the greening of frequency converters through technical means in the design, thereby fundamentally solving the harmonic problem. 3. Conclusion In summary, the causes of harmonic generation can be clearly understood. Specific mitigation methods include using passive filters and active filters to reduce loop impedance, cutting off harmonic transmission paths, and developing and using green frequency converters that eliminate harmonic pollution. These methods aim to control the harmonics generated by frequency converters to a minimum, achieving scientific and rational electricity use, suppressing grid pollution, and improving power quality.