Abstract : With the development of power electronics technology, the harm caused by harmonics has become increasingly serious, making harmonic control an urgent issue. This paper analyzes and summarizes the harmonic sources of power electronic devices, points out their hazards and corresponding harmonic management principles and comprehensive control methods, and provides an outlook on harmonic control work. Keywords : Power electronics; Harmonics; Hazards; Suppression Introduction With the development of power electronics technology, the widespread application of power electronic devices has brought serious harmonic pollution to the power system. The widespread application of various power electronic devices in transportation, metallurgy, chemical industry, and many other industrial and transportation fields has made the harmonic problem in the power grid increasingly serious. Many low-power-factor power electronic devices place an additional burden on the power grid and affect power supply quality. Therefore, harmonic pollution from power electronic devices has become a major obstacle to the development of power electronics technology. Thus, research on suppressing harmonic pollution and improving the power factor has become a major topic in power electronics technology. This paper focuses on this key issue, through understanding and analyzing the harmonic sources of power electronics and their hazards, considering the relationship between pollution and prevention, exploring comprehensive control methods, and finally providing an outlook on the development trend of comprehensive harmonic control. 1. Power Electronic Devices – The Primary Harmonic Source Nonlinear loads are harmonic sources, causing voltage distortion in the power grid and introducing components with frequencies multiples of the fundamental frequency. The primary harmonic sources in power electronic devices are various AC/DC converters (rectifiers, inverters, choppers, frequency converters) and bidirectional thyristor-controlled switching devices, as well as internal power system converters such as rectifier valves and inverter valves in DC transmission. The harmonics they generate are analyzed below. 1.1 Rectifiers As DC power supply devices, rectifiers are widely used in various applications. Figures 1(a) and 1(b) show typical single-phase and three-phase circuits, respectively. In a rectifier, the AC current is a rectangular wave, a composite waveform of the fundamental current and higher-order harmonic currents that are odd multiples of the fundamental frequency. The ratio of the higher harmonic component In to the fundamental component I1 in the rectangular wave is found to be 1/n by Fourier series. As the trigger control angle α decreases and the commutation overlap angle μ increases, the harmonic component tends to decrease. In addition, existing research results show that the operating mode of the rectifier also has a direct impact on the magnitude of the harmonic current. Therefore, when considering adjusting the rectifier voltage and current, it is best to perform overlap angle, commutation voltage drop and harmonic calculation in order to determine a safe and economical operating mode. When the control angle α is close to 40° and the overlap angle μ is around 8°, the situation is often the most severe state of harmonics. Therefore, it is necessary to calculate and try to avoid the most severe harmonic point by correctly selecting the tap of the voltage regulating transformer [1]. 1.2 AC voltage regulator AC voltage regulators are mostly used in lighting dimming and induction motor speed regulation. Figure 2(a) and Figure 2(b) are typical circuits of single phase and three phase, respectively. The harmonic order generated by the AC voltage regulator is basically the same as that of the rectifier. 1.3 Frequency Converter Frequency converters are representative AC/AC converters. When used as speed control devices for motors, they contain sidebands that vary with the output frequency. Due to the continuous frequency variation, the harmonic content is quite complex. 1.4 General-Purpose Frequency Converter The input circuit of a general-purpose frequency converter typically consists of a diode full-bridge rectifier circuit and a DC-side capacitor, as shown in Figure 3(a). The input current waveform of this circuit varies greatly depending on the impedance. When the power supply impedance is relatively small, the waveform is a narrow and tall elongated waveform, as shown by the solid line in Figure 3(b); conversely, when the power supply impedance is relatively large, the waveform is a short and wide flat waveform, as shown by the dashed line in Figure 3(b). Besides the typical converters mentioned above generating a large number of harmonics, household appliances are also significant sources of harmonics. Examples include televisions and battery chargers. Although their individual capacities are small, their large numbers mean that the harmonic components they inject into the power supply system are not negligible. 2. Harms of Harmonics The main harms of harmonics to the public power grid include: 1) Increasing harmonic losses in components of the power grid, reducing the efficiency of power generation and transmission equipment. Large amounts of third harmonics flowing through the neutral line can cause overheating and even fires; 2) Affecting the normal operation of various electrical equipment. Besides causing additional losses, it can also cause mechanical vibration, noise, and overvoltage in motors, severe local overheating in transformers, and overheating, insulation aging, and shortened lifespan of capacitors, cables, and other equipment, leading to damage; 3) Causing local parallel and series resonances in the public power grid, thus amplifying harmonics and greatly increasing the aforementioned harms, even causing serious accidents; 4) Leading to malfunctions of relay protection and automatic devices, and inaccurate measurement by electrical measuring instruments; 5) Interfering with nearby communication systems, ranging from noise and reduced communication quality to information loss and system malfunction. 3. Harmonic Management Principles To improve power quality, it is essential to strengthen the management of harmonics. The principle is to limit the injection of harmonic current into the public power grid and keep harmonic voltage within permissible limits. First, it is essential to understand the harmonic sources and their distribution within the system, restricting their harmonics to within permissible limits before allowing them into the grid. Those failing to meet the standards must be addressed with mitigation measures to prevent harmonic propagation. For this purpose, both the International Electrotechnical Commission (IEC) and the American Institute of Electrical and Electronics (IEEE) have recommended standards. For example, the IEEE's current harmonic limit standards are shown in Table 1. China's voltage sinusoidal waveform distortion rate regulations, formulated based on the actual level of the power grid and drawing on other national standards, are shown in Table 2. Table 1 Harmonic Current Limits (IEEE 519-1992) Table 2 Voltage Sinusoidal Waveform Distortion Rate Limits 4 Comprehensive Harmonic Management Currently, China's power system management of harmonics presents a passive situation of "polluting first, then treating," so how to comprehensively manage harmonics has become an urgent research topic. Regarding the connotation of "comprehensive," some believe it should be described as having a wide scope and being widely promoted; others believe that using a collective and integrated approach is more practical. This author believes that comprehensive governance should include the following two aspects: —Strengthening scientific and legal management; —Taking effective technical measures to prevent and suppress harmonics. 4.1 Strengthening Scientific and Legal Management Management should be strengthened mainly in two aspects: —Using legally and economically binding means to change the passive situation of "pollute first, then treat." This means strictly evaluating the harmonic content of various power equipment and power electronic equipment according to the technical specifications. If the content exceeds the national standards, the equipment should not be allowed to leave the factory or be put into use in the power system; —Power supply departments should take a holistic approach, plan comprehensively, and take effective measures to strengthen technical supervision and management. On the one hand, they should review the harmonic levels of loads yet to be put into operation; on the other hand, they should require users to install filter devices for harmonic source loads already in operation. 4.2 Adopting Effective Technical Measures Currently, there are two main technical approaches to solving harmonic pollution from power electronic equipment: —Active harmonic suppression schemes, which involve improving the power electronic devices themselves to prevent them from generating harmonics, or controlling their power factor as needed; —Passive harmonic suppression schemes, which do not change the harmonic load itself, but instead add passive filters (PF), active filters (APF), or hybrid filters (HAPF) to the AC side of the power system or harmonic load to compensate for harmonics in the power grid through external equipment. 4.2.1 Active Harmonic Suppression Schemes These schemes mainly start from the converter device itself, reducing or eliminating harmonics through the structural design of the converter device and the addition of auxiliary control strategies. Currently, the main technologies used include: —Multi-pulse converter technology: High-power power electronic devices often design the original 6-pulse converter to a 12-pulse or 24-pulse converter to reduce the harmonic current content on the AC side. Theoretically, the more pulses, the better the harmonic suppression effect. However, a higher pulse count leads to a more complex rectifier transformer structure, larger size, and increased difficulty and cost in controlling and protecting the converter. —Pulse Width Modulation (PWM) Technology: The basic idea of ​​PWM technology is to control the transition times of the PWM output waveform to ensure the symmetry of the quarter-wavelength. Based on the Fourier series expansion of the output waveform, the amplitude of the harmonics to be eliminated is made zero, and the fundamental amplitude is set to a given value, thus achieving the purpose of eliminating specified harmonics and controlling the fundamental amplitude. Currently used PWM technologies include optimal pulse width modulation, improved sinusoidal pulse width modulation, delta modulation, tracking PWM modulation, and adaptive PWM control. —Multilevel Converter Technology: For various power electronic converters (voltage-type converters must be connected to the AC power supply using a connecting inductor), methods such as phase-shifting multiplexing, sequential control, and asymmetric control multiplexing are used to superimpose square wave currents or voltages, making the current or voltage generated by the converter on the grid side a near-sinusoidal stepped wave, maintaining a certain phase relationship with the power supply voltage. —A power factor pre-regulator is added to the power electronic device. On the DC side of the pre-regulator, the input current is controlled by a DC/DC converter to ensure that the current drawn from the power electronic device from the grid is sinusoidal and in phase with the grid voltage. This method is simple to control and can simultaneously eliminate high-order harmonics and compensate for reactive current, bringing the power factor at the input of the power electronic device close to 1. The main problems with active harmonic suppression schemes are high cost and low efficiency. Furthermore, the high switching frequency in power electronic systems causes high-order harmonics in the PWM carrier signal, leading to high-level conducted and radiated interference. Therefore, when designing an active harmonic suppression scheme, EMI filters must be used to filter out high-order harmonic signals from the system to prevent them from entering the grid as conducted interference; shielding must also be used to prevent them from entering free space as radiated interference, causing electromagnetic pollution. Therefore, for higher-power power electronic devices, in addition to active harmonic suppression methods, passive or active filters are generally used to further suppress high-order harmonics. 4.2.2 Passive Harmonic Suppression Scheme – Passive Filter (PF) Passive filters typically use a combination of power capacitors, reactors, and resistors according to functional requirements to provide a parallel low-impedance path for harmonics in the system, thus achieving a filtering effect. The advantages of passive filters are low investment, high efficiency, simple structure, reliable operation, and convenient maintenance. Therefore, passive filtering is currently the widely used main method for suppressing harmonics and performing reactive power compensation. The disadvantages of passive filters are that their filtering characteristics are determined by the impedance ratio between the system and the filter; they can only eliminate specific harmonics, while amplifying other harmonics; under certain conditions, they may resonate with the system; the filter load increases with the increase of harmonic current, potentially causing filter overload; and they consume more effective materials, resulting in a larger size. – Active Filter (APF) Figure 4 shows the APF schematic. The APF detects the harmonic current in the power grid through a detection circuit, then controls the inverter circuit to generate a corresponding compensation current component, which is injected into the power grid to achieve harmonic elimination. The filtering characteristics of the APF are not affected by the system impedance, eliminating the risk of resonance with the system impedance. Compared to passive filters, APFs offer high controllability and fast response, not only compensating for harmonics but also suppressing voltage flicker and compensating for reactive current, making them cost-effective. Furthermore, APFs have adaptive capabilities, automatically tracking and compensating for changing harmonics. APFs are classified according to their connection method to the system, including series, parallel, hybrid, and series-parallel types. Parallel APFs can be considered equivalent to a controlled current source, primarily suitable for harmonic compensation of inductive current source loads. They can dynamically compensate for harmonics and reactive current, and their compensation characteristics are unaffected by grid impedance. This type of APF technology is currently quite mature, and most industrial APFs are of this type. Series APFs can be considered equivalent to a controlled voltage source, mainly used to eliminate the influence of voltage-type harmonic source loads such as diode rectifier circuits with capacitor filters on the system, as well as the influence of system-side voltage harmonics and voltage fluctuations on sensitive loads. Since the current flowing through this type of APF is a nonlinear load current, the loss is relatively large. In addition, the switching of series APF and the exit after a fault are more complicated than those of parallel APF. Therefore, there are few cases of using this type of APF alone. Domestic and foreign research focuses on the hybrid APF composed of it and LC passive filter [2]. The hybrid APF removes the fundamental voltage on the conventional APF, so that the active device only bears the harmonic voltage, thereby significantly reducing the capacity of the active device and achieving the purpose of reducing cost and improving efficiency. Among them, the LC filter is used to eliminate high-order harmonics, and the APF is used to compensate for low-order harmonic components. The series-parallel type APF is also called the power quality regulator (UPQC) [3]. It has the functions of series and parallel APF and can solve most of the power quality problems that occur in the power distribution system. It has a high cost performance. Although it is still in the experimental stage, from a long-term perspective, it will be a very promising active filter device. As a key technology for improving power supply quality, active filter technology has been highly valued and increasingly widely used in industrialized countries such as Japan, the United States, and Germany. However, there are still some problems that need to be solved for active filters, such as improving compensation capacity, reducing cost and loss, further improving compensation performance, and improving device reliability. At the same time, APF failure can easily cause system failure, so countries still maintain a certain cautious attitude towards this technology [4]. ——Active Circuit Regulator (APLC) Figure 5 is the schematic diagram of Active Circuit Regulator (APLC). Its structure is similar to that of APF, so many documents in the past have equated it with APF. In fact, in principle, compared with APF single-node harmonic suppression, APLC injects compensation current into one (several) preferred nodes in the network. Through the flow of compensation current in a certain range in the network, the comprehensive suppression of harmonic voltage of all nodes in that range is achieved. That is, through the installation of a single node and a single device, the function of comprehensive management of multi-node harmonic voltage is achieved. The emergence of APLC indicates that the harmonic management of the power system is developing towards a dynamic, intelligent and economically efficient direction. 5 Prospect of Comprehensive Harmonic Management The increasingly serious harmonic pollution has attracted great attention from all parties. With a deeper understanding of the mechanisms and phenomena of harmonic generation, more effective methods will be found to suppress and eliminate harmonics, which will also help in formulating more reasonable harmonic management standards. Increased investment in harmonic research will greatly accelerate the solution to the harmonic problem. Of course, the ultimate solution to the harmonic problem will depend on the development of related technologies, especially power electronics technology. With the further development of the national economy, harmonic suppression technology, the further improvement of the legal system, and the increasing demands for efficient energy utilization, the harmonic governance problem will eventually be properly resolved. With the development of electronic computers and power semiconductor devices, the performance of active power filters will improve, and their prices will decrease. However, the prices of capacitors and reactors used for passive filtering are increasing. Therefore, active power filters will be the main development direction for harmonic suppression devices in the future. In addition, active power factor correction technology in power electronics is also highly promising. 6 Conclusion Comprehensive harmonic governance is imperative. The work of eliminating harmonic pollution from power electronic devices can be called a "green project" for the application of power electronics technology. The development of power electronics technology must proceed in tandem with this project. Only in this way can we open up important avenues for the efficient and low-pollution utilization of electrical energy, promoting the development of our national economy and the innovation of electrical equipment. Simultaneously, the promotion and application of power electronics technology will have a broader prospect for development.