1. Introduction Due to the significant energy-saving effects and substantial economic benefits of high-power variable frequency speed control schemes for fans and pumps, research on high-voltage, high-power variable frequency speed control technology has become one of the leading directions in energy conservation efforts worldwide. Power electronic converter circuits remain the core of frequency conversion technology. Since power electronic devices operate in a switching state, devices composed of these circuits have become major harmonic sources in power systems. The harmonic current output by the frequency converter can cause resonance and harmonic current amplification, damaging rotating motors and transformers, and affecting the accuracy of relay protection and power measurement. In recent years, researchers have proposed different rectification and inversion schemes in terms of circuit structure and control technology to suppress harmonic currents, resulting in diverse high-power frequency conversion technologies. This paper systematically summarizes the structure of high-voltage, high-power frequency converters, studies the harmonic suppression principles of various frequency converters, deeply analyzes the input and output harmonics of high-voltage, high-power frequency converters, and conducts a comparative study based on IEEE-519 standards, providing a reference for frequency converter selection. 2. Introduction to Harmonic Suppression Standard (IEEE-519) To limit harmonic interference from converters and nonlinear loads to power systems, countries and organizations worldwide have developed relevant standards to ensure power supply quality. The most authoritative standard is IEEE-519, developed by the Institute of Electrical and Electronics Engineers (IEEE) and adopted as an American National Standard (ANSI). This standard analyzes in detail the causes and effects of waveform distortion; identifies parameters for judging the degree of distortion; sets limits on waveform distortion in power systems; and introduces analysis methods and control measures for waveform distortion. It provides guidance for engineers developing and applying high-power variable frequency speed control systems. The limits in IEEE-519 are all based on the "worst-case" conditions for steady-state operation; exceeding these limits is permitted during transient processes. Table 1 lists the IEEE-519 limits for voltage harmonics. [align=center]Table 1 IEEE-519 Limits on Voltage Harmonics[/align] Table 2 lists the limits on harmonic current values ​​and total harmonic distortion (THD) values ​​in power supply systems below 6.9kV under different short-circuit ratios (short-circuit ratio SCR is defined as the ratio of the maximum short-circuit current Is to the average set maximum load current Il), while even harmonics are limited to less than 25% of odd harmonics. Therefore, it is very important to correctly select the main circuit connection form (equivalent number of phases, number of pulses) and control method according to the ratio of power electronic device capacity to power system short-circuit capacity. [align=center]Table 2 IEEE-519 Limits on Current Harmonics[/align] 3 High-Voltage Frequency Converter Input Harmonic Analysis 3.1 Basic Principle of Multi-Pulse Rectification for Suppressing Input Harmonics The multi-phase superposition technology was proposed by A. Kernick et al. as early as 1962. This technology uses a 6-pulse three-phase full-wave rectifier (or equivalent three-phase full-wave rectifier) ​​with a pulsation width of 60° as the basic unit. The AC side voltages of m rectifier circuits are sequentially phase-shifted by α = 60°/m, thus forming a multi-pulse rectifier with a pulsation number of p = 6m. The relationship between the pulsation number p, the number of groups m, the phase shift angle α, and the corresponding harmonic order h is shown in Table 3. [align=center]Table 3 Composition of Multi-Pulse Rectifier[/align] For a 12-pulse rectifier, the rectifier transformers are conventionally connected y/y-12 (or δ/δ-12) and y/δ-11 or (δ/y-1), with their AC side secondary voltages phase-shifted by 30°. The DC sides are connected in parallel (or in series) to form a 12-pulse rectifier. For rectifiers with 18 pulses or more, the rectifier transformer windings are implemented using a zigzag connection (z-connection), with each rectifier unit connected in parallel (or in series) to supply power to the load. As long as the voltage u(n) (n=1,2,……,m) on the AC side of the m-group 6-pulse rectifier is shifted by α=60°/m, a multiphase rectifier with p=6m pulses can be obtained. The specific transformer group selection is shown in Table 4. 3.2 Simulation Analysis of Input Harmonics in Multi-Pulse Rectifier The Simulink/Power System toolbox in MATLAB was used to conduct simulation research on multi-pulse rectification. This paper constructs a unified analysis module for multiplexed rectification. After setting the parameters, it can realize the working characteristics of 12, 18, 24, 30, and 36 pulse rectifier circuits. Following the relevant instructions in the parameter panel, selecting the appropriate transformer connection and inputting the phase shift angle allows for the simulation analysis of multiplexed rectification with the corresponding number of pulses. The parameter setting panel of the multi-pulse rectifier input simulation circuit is shown in Figure 1. [align=center]Figure 1. Parameter setting dialog box for multi-pulse rectification simulation circuit[/align] Taking the 12-pulse simulation as an example, the waveform and spectrum are shown in Figure 2. It can be seen that the main harmonics in 12-pulse rectification are 12k±1, which is consistent with the theoretical analysis. [align=center]Figure 2. Waveform and spectrum of 12-pulse rectification[/align] Based on the IEEE-519 standard, the rectification of each pulse number is compared as shown in Table 5. It can be seen that without adding other filtering devices, 12-pulse rectification can meet the requirements of IEEE-519, and the harmonic content exceeds the standard in all ranges. The 36-pulse situation is much better. The harmonics below the 35th order and the THD can meet the requirements of IEEE-519, but it still contains relatively large harmonics of the 35th and 37th orders. The analysis shows that multi-pulse rectification effectively solves the harmonic suppression problem at the input of the frequency converter, especially the suppression effect of low-order harmonics, and the input waveform is approximately sinusoidal, which well meets the requirements. However, compared with the IEEE-519 standard, without adding other filtering devices, multi-pulse rectification cannot meet the requirements of IEEE-519 for all harmonics. The influence of higher harmonics is still significant, requiring its use in conjunction with other filters. 4. High-Voltage Inverter Output Harmonic Analysis As the output stage of a high-voltage, high-power inverter, a high-performance inverter is essential to its performance. However, unlike low-voltage inverters, high-voltage, high-power inverters do not have mature, unified technology; various topologies and control schemes each have their own advantages and disadvantages. 4.1 Harmonic Analysis of Transformer-Coupled Output Inverter In 1999, Cengelci et al. proposed this topology. Its main idea is to superimpose the outputs of three conventional two-level three-phase inverters (composed of high-voltage IGBTs or IGBTs) using a transformer, achieving high-quality three-phase high-voltage output, low DV/DT PWM waves, and excellent balanced operation. The utilization rate of each three-phase inverter is close to 100%, making it particularly suitable for driving constant torque and variable torque loads. Furthermore, these three conventional inverters can be controlled using ordinary low-voltage frequency converter control methods, greatly simplifying the inverter's circuit structure and control method. This structure is shown in Figure 3. [align=center]Figure 3 Transformer-coupled output inverter topology[/align] A transformer-coupled output inverter requires only three independent three-phase inverters to generate medium-to-high voltage output. During operation, each inverter operates in parallel, providing one-third of the output power. This facilitates the use of low-voltage IGBT devices in high-voltage systems. This balanced operation also reduces the need for the DC-side capacitors to store excessive energy. The presence of the output transformer helps generate higher output voltage and eliminates circulating current between inverters. The simulation waveforms and spectra of this structure in MATLAB are shown in Figures 4 and 5. [align=center]Figure 4 Output voltage and spectrum of transformer-coupled output inverter[/align] [align=center]Figure 5 Output current and spectrum of transformer-coupled output inverter[/align] The output waveform of the transformer-coupled output inverter can be equivalent to a 7-level line voltage PWM wave, which is better than that of a regular two-level inverter. Its dv/dt is also lower, containing only very small low-order harmonics, and the THD value is also very low. However, high-order harmonics still exist, such as the 23rd and 25th harmonics. This is mainly due to the use of PWM modulation by each independent inverter. This problem can be solved by adopting a better modulation strategy or adding a small-capacity low-pass filter. 4.2 Multilevel Inverters In 1980, A. Nabae et al. of Nagaoka University of Technology in Japan proposed a three-level inverter, also known as a neutral point clamped (NPC) inverter. After years of research, two main topologies have emerged: diode clamped and flying capacitor. The diode clamped topology is shown in Figure 6. [align=center]Figure 6 Three-level inverter topology[/align] Compared with the traditional two-level topology, the midpoint clamped three-level inverter is more suitable for the high voltage and large capacity characteristics of medium and high voltage frequency converters. The special topology gives the devices twice the forward blocking voltage capability. Its multi-layered stepped output voltage, theoretically, can make the output voltage waveform closer to a sine wave by increasing the number of stages, reducing harmonics. Under the same output performance indicators, the switching frequency of the three-level inverter will be 1/5 of that of the two-level inverter, thus reducing system losses. As the number of levels increases, the amplitude of each level decreases relatively, dv/dt becomes smaller, the pulsating component in the main circuit current decreases, and torque pulsation and electromagnetic noise are effectively suppressed. Although the three-level inverter has a simple structure and can achieve four-quadrant operation, due to the current limitations of device withstand voltage levels, it can only reach medium and high voltage conditions such as 4.16kV. To output higher voltages, devices must be connected in series, but this will bring problems such as voltage equalization. Figures 7 and 8 show the output voltage and current waveforms and their spectra of the three-level inverter. [align=center]Figure 7 Line voltage waveform and its spectrum Figure 8 AC side current waveform and its spectrum[/align] 4.3 Cascaded Multiplexed Inverter Units The multiplexed structure is a promotion and application of multiplexed technology and is a type of multiplexed frequency converter. As shown in Figure 9, the cascaded multiplexed frequency converter uses several low-voltage PWM frequency converter power units connected in series to achieve direct high-voltage output. The increase in the number of levels effectively suppresses output harmonics. Since each power unit module contains not only an inverter output structure but also a rectification function, the rectification section is multiplied accordingly, so that the input and output harmonic suppression of the frequency converter is completed synchronously. Its harmonic suppression principle is similar to that of ordinary multiplexing, which also uses phase shift technology to eliminate certain output harmonics of each power module by shifting them at a certain angle. [align=center]Figure 9 Cascaded Multiplexed Frequency Converter[/align] Although it is a series structure, there is no voltage balance problem because the DC side uses separate DC power supplies. Without the limitation of diodes and capacitors, the number of levels in the series structure can be large. Generally, diode and capacitor clamping structures are limited to 7 or 9 voltage levels, while series structures have no such limitation. Since each stage of the inverter bridge has the same structure, it facilitates modular design and manufacturing. However, due to the large number of power units and devices used—for example, a 6kV system with three units in series per phase requires 90 power devices (54 diodes and 36 IGBTs)—the device becomes too large, and installation location becomes a problem. The simulation waveforms and spectra of this topology in MATLAB are shown in Figures 10 and 11. [align=center] Figure 10: Cascaded Multiplexed Output Voltage and its Spectrum[/align] [align=center] Figure 11: Cascaded Multiplexed Output Current and its Spectrum[/align] 4.4 Comparison and Analysis of High-Voltage Frequency Converter Output Harmonics with IEEE-519 The ratios of the main subharmonics to the fundamental frequency in the three inverter structures are shown in Tables 6 and 7. All data were obtained using MATLAB/Simulink simulation without additional filtering devices. A comparative analysis was conducted based on the IEEE-519 standard. Transformer-coupled output inverters generally meet the requirements of IEEE 519, especially in terms of harmonic content below the 23rd order, which is completely less than the values ​​specified in IEEE 519. Only due to the influence of the PWM carrier ratio do larger harmonics appear around the 23rd and 25th orders. Three-level inverters also generally meet the requirements of IEEE 519, with low-order harmonics not exceeding the standard. Cascaded multi-level inverters offer the best performance, perfectly meeting the requirements of IEEE 519, with harmonic content within all ranges not exceeding the standard. Regarding total harmonic distortion (THD), all three output structures meet the requirements of the IEEE 519 standard. Each of the three topologies has its advantages and disadvantages, but all effectively solve the problem of inverter output harmonic suppression, making the output waveform closer to a sine wave. Comparison with the IEEE 519 standard reveals that, without additional filtering, the cascaded multi-level inverter performs best, fully meeting the IEEE 519 requirements. Other structures cannot fully meet the limits specified in the standard and require the assistance of filtering circuits of a certain capacity. 5. Conclusion With the widespread application of high-voltage, high-power frequency converters, improving their topology to effectively suppress and reduce AC side harmonics has become a goal pursued by engineers. The multi-level structure, a commonly used topology on the rectifier side of frequency converters, can meet the needs of different applications and voltage levels. However, as the number of levels increases, although the harmonic suppression effect is significant, the structure of the device becomes more complex, and transformer losses increase. Therefore, 12- or 18-pulse structures are commonly used. This way, adding a small-capacity filter can well meet the IEEE-519 requirements. On the inverter side, the unit-cascaded multi-level structure has the best input and output harmonic suppression effect, but the system structure is complex, with a large number of components and a large size. The three-level structure is simple and requires the fewest components, but its harmonic suppression effect is slightly worse due to the limitation of the number of levels, and there is a midpoint potential balance problem, which is also one of the factors hindering the harmonic suppression effect. Implementing the NPPC on the rectifier side can solve this problem. The transformer-coupled output structure can meet the needs of high voltage and high power while using current voltage-resistant devices. It is simple in structure, has a relatively small number of components, and its harmonic suppression effect is between the two structures mentioned above, making it a compromise solution.