With the accelerated marketization of AC-DC-AC frequency converters, an increasing number of medium and high voltage frequency converters are entering the market. While traditional electronic components, primarily switching devices, have made significant progress, the advancements in other components have lagged far behind. DC support and filter capacitors are a particularly significant issue. Traditional frequency converters almost exclusively use aluminum electrolytic capacitors for DC support and filter capacitors. Early selection criteria primarily considered the excellent capacitance-to-volume ratio and relatively low price of aluminum electrolytic capacitors. Furthermore, early issues with power device stability and system protection, coupled with a blind pursuit of large capacitance and lower ripple coefficients, led to the widespread use of aluminum electrolytic capacitors despite their numerous drawbacks. However, with the maturation and cost reduction of new metallized film technology, DC support and filter capacitors (hereinafter referred to as metallized capacitors) suitable for medium and high voltage converters, based on metallized film technology, have entered the civilian market. The intermediate DC link support capacitors of the GVF series high-voltage frequency converters produced by Zhuzhou Converter Technology National Engineering Co., Ltd. use a new type of stepless metallized polypropylene self-healing safety film capacitor. This type of capacitor has a long service life, is completely maintenance-free throughout its entire lifespan, and comprehensively improves upon the shortcomings of traditional aluminum electrolytic capacitors, such as large capacitance deviation, low voltage, low current, high loss, and poor frequency characteristics. A basic performance comparison is shown in the table below. Performance Comparison: Due to the inherent material and structural characteristics of aluminum electrolytic capacitors, their capacitance deviation is significantly different from that of metallized capacitors in terms of rated capacitance and capacitance temperature stability. Aluminum electrolytic capacitors cannot accurately control the capacitance because the oxidation area of ​​the aluminum plates cannot be controlled. In contrast, metallized capacitors can precisely control the capacitance by directly controlling the plate area, with deviations controlled within -2% to +2%, providing a reliable guarantee for circuit accuracy. Because the insulating dielectric material in metallized film capacitors is a polymer film (polypropylene, polyester, etc.), the rated voltage of the capacitor can be increased by controlling the film thickness (with an accuracy of 0.2 micrometers or higher). In contrast, aluminum electrolytic capacitors, whose insulating dielectric is an aluminum oxide layer on the surface of aluminum foil, have a difficult-to-control oxide layer thickness and uniformity, making it difficult to achieve high withstand voltage and overvoltage ratios. Aluminum electrolytic capacitors have lower rated voltages (typically ≤450V DC), while metallized film capacitors used as DC filter capacitors in GVF series high-voltage frequency converters have a single rated voltage reaching 20000V DC. The company previously used 52 segments of aluminum electrolytic capacitors connected in series, which was very cumbersome. Using metallized film capacitors significantly reduced the tedious voltage equalization work during the series connection of aluminum electrolytic capacitors, saving material and labor costs and improving efficiency. This new type of metallized capacitor can achieve a rated ripple current several to tens of times higher than that of an aluminum electrolytic capacitor of the same capacity. Furthermore, the overcurrent capacity of an aluminum electrolytic capacitor is only 1.1in, while that of a metallized capacitor can reach 1.3in. This avoids the unnecessary waste caused by increasing the number of aluminum electrolytic capacitors in parallel to achieve the system's rated current in medium to high power systems, sacrificing capacity. Especially when dealing with rapid voltage changes (high DV/DT) requiring capacitors to provide instantaneous large currents for voltage support or absorption, aluminum electrolytic capacitors are almost useless. When the load motor operates under four-quadrant conditions, or the inverter bridge operates in rectification mode requiring capacitors to absorb lost energy, metallized film capacitors show a significant advantage over aluminum electrolytic capacitors. Simultaneously, the capacitive reactance of a capacitor is inversely proportional to the frequency, while the active power loss of a capacitor increases with increasing current, and is directly proportional to the square of the current. Therefore, capacitor losses become a crucial factor for safe and reliable operation. As shown in Figure 1, the capacitance temperature stability of metallized capacitors exhibits a capacitance temperature change rate that is only about 10% of that of aluminum electrolytic capacitors. Furthermore, the losses of metallized capacitors are only about 1% of those of aluminum electrolytic capacitors, resulting in lower heat generation and temperature rise, thus further ensuring system stability. In contrast, aluminum electrolytic capacitors have very high losses, with active power losses accounting for up to 20% of the total reactive power. Moreover, the active power losses of aluminum electrolytic capacitors increase with frequency; the rate of change gradually increases above 1000 Hz, and increases sharply above 10000 Hz, leading to increased active power losses and heat generation. Figure 2 shows the frequency characteristics of aluminum electrolytic capacitor losses (provided by a well-known international aluminum electrolytic capacitor manufacturer). This is precisely the fundamental reason why aluminum electrolytic DC filter capacitors have long been used in medium- and high-frequency high-power environments, resulting in high heat generation and high failure rates. Metallized capacitors have very low inherent losses, with the overall capacitor loss less than 2‰, only 1% of that of aluminum electrolytic capacitors. Furthermore, they use polypropylene film as the dielectric, resulting in very stable frequency characteristics, as shown in Figure 2. Therefore, using metallized DC filter capacitors in medium- and high-frequency environments can significantly reduce line losses, prevent overheating, and provide extremely high stability. Based on these factors, aluminum electrolytic capacitors have a very high failure rate and a very short lifespan in medium- and high-frequency environments, with an expected lifespan of ≤50,000 hours under rated operating conditions (as shown in Figure 3). The capacitance changes drastically with varying operating conditions, reducing the lifespan; some companies even use aluminum electrolytic capacitors in medium- and high-frequency environments with a lifespan of less than 10,000 hours, often causing capacitor failures that affect the entire equipment's operation. Conversely, metallized capacitors, due to their excellent frequency and current characteristics, ensure a safe and long lifespan in medium- and high-frequency environments, with an expected lifespan of ≥80,000 hours under rated operating conditions. Moreover, their electrical performance remains essentially unchanged throughout their lifespan regardless of environmental changes (as shown in Figure 3), ensuring the safe and reliable operation of the equipment. However, metallized film capacitors are not perfect compared to aluminum electrolytic capacitors, mainly in terms of capacitance-to-volume ratio and price. The GVF series high-voltage frequency converters produced by Zhuzhou National Engineering Research Center for Converter Technology Co., Ltd. utilize new materials and structures, significantly improving the capacitance-to-volume ratio. Their volume and capacitance are reduced by more than 40% compared to traditional metallized film capacitors, but they still lag behind aluminum electrolytic capacitors. Advanced foreign capacitor and frequency converter manufacturers have proposed and applied methods to determine the range of supporting capacitance and rationally select supporting capacitors based on the current supply capability, reducing the supporting capacitance used in frequency converters compared to aluminum electrolytic capacitors to address the shortcomings of metallized capacitors. For example, when using a three-phase bridge rectifier circuit as the DC source for the inverter circuit, the GVF series high-voltage frequency converters have performed well in actual user applications, achieving a capacitance reduction of approximately 40% compared to aluminum electrolytic capacitors. Based on this principle, the volume and price of film capacitors are now very close to those of aluminum electrolytic capacitors. In conclusion, with the increase in power, voltage, and frequency of equipment, traditional aluminum capacitors have lost their original cost-effectiveness and may even be completely unsuitable for new equipment requirements. The development of a new type of metallized polypropylene self-healing DC filter capacitor comprehensively solves the problems faced by aluminum electrolytic capacitors in medium-high frequency, medium-high power, and medium-high voltage applications. It is particularly suitable for medium-high voltage high-power frequency converters, AC 380V～1000V medium-high frequency electric arc furnaces, and other medium-high power, medium-high frequency, and medium-high voltage applications.