Abstract High-voltage frequency converters (VFDs) have a wide range of applications and broad market prospects in China. This article mainly analyzes and reviews the current application status and characteristics of VFDs in the power and metallurgical industries, as well as their market prospects and capacity. Keywords : High-voltage frequency converter, power, metallurgical application market. With the continuous maturation of high-voltage frequency conversion technology, VFDs have been widely used in various fields. The huge energy-saving potential and excellent speed regulation performance of VFDs have given them strong development momentum and a broad market space. Currently, high-voltage frequency conversion technology has become one of the hot topics in the field of electric drive. High-voltage frequency conversion retrofitting of large-capacity electric drive systems, especially large-capacity fan and pump systems, has become a trend, bringing significant energy-saving benefits to enterprises and industries using high-power transmission equipment. 1. Current Status of High-Voltage Frequency Converter Applications in the Power and Metallurgical Industries 1.1 Current Status of Power Industry Applications According to statistics, the total capacity of motors supporting primary air fans, forced draft fans, induced draft fans, feedwater pumps, circulating water pumps, and slurry pumps used in China's thermal power plants reaches 15,000 MW, with an annual total electricity consumption of 52 billion kWh, accounting for 5%-8% of the country's total thermal power generation. Currently, except for a small number of steam-driven feedwater pumps, hydraulic couplings, and dual-speed motors, most fans and pumps in China's thermal power plants are driven at constant speed. These constant-speed driven fans use inlet dampers, and pumps use outlet valves to regulate flow, both resulting in significant throttling losses. Especially during unit load changes, the operating efficiency decreases because the fans and pumps deviate from their high-efficiency points. This is because the flow rate of fans and pumps is directly proportional to the motor speed, the pressure is directly proportional to the square of the speed, and the power is directly proportional to the cube of the speed (as shown in Figure 1). Existing methods for regulating flow do not change the motor speed, therefore the power consumed by the motor remains unchanged. If a frequency converter is used to regulate the motor speed to achieve the user's desired flow rate, a significant amount of energy can be saved. Foreign thermal power plants mostly add speed control devices to their fans and pumps, while the use of frequency converters for auxiliary equipment in domestic thermal power plants is on the rise. For thermal power plants, the economical operation of auxiliary motors directly affects the plant's power consumption rate; in other words, the effective speed regulation of auxiliary motors directly affects the plant's profitability. [align=center]Figure 1 Relationship between flow rate, power, and speed of fans and pumps[/align] The operating efficiency of boiler fans in my country's 50MW and above units is generally very low, around 45-65%. Generally speaking, using a high-voltage frequency converter will increase fan operating efficiency by 20-30%, and the investment cost can be recovered in approximately 1.5 to 2 years. With the deepening of power industry reforms and the gradual implementation of policies such as the separation of power plants and grids and competitive bidding for grid connection, reducing power consumption, lowering generation costs, and improving electricity price competitiveness have become the economic goals pursued by power plants. Practical application shows that thermal power plants are major users of high-voltage frequency converters, with foreign brands such as Siemens, ABB, Robicon, and AB accounting for over 40% of power users. Extensive engineering practice has proven that promoting frequency converter technology can improve the operation of many production systems, such as flow regulation and control in circulating water and condensate systems, and is a crucial means of improving the safe and economical operation of generating units. For a 300MW thermal power unit, the main equipment配套 with each boiler unit and requiring high-voltage variable frequency speed regulation includes: 4 ball mills, with main motor power ranging from 280kW to 1100kW depending on the unit's power generation and coal quality; a common (average) power of 4 x 560kW is taken, with a voltage of 6kV (all data below are average values, and the operating voltage is 6kV); 2 exhaust fans in the pulverizing system, with matching motor power ranging from 2 to 710kW; 3 slurry pumps in the ash removal system (1 in operation, 1 on standby, and 1 under maintenance), with a motor power of 3 x 250kW; 2 boiler forced draft fans, with motor power of 2 x 1000kW; 2 boiler induced draft fans, with motor power of 2 x 2000kW; and 2 boiler primary air fans, with motor power of 2 x 630kW. The total load capacity of all the above is 11.67MW, accounting for approximately 4% of the rated total power generation of the 300MW unit. In addition, some large power plants with a large number of generating units (such as 4x300MW, 600MW, and 2x600MW thermal power plants) also have equipment such as circulating water pumps (single unit capacity 500kW～2300kW), condensate pumps (single unit capacity 560kW～2950kW), and feedwater pumps (single unit capacity 800kW～2300kW) that require variable frequency speed control. These devices typically require at least two units to operate as backups for each other. If the capacity of these devices is calculated at full load, the total electricity consumption is almost equal to the actual total plant power consumption, and in reality, the electricity consumption of these large auxiliary machines accounts for about 80% of the plant's total power consumption. Currently, only a portion of thermal power plants have carried out high-voltage variable frequency retrofits on their forced draft fans and induced draft fans, and a few power plants have retrofitted various water pumps and primary air fans, possessing 10-20 high-voltage variable frequency drives (VFDs). Most power plants have no more than 5 high-voltage VFDs, and many power plants do not use high-voltage VFDs at all. According to data from the State Grid Corporation of China, by the end of 2002, there were a total of 3,984 thermal power units with a capacity of 6MW or above nationwide, with a total installed capacity of 244,468,600 kW. There were a total of 313 thermal power units with a capacity of 300MW or above nationwide, with a total installed capacity of 110,753.0MW. This included 191 300MW units with a total installed capacity of 57,300.0MW; 7 320MW units with a total installed capacity of 2,240.0MW; 44 350MW units with a total installed capacity of 15,600.0MW; and 26 600MW units with a total installed capacity of 15,600.0MW. There were a total of 205 thermal power units with a capacity of 200MW to 300MW, with a total installed capacity of 41,850.0MW. There were a total of 343 thermal power units with a capacity of 100MW to 200MW, with a total installed capacity of 39,798.7MW. If we calculate based on a 50% conversion rate for speed regulation, approximately 4000MW of high-voltage frequency converters would be needed. Currently, only about 500-600 high-voltage frequency converters are used in thermal power plants in my country, indicating a vast development potential for high-voltage frequency converters in the power industry. 1.2 Current Application Status in the Metallurgical Industry In steel plants, important fans and pumps such as steelmaking dust removal fans, blast furnace dust removal fans, sintering waste heat fans, cold rolling hydrogen pressurizers, and boiler feedwater pumps are typically driven by AC asynchronous motors. These devices have a wide speed range, significantly impacting processes and environmental protection. In the past, most steel plants did not adjust the speed of their fans and pumps, directly using industrial frequency power supply and fixed-speed drive, controlling airflow and water supply by adjusting dampers and valves, thus wasting energy. Using high-voltage frequency converters for variable frequency speed regulation can save a significant amount of electricity, bringing benefits to the enterprise. In addition, large fans in steel plants, such as blast furnace blowers, sintering main exhaust fans, and oxygen generators, have relatively high power and are generally driven by synchronous motors, but can also be retrofitted with frequency converters. Furthermore, steel plants typically use lifting equipment such as ship loaders, ship unloaders, stacker-reclaimers, converter oxygen generators, auxiliary lances and tilting devices, and large overhead cranes. These lifting devices are characterized by potential energy loads, requiring high starting torque and strong overload capacity. Early speed control equipment generally used DC speed regulation. Although this method is technically mature, the daily maintenance workload is very large. Therefore, with the continuous development and increasing maturity of high-voltage frequency conversion technology, AC frequency conversion speed regulation has been widely applied to lifting equipment. Currently, high-voltage frequency converters are also widely used in the main rolling mills of hot and cold rolling mills in steel plants. The main rolling mills for hot and cold rolling have high requirements for speed regulation, strong overload capacity, high control precision, and fast dynamic response; therefore, foreign brands of high-voltage frequency converters are predominantly used. In recent years, the energy-saving effect and speed regulation performance of high-voltage frequency converters have been recognized by enterprises, and many steel plants in China have carried out high-voltage frequency conversion speed regulation retrofits. For a steel rolling mill with an annual output of 300,000 tons, the total capacity of its electrical equipment is approximately 18,500 kW, of which the capacity of the high-voltage main drive motors is approximately 10,200 kW. This includes: one roughing mill (1x2000 kW), four intermediate rolling mills (4-550 kW each), four secondary rolling mills (4-500 kW each), and eight finishing mills (8-500 kW each). In early steel rolling mills, most of these main drive motors were DC motors; with the maturity of high-voltage frequency conversion technology, these motors have gradually been replaced by AC speed regulation. A simple calculation shows that these high-voltage motors account for about 55% of the total electricity consumption of the steel rolling mill. For steel plants, the main peripheral equipment of the blast furnace includes hot blast stoves, dust removal equipment, and slag treatment equipment. Generally, each blast furnace has 3-4 hot blast stoves, one for supplying hot blast, one for heat storage, and one for combustion heating. Some steel plants have a spare hot blast stove. Each hot blast stove requires one blower. Some steel plants use large blowers directly driven by steam turbines for their hot blast stove blowers, while many others have converted to high-voltage electric motors driving the blowers. Dust removal equipment includes charging dust fans and tapping dust fans. Due to the particularly large speed range of tapping dust fans, variable frequency speed control is now widely used. Generally, each blast furnace is equipped with at least 2 dust fans and 3-4 slag flushing water pumps. According to data released by the China Iron and Steel Association website in January 2006: In 2005, China's crude steel output was 349.3615 million tons, pig iron output was 330.4047 million tons, and steel output (including re-rolled steel) was 371.1702 million tons. In 2005, the scale of my country's steel enterprises expanded rapidly. Among them, eight enterprises had an annual crude steel output of over 10 million tons, including Baosteel, Ansteel, Wuhan Iron and Steel, Shougang, Shagang, Laiwu Steel, Jinan Steel, and Tangshan Steel; ten enterprises had an annual crude steel output of 5 million to 9.7 million tons, including Maanshan Iron and Steel, Valin Steel, Baotou Steel, Handan Steel, Benxi Steel, Panzhihua Steel, Anyang Steel, Taiyuan Steel, Jiuquan Steel, and Jianlong Steel; and sixteen enterprises had an annual crude steel output of 3 million to 5 million tons. The 18 steel enterprises with an annual crude steel output of over 5 million tons had a combined crude steel output of 161.9716 million tons, accounting for 46.36% of the national crude steel output. Currently, high-voltage frequency converters in the metallurgical industry are mainly used in fans and pumps, and rarely in rolling mills. Therefore, high-voltage frequency converters have broad prospects in the metallurgical industry. 2. Application Characteristics of High-Voltage Frequency Converters in the Power and Metallurgical Industries 2.1 Application Characteristics in the Power Industry The application of high-voltage frequency converters in the power industry is mainly concentrated on fan and pump loads, and is characterized by large single-unit capacity. The power industry has always been conservative in applying new technologies because the power sector has very high requirements for power supply safety; therefore, large-scale high-voltage fan retrofitting is still not common. The main purpose of using high-voltage frequency converters in thermal power plants is speed regulation and energy saving. After several years of operational practice, the reliability of high-voltage frequency conversion speed regulation systems can now basically meet the requirements of power generation enterprises, and thermal power plants have gradually accepted this new technology. For power plants, reliable equipment operation is one of the most important factors. No matter how good the performance or how significant the energy saving of any auxiliary speed regulation equipment, if reliability is not guaranteed, everything is meaningless. Because once these critical auxiliary equipment fail due to speed regulation device malfunctions and stop normally, it often leads to a significant drop in the main load, and in severe cases, it may even cause the main unit to shut down and the boiler to shut down, resulting in a power generation accident. The losses caused will far outweigh the energy saving benefits. Therefore, the application of high-voltage frequency converters in power plants places extremely high demands on their reliability. For example, consider the forced draft fans and induced draft fans in thermal power plants. Because the unit load frequently changes, it's crucial to maintain stable boiler furnace negative pressure, flue gas oxygen content, and corresponding steam temperature and pressure. This necessitates timely adjustments to the forced draft and induced draft fan volumes and pulverized coal quantities. Forced draft fans typically adjust airflow by adjusting inlet guide vanes, with the vane opening frequently varying between 60% and 80%. Induced draft fan dampers, on the other hand, use baffles for adjustment, with a daily operating opening typically around 50%. When using high-voltage frequency converters to regulate their speed, the typical speed range is approximately 40Hz to 50Hz. 2.2 Application Characteristics in the Metallurgical Industry The metallurgical industry holds a very important position in the national economy, and its enterprises are typically large-scale, highly profitable, and operate as continuous, efficient process flow enterprises. Steel enterprises of a certain scale have generally achieved automated process control within their workshops, resulting in a high degree of automation in the production process. Numerous field devices require control during production. Due to the extremely harsh environment at the production site—characterized by high dust levels, strong vibrations, and significant temperature fluctuations—and the need for continuous 24-hour operation, the requirements for on-site control equipment are much higher than in other industries. For example, the dust collector fan in a steel plant is the power hub of the dust removal and purification system. If it malfunctions, it not only affects production and causes substantial economic losses but also potentially threatens the personal safety of on-site personnel. Therefore, the high-pressure speed control system配套 with the dust collector fan requires extremely high reliability. Taking the dust collector fan in a blast furnace as an example, the iron tapping process in blast furnace ironmaking is not continuous but periodically intermittent. Utilizing a high-voltage frequency converter to perform variable frequency speed control on the dust collector fan in a timely manner can achieve significant energy savings. During iron tapping, a large amount of brownish-red smoke is generated, requiring the dust collector fan to operate at high speed, with the frequency converter set at approximately 45Hz; while during iron tapping, the fan only requires a very low speed, with the frequency converter set at approximately 20Hz. Similarly, in electric arc furnaces for steelmaking, the dust removal fan must adjust its airflow in a timely manner according to the varying amounts of flue gas produced by the furnace to meet the requirements of the production process. The general production cycle of an electric arc furnace consists of seven steps: charging, adding iron, power supply, power and oxygen supply, sampling, tapping, and plugging. In each process, the amount of flue gas produced by the furnace varies, as does the required furnace temperature. If the fan airflow is too large, it will result in heat loss within the furnace, prolonging the production cycle and wasting energy; if the airflow is too small, it will lead to excessively high furnace temperatures, potentially burning out the equipment. Furthermore, in the metallurgical industry, some high-power equipment requires the transmission system to have excellent low-speed starting performance, while also requiring the motor and frequency converter to have strong overload capacity, such as cranes and high-power tilting equipment. Other high-power equipment, such as rolling mills, requires the transmission system to have high control precision, rapid dynamic response, and a certain overload capacity. Coupled with the harsh operating environment, in addition to high requirements for the speed regulation accuracy and performance of the frequency converter, there are also high requirements for the reliability of the high-voltage frequency converter system and its control network. 3. Market Prospects for High-Voltage Frequency Converters According to surveys and forecasts, the total potential market for frequency converters in China should be 120-180 billion yuan, with low-voltage frequency converters accounting for about 60% of the market share and medium- and high-voltage frequency converters accounting for 40%. The Chinese frequency converter market is currently experiencing rapid growth, maintaining a high growth rate of 12% to 20% in the past few years. In 2003, the market size even exceeded 5.5 billion yuan. Experts believe that it will take at least 10 years for the Chinese frequency converter market to become saturated and mature. High-voltage frequency converters are constrained by high-voltage power electronic devices, and their application in various industries is still in its early stages and requires further promotion. Currently, the industries with the most applications of high-voltage frequency converters are metallurgy, petrochemicals, power plants, urban water supply, and building materials. Depending on the usage environment, the service life of frequency converters ranges from 4 to 15 years, resulting in a large number of product replacements every year. Currently, major international high-voltage frequency converter brands include Siemens, ABB, Rockwell, Mitsubishi, and Robicon, while domestic manufacturers include Dongfang Hitachi and Lide Huafu. The total number of high-voltage frequency converters in my country has exceeded 6,000 units. Figure 2 shows the development history of my country's high-voltage frequency converter market, clearly demonstrating its strong upward trend. 3.1 Market Prospects in the Power Industry According to rough estimates, my country's power industry currently has approximately 350,000 fans and pumps, with a total capacity of nearly 7.5 million kW and an annual electricity consumption of over 40 billion kWh. Among them, approximately 30,000 fans and pumps are driven by high-voltage motors of 6kV or higher, with a capacity of 6.5 million kW. The total capacity of equipment requiring speed regulation in thermal power plants accounts for 50% to 70% of the plant's electricity consumption. The economical operation of this equipment directly affects the plant's power consumption rate. Upgrading thermal power plants using high-voltage frequency converters will significantly reduce the plant's power consumption rate, bringing substantial economic benefits to enterprises. Furthermore, frequency converters can be integrated with the unit's automatic control system to improve the automatic circuit availability rate, forming a complete optimized control system for the unit. This achieves the goals of saving manpower, labor, and equipment, and improving the overall automation level of the unit and the overall management level of the power plant. With the development of high-power power electronic device technology, the technical indicators of high-voltage, high-capacity frequency converters have been improved, their functions are becoming stronger, their prices are decreasing, and their investment payback period is shortening. All of this indicates that the application of high-voltage frequency converters in thermal power plants will be rapidly promoted. 3.2 Market Prospects in the Metallurgical Industry In the metallurgical and steel industry, although the number of large electrical equipment such as rolling mills, large fans, and water pumps is not large, accounting for only about 20%, their electricity consumption accounts for 80%. Typically, after using frequency conversion speed regulation technology, the efficiency of square-root torque equipment such as fans and pumps can be improved by about 30%. The efficiency of constant torque equipment such as rolling mills can be improved by 15-30%, averaging about 20%. Furthermore, the application of variable frequency speed control technology can improve the power factor of motors (generally reaching 0.98), reduce equipment maintenance, extend equipment lifespan, and reduce significant production losses caused by accidental shutdowns. These all bring great benefits to reducing operating costs. For example, the use of variable frequency speed control technology in steel rolling heating furnaces to automatically control and adjust the gas/air ratio not only reduces the power consumption of air blowers but also ensures the gas-air combustion ratio, guaranteeing complete gas combustion and reducing gas consumption. The economic benefits of reduced gas consumption far outweigh the energy savings, while also reducing the workload of workers. 4. Market Capacity of High-Voltage Variable Frequency Drives According to statistics, the current capacity of AC motors in use in China is approximately 400 million kW, with motors used for fans and pumps accounting for about 40%, totaling 160 million kW. Among them, the high-voltage motor capacity of large fans and pumps accounts for 40% of all fans and pumps (i.e., 64 million kW). Conservatively estimated, if 20% of these are retrofitted with variable frequency technology, this would reach 12.8 million kW, and the energy savings would be substantial. If calculated based on an average energy saving rate of 20%-30%, the energy saving power would be 2.56-3.84 million kW. Excluding the high-voltage frequency converters配套 with newly built large-scale fans and pumps, and only considering the high-voltage frequency conversion speed regulation retrofit of existing large-scale fans and pumps: if the retrofit of 12.8 million kW is completed over 5 years, then approximately 2.5 million kW will be retrofitted annually. Based on the current market price of 1500-2000 yuan/kW, this would generate an annual market revenue of 3.75-5 billion yuan. Currently, the high-voltage frequency converter market is still in the development and exploration stage. In 2004, the market size of high-voltage frequency converters was 850 million yuan, accounting for approximately 13% of the total frequency converter market size (6.63 billion yuan). The market share of high-voltage frequency converters in various industries is shown in Figure 3. [align=center]Figure 3. Operating Proportion of High-Voltage Frequency Converter Industry in 2004 (Unit: 100 Million Yuan)[/align] 4.1 Market Capacity of the Power Industry The power industry has the largest demand and market capacity in the high-voltage frequency converter market, accounting for more than 50%. In existing thermal power plants, more than 5,000 high-voltage, large-capacity motors require frequency conversion speed regulation upgrades, with a market potential of 6.5 billion yuan in this area alone. According to the national economic development plan, the power construction development speed in the first decade of the 21st century is approximately 7% to 8%, with an average annual increase of 24 to 25 million kW in installed capacity. The annual demand for high-voltage, large-capacity rotating equipment such as fans and pumps配套 with these units is 500 to 600 units. Just estimating the high-voltage frequency converters配套 with these new equipment, the annual market potential can reach 650 to 780 million yuan. 4.2 Market Capacity of the Metallurgical Industry The electricity consumption of the metallurgical industry accounts for 14.18% of the total national industrial electricity consumption. In my country, the energy utilization rate of electric motor drive systems is about 20% lower than the international average. The technology of these motors is only equivalent to that of foreign technologies from the 1950s and 60s, indicating significant energy-saving potential. Currently, most mining drills and shovels used in the metallurgical industry employ DC speed regulation, with motor capacities ranging from 100 to 500 kW. Only a small number of imported devices use AC variable frequency speed regulation. The drive motors of mine hoists have capacities between 500 and 4000 kW, using DC drives, although some have been converted to AC speed regulation. The main equipment for ensuring mine safety, such as mine ventilation fans and drainage pumps, have motor capacities between 200 and 800 kW, and are numerous, representing huge energy-saving potential. The total installed capacity of fans and pumps used in the steel industry alone exceeds 2 million kW. With the adjustment of the product structure in the metallurgical industry and the environmental requirements of metallurgical production, the metallurgical industry will build a large number of new steel rolling production lines and extensively use dust removal fans and cooling water pumps, resulting in a considerable annual increase in capacity. 50% of wind turbines and water pumps need to be retrofitted with AC variable frequency speed regulation or newly installed units should be equipped with high voltage frequency converters. The market capacity will also exceed one million kW. The development of high voltage frequency converters is booming. With the development of power electronics technology itself and the needs of the rapid development of the national economy, the application of high voltage frequency converters in China's power, metallurgy and other industries will also have a greater development. 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