Abstract: High-voltage frequency converters 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 high-voltage frequency converters in the power and metallurgical industries, as well as their market prospects and capacity.
Keywords: High-voltage frequency converter, power and metallurgical application market
Analysis About the High Voltage Inverters' Application Status in Power and Metallurgy Industry Xuesong Wang ZhengmingZhao (Department of Electrical Engineering of Tsinghua Universicy, State Key Laboratory of Power System Beijing, China 100084)
Abstract : The high voltage inverter is widely applied in the future market in our country． In the paper not only the applied status and characteristics ale mainly presented, and also the market foreground and market capacity of the medium voltage inverter applied in power and metallurgy industry are given here．
Keywords: high voltage inverter, power industry, metallurgy industry, application status, market
With the continuous maturation of high-voltage frequency conversion technology, high-voltage frequency converters have been widely used in various fields. The enormous energy-saving potential and excellent speed regulation performance of high-voltage frequency converters have given them strong development momentum and a broad market space. Currently, high-voltage frequency conversion technology has become one of the hottest 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 drive equipment.
1. Current Status of High Voltage Frequency Converter Applications in the Power and Metallurgical Industries
1.1 Current Application Status in the Power Industry
According to statistics, the total capacity of motors for primary air fans, forced draft fans, induced draft fans, feed water pumps, circulating water pumps and slurry pumps used in China's thermal power plants is 15,000 MW, with an annual total electricity consumption of 52 billion kWh, accounting for 5% to 8% of the country's thermal power generation [1] . At present, except for a small number of steam-driven feed water pumps, hydraulic couplings and dual-speed motors used in China's thermal power plants, most of the other fans and pumps are driven by constant speed. The fans driven by constant speed use inlet dampers and the pumps use outlet valves to regulate the flow, both of which have serious throttling losses. Especially when the unit is running under variable load, the operating efficiency is reduced because the operation of the fans and pumps deviates from the high efficiency point. The reason is that the flow rate of the fans and pumps is proportional to the motor speed, the pressure is proportional to the square of the speed, and the power is proportional to the cube of the speed (as shown in Figure 1). The existing method of regulating the flow rate does not change the speed of the motor, so the power consumed by the motor remains unchanged. Using frequency converters to regulate the speed of motors to achieve the user's desired flow rate can save a significant amount of electrical energy. While many foreign thermal power plants have added speed control devices to their fans and pumps, the adoption of frequency converters for auxiliary equipment in domestic thermal power plants is gaining popularity. 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 impacts the plant's profitability.
In my country, the operating efficiency of boiler fans in units with a capacity of 50MW or higher is generally very low, ranging from approximately 45% to 65%. Generally speaking, using high-voltage frequency converters (VFDs) can improve fan efficiency by 20% to 30%, allowing for a recovery of the investment cost in about 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 plant power consumption, lowering power 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 VFDs, with foreign brands such as Siemens, ABB, Robicon, and AB accounting for over 40% of power users. Extensive engineering practice has proven that promoting VFD technology can improve the operation of many production systems, such as flow regulation and control in circulating water and condensate systems, making it a crucial means of improving the safe and economical operation of power 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 of 2 x 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, 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 regulation. These devices usually require at least two generating 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, while in reality, the electricity consumption of these large auxiliary machines accounts for about 80% of the plant power consumption [2-3] .
At present, only some thermal power plants have carried out high-voltage frequency conversion retrofits on blowers and induced draft fans, and a few power plants have carried out retrofits on various water pumps and primary air fans, with 10 to 20 high-voltage frequency converters in the plant. However, most power plants have no more than 5 high-voltage frequency converters, and many power plants have not used high-voltage frequency converters. According to data from the State Power Information Network [4] , as of the end of 2002, there were a total of 3,984 thermal power units with a capacity of 6MW or above in the country, 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 in the country, with a total installed capacity of 110,753.0MW. This includes 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 are a total of 205 200MW-300MW thermal power units with a total installed capacity of 41,850.0MW. There are a total of 343 100MW-200MW thermal power units with a total installed capacity of 39,798.7MW. If we calculate based on 50% frequency conversion speed regulation transformation, we would need about 4,000MW of high-voltage frequency converters. Currently, there are about 500-600 high-voltage frequency converters already in use in China's thermal power plants [5] , so high-voltage frequency converters have a broad development space 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 feed pumps are typically driven by AC asynchronous motors. These devices have a wide speed range, significantly impacting process and environmental performance. Previously, most steel plants used fixed-speed drives for their fans and pumps, relying on industrial frequency power and adjusting dampers and valves to control airflow and water supply, thus wasting energy. Using high-voltage frequency converters for variable frequency speed control can save significant amounts of electricity, bringing benefits to the enterprise. Furthermore, large fans in steel plants, such as blast furnace blowers, sintering main exhaust fans, and oxygen generators, have considerable power and are generally driven by synchronous motors, which can also be retrofitted for variable frequency speed control.
In addition, steel plants typically use various 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 being potential energy loads, requiring high starting torque and strong overload capacity. Early speed control equipment generally used DC speed regulation. While this method is technically mature, it involves a large amount of daily maintenance. 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 domestic steel plants have already 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 with a capacity of 1 x 2,000 kW; four intermediate rolling mills with a capacity of 4 x 550 kW; four intermediate rolling mills with a capacity of 4 x 500 kW; and eight finishing mills with a capacity of 8 x 500 kW. 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 were gradually replaced by AC speed control. A simple calculation shows that the electricity consumption of these high-voltage motors accounts for about 55% of the total electricity consumption of the steel rolling mill [6-7] .
For steel plants, the main peripheral equipment of blast furnaces includes hot blast stoves, dust removal equipment and slag treatment equipment. Generally, each blast furnace has 3-4 hot blast stoves, one of which is in the state of supplying hot blast to the blast furnace, one in the state of heat storage, and one in the state of combustion heating. Some steel plants will have a spare hot blast stove. Each hot blast stove needs to be equipped with a blower. Some steel plants use a method where the hot blast stove blower is directly driven by a steam turbine to drive a large blower. More steel plants have been transformed to drive blowers with high-voltage motors. The dust removal equipment includes charging dust removal fans and tapping dust removal fans. Because the speed range of the tapping dust removal fan is particularly large, the tapping dust removal fan currently uses a lot of frequency conversion speed regulation. Generally, each blast furnace is equipped with at least 2 dust removal fans and 3-4 slag flushing water pumps [7]. According to data released by the China Iron and Steel Association website in January 2006: In 2005, the national 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 China's steel enterprises expanded rapidly. Among them, there were 8 enterprises with an annual output of more than 10 million tons of crude steel, including Baosteel, Ansteel, Wuhan Iron and Steel, Shougang, Shagang, Laigang, Jinan Iron and Steel, and Tanggang; there were 10 enterprises with an annual output of 5 million to 9.7 million tons of crude steel, including Maanshan Iron and Steel, Valin Steel, Baotou Steel, Handan Steel, Benxi Steel, Panzhihua Steel, Anyang Iron and Steel, Taiyuan Iron and Steel, Jiuquan Iron and Steel, and Jianlong Steel; there were 16 enterprises with an annual output of 3 million to 5 million tons of crude steel. The 18 steel enterprises with an annual output of more than 5 million tons of crude steel had a total crude steel output of 161.9716 million tons, accounting for 46.36% of the national crude steel output [8]. Currently, high-voltage frequency converters in the metallurgical industry are mainly used in fans and water pumps, and are rarely used 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 in fan and pump loads, and is characterized by large single-unit capacity. The power industry has historically been conservative in adopting new technologies due to its extremely high requirements for power supply safety; therefore, large-scale high-voltage fan retrofitting is still not widespread. The primary 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 variable frequency speed control systems can now basically meet the requirements of power generation companies, and thermal power plants have gradually accepted this new technology.
For power plants, reliable equipment operation is one of the most critical factors. No matter how good the performance or how significant the energy savings of any auxiliary speed control equipment, if reliability is not guaranteed, everything else is meaningless. If these critical auxiliary equipment shuts down abnormally due to speed control device failure, it often leads to a significant drop in the main unit load, and in severe cases, even causes the main unit to shut down, the boiler to shut down, and power generation accidents, resulting in losses far exceeding 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, maintaining stable boiler furnace negative pressure, flue gas oxygen content, and corresponding steam temperature and pressure requires timely adjustments to the forced draft and induced draft fan volumes and pulverized coal quantities. Forced draft fans typically adjust airflow by adjusting the inlet guide vanes, with the vane opening often varying between 60% and 80%, while induced draft fan dampers use baffles for adjustment, with the daily operating opening generally only around 50%. When using a high-voltage frequency converter to regulate its speed, the general speed regulation range is approximately 40Hz-50Hz.
2.2 Application Characteristics in the Metallurgical Industry
The metallurgical industry plays a vital role in the national economy, and its enterprises are typically large-scale, highly profitable, and operate on a continuous, efficient production process. Steel companies of a certain size have generally achieved automated process control within their workshops, resulting in a high degree of automation in production processes. Numerous on-site devices require control. Due to the harsh environment—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 higher than in other industries. For example, the dust collector fan in a steel plant is the power center of the dust removal and purification system. If it malfunctions, it not only affects production and causes significant 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 of a blast furnace as an example, the iron tapping process in blast furnace ironmaking is not continuous but intermittent. Using a high-voltage frequency converter to control the speed of the dust collector fan in real time can achieve significant energy savings. During iron tapping, a large amount of brownish-red dust is generated, requiring the dust collector fan to operate at high speed; the frequency converter setting is then set to around 45Hz. When iron is not being tapped, the fan only needs a very low speed, and the frequency converter setting is then set to around 20Hz. Similarly, in electric arc furnaces (EAFs), the dust collector fan's airflow must be adjusted in real time according to the amount of flue gas produced by the furnace to meet the requirements of the production process. Generally, the production cycle of an EAF consists of seven steps: charging, iron charging, power supply, power and oxygen supply, sampling, steel tapping, and plugging. In each process, the amount of flue gas produced by the EAF varies, and the temperature requirements also differ. If the fan's airflow is too large, it will cause heat loss inside the furnace, prolong the production cycle, and waste energy; if the airflow is too small, it will cause the furnace temperature to be too high, which may burn 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 demanding strong overload capacity from the motor and frequency converter, 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 RMB 1200-1800 billion, with low-voltage frequency converters accounting for about 60% of the market share and medium- and high-voltage frequency converters accounting for 40%. The frequency converter market in China is currently in a period of rapid growth, maintaining a high growth rate of 12% to 20% in the past few years. In 2003, the market size even exceeded RMB 5.5 billion. Some people believe that it will take at least 10 years for the frequency converter market in China to become saturated and mature [9] . Due to the constraints of high-voltage power electronic devices, the application of high-voltage frequency converters in various industries has just begun and needs further promotion. At present, the industries that use high-voltage frequency converters more often 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, and a large number of products will be replaced 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 China has exceeded 6,000 units. Figure 2 shows the development history of China's high-voltage frequency converter market [10-11] . The strong upward trend of the high-voltage frequency converter market is easily seen in the figure.
3.1 Market Prospects of 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 kilowatt-hours. Among these, about 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 power consumption, and 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 the enterprise. 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, achieving 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 specifications of high-voltage, high-capacity frequency converters have improved, their functions have become more powerful, their prices have become lower, and their investment payback period has been shortened. All of this indicates that the application of high-voltage frequency converters in thermal power plants will be rapidly promoted.
3.2 Market Prospects of 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%. Usually, after using variable frequency speed control technology, the efficiency of square root torque equipment such as fans and pumps can be increased by about 30%. The efficiency of constant torque equipment such as rolling mills can be increased by 15-30%, with an average of about 20%. In addition, the application of variable frequency speed control technology can also improve the power factor of the motor (generally up to 0.98), reduce the amount of equipment maintenance, extend the service life of the equipment, and reduce the major losses caused by accidental shutdowns to production. These will bring great benefits to reducing operating costs [12] . For example, the use of variable frequency speed control technology on the steel rolling heating furnace to realize the automatic control and adjustment of the gas/air ratio not only reduces the power consumption of the air fan, but also ensures the combustion ratio of gas and air, ensures the full combustion of gas, and reduces the consumption of gas. The economic benefits brought by the reduction of gas consumption are far greater than the power saving benefits, and at the same time, it also reduces the production operation intensity of workers.
4. Market capacity of high voltage frequency converters
According to statistics, the current AC motor capacity in use in China is about 400 million kW, and the motors used for fans and pumps account for about 40%, with a total capacity of 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% frequency conversion is achieved, it will reach 12.8 million kW, and it is not difficult to calculate that the power saving benefits will also be very significant. If calculated based on an average power saving rate of 20%-30%, the power saving will be 2.56-3.84 million kW. In addition to the high-voltage frequency converters that are matched with the newly built large fans and pumps, only the high-voltage frequency conversion speed regulation of the existing large fans and pumps is used for calculation and analysis: if 12.8 million kW is completed in 5 years, then about 2.5 million kW will be converted each year. Based on the current market price of 1,500-2,000 yuan/kW, there will be a market sales of 3.75-5 billion yuan per year [12] . At present, the high voltage frequency converter market is still in the development and exploration stage. In 2004, the market size of high voltage frequency converter was RMB 850 million, accounting for about 13% of the total frequency converter market size (RMB 6.63 billion). The market share of high voltage frequency converter in various industries is shown in Figure 3 [11] .
4.1 Market Capacity of the Power Industry
In the market for high voltage frequency converters, the power industry has the largest demand and market capacity, accounting for more than 50%. In the thermal power plants that are already in operation, there are more than 5,000 high voltage and large capacity motors that need to be retrofitted for frequency conversion speed regulation. The market potential in this field alone is as high as RMB 6.5 billion. According to the national economic development plan, the power construction development speed in the first ten years of the 21st century is about 7% to 8%, with an average annual increase of 24 million to 25 million kW of installed capacity. The annual increase of high voltage and large capacity rotating equipment such as fans and pumps that are matched with the units requires 500 to 600 units. Just by estimating the high voltage frequency converters that are matched with these new equipment, the annual market potential can reach RMB 650 million to RMB 780 million [13] .
4.2 Market Capacity of the Metallurgical Industry
The metallurgical industry accounts for 14.18% of the nation's total industrial electricity consumption. In my country, the energy utilization rate of motor drive systems is about 20% lower than the international average, and the technology of these motors is only equivalent to that of foreign countries in the 1950s and 60s, indicating significant potential for energy conservation. 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 utilize AC variable frequency speed regulation. Mine hoists use drive motors with capacities between 500 and 4000 kW, employing DC drives, although some have been converted to AC speed regulation. Mine ventilation fans, drainage pumps, and other key equipment for ensuring mine safety have motor capacities between 200 and 800 kW, and are numerous, representing substantial 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 product structure and environmental protection requirements in metallurgical production, the metallurgical industry will build a large number of new steel rolling production lines and make extensive use of dust removal fans and cooling water pumps, resulting in a considerable increase in capacity each year. Among these, 50% of the fans and pumps will require AC variable frequency speed control retrofitting or new units equipped with high-voltage frequency converters, and the market capacity for this will exceed one million kW.
The development of high-voltage frequency converters is booming. With the advancement of power electronics technology and the needs of rapid national economic development, the application of high-voltage frequency converters in my country's power, metallurgy, and other industries will undoubtedly see even greater growth. The reliable application of high-voltage frequency converters will become one of the main measures for energy conservation in my country's power sector.
References
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