Abstract: In the field of rolling mill transmission, the most widely used current transmission systems are AC-AC frequency control systems and AC-DC-AC frequency control systems. By comparing the differences between these two transmission methods in terms of system structure, working principle, and performance indicators, their respective application scenarios are determined. The development direction of AC variable frequency speed control systems is also discussed by analyzing their technological advantages. Keywords: AC /AC frequency converter, AC/DC/AC frequency converter, hot rolling mill line, driving motion 1. Introduction In the field of transmission today, electric motors used include DC motors and AC motors. In the past, DC motors dominated because AC motor control systems could not meet the requirements of production machinery for electrical control systems in terms of speed range, dynamic response, and static error. However, DC motors have a series of drawbacks, such as commutation and commutator issues, and the large maintenance workload of components like brushes. This limits their ability to increase single-machine capacity, improve overload capacity, reduce rotational inertia, and simplify maintenance. Compared with AC motors of the same capacity, they are larger, heavier, have higher rotational inertia, are more expensive, and have lower efficiency, making them unsuitable for the development of rolling mills towards larger and higher speeds. With the rapid development of power electronics and microelectronics technologies and the application of modern control theory, AC motors have developed rapidly, and their speed regulation characteristics and performance even surpass those of DC motors. Therefore, current hot strip rolling mills mainly use AC motor drives and AC speed regulation. There are various methods for AC speed regulation: pole-changing speed regulation of asynchronous motors, voltage regulation speed regulation, rotor series resistance speed regulation, electromagnetic slip clutch speed regulation, hydraulic coupling speed regulation, mechanical differential speed regulation, frequency conversion speed regulation, etc., but frequency conversion speed regulation is the mainstream. Currently, the AC speed regulation technologies used in the main drive of hot strip rolling mills are mainly AC-AC frequency conversion speed regulation and IGCT/IGBT three-level AC-DC-AC frequency conversion speed regulation. AC-DC-AC frequency conversion can be further divided into two main categories: voltage-type and current-type. AC-AC frequency conversion is mostly voltage-type, with a small number using current-type. Frequency conversion control methods include voltage-type, current-type, and pulse width modulation, etc. The main circuit offers a variety of topologies and control strategies to choose from. Power devices include SCR (thyristor), GTO (gate turn-off thyristor), IGBT (insulated gate bipolar transistor), and IGCT (integrated gate commutated thyristor). Main circuit topologies can be selected from two-level, three-level, and load-commutated SCR current-source inverters. Control strategies include V/F control, vector control, direct torque control, pulse width modulation (PWM), and pulse amplitude modulation (PAM). Voltage options include high voltage (3 to 6 kV, mainly for large-capacity synchronous or asynchronous motors), medium voltage, and low voltage (such as general small-power 380V motors and auxiliary drive motors in steel rolling mills). Furthermore, variable frequency speed control also includes pole-changing speed regulation, and stepless speed regulation includes vector control and variable voltage variable frequency (VVVF) control. 2. Introduction to Conventional Hot Continuous Rolling Mill Production Line A conventional hot continuous rolling mill production line generally consists of areas such as a slab silo, heating furnace, roughing mill, finishing mill, coiling area, and conveyor chain. A schematic diagram of its process equipment is shown in Figure 1. The main motor-driven mechanical equipment requiring speed adjustment includes: slab or strip transport rollers for the entire rolling line, slab silo receiving turntable, crossover trolley, holding furnace or pit cover, heating furnace walking beam lifting device, steel loading/unloading translation or lifting device, width-fixing press or vertical rolling mill in the roughing mill, vertical roll gap adjustment device, upper and lower rollers of the roughing mill main unit, low-speed and high-speed electric pressing devices, flying shear main drive, finishing mill main drive, cross roll PC device, electric looper, upper and lower clamping rollers for coiling, auxiliary coiling rollers, coiling drum, conveyor chain, quality inspection station floor rollers, and electric side guides before and after the mill. Although some of the aforementioned devices have evolved from electric drive to hydraulic drive due to advancements in process technology and mechanical hydraulic technology, such as side guide plates, finishing mill loopers, roughing mill vertical rolls, and finishing mill roll gap adjustment devices, the vast majority of equipment on hot strip mill production lines is still driven by variable frequency drives (VFDs). [align=center]Figure 1 Schematic diagram of conventional hot strip mill process equipment configuration[/align] Since the main hot rolling mill is primarily a large-capacity, low-speed mechanical motion device, such as the roughing mill's upper and lower roll motors with power ranging from 2500KW to 9000KW and speeds between 20rpm and 100rpm; and the finishing mill's motors with power ranging from 5000KW to 10000KW and speeds between 200rpm and 550rpm, and considering that synchronous motors are superior to asynchronous motors in terms of power factor, motor size and moment of inertia, working efficiency, connected VFD capacity, control accuracy, and field weakening ratio, the motors driven by synchronous motors are all synchronous motors. Medium-capacity high-speed motors or small-capacity motors are all AC squirrel-cage asynchronous VFD motors. 3. Introduction to Two AC Variable Frequency Speed ​​Control Systems Currently, the main drive speed control systems for large hot-rolled mills mainly include two types: AC-AC variable frequency speed control systems and AC-DC-AC variable frequency speed control systems. Both of these drive systems are high-performance, advanced AC drive methods that can meet the technical performance requirements of the main drive of large hot-rolled mills, each with its own characteristics. 3.1 AC-AC Variable Frequency Converter 3.1.1 Characteristics of AC-AC Variable Frequency Converter The AC-AC variable frequency speed control system, as shown in Figure 2, consists of three sets of anti-parallel thyristor reversible three-phase bridge converters, corresponding to the A, B, and C phases of the synchronous motor stator. It follows the natural commutation principle of the power grid used in thyristor converters. The three-phase AC-AC converter adopts a logic-free circulating current and a three-phase neutral point configuration, powered by a secondary-side three-split rectifier transformer. The connections of the rectifier transformers for each motor are staggered to reduce high-order harmonics in the power supply. The output terminal uses a star-point connection, and the motor stator windings are three-phase star-connected. The motor star point and the frequency converter star point are independent. It has advantages such as strong overload capacity, high efficiency, and good output waveform, but it also has disadvantages such as low output frequency (the highest frequency is less than 1/2 of the grid frequency), low grid power factor, and side-frequency harmonic influence. AC-AC frequency converters are divided into those with and without circulating current, and can drive synchronous or asynchronous motors. For the main drive system, the controller generally uses vector control to drive the synchronous motor. [align=center]Figure 2 AC-AC frequency converter synchronous motor speed control system[/align] 3.1.2 Application of AC-AC frequency converters in hot strip mills In 1993, the first domestically produced 2500kW AC-AC frequency converter synchronous motor speed control system was successfully developed and applied to the 850-type steel rolling mill at Baogang Rail and Beam Plant; in 1996, the first domestically produced 4000kW fully digital control AC-AC frequency converter speed control system was launched and applied to the main drive of the medium plate rolling mill at Chongqing Iron and Steel Group; in 1999, the first domestically produced dual-motor drive AC-AC frequency converter speed control system was successfully developed and applied to the medium plate rolling mill at Wuhan Iron and Steel Group; in 2000, the first AC-AC frequency converter speed control system for hot strip mills was put into operation at Panzhihua Iron and Steel Company. According to statistics, from 1996 to 2005, there were a total of 263 sets of high-power AC-AC frequency converter rolling mill drive systems in my country, of which 171 sets were domestically manufactured, accounting for 65%. my country's high-power AC-AC frequency converter technology and application scale have surpassed those of GE in the United States, Alstom in France, and Ansaldo in Italy, reaching the world's advanced level; completely reversing the long-standing reliance on imports for large-scale industrial rolling mill transmission equipment. 3.2 AC-DC-AC Frequency Converters 3.2.1 Characteristics of AC-DC-AC Frequency Converters Since the 1980s, self-turn-off power semiconductor devices, breaking the dominance of thyristor components, such as high-power transistors (GTRs), turn-off thyristors (GTOs), and field-controlled insulated-gate bipolar transistors (IGBTs), have emerged, ushering in a new era centered on self-turn-off power semiconductor devices. Compared with traditional semi-controllable thyristor devices, electrical drive devices using self-turn-off power semiconductor devices have significant advantages such as saving raw materials, simple converter structure, small size, light weight, high power factor, and low harmonic pollution. An AC-DC-AC frequency conversion speed control system is shown in Figure 3. [align=center]Figure 3 AC-DC-AC Variable Frequency Synchronous Motor Speed ​​Control System[/align] 3.2.2 Application of AC-DC-AC Variable Frequency Converters in Steel Rolling Drives: In the field of high-power, high-voltage variable frequency speed control, GTO components once held a dominant position. In the 1990s, Mitsubishi Corporation of Japan pioneered the development of 6000V/6000A high-power GTO components and successfully applied the world's largest power 7000kW, 3kV GTO synchronous motor variable frequency speed control to the Baosteel 1580mm hot strip mill, Ansteel 1780mm hot strip mill, and Shanghai Stainless Steel Hot Strip Mill in my country. Figure 4 shows the GTO AC-DC-AC multi-level PWM variable frequency speed control system. This system is a voltage-type frequency converter, and the power supply current converter also adopts GTO pulse width modulation technology. The phase angle of the input current can be controlled to achieve a power factor of 1 at all times and reduce harmonics of the input current. This frequency converter adopts three-level GTO component series control technology, enabling the input and output voltages of the frequency converter to reach 3300V. Compared to AC-AC frequency converter speed control systems using thyristor elements, GTO frequency converters have significant advantages such as unrestricted output frequency, low grid harmonic pollution, and high power factor. However, they also have drawbacks such as high switching losses, low efficiency, the need for water cooling, and difficult maintenance. [align=center]Figure 4 Main circuit of GTO AC-DC-AC three-level PWM frequency converter speed control system[/align] Due to the aforementioned shortcomings of GTO elements, countries around the world have begun to compete in researching new high-voltage, high-power power semiconductor devices. The gate turn-off thyristor (IGCT) developed by ABB in Switzerland is a new type of high-power power semiconductor device that innovates on the GTO element. In its structural design, it reduces the control gate circuit inductance and integrates the drive circuit next to the device, reducing the switching losses of IGCT by an order of magnitude compared to GTO, improving the switching speed, eliminating the buffer absorption circuit, greatly simplifying the frequency converter structure, and improving system efficiency. ABB, GE, and Siemens have successfully developed high-power three-level PWM frequency converters using IGCT elements for rolling mill main drives. The 1700 rolling mill of Benxi Iron and Steel Company in my country adopted GE's IGCT three-level frequency converter, with a motor power of 7MW/6kV. IGCT has become the replacement device for GTO. Toshiba Corporation of Japan has successfully developed a high-voltage, high-power IEGT element, namely electronically promoted insulated gate bipolar transistor, 4000A/4500V. IEGT is a form of IGBT, which has the advantages of voltage drive, fast switching speed and self-protection of IGBT element. Toshiba Corporation has applied the 7MW/3kV high-power three-level frequency converter with IEGT element to the main drive of thin slab continuous casting and rolling of Lianyuan Iron and Steel Company in my country. 3.2.3 Problems of AC-DC-AC frequency converters Although AC-DC-AC frequency converters have the advantages of high output frequency and high power factor, there are still many problems to be improved: (1) At present, high-power, high-voltage power electronic devices are in the development stage, GTO elements are facing elimination, and IGBT and IGCT are still immature. (2) For converters using IGCT (or GTO) and IECT, protection against short circuits caused by device failure is still a challenge; if a short circuit occurs in the power supply-side converter, it will cause a short circuit in the power grid, so the converter must use a high leakage reactance input transformer, generally requiring 15%, or even as high as 20%. (3) The overload capacity of AC-DC-AC converters is reduced when operating at low frequencies, and the overload capacity of the converter is generally halved when operating below 5Hz. (4) The voltage change rate du/dt of the output PWM modulation voltage waveform of AC-DC-AC converters is very high, which can easily cause insulation fatigue damage to motors and electrical appliances; when the output wire is long, the common-mode reflected voltage will generate a very high voltage on the motor side. If it is a two-level converter, the peak value of this voltage is twice that of the DC voltage. If it is a three-level converter, the peak value of this voltage is three times that of the middle half of the voltage. (5) PWM modulation of AC-DC-AC converters will generate harmonics, noise, shaft current and other problems. 4. Comparison of Two AC Variable Frequency Speed ​​Control Systems 4.1. Differences in Power Electronic Devices: AC-AC Variable Frequency Drive (AC-AC): Uses thyristors, a very mature power electronic device. AC-DC-AC Variable Frequency Drive (AC-DC-AC): Uses IGBTs, GTOs, IGCTs, IEGTs, etc., which are newer turn-off devices. These devices are still being modified and updated, and there is insufficient experience in their use; they are transitional. 4.2. Differences in Control Methods: AC-AC Variable Frequency Drive (AC-AC): Uses a fully digital vector control method. AC-DC-AC Variable Frequency Drive (AC-DC-AC): Uses pulse width modulation (PWM) fully digital vector control or direct torque control. 4.3. Overload Capacity: AC-AC Variable Frequency Drive (AC-AC): Higher, capable of withstanding 250% overload. Especially at low speeds, it possesses and exceeds the characteristics of a DC motor. AC-DC-AC Variable Frequency Drive (AC-DC-AC): Lower, typically 136%. To improve overload capacity, the device capacity must be increased to meet maximum output. At low speeds (frequency below 5 Hz), the overload capacity decreases, generally by 50%. 4.4. Speed ​​Range: AC-AC frequency converter: Generally, the maximum operating frequency is 22Hz. Above 22Hz, system characteristics deteriorate. Suitable for medium and low speed drives. Therefore, when used in rolling mills, the maximum motor speed is 660 r/min. AC-DC-AC frequency converter: Suitable for medium and high speeds, with a minimum operating frequency of 5Hz. At lower frequencies, torque pulsation increases, and output decreases. The maximum frequency can reach 100Hz, and the maximum motor speed can reach 6000 r/min. 4.5. Rectifier Transformer Capacity: For AC-AC frequency converters, the rectifier transformer capacity is selected based on the peak value of the frequency converter voltage, approximately 1.8 to 2 times the motor capacity. For AC-DC-AC frequency converters, the rectifier transformer capacity is selected to be 1.3 to 1.4 times the motor capacity. AC-AC frequency converters require larger transformer capacities than AC-DC-AC frequency converters, resulting in higher transformer iron losses (i.e., higher no-load losses). 4.6. Cables Used: AC-AC frequency converters have low output voltage and high operating current, requiring correspondingly large cable dimensions; AC-DC-AC frequency converters have high output voltage, necessitating higher cable withstand voltage ratings. 4.7. Motors: Low-speed synchronous motors used in AC-AC drives have large rotational inertia, resulting in significant weight and higher investment costs. 4.8. Cooling Methods: AC-AC frequency converters: SCR components have low forward voltage drop, only around 1.2-1.5V. Due to low component losses, air cooling can be used. AC-DC-AC frequency converters: Converter components have large forward voltage drop, typically around 3.5-4V. Due to high forward voltage drop and switching losses, air cooling is usually not feasible; water cooling is required. Therefore, a water cooling system with a large amount of secondary circulating cooling water is necessary. The initial investment is higher, but the cooling effect is excellent. 4.9. Power Factor: The power factor on the motor side of an AC-AC frequency converter is 1; however, the power factor on the grid side is lower, typically below 0.7, requiring reactive power compensation. A reactive power compensation device (SVC) is needed to improve the power factor. Due to the intermediate capacitor, the power factor of AC-DC-AC converters is 1 on both the motor and grid sides, requiring no reactive power compensation. 4.10. Harmonics: AC-AC converters: High harmonics are generated on the grid side, typically around 8% when using a 6-pulse design, requiring a filter. AC-DC-AC converters: Low harmonics are generated on the grid side, typically around 4.5%. Filters are also needed when the active power impact of the rolling mill is large or the grid short-circuit capacity is small. 4.11. Supporting Machinery: AC-DC-AC converters have high output frequencies and high motor speeds. To meet the requirements of the rolling mill process, a reducer and water cooling system are required, increasing investment in machinery and installation costs; the reducer is large, affecting process layout. 4.12. Frequency Converters: The main power equipment of AC-AC converters has been domestically produced, and domestic manufacturers have successful supply and engineering application experience, resulting in lower prices; high-power AC-DC-AC converters relied entirely on imports for a long time, resulting in expensive equipment and high maintenance costs. 4.13. Energy Consumption: The energy consumption of AC-AC frequency converters for fan and air conditioning cooling, as well as SVC, is higher than that of AC-DC-AC frequency converters. In summary, we can see that: 1. Both AC-AC and AC-DC-AC frequency conversion speed control systems are advanced and high-performance speed control methods, each with its own characteristics. Both can meet the technical performance requirements of the main drive of large hot-rolling mills. 2. AC-AC frequency conversion technology has significant advantages such as large overload capacity, high efficiency, simplicity, reliability, and complete domestic production capability. However, its main disadvantages are low frequency, low speed, low power factor, and the need for reactive power compensation. It is suitable for high-power, medium- and low-speed rolling mill drives with large overload capacity, such as large hot-rolling mills, medium-thick plate mills, and section steel mills. 3. AC-DC-AC frequency conversion has significant advantages such as high frequency, high speed, high power factor, and no need for reactive power compensation overload. However, its significant disadvantages are lower efficiency, poor overload capacity, insufficient user experience, inability to be domestically produced, and significant issues with spare parts and technical services. Suitable for high-power, medium- and high-speed fans, pumps, and rolling mill drives with low overload capacity, such as high-speed cold rolling mills and seamless steel pipe rolling mills. 5. Conclusion From the perspective of the main drive systems of large-scale hot-rolling mills already in operation and under construction in China, AC-AC variable frequency speed control systems remain the mainstream in terms of both technology and quantity. The main reasons are: lower investment, higher efficiency, stronger overload capacity, and complete domestic production capability. The significant progress made in China's rolling mill drive AC-AC variable frequency speed control systems is due to the foundation of thyristor technology accumulated over many years. Domestic AC-AC variable frequency speed control technology is very mature, and many key technologies of the main drive speed control system have been successfully solved, such as: high precision and high dynamic response of rolling mill drives; high-power AC-AC frequency converters; design and manufacturing of AC-AC variable frequency synchronous motors; dynamic reactive power and harmonic calculation and harmonic mitigation of the power grid by AC-AC frequency conversion; and torsional vibration calculation and suppression of the rolling mill. However, AC-AC systems generate a large number of harmonics, causing a certain degree of pollution to the power grid. Therefore, additional filtering and reactive power compensation devices are required on lines with high power grid quality requirements. This shortcoming cannot be completely overcome at present. Currently, the transmission field needs the domestic development of high-power AC-DC-AC frequency converters. AC-DC-AC frequency converter technology and equipment are gradually being promoted due to their excellent control performance, high power factor, and good energy-saving effect. With the improvement of the performance of high-voltage, high-power power semiconductor devices such as GTO, IGCT, and IEGT, the size of equipment has decreased accordingly. Furthermore, factors such as the low output frequency of AC-AC frequency converters (the highest frequency is less than 1/2 of the grid frequency), low grid power factor, side-frequency harmonic influence, the need for SVC, and large footprint have led to the replacement of main drive frequency converters in hot-rolled strip steel production lines in the new century with three-level voltage-type AC-DC-AC PWM vector control frequency converters or direct torque control frequency converters. References [1] Chen Boshi, Chen Minxun. AC speed control system. Beijing: Machinery Industry Press, 2004 [2] Ma Xiaoliang. High power AC-AC frequency conversion speed regulation and vector control technology. Beijing: Machinery Industry Press, 2004 [3] Wang Shu. Design and application of frequency conversion speed regulation system. Beijing: Machinery Industry Press, 2005