Abstract: Based on practical experience, this article describes the configuration of frequency converters of various capacities (large, medium, and small) for conventional hot strip mill production lines. It also discusses the considerations for negotiating, designing, selecting, and applying imported frequency converters, providing suggestions for the design of similar hot strip mill frequency converters in the future. Keywords: Hot Rolling Mill Line; Frequency Converter; Design; Application 0 Introduction Conventional hot rolling mill lines are one of the important production lines in the metallurgical industry. In recent years, similar production lines have been continuously launched, with 12 new lines added in China in 2007 alone. As is well known, most of the mechanical equipment in general hot rolling mills is driven by AC speed-regulating motors, and the speed regulation method is mainly frequency conversion speed regulation. Therefore, AC variable frequency speed control devices occupy an extremely important position in hot rolling electrical equipment. Due to the requirements of hot continuous rolling production lines for speed, stability, and continuity, and the immaturity of similar domestic frequency converter products, most domestic hot rolling production lines import products from foreign companies, whether for low-voltage, small-capacity frequency converters or large-capacity, high-voltage, high-performance main drive devices. Currently, hot rolling, especially, holds an absolute market share with products from Siemens (Germany), Mitsubishi Electric (Japan), and Toshiba (Japan; the latter two merged some of their businesses in 2003 to form TMEIC, a company mainly engaged in steel and paper manufacturing). Although the above-mentioned manufacturers have mature and series of variable frequency speed control devices for the metallurgical industry, in actual engineering, due to factors such as cost and design experience, there are still many practical problems to be discussed, explored, and solved in the negotiation, design, selection, and application of frequency converters used in hot rolling production lines. 1 Equipment Overview A conventional hot continuous rolling production line generally consists of six major areas: slab warehouse, heating furnace, roughing mill, finishing mill, coiling, and transport chain. Its process equipment flow diagram is shown in Figure 1 (the figure does not include the slab warehouse and transport chain areas). The main motor-driven mechanical equipment requiring speed regulation includes: slab or strip transport roller conveyors for the entire rolling mill, slab warehouse receiving turntables, cross-span trolleys, holding furnaces or pit covers, walking beam lifting devices for heating furnaces, steel loading and unloading translation or lifting devices, the main side pressure unit (SSP) in the roughing mill zone, SSP gap adjustment devices, SSP synchronization devices, SSP pinch rolls, upper and lower rolls of the roughing mill main unit, low-speed and high-speed electric pressing devices, vertical roll main unit, vertical roll gap adjustment devices, flying shear main drive, finishing mill main drive, cross roll PC device, electric looper, upper and lower pinch rolls for coiling, auxiliary coiling rolls, coiling drums, conveyor chains, quality inspection station ground rolls, and electric side guides before and after the mill. Although with the development of process technology and mechanical hydraulic technology, very few of the above devices have evolved from electric drive to hydraulic drive, such as side guides, finishing mill loopers, and roughing mill vertical roll gap adjustment devices. [b][align=center] Fig1 The equipment layout schematic diagram of normal hot rolling mill line [/align][/b] Since the main hot rolling mill, except for the SSP, consists of large-capacity, low-speed mechanical motion equipment, such as the upper and lower roll motors of the roughing mill with power ranging from 2500KW to 9000KW and speeds between 20rpm and 100rpm; and the finishing mill motors with power ranging from 5000KW to 10000KW and speeds between 200rpm and 550rpm. In addition, synchronous motors are superior to asynchronous motors in terms of power factor, motor size and moment of inertia, working efficiency, connected frequency converter capacity, control accuracy, and field weakening ratio. Therefore, the motors driven by them are all synchronous motors. Medium-capacity high-speed motors or small-capacity motors all use AC squirrel-cage asynchronous frequency converters [1]. Although there are many of the above-mentioned devices, the voltage and capacity of the motors range from AC 380V/2KW to 3300V/10000KW, and the capacity of the inverters ranges from 2KVA to 16MVA. However, according to the product series provided by the frequency converter manufacturer, they are generally divided into three categories. That is, according to the capacity of the motor and the frequency converter: main motor - main drive device, important main and auxiliary motors - medium capacity frequency converter, and auxiliary motor - small capacity frequency converter. Table 1 is an explanation of the frequency converters supplied by Mitsubishi Electric in the mid-to-late 1990s and Toshiba in the early 21st century for hot rolling mills. [b][align=center]Table 1 A list of corresponding models of speed control equipment and frequency converters for hot rolling mill lines[/align][/b][/align] 2 Frequency Converter Design and Selection Main Drive Frequency Converter With the emergence and application of high-voltage, high-power power semiconductor devices such as GTO, IGCT, and IEGT, as well as the influence of 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 effects, the need for SVC, and large footprint. In the 21st century, the main drive frequency converters of hot-rolled strip steel production lines are being replaced by three-level voltage-type AC-DC-AC PWM vector control frequency converters or direct torque control frequency converters. Even Siemens, the German company that has always advocated and insisted on AC-AC frequency converter vector control technology with thyristors as power elements, has recently had to launch voltage-type AC-DC-AC frequency converters with IGCT as power elements to power the main mill of hot rolling production lines. Table 2 shows the voltage-type AC-DC-AC frequency converter products and application examples that have been operating in the hot rolling field for many years in recent years: [b][align=center]Table 2 Typical foreign production parameter and typical application user of main driver in Hot Rolling Mill[/align][/b] Taking the Toshiba TM-70 product as an example to illustrate the main drive of hot rolling, Figure 2 is a schematic diagram of the main circuit principle of a single-loop (1 bank) voltage-type AC-DC-AC three-level frequency converter. As can be seen from Figure 2, the rectifier and inverter of this structure are typical symmetrical structures, and the main circuit device can be replaced. Figure 2 illustrates the TM-70 with an 8000-frame configuration as an example: If the motor is a squirrel-cage induction motor, its input voltage is AC 3550V, 50Hz, and its output is 3400V, 0-60Hz, with a device capacity of 8MVA. If the motor is a synchronous motor, its input voltage is AC 3550V, 50Hz, and its output is 3200V, 0-60Hz, with a device capacity of 7.5MVA. If the single-circuit power supply capacity is insufficient, a dual-circuit power supply method can be used. Generally, there are two modes: one is a dual-frequency converter for a single dual-winding motor (see Figure 3); the other is that the output of the dual-frequency converter can be output to a single-winding motor through an output reactor. Practice has proven that the TM-70 device has the characteristics of high efficiency, high performance, no pollution to the power grid, and small footprint. [b][align=center]Fig 2 Main circuit configuration diagram of 3-level voltage source converter[/align][/b] [b][align=center]Fig 3 The single line diagram of double converters to twin-winding synchronous motor[/align][/b] 2.1.1 Selection of Variable Frequency Drive Capacity Under the process product outline and related parameters, the required rolling torque, speed, overload multiple, rolling rhythm, etc., of the motor are determined by the machinery supplier or design institute after calculation. Once the motor parameters are determined, the motor supplier, the variable frequency drive supplier, and the design institute jointly select the relevant parameters such as the variable frequency drive, rectifier transformer, high-voltage switch, inverter, and isolating switch between the motor and the variable frequency drive. Due to uncertainties in the process during negotiations and the fact that many factors are not considered in theoretical calculations or simulations, or the calculations are complex and the relevant coefficients vary, inaccurate calculations or even serious deviations may occur, such as the calculated value of the rolling force of the hot rolling mill. As a result, the electromagnetic power of the motor may be calculated incorrectly, and subsequently, the wrong inverter, other switches, transformers, etc., may also be selected. Furthermore, foreign bidders, aiming to reduce costs, may design the capacity margin of motors or inverters to be too small, especially for equipment with capacities exceeding the standard range. For example, assuming a single inverter has a capacity of 8MVA and an overload factor of 1.5, if the load is an AC synchronous motor with a rated power of 4800KW, under an overload factor of 2.5 for 60 seconds, one unit would be insufficient. However, to win the bid, foreign bidders often persuade the buyer by emphasizing that their unit is sufficient or by highlighting improvements in motor efficiency. Based on practical experience, due to future capacity increases, faster pace, and the expansion of new product varieties, especially the increased proportion of difficult-to-roll varieties, the increase in extreme specifications, and changes in process conditions (such as lower-limit steelmaking and low-temperature tapping of slabs for energy conservation, poor slab temperature uniformity, certain deviations in mill load distribution, or longer slab placement time before the mill), insufficient rolling torque may cause overcurrent tripping of the equipment during production. Therefore, when selecting the main drive unit for the rolling line, it is recommended to select according to design needs as much as possible, while considering the uniformity and interchangeability of frequency converters throughout the plant and reducing the variety of equipment specifications. If similar selection phenomena occur, it is preferable to sacrifice cost or uniformity to ensure that the unit has sufficient redundancy. The same principle applies to the corresponding motors. In some situations, it is better to use a "large horse pulling a small cart" approach; otherwise, the losses will be greater or modifications will need to be made in advance. [b]2.1.2 Selection of Auxiliary Equipment Related to Frequency Converter (1) Pure Water Cooling Device[/b] Due to the use of high-power drive components and the tight installation of components inside the cabinet, in addition to forced air cooling, a pure water cooling system is generally used. Each pure water cooling device consists of a water tank, filter, ion exchanger, conductive ion detection device, industrial circulating cooling water and related pipelines, flow and pressure detection, valves, etc. This cooling device is divided into cabinet type and non-cabinet type, and independent and non-independent types. Due to the large number of hot rolling main units, the selection principle is to configure them according to the principle of proximity and similar or identical cooling capacity. For example, a hot rolling plant has 13 sets of main drive voltage type AC-DC-AC frequency converters, and only 4 sets of pure water cooling devices are equipped, namely, 1 set is used for the large side pressure SSP and the upper and lower roll devices of roughing mill R1, 1 set is used for the upper and lower roll devices of roughing mill R2, and 1 set is used for each of flying shear CS and finishing mills F1 to F3, and finishing mills F4 to F7. The installation location can be selected according to the size of the electrical room. If there is no space, the cable interlayer can be selected; if there is space, it can be placed in the electrical room. (2) Configuration of other related equipment or devices Because the buyer has little experience or understanding of the foreign supplier's equipment and functions, they are at a disadvantage in technical negotiations. The seller also takes advantage of this by setting many traps in the appendices to the technical contract. At this time, the buyer still thinks that such devices are standard series products. Therefore, in actual projects, there are often shortages or inferior products, especially in the optional accessories: discharge devices or circuits, water leakage detection sensors and alarm devices in the cabinet, lighting devices in the cabinet, temperature detection and alarm devices for the main excitation circuit, spare display circuits, simple maintenance panels, trolleys for disassembling and installing STACK components, professional tools and testing devices for replacing power components of STACK components. In addition, attention should also be paid to the cooling fans on the top of the cabinet, the intermediate circuit filter electrolytic capacitors, the rectifier side inlet reactors and surge absorption devices, control power supplies and other components to avoid inferior products. (3) Communication devices First of all, there is the communication device between the transmission device itself and its upper-level basic automation. It is required to be open or to disclose its communication protocol to the user, such as Profibus-DP. The communication medium is generally optical fiber. Secondly, the local area network composed of each transmission device system should use a common TCP/IP Ethernet to enable remote maintenance and monitoring. In addition, the entire rolling line should be configured with 3 to 4 sets of transmission systems and PLC systems combined into one or independent data acquisition and processing systems according to the area, so as to facilitate online information or trigger tracking. The fastest sampling time is no more than 0.1ms. Finally, the transmission maintenance tools (PC computers, which can realize functions such as program or parameter download and upload, parameter setting and modification, real-time monitoring, fault waveform or information reading) can generally be connected using RS232 or RS485 or general Ethernet cables. [b] (4) The protection function of the transmission device should be complete and effective. (5) Software or special function configuration of the transmission device[/b] After the transmission device is debugged, the software and parameters are basically unchanged. However, with the increasing requirements of the process, such as the expansion of product material specifications, or when the actual system has abnormalities such as electromechanical resonance and torsional vibration during the rolling of some slabs, or when the mechanical system is upgraded or aged, or the electrical system is aged, the system is required to have convenient software reengineering function or simple hardware addition and modification. For example, internal interlocking combinations or additions, optimization of power outage or control logic or timing, free combination of typical loop (proportional, integral, derivative) links of current loop and speed loop PID controllers, variable gain and variable time parameters of the same controller under different operating conditions, multiple variable amplitude control, programming of loop additions such as state observation loop and filter loop. In addition, in the control function of hot continuous rolling mill, there are also special functions such as the upper and lower roll load balancing function of roughing mill, the Drop function, and the SFC function to suppress mill torsional vibration. In addition to the above functions, the system should preferably have a powerful self-diagnostic function, and the prediction should be accurate and timely, and it should be convenient for users to quickly find and solve problems under the fault code prompts and work instructions. The system software upgrade should be convenient and fast. (6) Relationship with upstream and downstream equipment The capacity of the rectifier transformer, high voltage cabinet and disconnect switch and cable should be appropriate, the protection should be matched, and the interlocking should be thorough. 2.2 Medium-Capacity Variable Frequency Drives The electromagnetic power of the motors in the vertical rolling mill E before the roughing mill and the flying shear CS and winding drum device before the finishing mill ranges from 500KW to 1200KW. Due to their moderate capacity and high control precision requirements, practical engineering uses variable frequency drives that differ from low-capacity and high-capacity drives. These drives use medium-voltage and medium-capacity drives. The rectifier is a shared SCR (Silicon Controlled Rectifier) ​​and outputs through a shared bus to a three-level AC-DC-AC PWM variable frequency drive using IGBTs as power elements. If the inverter capacity is large, multiple variable frequency drives of the same capacity are typically connected in parallel for power supply. The control principle diagram is shown in Figure 4, and the actual configuration of a certain plant is shown in Table 3. Since this type of drive inverter uses IGBTs, water cooling is not required. Other selection principles are basically the same as for the main drive. However, Toshiba Corporation of Japan is about to launch the TM-50, a medium-capacity rectifier-inverter with a symmetrical structure similar to the main drive. [b][align=center]Fig 4 The control sketch diagram of middle capacity TM-30T/30 frequency converter[/align][/b] As shown in Fig. 4, the rectifier section consists of two sets of parallel ordinary thyristor rectifiers, which send ±900V DC voltage to the two inverter units through an LC DC filter circuit. The inverters are three-level voltage type AC-DC-AC frequency converters, and the power components are IGBTs. Since the working side and drive side of the vertical rolling mill are independently driven and connected by a mechanical intermediate shaft, and are close to each other, the two inverters often share a common rectifier. However, the coiler or flying shear motor is often driven independently by a single motor. Considering that the loss should be minimized in the event of a rectifier failure, the inverters of the coiler are installed in close proximity, but often two independent rectifiers are set up, that is, one coiler uses one rectifier. Although the initial investment cost increases, the risk of future plant shutdown is minimized. The parameters of the motor, rectifier, and inverter are shown in Table 3. [b][align=center]Table 3 The main parameters and allocation of middle capacity frequency converter in a hot rolling production line[/align][/b] 2.3 Low-voltage, small-capacity frequency converter For other speed-regulating loads on the hot rolling line, due to the low voltage and small power of the motors, but their large number, a common rectifier is generally used for rectification. Then, the DC voltage is sent to many inverter cabinets through the DC bus, and then the motor is powered by an inverter with IGBTs as the main circuit power element. The control technology adopts two-level PWM vector control technology. To facilitate motor maintenance or insulation testing, an air switch is generally added between the inverter and the motor, or an output contactor is added inside the inverter. The schematic diagram of the low-voltage, small-capacity frequency converter is shown in Figure 5. [b][align=center]Fig 5 A real example of one common converter with many inverters[/align][/b] As shown in Fig. 5, unit T1212 represents a common rectifier using thyristors as power elements. Its capacity is 1440KW, input AC voltage is 630V, frequency is 50Hz, and the rectifier output is a low-voltage DC of 600V with a rated current of 2400A. The device overload capacity is 150% for 60 seconds. Its output DC voltage is sent to each inverter through a 600V DC bus. Depending on the inverter power, those less than 125KVA do not have a DC input switch, while those greater than 125KVA do. Each INV unit has an output circuit breaker for easy maintenance. Fig. 5 only shows some of the inverter loads connected to this common rectifier. These loads include both independent inverters supplying power to a single motor and groups of roller conveyors. In actual engineering, this rectifier drives three types of inverters (Type 125, Type 200, and Type 300), totaling 12 inverter sets, and two types of motors (22KW and 37KW), totaling 103 roller conveyor motors and 2 turntable drive motors. 2.3.1 Rectifier Configuration The principle of configuration is that the single-unit capacity should not be too large (otherwise it will increase investment and land occupation) nor too small (making maintenance and power outages inconvenient). The following factors should be considered: the electrical room and frequency converter should be located in or near the user's center to reduce cable length and voltage drop; the rectifier's own capacity; minimal impact from faults; and the area of ​​the process equipment and the convenience of maintenance and power outages. Case 1 involves a hot rolling mill sharing a common rectifier with auxiliary equipment near the slab warehouse and heating furnace, as well as the roughing mill's SSP and R1 mills. This frequently results in a conflict: while the auxiliary equipment in the SSP and R1 mills is under maintenance, the auxiliary equipment in the slab warehouse and heating furnace area must be received and loaded with steel more than four hours earlier after maintenance, creating a situation where half the load needs to be shut down for maintenance while the other half needs to operate. Case 2 fails to consider the possibility of a complete line shutdown in the event of a rectifier failure. A hot rolling line's coiling area has two independently operating coiling machines. If these two auxiliary machines share a common rectifier, a failure of this device for more than six hours has resulted in a six-hour line shutdown. Therefore, it is necessary to separate the loads to ensure that the coiling machine powered by the other machine can continue coiling steel after the failure of one common rectifier. If the load is far from the load center, it is preferable to add a small rectifier power supply near the load, such as for roller conveyors or auxiliary loads like the slab warehouse and conveyor chains, to ensure the motor power supply voltage and significantly reduce the length and cross-sectional area of ​​the motor cables. A hot rolling production line is equipped with 10 sets of common rectifier devices (3 sets of slab storage heating furnaces, 2 sets of roughing mills, 1 set of finishing mills, and 4 sets of coiling and conveyor chains). There are three types of devices, and the rated capacity-DC voltage-rated voltage parameters are 720KW-600V-1200A, 1440KW-600V-2400A, and 2880KW-600V-4800A, respectively. 2.3.2 Inverter Configuration or Selection Principles for selecting inverters for hot rolling auxiliary machines: (1) The device capacity should have a certain margin; (2) Important loads should be powered independently, while ordinary roller conveyors can be powered in groups. The motors of the grouped roller conveyors should be in the same process speed range, with the same speed and linear speed. The parameters of the motors in the group should be as close as possible or the same; (3) The number of inverter types in the whole plant should be reduced as much as possible to reduce the number of spare parts; (4) Small capacity inverters INV should adopt a drawer-type structure similar to the motor control center as much as possible. Japanese inverters often feature multi-stage drawer-type, three-phase integrated independent cabinet-type, and three-phase independent cabinet-type designs, while Siemens of Germany typically uses stationary types. Toshiba's auxiliary inverters are available in as many as 17 different capacities: 2, 4, 8, 15, 25, 45, 75, 125, 200, 300, 400, 500, 700, 900, 1000, 1400, and 1800, covering all auxiliary loads in hot rolling mills, offering a complete range of options. Mitsubishi Electric's Melvc-1200 series, launched in the early 1990s, and Siemens' 6SE70 general-purpose inverter series both offer numerous models with varying capacities. A certain hot rolling production line actually used 209 sets of inverters from 15 of the 17 models in Toshiba's TM-10 series. The largest inverter, the Model 1800, has an output capacity of 979 KVA, an output AC voltage of 440V, and a current of 1285A. It operates continuously at 100% load, under 225% overload for 60 seconds, and under 250% overload for 15 seconds. The smallest inverter, the Model 2, has an output capacity of 2 KVA, an output AC voltage of 440V, and a current of 2.6A. It also operates continuously at 100% load and under 150% overload for 60 seconds. The principle for calculating the device capacity is that the product of the capacity of the independent drive inverter's INV unit multiplied by the inverter's overload factor (1.5 times for Toshiba and Mitsubishi Electric inverters, and 1.36 times for the Siemens 6SE70 series) must not be less than the motor's output electromagnetic power multiplied by its maximum permissible overload factor. For certain special loads, such as hot rolling production lines for high-temperature silicon steel, sufficient capacity margin must be reserved for the rollers from the furnace exit roller table to the roughing mill, the inlet and outlet stand rollers of the roughing mill, the outlet stand rollers of the finishing mill, the rollers with roller lifting devices in the SSP (Special Purpose Roller Supply System), and the ground rollers of the quality inspection station. Otherwise, frequent power outages of such inverters will occur. This is because, in addition to considering the slippage of some motors during startup, it is also necessary to consider the uneven wear of the rollers, their elevation, slab deformation, the head and tail collapse of the silicon steel, the poor control of the roughing mill's head lifting, and the characteristics of slab incline. Sometimes, in a group of 13 rollers, only 2 to 3 rollers are in contact with the slab. Therefore, the inverter capacity calculation must leave a margin, and its demand factor cannot be considered too small. Generally, the maximum output current of a hot-rolled assembly line inverter should satisfy the following formula: IINVmax ≤ uINV＊IINVN 〔2〕 Where IINVmax is the maximum output current of the inverter when slippage occurs during startup, u is the allowable overload multiple of the inverter's output current; IINVN is the rated value of the inverter's output current, and IINVmax can be calculated using the following formula: IINVmax＝n1＊IM1+n2＊IM2 In this inverter, n1 motors are in no-load starting state, with each motor having a current of IM1. Meanwhile, n2 motors are in load starting state with slippage, with each motor having a current of IM2. Besides calculating the inverter's INV capacity, the configuration of other inverter components must also be carefully considered. This includes whether a DC circuit breaker is added before the inverter, whether a contactor or AC circuit breaker is installed after the INV output, whether an output reactor is included, whether a temperature protection device is installed within the device, whether there is a built-in fan, and the selection of other components such as capacitors and power supplies. Furthermore, if the motors require forced cooling fan power, the additional fan power supply and interlocking conditions must be considered. In this type of small-capacity frequency converter, some adopt speed feedback vector control technology, while others adopt non-speed feedback vector control technology. It is essential to differentiate based on the precision of the controlled object to prevent improper selection that fails to meet control requirements. 3. Conclusion Frequency converters are among the most important electrical equipment in hot rolling mill production lines. Besides proper equipment maintenance during normal production, careful calculation and verification are crucial during product selection and design. Data provided by machinery suppliers should not be entirely trusted. For the aforementioned loads, careful verification is essential to prevent under-caliber selection. Terms or coefficients affecting costs must be clearly defined before signing the contract; otherwise, endless problems may arise. Of course, in actual production, the frequency converter must be carefully maintained. This includes strictly controlling the operating environment, temperature, humidity, and dust levels; regular cleaning (cleaning filters and cleaning the cabinet interior) and tightening; regularly checking the appearance of electrolytic capacitors for deformation or leakage; checking the operating status of the cabinet fans; and performing precise tests on the power supply and pulse checks, or performance testing on some components, using a proactive approach to deterioration management. Currently, only the rectifier unit in the hot rolling mill can be domestically produced; the others still need to be imported. Therefore, it is necessary to prepare appropriate spare parts or key components. [References] [1] Li Chongjian, Duan Wei. AC Speed ​​Regulation Electromechanical Vibration Control of Rolling Mill Drives [M]. Beijing: Metallurgical Industry Press, 2003. [2] Tianjin Electrical Drive Design Institute, Electrical Drive Automation Technology Handbook [M]. Beijing: Machinery Industry Press, 2005. Author Biography: Ou Xianggui, born in 1965, male, from Hengyang City, Hunan Province, senior engineer, undergraduate degree, engaged in metallurgical automatic control. Mailing address: Room 702, No. 19, Lane 667, Haijiang Road, Shanghai, 201900, China; E-mail address: [email protected]. Contact number: 13391094435, 021-56129030.