Application of high performance and high power inverter in hot rolling field

Nowadays, the world is short of energy. How to protect resources and reduce energy consumption has become a top priority for scientific research and industry, especially
in the fields of metallurgy, steel, petrochemicals, traditional manufacturing, etc., which are energy-intensive. Therefore, using frequency converters to transform industrial motors, fans, pumps, compressors and other loads has become an effective way to save energy. This paper mainly introduces the application of high-performance and high-power frequency converters in the field of hot rolling, and puts forward some thoughts.

Research object process description


Figure 1 AC synchronous motor single-line diagram of AC frequency conversion speed regulation drive

(1) Hot rolling field: This paper mainly introduces the hot rolling strip field of the ferrous metallurgical industry (steel industry), including high-performance and large-capacity frequency converters used in conventional hot rolling, short-process CSP, furnace coil mills, medium and thick plate mills, etc. However, since the conventional strip hot rolling technology and product requirements are the most stringent, this paper focuses on the hot rolling production line with a strip product width of 1000mm or more and a thickness of 1.2mm-21mm.

(2) Main machine: In the hot strip rolling production line, the roughing mill and finishing mill (continuous rolling mill) are generally defined as the main rolling mill (or main machine), and the corresponding variable frequency speed control device is called the main transmission device.

(3) Large capacity: refers to the rated power of the motor being above 2500kW, or the rated capacity of the variable frequency device being above 6MVA (there is no unified statement).

(4) High performance: Generally, the above-mentioned equipment has a large capacity and is the core equipment for product production (including output and quality), so its device control performance requirements are very high; fast response speed; high static speed control accuracy (less than one ten-thousandth); small motor output torque pulsation; small dynamic speed drop (less than 0.3%S); high device efficiency (greater than 97%).

Motor: Generally speaking, the motors used in current main machines are all AC synchronous motors, and the motor capacities are as follows: Roughing mill motor rated power: 2500kW-9000kW, Finishing mill motor rated power: 6000kW-11000kW:

Device: After 2008, several large-capacity and high-performance frequency conversion speed regulation device manufacturers generally use AC-DC-AC voltage type frequency conversion devices. The rated output capacity of the device varies according to the rated power of the motor and the overload multiple, but the device is basically graded into standard main circuit components, and there are 4 corresponding rated large capacities: 6MVA, 8MVA, 10MVA, and 12MVA.

According to incomplete statistics, from the introduction of the Anshan Iron and Steel 2800/1700mm plate and strip continuous rolling mill from the former Soviet Union in 1957 to the Baosteel Zhanjiang project that is being prepared for construction today, China has introduced and independently designed no less than 60 wide and thin hot continuous rolling mill (width greater than 1000mm) production lines, including 8 from Baosteel (2050, 1580, 1780 (Shanghai), 1422, 1880, 1780 (Meizhou)); 4 from Anshan Iron and Steel
(2800/1700, 1780, 2150, 1700 (Anling)); 3 from Wuhan Iron and Steel (excluding Liuzhou Iron and Steel, 1700, 2250, 1580).

There are 10 hot continuous rolling mill production lines with a strip width of 2000mm in the country: Baosteel 2050, Wuhan Iron and Steel, Taiyuan Iron and Steel, Masteel, Handan Iron and Steel, Liangang 2250, Benxi Steel 2300, Shougang 21 60, 2150 of Nippon Steel and Angang Steel, etc. In addition, there are various hot rolling production lines such as CSP and medium and thick plate mills.

Device technology and product development history

We mainly understand the technology and development history of these devices from the aspects of motors, power devices, frequency conversion methods, and control technology.

Motors: have developed from DC motors to synchronous motors;

power devices: have developed from thyristors to gate-controlled thyristors (GTO), and then to IGCT (H-IGBT) and IEGT;

frequency conversion methods: have developed from AC-AC frequency conversion to today's AC-DC-AC voltage-type frequency conversion;

control technology: from vector control to direct torque control.

Representative products and manufacturers of main drive frequency conversion devices currently in use:

(1) Application configuration of AC-AC frequency conversion (Baosteel Branch) (1 6 sets) (all components are SCR) (see Figure 1 for the single-line diagram of AC-AC frequency conversion speed regulation driving AC synchronous motor);

(2) Application configuration of AC-DC-AC three-level PWM frequency conversion (Baosteel Branch) (46 sets).

3. Typical product/technology introduction

Now take Baosteel 1880 hot rolling mill and Japan TMEIC product TM-70 as examples for introduction.

The main motors include the upper and lower rollers of the roughing mill R1, the upper and lower rollers of R2 (synchronous motors), the finishing mill Fl-F7 (synchronous motors), the flying shear CS (asynchronous motors), and the slab large side pressing device SP (asynchronous motors), a total of 1 1 units.

The rated power of the motors is as follows:

R1: 2500kW×2;

R2: 9000kW×20

F1 - F5: 10000kW×5;

F6 - F7: 9000kW×20

SP: 3300kW×1;

CS: 2500kW×1.

The above main transmission devices all use the 8MVA drive device of Japan TMEIC TM-70, and its rectifier and inverter are AC-DC-AC voltage type 3-level PWM full digital vector control transmission system using IEGT as its power element .


Figure 2 IEGT inverter cabinet

This control method truly realizes sinusoidal wave output, reduces high-order harmonics and torque ripple, and does not require the use of SVG filtering devices; at the same time, high-voltage output reduces the cost of cables and wires.

Since the rated capacity of the motors of R2 upper and lower rollers and finishing mills F1-F7 is between 9000kW and 10000kW, one set of devices is not enough, and two sets of IEGT devices, TM-70 devices with a capacity of 8MVA, must be used at the same time.

However, since the motor capacity of R1 upper and lower rollers, GS, and SSP is between 2500kW and 3300kW, only one set of IEGT devices, TM-70 devices with a capacity of 8MVA, is required.

The specification comparison table of IGCT and IEGT inverters (see Figures 2 and 3 for actual pictures) is shown in Table 1.

Nowadays, IEGT three-level rectifier inverter is more widely used, and its main performance is:

high efficiency: greater than 98.5%;

high reliability: because it does not use electrolytic capacitors, it is not easy to fail;

easy maintenance: easy to disassemble, and can be quickly disassembled without professional tools



Figure 3 Single inverter

Power IEGT Stack;

small impact on the power grid.

4. Some thoughts

4.1 Dual system parallel drive

The metallurgical industry (especially the hot rolling production line of wide strip steel) requires the motor rated power to be in when rolling high-strength thin strip steel at high speed.
Above 10000kW, for the extra-large traction traffic load, the drive motor will be larger. Under the existing large-capacity power devices, one power circuit (bank) cannot meet the needs of the above load. Therefore, there are two parallel or even three or four circuits in parallel. How to solve this problem, there are two typical methods: multi-winding motor method and drive circuit parallel method.

The multi-winding motor method is typical of TM-70 of Toshiba Mitsubishi Corporation of Japan, and the drive circuit parallel method is the most typical of the MELVEC-3000 series of Mitsubishi Electric products and Toshiba 4 parallel system. Table 1 IGCT, IEGT inverter specification comparison table

4.2 Synchronous motor excitation system

When the MELVEC3000 system of Mitsubishi Electric of Japan, the SM-150 of Siemens of Germany, and the TM-70 products of TMEIC of Japan supply power to the synchronous motor, the synchronous machine excitation drive device generally adopts thyristor full-wave rectification.

The excitation main circuit topology used by these companies is similar, but in order to prevent overvoltage caused by energy release of the excitation circuit, the overvoltage protection circuits designed by each party are quite different.

The excitation protection overvoltage circuit of the TM-70 designed by Japan's TMEIC company is very complicated. Due to its poor hardware design and programming errors in its control PLC software, the diodes and resistors have been burned out many times. However, the design of Japan's Mitsubishi Electric is very simple and effective. The system has been running for nearly 16 years and has never failed.

4.3 Electromechanical resonance control function of rolling mill

Since the speed of the rolling mill is not very high (20rpm-6580rpm), the control frequency of its corresponding frequency conversion device will not be too high. According to N=60×f/P, considering the difficulty of motor manufacturing and the working efficiency and cost of the motor, the number of motor pole pairs is between 4 and 8. Therefore, the output frequency of the inverter that controls the speed is between 2.6 and 44Hz. Since the inherent frequency of the machine itself is also relatively low, when rolling high-strength steel, the system resonates through the rolled slab, which causes the system to trip and fail to roll steel, and the equipment will be damaged in severe cases. Baosteel's 1880 hot rolling roughing mill R1 is a typical example. Although TM-70 has the function of suppressing system torsional vibration SFC, due to various reasons, even after multiple debugging and system optimization by Japanese experts and the use of SFC function, its effect still cannot



meet the actual needs of the site. The rolling mills that are prone to resonance are: R1 rolling mill, F3 and F4 rolling mills.

4.4 Others

The use of high-performance and high-power inverters also needs to consider the purchase cost, accessories cost, maintenance and repair cost. In terms of technology, comprehensive factors such as power device and device manufacturing technology, assembly technology, debugging technology, and maintenance technology should be considered.