AJIT KUMAR CHATTOPADHYAY © PHOTODISC W ith the rapid developments of high-power semiconductors and microprocessor/digital signal processor (DSP)-based control and estimation technologies, high-power, high-performance ac drives using either induction motors (IMs) or synchronous motors (SMs) with cycloconverters or inverters have replaced the earlier dc drives for applications in the steel industry during the last 30 years In this article, a review of the state-of-the-art high-power devices, such as siliconcontrolled rectifiers (SCRs), gate turn-off thyristors (GTOs), insulated-gate bipolar transistors (IGBTs), integrated Digital Object Identifier 10.1109/MIE.2010.938719 30 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n DECEMBER 2010 Advancements in the Last 30 Years gate-commutated thyristors (IGCTs), and injectionenhanced gate transistors (IEGTs), converters, such as cycloconverters and three-level inverters, and control technologies adopted for such drives, such as the vector control (VC) and the direct torque control (DTC), is presented with brief features of the industrial ac drives developed for the steel industry by the leading drive manufacturers worldwide The steel industry continues to play an indispensable role in supporting the abundance of human life by providing the basic material for construction and economic development of a country The process of manufacturing the steel products from iron ore involves raw material handling, primary steel making (coke oven, blast furnace, and steel melting), refining, casting, hot and cold rolling, 1932-4529/10/$26.00&2010IEEE Coal from Mines Iron Ore from Mines Coke Oven Sinter Plant The steel industry continues to play an indispensable role in supporting the abundance of human life by providing the basic material for construction and economic development of a country Blast Furnace Basic Oxygen Furnace Continuous Casting Roughing Mill Finishing Mill Finished Product Cold Rolling Mill Finished Product FIGURE – Material flow in an integrated steel plant and finishing as shown in the material flow diagram (Figure 1) and a pictorial manufacturing process diagram (Figure 2) [1] of an integrated steel plant As shown in Figure 2, after the Iron Making Pellet blast furnace and basic oxygen furnace process, the molten steel is cast by continuous casting machine to produce slabs, blooms, and billets These castings are rolled to the required dimensions by the rolling mills to produce the steel products The steel shapes, bars, and wire rods are processed on section and bar mills and wire-rod mills, plates are worked on reversing mills, and hotrolled steel sheets are worked on hotstrip mills After pickling to remove scale from the surface, the hot-rolled steel sheets are transformed to coldrolled steel sheets on reversing mills or tandem rolling mills, and then they are tinned or galvanized to produce finished steel products Steel Making Continuous Casting Coke Since the early 1960s, the integrated steel plants around the world having large capacity motors have progressively introduced new technologies in drive control through power electronics for the processing of steel Although motors used in the primary area of steel making, such as the coke oven, blast furnace, and steel melting shop, not need very accurate speed or torque regulation, the motors used in roughing mills, finishing mills, plate mills, tube mills, run-out tables, coilers/uncoilers, and pinch roles need speed and torque regulation of higher accuracy [2] In most of the early steel mill applications, dc motors driven by four-quadrant converters (thyristor Leonard drives) Rolling Section Mill Rail Sheet Pile Shape Bar Wire Rod Iron Ore Sintered Limestone Ore Wire Rod Mill Billet Hot Metal Basic Oxygen Furnace (BOF) Hot Direct Rolling (HDR) Plate Plate Mill Hot Rolled Coil and Sheet Hot-Strip Mill Bloom Cold Rolling Tandem Mill Slab Blast Furnace (BF) Welded Pipe Mill Scrap Main Products Electric Arc Furnace (EAF) Cold-Rolled Coil and Sheet (Also for Plating) Welded Pipe Butt Welded Pipe Seamless Pipe Reheating Furnace Seamless Pipe Mill Steel Castings FIGURE – Manufacturing process diagram for iron and steel Figure reprinted with permission from JFE 21st Century Foundation [1] DECEMBER 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 31 10 Roughing Mill Power (MW) 15 Ov er Slabbing Mill Blooming Mill dC ap To rq ue ac Qu ity ali Hot Rolling Mill Cold Rolling Mill loa Skin Pass Mill 500 ty Cold Coiler Hot Coiler Bar and Rod Mill 1,000 Speed (r/min) 1,500 2,000 FIGURE – Speed and power requirement in various steel mills having high performance were used for variable-speed applications, where they have been almost substituted— since the 1970s—by ac motor drives having either IMs or SMs fed from either direct ac/ac cycloconverters or ac/dc/ac link inverters The ac drive realizes higher efficiency, less maintenance, and a smaller motor SM drives have the advantages over the IM drives in that these can be operated in near unity or even leading power factor with excitation control, reducing armature copper loss and permitting simplicity of commutation with thyristors or SCRs as switches [as in a load-commutated inverter (LCI) or LCI-fed drive], and it runs at a precisely set speed independent of load and voltage fluctuations Thyristors or SCR-based cycloconverterfed SMs with field orientation control Source Fixed Voltage (FOC) or VC [3], [4] have been extensively used in main rolling mill drives, and cycloconverter-fed IM drives with scalar V/Hz control have been used in roller/run-out table drives In the steel plants, ac drives of very high capacities (e.g., MW in a sinter plant and 14.4 MW in a plate mill) are used Critical applications are in rolling mills In roughing mills (e.g., blooming mill and slabbing mill), power requirements are very high (in the order of 10–15 MW), but the speed is low (60–300 r/min) and the overload capacity is very high, whereas in the finishing mill (e.g., wire rod mill), the power requirement is relatively low (in the order of 2–3 MW), but the speed requirement is comparatively high (1,000–2,000 r/min) The power and speed requirement in different mills are shown in Figure [5] Drive Variable Voltage Converter Motor Variable Frequency Fixed Frequency Speed/Position Controller Sensor Reference/Command (Speed/Position) FIGURE – Block diagram of a typical steel mill drive system 32 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n DECEMBER 2010 Steel Mill Since the 1980s, the trends in the steel mill drives are to use pulse width modulated (PWM)-voltage source inverters (VSIs) with self-commutated power semiconductor devices, such as IGBTs, GTOs, IGCTs, and IEGTs, for efficient variable-voltage variablefrequency (VVVF) control with harmonic reduction The development of new high-power semiconductors, such as 3.3/4.5-kV, 1.2-kA IGBTs, 6-kV, 6-kA GTOs, 6-kV, 6-kA IGCTs, and 4.5-kV, 5-kA IEGTs, and three-level (or multilevel) inverter topologies, in contrast to the earlier two-level ones, has led to an increased application of PWM-controlled voltage source converters (VSCs) ranging from 0.5 MVA to about 30 MVA The converters for drives such as steel mills meeting the high-performance requirements must n generate smoothly variable frequency and voltage n produce nearly sinusoidal current waveforms throughout the operating range to avoid undesirable torque oscillations n permit highly dynamic control both in forward and reverse motoring and braking applications n provide near performance or even better performance than that of the dual converter-fed dc drives as regards cost, service reliability, and harmonic effects on the system Besides the application of FOC in PWM inverter-fed motor drives with various PWM schemes, such as carrierbased, hysteresis band (HB) control and space vector modulation (SVM), the recent application of DTC to ac drives in plate rolling mills [6] has been claimed to achieve the highest torque and speed performance ever attained with variable speed drives, making it possible to control the full torque within a few milliseconds and reducing the impact of load shocks Thus, rapid and remarkable progress has been made over the years in the ac drive technology used in the steel industry Figure shows a block diagram of a typical ac drive system for a steel mill with its various components The objective of this article is to present a brief state-of-the-art review of the advances in ac drives in the steel industry in chronology of development of each of these components, such as high-power semiconductor devices, converter topologies, motors used, and their control The castings are rolled to the required dimensions by the rolling mills to produce steel products High-Power Semiconductor Devices Rapid advances in industrial ac drives and power conversion systems have been possible because of the continuous and astonishing development of the rating and performance of the power semiconductor devices over the last 50 years Two major types of high-power semiconductor devices are used in high-power converters in the steel industry: the thyristor-based (current switched) devices, which include SCR, GTO, and IGCT (or GCT), and the transistor-based (voltage switched) devices, which include IGBT and IEGT The voltage and current ratings of these devices, as commercially available today for high-power converters, are shown in Figure [7] High-Power Silicon-Controlled Rectifier Figure shows a 12-kV, 1.5-kA SCR, which is a high-power press-pack thyristor-based device with three terminals: gate, anode, and cathode Its turn-on process is initiated by applying a pulse of positive gate current, and it turns off when anode current becomes negative The turn-on time is 14 ls, and turn-off time is 1,200 ls The on-state voltage drop is about V This device blocks voltage in both forward and reverse directions Originally developed and marketed in 1958 by GE in the United States, it is the highest rated power device so far (especially with the light-triggered ones) for use with cycloconverter- and LCI-fed motor drives, besides high-voltage dc systems and static volt–amperes reactive (VAR) compensators (SVCs) High-Power Gate Turn-Off Thyristor The GTO is a self-commutated thyristor-based device that can be turned off by a negative gate current Figure shows a 6-kV, 6-kA press-pack GTO time is 2.5 ls, and turn-off time is 25 ls The on-state voltage drop is typically 4.4 V The GTO switching frequency is lower than that of IGBT’s and IGCT’s (which is described later) So, the GTO converters operating in PWM (high-frequency) mode use energy recovery snubbers consisting of a capacitor, a diode, and a resistor across each device in addition to a turn-on snubber consisting of an anode inductor in series with each device to (high-power GTOs that have been developed by the Japanese since the 1980s), which is turned on by a pulse of positive gate current and turned off by a negative gate current pulse However, the turn-off current gain is typically four to five, which means that a GTO with a 6,000-A anode current rating may require À1,500-A gate current pulse to turn off GTOs need bulky and expensive turn-off snubbers and complex gate driver The typical turn-on V (kV) 12 12 kV/1.5 kA (Mitsubishi) SCR 10 6.5 kV/0.6 kA (Eupec) (Toshiba) 7.5 kV/1.65 kA (Eupec) 6.5 kV/1.5 kA (Mitsubishi) kV/3 kA 6.5 kV/4.2 kA kV/6 kA (ABB) (ABB) (Mitsubishi) GTO/GCT IEGT 4.5 kV/1.5 kA (Toshiba, Press Pack) 4.5 kV/0.9 kA (Mitsubishi) 3.3 kV/1.2 kA (Eupac) (Toshiba, Press Pack) 4.8 kV /5 kA (Westcode) 2.5 kV/1.8 kA (Fuji, Press Pack) 1.7 kV/3.6 kA (Eupac) IGBT I (kA) FIGURE – Voltage and current ratings of high-power semiconductor devices Figure reprinted with permission from IEEE and John Wiley and Sons [7] FIGURE – A 12-kV, 1.5-kA SCR Figure used with permission from [8] FIGURE – A 6-kV, 6-kA GTO Figure used with permission from [8] DECEMBER 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 33 Very rapid and remarkable progress has been made over the years in the ac drive technology used in the steel industry FIGURE – A 6-kV, 6-kA IGCT/GCT Figure used with permission from [8] reduce di/dt of the anode current The GTO can be fabricated with asymmetrical structures suitable for VSIs or symmetrical structures suitable for current source inverters (CSIs) Integrated Gate-Commutated Thyristor IGCT (also known as GCT) is a harddriven GTO (developed by ABB in 1996) with unity current gain, which means that a 6,000-A (anode current) device is turned off by a À6,000-A gate current [9] However, the current pulse should be very narrow with low energy for fast turn off Figure shows an ABB press-packtype 6.5-kV, 6-kA IGCT with a built-in integrated gate drive circuit (consisting of several MOSFETs in parallel) on the same module The IGCTs have replaced the GTOs for the mediumvoltage drives over the past few years because of their special features, such as snubberless operation and low switching loss The snubberless operation is possible because of the FIGURE – A 3.3-kV HVIPM extremely low gate inductance (typically