Alternating current drives in the steel industry

In this article, a review of the state-of-the-art high-power devices, such as silicon-controlled rectifiers SCRs, gate turn-off thyristors GTOs, insulated-gate bipolar transistors IGBTs,

AJIT KUMAR CHATTOPADHYAY

Advancements

in the Last 30 Years

high-power semiconductors and microprocessor/digital signal pro-cessor (DSP)-based control and es-timation 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

silicon-controlled rectifiers (SCRs), gate turn-off thyristors (GTOs),

insulated-gate bipolar transistors (IGBTs), integrated

gate-commutated thyristors (IGCTs), and injection-enhanced 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 pre-sented 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 provid-ing 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, Digital Object Identifier 10.1109/MIE.2010.938719

© PHOTODISC

and finishing as shown in the material

flow diagram (Figure 1) and a

picto-rial manufacturing process diagram

(Figure 2) [1] of an integrated steel

plant As shown in Figure 2, after the

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 rolled steel sheets are worked on hot-strip mills After pickling to remove scale from the surface, the hot-rolled steel sheets are transformed to cold-rolled steel sheets on reversing mills

or tandem rolling mills, and then they are tinned or galvanized to produce finished steel products

Since the early 1960s, the inte-grated steel plants around the world having large capacity motors have progressively introduced new tech-nologies 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, do 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)

Coal from Mines

Coke Oven Sinter Plant

Blast Furnace

Basic Oxygen

Furnace

Continuous

Casting

Roughing Mill

Finishing Mill

Cold Rolling Mill

Finished Product

Finished Product

Iron Ore from Mines

FIGURE 1 – Material flow in an integrated

steel plant.

Iron Making Steel Making Continuous Casting Rolling Main Products

Pellet Coke

Iron Ore Sintered

Ore Limestone

Hot Metal

Blast Furnace

(BF)

Basic Oxygen Furnace (BOF)

Electric Arc Furnace (EAF) Scrap

Billet

Hot Direct Rolling (HDR) Bloom

Reheating Furnace

Section Mill

Wire Rod Mill

Plate Mill

Hot-Strip Mill

Welded Pipe Mill

Seamless Pipe Mill

Steel Castings Seamless Pipe

Welded Pipe Butt Welded Pipe

Cold-Rolled Coil and Sheet (Also for Plating)

Hot Rolled Coil and Sheet Plate Wire Rod Bar Shape Sheet Pile Rail

Cold Rolling Tandem Mill Slab

FIGURE 2 – Manufacturing process diagram for iron and steel Figure reprinted with permission from JFE 21st Century Foundation [1].

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.

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

mainte-nance, 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,

reduc-ing armature copper loss and

per-mitting 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

Thyr-istors or SCR-based

cycloconverter-fed SMs with field orientation control

(FOC) or VC [3], [4] have been exten-sively 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., 4 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 require-ments are very high (in the order of 10–15 MW), but the speed is low (60–300 r/min) and the overload ca-pacity is very high, whereas in the fin-ishing 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 3 [5]

Since the 1980s, the trends in the steel mill drives are to use pulse width modulated (PWM)-voltage source inver-ters (VSIs) with self-commutated power semiconductor devices, such

as IGBTs, GTOs, IGCTs, and IEGTs, for efficient voltage variable-frequency (VVVF) control with har-monic 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 con-verters (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 fre-quency and voltage

n produce nearly sinusoidal current waveforms throughout the oper-ating 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 carrier-based, 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 prog-ress has been made over the years in the ac drive technology used in the steel industry Figure 4 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

Hot Rolling Mill Cold Rolling Mill Skin Pass Mill

Cold Coiler

Hot Coiler Bar and Rod Mill

Slabbing Mill Blooming Mill

Ov

erload Capacity Torque Quality 15

10

5

Speed (r/min)

FIGURE 3 – Speed and power requirement in various steel mills.

Source Fixed

Voltage

Fixed

Frequency

Converter

Controller

Variable Voltage Variable Frequency Speed/Position

Motor

Sensor

Steel Mill

Reference/Command (Speed/Position)

Drive

FIGURE 4 – Block diagram of a typical steel mill drive system.

steel industry in chronology of

devel-opment of each of these components,

such as high-power semiconductor

devices, converter topologies, motors

used, and their control

High-Power

Semiconductor Devices

Rapid advances in industrial ac drives

and power conversion systems have

been possible because of the

continu-ous 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

(volt-age switched) devices, which include

IGBT and IEGT The voltage and

cur-rent ratings of these devices, as

commercially available today for

high-power converters, are shown

in Figure 5 [7]

High-Power

Silicon-Controlled Rectifier

Figure 6 shows a 12-kV, 1.5-kA SCR,

which is a high-power press-pack

thyristor-based device with three

ter-minals: gate, anode, and cathode Its

turn-on process is initiated by

apply-ing 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 4 V

This device blocks voltage in both

forward and reverse directions

Origi-nally developed and marketed in 1958

by GE in the United States, it is the

highest rated power device so far

(espe-cially with the light-triggered ones) for

use with cycloconverter- and LCI-fed

motor drives, besides high-voltage dc

systems and static volt–amperes

reac-tive (VAR) compensators (SVCs)

High-Power Gate

Turn-Off Thyristor

The GTO is a self-commutated

thyris-tor-based device that can be turned

off by a negative gate current Figure 7

shows a 6-kV, 6-kA press-pack GTO

(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 How-ever, the turn-off current gain is typi-cally four to five, which means that a GTO with a 6,000-A anode current rat-ing may require1,500-A gate current pulse to turn off GTOs need bulky and expensive turn-off snubbers and complex gate driver The typical turn-on

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

V (kV)

I (kA)

12

10

8

6

4

2

12 kV/1.5 kA (Mitsubishi)

6.5 kV/1.5 kA (Mitsubishi)

6 kV/3 kA (ABB)

6.5 kV/4.2 kA (ABB)

6 kV/6 kA (Mitsubishi)

4.5 kV/0.9 kA (Mitsubishi)

4.5 kV/1.5 kA (Toshiba, Press Pack)

3.3 kV/1.2 kA (Eupac) (Toshiba, Press Pack)

2.5 kV/1.8 kA (Fuji, Press Pack) 1.7 kV/3.6 kA

(Eupac)

4.8 kV /5 kA (Westcode) GTO/GCT

IGBT IEGT

6.5 kV/0.6 kA (Eupec) (Toshiba)

7.5 kV/1.65 kA (Eupec) SCR

FIGURE 5 – Voltage and current ratings of high-power semiconductor devices Figure reprinted with permission from IEEE and John Wiley and Sons [7].

FIGURE 6 – A 12-kV, 1.5-kA SCR Figure used with permission from [8].

FIGURE 7 – A 6-kV, 6-kA GTO Figure used with permission from [8].

The castings are rolled to the required dimensions by the rolling mills to produce steel products.

reduce di/dt of the anode current The

GTO can be fabricated with

asymmet-rical structures suitable for VSIs or

symmetrical structures suitable for

current source inverters (CSIs)

Integrated

Gate-Commutated Thyristor

IGCT (also known as GCT) is a

hard-driven 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 6,000-A

gate current [9] However, the

cur-rent pulse should be very narrow

with low energy for fast turn off

Figure 8 shows an ABB

press-pack-type 6.5-kV, 6-kA IGCT with a built-in

integrated gate drive circuit

(consist-ing of several MOSFETs in parallel) on

the same module The IGCTs have

replaced the GTOs for the

medium-voltage 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

extremely low gate inductance (typi-cally <3 nH compared with <30 nH for GTOs) by special construction

The rate of the gate current change

at turn off is normally greater than 3,000 A/ls compared with around

40 A/ls for GTO The turn-on and turn-off times are much faster than those of the GTO Although the IGCT does not require a turn-off snubber,

it requires a simple turn-on snubber

or a clamping circuit, since the di/dt capability of the device at turn between on is around 1,000 A/ls only

The on-state voltage of IGCT at 6,000 A

is only 4 V compared with 4.4 V for a GTO at 4,000 A As the storage time of IGCT is reduced to one tenth com-pared with GTO, a high switching speed is obtained IGCTs have a higher switching frequency (typically 1.0 kHz) than GTOs (typically 0.5 kHz)

Besides the asymmetrical IGCT (suita-ble for VSI as shown in the Figure 5, marketed by ABB), symmetrical IGCTs called SGCTs (suitable for CSI) are available from Mitsubishi for smaller ratings IGCTs are simple to use and easily available; they have demon-strated their reliability in many appli-cations, which include the rolling mill drives (e.g., ACS 6000 by ABB and SIMOVERT-ML2 by Siemens)

Insulated-Gate Bipolar Transistor After completely dominating the low-voltage converters, IGBTs are

increasingly used for medium-voltage converters It is a voltage-controlled hybrid device (developed by GE in 1983), combining the advantages of MOSFET’s high-gate circuit resistance and bipolar junction transistors’ small collector–emitter drop at saturated condition The ratings of these devi-ces have reached as high as 6.5 kV, 0.6 kA or 3.3 kV/4.5 kV, 1.2 kA It can be turned on with a þ15-V gate voltage and turned off when the gate voltage

is zero or negative The majority of high-power IGBTs are of modular design as shown in Figures 9 and 10

It can be turned on within 1 ls and turned off within 2 ls The main advantages of IGBT are the simple gate driver, snubberless operation, high switching speed, modular design, and controllability of switching be-havior providing reliable short-circuit protection Press-pack devices are also available, which are suitable for series operation The device has only forward blocking capability and can

be used in a VSI with a feedback diode However, reverse-blocking IGBTs have recently become available High-voltage IGBTs (HVIGBTs) have a higher voltage drop (e.g., 4.3 V for a 3.3-kV, 1.2-kA device) during conduction compared with thyristors or GTOs IGBT devices can be available in intelligent power module (IPM or HVIPM in Figure 9) form with gate drivers and built-in protection features to provide lower size and cost, improved reliability, and fewer electromagnetic interfer-ence problems

Injection-Enhanced Gate Transistor IEGT is basically an advanced high-voltage, high-power IGBT with spe-cial gate construction, commerspe-cially developed by Toshiba in 1999 [10], [11] It is designed in such a way that

a large number of electrons accumu-late at its electrodes, and it exhibits low on-state voltage (compared with IGBTs and GTOs of the same rating) Figure 11 shows a 4.5-kV, 2.1-kA (turn-off current 5.5 kA) IEGT and its gate driver, which is less than 1/200 in gate power compared with that for GTO/IGCT and more reliable It can

FIGURE 9 – A 3.3-kV HVIPM FIGURE 10 – A 4.5-kV HVIGBT.

FIGURE 8 – A 6-kV, 6-kA IGCT/GCT Figure

used with permission from [8].

Very rapid and remarkable progress has been

made over the years in the ac drive technology

used in the steel industry.

be turned on by the gate voltage

of þ15 V and turned off by that of

15 V The transistor-based IEGT has

the potential to achieve higher

out-put frequencies than the IGCT/GCT

Another advantage over the IGCT is

the power required to turn the device

on and off Figure 12 [12] shows the

comparison of typical gate trigger

pulses required for equivalent power

devices As a transistor-based device,

the gating power of IEGT is low and

approximately equal for both turn on

and turn off The on-state voltage

drop across this device is of the

order of 3.0 V (much less than that of

IGBT or GTO of similar rating) In the

IEGT-based system, neither turn-on

nor turn-off snubber is required for

each IEGT, as in the case of a GTO

However, each IEGT leg needs simple

and efficient clamp circuits to

elimi-nate the snubbers In 2000, Toshiba

supplied 8-MVA IEGT-based

three-level inverter systems for rolling mill

drives with an efficiency of 98.5%,

which is 2% more than that of an

equivalent GTO-based system, thus

saving a lot of energy

High-Power Converters and

Drive Control Strategies

The power converter topologies for

medium-voltage (2.3–13.8 kV),

high-power ac drives for applications such

as steel mills can be classified as direct

ac/ac cycloconverters and indirect

dc-link inverters Inverters may be

cur-rent source or voltage source type

While the CSIs may be either PWM-CSI

or LCI, the VSIs may be two-level PWM

with switches in series or three-level

PWM neutral-point clamped (NPC) as

developed in 1981 [13]

Cycloconverters and

Cycloconverter-Fed Drives

Cycloconverters with thyristors for

control of ac motors replaced the

thyristor converters used to control

dc motors for rolling mills (>1 MW)

applications in the early 1970s, and

with the introduction of FOC, a

high-performance 4-MW blooming mill

with a cycloconverter–SM drive

hav-ing a speed of 60–120 r/min was

commissioned by Siemens in 1981

together with a 4-MW roughing stand

of a strip mill [14] The cycloconverter consists of the same six-pulse antipar-allel thyristor converter bridges as used to control the dc motors Three-phase, full-wave or six-pulse, dual-bridge configuration with 36 SCRs is quite popular Basic cycloconverter configurations (both for noncirculat-ing and circulatnoncirculat-ing current type) and the output voltage and current wave-forms for a noncirculating current cyclo-converter as obtained by the adequate variation of the firing angle are shown in Figure 13 For low output frequencies, the sinusoidal voltage waveform can

be realized easily The circulating current type, though expensive, is

commonly employed for its simplic-ity, less torque ripple, and higher maximum output frequency (about 0.4 times the line frequency) com-pared with the noncirculating type, though more efficient (99% com-pared with 98.5% for circulating cur-rent one [15]), which has a dead time

of 1–3 ns for switching between for-ward and reverse current resulting in

a higher torque ripple Cycloconvert-ers operate with a lagging power fac-tor because of the phase control, and when applied to a processing line with large load variations such as a hot-strip mill, an SVC or static VAR generator (SVG) is needed for reactive power compensation Asymmetrical

First Quadrant Second Dead Interval 1 Dead Interval 1

Component Converter I

Component Converter II Third

VA

IA

(c)

FIGURE 13 – (a) Noncirculating current-type cycloconverter, (b) circulating current-type cycloconverter, and (c) voltage and current waveforms for (a) Part (c) is used with permission from [14].

IGCT/GCT

50 A

1.5 A

4,000 A

1.5 A

IEGT 1.5 A

4,000 A

1.5 A

FIGURE 12 – Typical gate trigger signals for IGCT/GCT [12] Figure courtesy of

R Tessendorf, TMEIC.

FIGURE 11 – IEGT with gate driver Figure used with permission from [10].

voltage control has also been used

earlier to reduce the reactive power

by approximately 15% in comparison

with that of symmetrical control of

two bridges [16]

The six-pulse, 36-SCR

cyclocon-verter concept is expanded to a 12-pulse,

72-SCR cycloconverter (Figure 14) by

connecting two dual bridges in series

for each phase of very large capacity

cycloconverter drives with improved

characteristics for seamless tube

pierc-ing mill [16] and other Nippon steel

mills [15] in Japan The torque and

speed control response of the

Nip-pon cold strip mills [15] in the case

of the noncirculating current-type

cycloconverter-fed drive were found

to be 600 rad/s and 40 rad/s,

respec-tively, while those for circulating

cur-rent type were 800 rad/s and 60 rad/s

Motor Types

Cycloconverters for the steel mill

drives can be used with either IMs or

SMs, and some of the early motors

used were IMs [15], [17] However,

later, SMs were used more in view of

their high-capacity availability and

reduced kVA requirements for a

desired shaft kW output as well

as their higher efficiency Table 1

lists the key figures for comparison

between the motor types for a

6-MW, 60/120 r/min reversing

roll-ing mill drive [18] on the basis of

required load at 200% base speed

In the early 1960s,

cycloconverter-fed multiple IMs (e.g., 300 motors of

2.6 hp, 212 r/min each with speed

range 13-0-13 Hz with six-pole) were

used to drive hot-strip mill run-out tables [19]

Field Orientation Control

or Vector Control

In a separately excited dc machine, the use of power electronic convert-ers with current feedback provides a direct control of the magnitude of the armature current and, in proportion, the torque However, for an ac machine, this control is to be achieved in terms

of both amplitude and phase, which has led to the generic term ‘‘VC.’’ In addition, unlike the dc machine, where the orientation of the field flux and the armature magnetomotive force is fixed

by commutator and brushes, the ac machine requires external control to fix this orientation without which the space angle between various fields vary with load (and during transients), giving rise to oscillatory dynamic response FOC directly controls this space angle and, in particular,

attempts to make it 90° between the specifically chosen field components

so as to emulate a dc machine and provide decoupling control The tech-nique can be applied to either IMs

or SMs fed from VSI/CSI or cyclo-converter Early conceptual works on

VC were proposed in Germany [20], [21] in the beginning of the 1970s and initially implemented by Siemens as patented transvector control with ana-log ana-logic only, resulting in poor current control response due to vector com-putation errors The remarkable devel-opment of microprocessors (now DSPs), computer power, and digital control in the 1980s allowed vector computation to be performed with higher speed and accuracy and helped

ac drives to outperform dc drives Figure 15 shows the vector dia-gram of the synchronous machine (as preferred for a high-capacity steel mill) used to develop FOC required to adjust speed and torque, where esis the air-gap electromotive force (emf),

iq (torque producing component) is the quadrature axis component of the current is, id (flux producing component) is the direct axis compo-nent of current is, W is the magnetic flux, ilis the magnetizing current, jL

is the load angle, jS is the flux axis angle, and k is the rotor axis angle The currents idand iq of the stator current in synchronously rotating reference frame are analogous to the field current If and to the armature current Iaof the dc machine, and the torque can be expressed as Te¼ KtW

iq ¼ Kt 0If Ia ¼ Kt 00id iq These two

TABLE 1–COMPARISON OF ALTERNATIVE MOTOR TYPES FOR A 6-MW, 60/120-R/MIN REVERSING ROLLING MILL [18].

Required kVA for excitation rectifier (peak load) – 10%

iq

is

isXh

id

ie

is

iμ

es

ϕs

ψ

α λ

β

ϕL

eL

Flux Axis

Rotor Axis Stator Axis

is

s h

id

ie

is

iμ

es

ϕs

ψ

α λ

β

ϕL

eL

F A

Ro

R to Axis Stator Axis

FIGURE 15 – Vector diagram of the synchronous machine Used with permission from [22].

N-Converter

N-Converter

P-Converter

P-Converter

N-Converter

N-Converter

P-Converter

P-Converter

Load

FIGURE 14 – One phase of a three-phase,

12-pulse, 72-SCR cycloconverter.

components can be independently

controlled with VC Unlike the IM, the

space position of the SM field is

located by the position of the rotor

For a self-controlled SM with rotor

position feedback and VC, the

imple-mentation calls for control of the

magnitude and phase of the stator

current with respect to the location

of the field winding axis The response

of the field current is sluggish because

of large time constant, and, as a result,

the response is slow The response

can be improved considerably by

using VC where the transient

magnet-izing current demand to maintain the

rated flux can be temporarily

sup-plied from the stator side

Figure 16 shows the simplified

block diagram [22] of the speed and

torque control system adopted by

Siemens, which includes a

propor-tional-integral (PI) controller for the

speed n and another PI controller for

the flux W The speed controller

delivers the reference value of the

torque producing current iq*, while

the flux controller delivers the

refer-ence value of the field controlling

cur-rent i* The stator currents id L1, iL2,

and iL3and the voltages vL1, vL2, and

vL3are measured and used in voltage

model block M1 to calculate the

magnitude jWj and position (sin jS,

cos jS) of the flux The position of the

flux is used to transform from d–q to

a–b reference axis in block 2 Block 4

transforms the two-phase currents

iLaand iLbinto three-phase reference

currents i* , iL1 L2* , and iL3* , which are

delivered to the current controllers

of the cycloconverter M2 in block 6

is the current model, which uses the

current components in field

coordi-nates (id, iq) to determine the flux

position with respect to the rotor

axis Then, the field position with

respect to the stator axis is obtained

by adding jLto the rotor position k

The current model is useful during

low operating speeds as needed at

starting and positioning of the mill

when the machine voltage terminals

are very noisy for using voltage

model Block 2 is the field flux

con-troller used to generate the

refer-ence value of the rotor current i*e

fed to the controlled rectifier of block 8

Nakano et al [23] reported the development of a high-performance

SM drive for a rolling mill by Fuji, Japan, with an open-loop flux estima-tor and PI current controller Here, the flux linkage was kept constant by feeding part of the field current to the armature windings transiently, and the power factor could be controlled

to unity An improved personal com-puter (PC)-based VC scheme for a six-pulse noncirculating current cyclo-converter-fed SM with a closed-loop

flux observer and operating with unity power factor for a rolling mill drive as developed at the Indian Institute of Technology (IIT) Kharagpur, India, in

1996 and a prototype made by C-DAC, Trivandrum, India, is reported in [24] and [25]

Major manufacturers of cyclo-converter drives above 10 MVA are Siemens (Simovert D), Toshiba (Tos-vert-l/S850), ABB (ACS 6000C), and Alstom (ALSPA CL9000) Cold rolling mills, such as tandem mills, require high-dynamic response, accurate speed, and torque control of main and

Control Scheme for Synchronous Motor

Current Controller

Power Circuit

Flux Controller 1

ψ ∗

ψ ∗

n∗

ψ

ψ

ψ

–

–

– 1

sin ϕs

sin ϕL

sin λ cos λ

cos ϕs

cos ϕL

I∗d

I∗

e

I

e

I∗

L1

I∗L2

I∗

L3

I

L1

IL1

IL2

IL2 I

L3

VL1 VL2 V

L3

I∗q

I

q I

d

ILα

IL1, IL2,

IL3

VL1,

VL2,

VL3

ILβ

2 3 dq

dq αβ

αβ

n

5

3 M1

M2 6

S

λ

Speed Controller

Flux Controller 2

sin cos

–

–

–

M

3Φ,

50 Hz

7

–

Control Scheme for Synchronous Motor

Current Controller

Power Circuit

Flux Controller 1

ψ ∗

ψ∗

n∗

ψ

ψ

ψ

–

–

– 1

sinϕs

sinϕL

sin λ cosλ

cos ϕs

cos ϕL

I∗d

I∗

e

I

e

I∗

L1

I∗L2

I∗

L3

I

L1

IL1

IL2

IL2 I

L3

VL1

V VVL2 V

L3

V

I∗q

I

q I

d

ILα

IL1, IL2,

IL3

VL1,

VL2,

VL3

ILβ

2 3 dq

dq αβ

αβ

n

5

3 M1

M2 6

S

λ

Speed Controller

Flux Controller 2

i

sin coso

–

–

–

M

3Φ,

50 Hz

7

–

8

FIGURE 16 – Block diagram of the speed and torque control for the mill with cyclo-converter Used with permission from [22].

Recently, two kinds of three-level inverters with high-power switching devices, such as IGCTs and IEGTs, having minimum conduction and switching loss as well as being smaller

in size and footprint (50% smaller), have replaced GTOs/IGBTs.

auxiliary drives, while hot rolling

mills, such as roughers, and hot-strip

mills require good torque control

and momentary overloadability; all

such performance criteria are met by

these drives

Three-Level Gate Turn-Off

Thyristor/Insulated-Gate

Bipolar Transistor Pulse Width

Modulated Inverter-Fed Drive

Because of the limitations of

cyclo-converters, such as low power

factor, presence of low-frequency

interharmonics, and less maximum output frequency, advances in IGBT and GTO technology cleared the way for their application to the steel mill drives with PWM two-level and later three-level neutral clamped convert-ers invertconvert-ers with high switching frequency since the 1990s A three-level inverter compared with a two-level inverter (Figure 17 with IGBT switches) either with IGBT or GTO can achieve a significantly higher output voltage (about twice) without series connection of the devices and

an improved output waveform In two-level inverters, the output voltages consist of pulses of either þVd/2 or

Vd/2, whereas with a three-level inverter these areþVd/2, 0, andVd/2

as shown in Figure 17

Hitachi and Mitsubishi of Japan reported the development of high-performance three-level GTO-based 6.4 MW and 10 MVA inverter IMs and SMs, respectively, for the steel main rolling mill drives in 1996 One such PWM rectifier–inverter configuration with GTOs developed by Mitsubishi is shown in Figure 18 [26] With the PWM rectifier in the front end, it has been possible to achieve sinusoi-dal input current and programmable input power factor (to unity) without

a compensating device The regenera-tive snubber circuit, which is devel-oped to have high efficiency, and an SVM method to minimize harmonic distortion are discussed in [26] Hita-chi [27] developed similar GTO-based three-level inverters 5–6.4 MW and

2 MW IGBT-based three-level inver-ters for the steel rolling mills at the same time Siemens introduced the SIMOVERT-ML drive with three-level GTO converters of MW range with

VC for application to SMs and IMs These converters compete with cyclo-converters in the region of 10 MVA or less Three-level IGBT inverters with the same configuration as the three-level GTO inverters (shown in Figure 18) were introduced in many steel plants (up to 3 MW) of medium capacity, e.g., 1.5 MVA, in 1996 [27], where three inverters were driven by a common converter Hitachi has recently realized

a maximum capacity of 15 MVA [28] drive for a hot rolling mill with series/ parallel IGBT cell units Group drive applications, such as approach tables and run-out tables, are configured with one inverter supplying several motors

in a simple V/Hz mode The Mitsu-bishi MELVEC 2000N three-level IGBT inverter (1.5–3 MVA) is claimed to be 40% smaller than conventional equip-ment [29] With GTO/IGBT inverters, the torque and speed control response

of Nippon steel mills mentioned earlier [15] were found to be 1,000 rad/s and

80 rad/s, respectively, showing an

3-Ph

50 Hz

PWM

Rectifier InverterPWM

ac Motor

Reference Sine wave Carrier Wave

Fundamental Component Load

Voltage

S1U S1v

S2v

S3v

S4v S4w

S3w

S2w

S1w

w

S3U

S4U

wt

wt

Vd/2

Vd/2

Vd/2

S1U S1v

S2v

S3vv

S4v S4w

S3ww

S2w

S1w

w

o S2U

S3U

S4U

FIGURE 17 – Comparison of two-level and three-level PWM inverter topologies and output

waveforms [25] (a) Two-level PWM rectifier-PWM VSI inverter, (b) two-level carrier-based

sinusoidal PWM, (c) three-level PWM inverter topology, and (d) three-level PWM inverter

output waveforms.

Power Factor Control

dc Voltage Command

Current

dc Voltage

Speed Command Speed

PWM Control Gate Control

Rectifier Block Inverter Block

SM

Vector Control PWM Control Gate Control

Current

FIGURE 18 – Three-level GTO rectifier–converter system for steel rolling mills Used with

permission from [26].

improvement in control performance

over the cycloconverter-fed drives

Three-Level Integrated

Gate-Commutated Thyristor/

Injection-Enhanced Gate

Transistor Inverter-Fed Drive

Recently, two kinds of three-level

inver-ters with high-power switching devices,

such as IGCTs and IEGTs, having

mini-mum conduction and switching loss

as well as being smaller in size and

footprint (50% smaller), have replaced

GTOs/IGBTs

Integrated Gate-Commutated

Thyristor Inverter-Based Drive

In 1998, ABB developed IGCT-based

ACS 1000 (0.3–5 MVA) with a front-end

rectifier, the world’s first standard and

compact drive for medium-voltage

applications [30], and, later, the ACS

6000 (3–27 MVA) with a

front-end-controlled rectifier (active rectifier

unit), designed to meet the specific

challenges faced by plate mills/

reversing cold mills [6], such as rapid

load and torque changes, cyclic loads,

and fast alteration between driving,

reversing, and braking over a wide

speed range Figure 19 [30] shows the

configuration of AC 1000 used for

pumps, fans, compressors, conveyors,

and other auxiliary processes in the

steel industry, and Figure 20(a) and (b)

[6], [31] shows the application of

ACS 6000 to a cold reversing mill and

plate mill, respectively These schemes

result in higher switching frequency

(1 kHz) compared with GTO-based

schemes (0.5 kHz), higher efficiency

(98%), and higher input power factor

(0.97), and less space because of snubberless operation These drives employ DTC method, which allows accurate control of both rotor speed and torque without pulse encoder feedback from the motor shaft

Direct Torque Control DTC is an advanced computation-intensive method proposed as an alternative to field orientation/vector control for a high performance drive around 1986 [32], [33] and devel-oped/marketed as products by ABB with IGCT inverters since the 1997 [34] The well-known features of this control when compared with the FOC are simple direct control of torque and stator flux by selection of a voltage vector, no d–q axis/vector transformation, no traditional PWM algorithm, no feedback current con-trol, and no PI regulators It can be shown that the developed torque of the machine is proportional to the product of synchronously rotating stator flux Ws, rotor flux Wr, and the angle hs between them The main

variable to be controlled in the DTC scheme is Ws, which can be directly controlled by the stator voltage vs (neglecting stator resistance) DTC has torque and stator flux control loops as shown in Figure 21, while an optional speed loop (speed may be obtained through an encoder or esti-mated for a sensorless control) may

be added to generate the torque and flux reference The control loop errors are fed to the HB-type flux and torque comparators whose outputs

eW and eT are sent to the voltage switch logic unit (SLU) or a look-up switching table for proper selection

of the voltage vectors or switching states of the inverter to satisfy the flux and torque demands SLU also gets the information about the angle

hsin one of the 60° sectors shown in Figure 22 [35] The vector Wsrotates

in a circular orbit within a HB cover-ing six sectors as shown Figure 22 [35] shows six active voltage vectors and two zero vectors of a two-level inverter (e.g., relevant to the state vector PWM control) controlled by

Isolation Transformer

Rectifier dc Link Inverter Filter Motor

Induction Motor

b a

FIGURE 19 – Three-level IGCT inverter topology for ACS 1000 [30] Figure courtesy of ABB.

Motor WorkRolls

Gear Box

Motor Motor

ACS 6000

ACS 6000

Top Motor

Bottom Motor

Extension Shaft SpindlesJoint WorkRolls

Hot Material

Reversing Cold Mill

FIGURE 20 – (a) Cold reversing mill with ACS 6000 ABB [31] and (b) plate mill with ACS 6000 ABB [31] Figure courtesy of ABB.