Steel Authority of India (SAIL), established in 1954, is India's largest state-owned
steel producer. It manufactures and sells a wide variety of steel products such
as hot and cold rolled sheets and coils, galvanized sheets, electrical sheets, structural,
railway products, plates, bars and rods, stainless steel and other alloy steels.
The Need
The skip winch charging system is a critical mechanism for blast furnaces. Skips
are designed for transportation of burden materials from the skip pit via the blast-furnace
throat to the reception hopper of the charging device. SAIL has around 24 blast
furnaces below 4000 m3, equipped with skip winches for charging the burden into
the furnace. The skips of the blast furnace, travel upwards - 85 meters from skipping
pit to reach the discharging point. The velocity and rate of acceleration of skips
need to be varied at different points as the inclination of the skip bridge rail
track is not uniform throughout the length. In the skip, 98% of travel period is
pulled up by motoring action and is decelerated by regenerative action at creeping
speed to discharging position.
For the last six decades, skip winches of the blast furnaces are driven by DC motors
and DC drives in spite of many deficiencies. SAIL, DSP decided to upgrade its Skip
Winch system due to non-availability of spares for DC motors and support, as well
as to meet its stringent quality norms, power quality and energy conservation goal.
The main objectives in introduction of AC motors and Drives for SKIP operation were:
- To overcome non-availability of DC Motors - a constraint as most reputed motor manufacturers
have stopped production of DC motors.
- Continuous development in AC drive technology over the years has resulted in performance
of AC motors getting superior day by day. AC drive technology is replacing the DC
drive system worldwide in almost all areas of application.
- To have a system with less maintenance, which is a drawback of DC motors
- To have improved power factor and less harmonics.
For the above reasons, SAIL decided to switch to AC Drives based system and awarded
the project to L&T Electrical & Automation's Control & Automation (C&A) business
unit, which is a leading solutions provider.
The scope of work included design, engineering, manufacturing, testing, supply,
erection and commissioning of the AC motors with dual redundant regenerative AC
drives and dedicated programmable logic controller (PLC) system along with required
communication cables, field devices and HVAC system for skip winch of BF No. 3.
Solutions
The blast furnace No.3 has a useful volume of 1400m3 cubic. The skip cars are used
to feed burden materials from the top, which require high starting torque, steep
acceleration and regenerative braking. C&A took the challenge of migrating the existing
DC Drive based System to AC Drive, despite there being no reference of AC Drive
being deployed in Blast Furnace Skip Winch application in India and only 3 references
available globally. The major challenges included selection of suitable AC motors
to match velocity and the acceleration diagram for same or better throughput, availability
of sufficient mechanical motor ratings to select appropriate motors that would help
to achieve high starting torque and equivalent response to the DC motor being used.
In addition, another challenge was to install AC motors in place of DC Motors in
available space with minimum modification to the existing civil foundation. Also
SAIL-DSP team wanted to retain the existing DC Drive system, in the event of an
AC Drive system not performing as expected so that they can switch to the old system.
In line with customer's requirement, C&A supplied active front end, 4 quadrant,
AC drives with suitably derated induction motors, associated automation system to
protect customer's existing investment in hardware and software to the maximum extent
possible.
Sizing of new AC motors was critical since the angle of Skip Bridge was modified
from 520 to 540, travel distance exceeded by seven meters and filling capacity of
skip was enhanced. The C&A team along with SAIL-DSP studied the existing system
and recorded the existing loading pattern of skipping due to the absence of sufficient
mechanical data.
“C&A's strength in the field of design, engineering, project management and
implementation of Integrated (drives and motors) system brings sustainable benefit
to SAIL-DSP operations.” - S.C. Prasad, CET
The recorded data was extrapolated to its worst case loading pattern, which helped
to finalize AC Motor ratings by evaluating rope drum/torque requirements for different
burdens. The AC drives were selected to match the overload/acceleration torque requirement
of the motors. The DC motors were removed including its mounting plates and new
mounting plates were provided for AC motors keeping the foundations unchanged.
The performance of the new AC motors and AC drives were found satisfactory as these
were able to meet the process requirements keeping entire operation and control
philosophy, safety interlocks and protections intact, in line with the existing
arrangement. With the help of a new system, the burden charging capacity of skips
increased by 30%. Travel time of skips from skip pits to discharging point reduced
from 55 sec to 50 sec.
With the help of a new system, the burden charging capacity of skips increased by
30%. Travel time of skips from skip pits to discharging point reduced from 55 sec
to 50 sec. With introduction of bell less charging system in combination with skip
charging system with new drives, an increase from 1900 TPD to 2000+ TPD of Hot metal
production in Blast furnace no.3 was achieved. The end user is fully relieved from
spares planning and day to day maintenance related issues arising in the case of
DC motors.
The engineers of CET, DSP & C&A jointly as a team and made sincere efforts to ensure
that project was successfully commissioned. The project was successfully implemented
for the first time in any Indian Blast furnace and fourth Blast furnace in the world.
Benefits:
- Significant improvement in production
- Improved dynamic response and better speed regulation
- Improved power factor and power quality
- Energy efficiency
- Minimised spare requirements
- Reduced down time and maintenance time.
Conclusion:
Skip charging system in small capacity blast furnaces is expected to continue for
next 20-25 years. Hence it is recommended to replace the DC drives by energy efficient
AC drives technology not only to get the above benefits but to ensure that furnaces
do not face any crisis in future.
System Architecture: