To encourage “Energy Conservation & Environment
Protection” in the industrial sector PCRA has been continuously
upgrading the energy conservation knowledge through energy
audits and sharing the experience of energy efficiency improvement
and achievement of industries. Based on these information,
developing case studies for dissemination of information to
various industrial groups and organizations. The case study
detailed below is one of the modification carried out by Visakhapatnam
Steel Plant, Rashtriya Ispat Nigam Ltd. Which is being reproduced
for the benefit of steel plants & re-rolling mills.
Introduction
Rashtriya Ispat Nigam Ltd. is the corporate entity of Visakhapatnam
Steel Plant. The plant has a capacity to produce 2.656 Mt
of saleable steel of which 2.410 Mt is finished steel.
The product profile of the plant comprises of wire rods,
rounds, reinforcement bars (rebars), angles, channels, beams,
squares, billets and blooms. The product profile also includes
basic grade pig iron, granulated slag, coal chemicals and
other by products. The plant also exports power to AP Transco
from its captive power plant.
Steel making is energy intensive in nature and hence VSP
uses various types of energies for reduction of oxide and
process heating. In his energy policy RINL is committed to
reduce specific energy consumption by 1% per year upto 2010.
Background
There are 11nos. of 1700KW and 9 nos. of 1000KW main drive
DC motors installed in Medium Merchant Structural Mill (MMSM)
to run the mill stands. They are run at different speeds as
required for the profile being rolled. During normal operation,
the motors will be running at lesser field currents. However,
when the mill is stopped and the drive is kept ready for rolling,
the field current will remain at 100%. It was observed that
operating at 100% field currents during non rolling period
is leading to energy wastage. The duration of rolling is 18Hrs.
and non-rolling period is 6Hrs. per day.
Observations
When on full field, motor is ready to run if the speed computer
reference is released. Once the computer reference is released,
it will decide about the speed of the motor. If it is less
than or equal to the base speed then drive will have full
field current and if the speed is more than the base speed
field current is reduced to develop more speed. Normally drives
are running in field weakening mode.
Concept
It is proposed to keep the field current at 25% of rated
value during non rolling and increase it to 100% field when
speed reference is released. This can be implemented by modifying
the thyristor converter control circuit of the DC drives.
Modification
The Thyristor was of standard make. Thyristor converter interlocking
and control circuits were modified for all 1700KW (11nos.)
and 1000KW (9nos.) DC motors. The circuit is now having three
more additional relays with a timer in line. When Armature
auxiliaries are made ON, field will be ON, but field current
is limited to 25% of rated value only. The sequence is modified
as given below: |
