How to enhance drive safety features
Safety continues to be a major focus of steel mills, such that Russula is working closely with steel mills to enhance safety on each and every project.
Recently Russula has received an engineering contract to supply new drives to a long product mill in the USA. The project also has the goal to enhance the safety features of the drives. Phase one of the major drive upgrade will affect the DC drives for the stands and the shear as well as AC drives for the “auxiliaries”, with the safety design of SIL 3 being contemplated upon completion. The focus of the project will be to provide a cost effective solution that meets the requirements of both the mill operator and maintenance groups.
The operations group would like to improve safety for the rollers and people who need to make adjustments to the mill equipment. Depending on the product changes and other factors this may be needed many times a day. Disconnecting the motor power during these adjustments will enhance safety for all operational personnel.
The maintenance personnel are focused on reducing Arc-Flash risk. Remotely de-energizing the incoming power to the drives before approaching the drive cabinet will eliminate this arc-flash risk, which can harm personnel.
The existing DC drives are equipped with main 3 phase circuit breakers incorporating both over-current protection and disconnection means. Typically these large power circuit breakers have an expected life cycle of only 3000-5000 operations before major repairs or replacement are required, which make them unsuitable for the frequent disconnections desired by the rollers. The future design of these large DC drives is to provide each drive with a motor-operated main 3ph circuit breaker followed by a 3ph power contactor (typically over 100,000 operations are expected). A safety relay in each drive will monitor the auxiliary switches on the circuit breaker and contactor and provide a Safe Torque Off (STO) input to inhibit thyristor firing. Russula is taking an additional precaution by adding another safety rated voltage presence relay, which will monitor phase to phase and phase to ground voltage. The safety rated voltage presence relay will function as an extra safety monitor. This extra step is beyond the norm, especially for the case when one phase of the circuit breaker and/or contactor is not opening, an unsafe conditio not typically caught by the auxiliary switches. In addition the cabinet is designed to allow easy access to the lower voltage control and I/O cards without opening the main power section.
The entire drive train will be controlled by a programmable safety PLC. The mill operators will have kirk keys (trapped interlock keys) or other input device(s) that will command the safety PLC to open the drive contactors and verify disconnection, before entering the rolling mill area. The maintenance personnel will also have input device(s) to the safety PLC, which will open the main circuit breakers and verify disconnection before approaching the drive cabinet(s) for maintenance. The use of a safety PLC will allow the mill to program zones of operation and easily make changes as mill safety procedures evolve.
