Many small power plants have an outdated Programmable Logic Control (PLC) currently in place. As a PLC reaches the end of its useful life, an upgrade to a new automation system is required.
Once the decision has been made to upgrade, the upgrade strategy must be defined. In most cases, it’s necessary to perform the upgrade with as little downtime and risk as possible, and these requirements determine much of the upgrade strategy.
There are four main strategic decisions that must be made before an upgrade can take place.
First, it must be decided if the new automation system will just replicate the operation of the existing PLC—or improve upon the existing PLC in terms of throughput, quality and other factors. Replication is cheaper up front, but usually much more expensive over the entire life cycle of the new automation system, as many of the benefits of a modern automation system are forfeited.
Second, it must be determined if the upgrade will be vertical or horizontal. In a vertical upgrade, one particular process area is upgraded at a time. In a horizontal upgrade, all similar process units are upgraded simultaneously, generally across multiple process areas. For example, if a plant had 2(two) or more boilers, all would be upgraded at once in a horizontal upgrade, as opposed to upgrading only the boiler(s) in the vertical process unit.
Third, it must be determined if the upgrade will be done by replacing all automation system components simultaneously, or with a phased approach. With a phased approach, the Man Machine Interface (MMI) components are replaced first, followed by the controllers, and finally by the I/O. Replacing the automation system in phases takes longer, but will require less downtime and entail less risk.
The fourth and final strategic decision that must be made is hot versus cold cutover. With hot cutover, the old PLC and the new automation system operate simultaneously, with one control loop at a time migrated from the old PLC to the new automation system at the I/O level. With cold cutover, the old PLC is replaced by the new automation system, with the entire process being restarted at once.
The hot cutover option is more expensive in terms of upgrade costs, but with an overall lower cost in most cases when downtime is taken into account. Risk is also lower with hot cutover as only one loop is converted at a time, with the old PLC still available in case of any unforeseen difficulties with the new automation system.
