Redundancy in SCADA plant monitoring systems
November 1st 2007 Supervisory control and data acquisition systems (SCADA) are widely used in plants and factories. According to Citect, redundancy is an important aspect to consider but is all too often overlooked at the specification stage Control systems for both single node and network applications have a single point of failure, meaning that they will break down entirely if one piece of hardware fails (such as when the computer is connected to the control and monitoring units). Most modern computers are designed for reliability, but breakdowns still occur, especially with computers located in harsh environments. Consequently, if some or all of the plant processes are critical, or if the downtime costs are high, redundancy must be incorporated into the system to eliminate failures due to equipment failure. In the first instance it is important to determine what level of redundancy is required: hardware failure, catastrophic failure, energy failure or a natural disaster? Mission critical installations often have separate power sources in case of a power failure, and installations in areas prone to natural disasters or the threat of fire separate the servers in different geographic locations. However, whatever type of disaster recovery is planned for, it is possible to greatly reduce lost data and downtime by planning the proper system design, and by choosing a SCADA system with built-in redundancy. To provide redundancy, a second standby server can be added that is also dedicated to communication with plant control devices. If the primary server fails, the client's requests for data are channelled to the standby server. In very large installations, host pairs of servers are used with one host pair dedicated as a standby in a separate location from the primary host pair. If a dedicated file server is also added to the SCADA system, the user can centralise the databases and display screens; continuity is then maintained if the primary server fails. Another benefit is that ENTER 80752 Tel: 01675 466658 ipesearch.com/enquiry centralised databases are easier to manage and maintain: changes only need to be made to one database and are then automatically updated everywhere else. In addition, it is possible to support dual network paths to the centralised database, allowing dual file servers if required. If the LAN also fails, then control and monitoring by the display nodes is lost. In view of this, a second LAN and file server are crucial to help ensure system stability, even in the event of a network failure. Where it is important to ensure a plant's uninterrupted operation, beyond duplicating hardware, the plant engineer can employ Split-Task Redundancy. This goes further than simply maintaining continuous communication with the plant-floor devices: it also ensures that all alarm and trend data is maintained in the event of a failure. Finally, to ensure maximum system stability, where parallel PLCs are employed these units should be connected to the same field devices. This ensures that any hardware component in the system can fail without disrupting the control and monitoring of the plant. More articles from Citect Ltd: | 
