Compact PLCs installed at Eskom hydro plant
November 2001
PLCs, DCSs & Controllers
Eskom's Second Falls Hydro plant in the Eastern Cape consists of two hydro generators used to supplement the electricity supply for the National Grid.
Ram-Tec Systems (RTS) supplied, programmed and commissioned a Modicon Compact PLC to control each generator. The programming of the PLCs was done using the CONCEPT V2.1 programming software. A third PLC was supplied with the same configuration as a back-up. All PLCs contain the same version of code, making them swappable with each other.
Although the I/O for each generator is relatively small (<96), there is a need to have a number of different modes of operation, which make the system complex. For example, there are four control sub-systems for each generator (generator control, excitation control, synchronisation and protection) and two operational states (maintenance and in-service). In addition, the in-service state has a further three sub modes of operation (local auto, remote auto and local manual). Finally, plant shutdown can also be done in three modes (normal shutdown, quick shutdown or emergency shutdown).
The PLC's task is to co-ordinate and to interface all of the above tasks. To simplify the PLC programming, a 'blocking function' was developed to invalidate the various group states. This blocking function is provided by the PLC to prevent an action being performed if the pre-conditions are not met.
This approach allowed the use of a relatively low-cost PLC in terms of plant I/O requirements while ensuring that enough memory and processing power was available to cater for the different modes of operation.
Ram-Tec Systems
(011) 616 1328
info@ramtec.co.za
www.ramtec.co.za
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