PLC (Programmable Logic Controllers)-based Protection and Control System for Small-sized Steam Turbines at a Thermal Power Plant

A new control system has been designed when instead of the original relay control logic a PLC (programmable logic controller) is adopted for the protection system of a small-sized steam turbine destined for the feedwater pump of a 300 MW unit. The system employed a double redundant CPU (Central Processing Unit) hot standby and double power supply design. As the hardware structure is simple and the control logic is realized by a trapezoid graphic software instead of the original relay control logic, the safety and reliability of the system has been greatly enhanced. In the meantime a small turbine emergency-trip first-out memory function is additionally designed in the control logic software, thereby providing a basis for fault analysis of the system. The on-site operation shows that the system can operate steadily and comply with relevant design requirements.