Reliability considerations of overspeed protection for turbine generators

Triplex modular redundancy (TMR) systems have been used for many years in plant such as turbine governors in order to enhance the availability of the system. However, in pieces of equipment such as the overspeed trip system which is essentially a protection system, TMR is used as a last line of defence and therefore it is termed safety critical equipment. Great care and attention must therefore be taken in order to ensure that it is as reliable as possible. The article presents the NEI Parsons Overspeed Trip System. It is housed in a 3U high rack, contained in a stand-alone cubicle. Modules within its design include three speed/time measurement modules (EG573), one data acquisition module (EG592) and two independent power supplies. Techniques of failure to safety and the use of TMR for high integrity and availability are key design principles which aid serviceability and maintainability