Fault diagnosis, fault identification, and fault-tolerant control strategy for inverted U-tube steam generators with valve faults and disturbances

The main feedwater valve acting on the inverted U-tube steam generator (UTSG) in the pressurized water reactor-type nuclear power plant (NPP) is practically prone to encounter faults (like opening deviation and drift) due to aging and wear as well as external disturbances. The faults of the main feedwater valve not only result in the feedwater flow difference, but also heavily deteriorate the water level control response, where overlarge water level fluctuations even may trigger the NPP shutdown. On the other hand, nearly all existing studies either ignore the actuator implementations of the UTSG or focus solely on individual UTSG water level control without considering the coordinated control of all UTSGs. Hence, this paper develops a fault-tolerant sliding mode coordinated (FTSMC) control strategy. In the control framework, the developed strategy is composed of a fractional-order sliding mode water level controller to determine the desired feedwater flow rate for each UTSG, a fault diagnosis and identification scheme to detect the valve faults occurrence and identify the fault location, and a fault-tolerant coordinated control scheme capable of coordinately regulating the feedwater pumps power and the normal feedwater valves opening across all UTSGs. Such design enables the control system to deal with the valve faults effectively, while guaranteeing the superior coordinated water level control performance across all UTSGs in the NPP. Furthermore, rigorous theoretical validation, based on Lyapunov’s direct approach, reveals that the water level is predefined-time stable once their feedwater flow rate reaches the desired value determined by the developed strategy. Finally, numerical simulation results present the superiority of the proposed FTSMC control strategy in both enhancing fault-tolerant capability and improving water level control response.