Minimum Throttling Feedwater Control in VVER-1000 and PWR NPPs

摘要

This paper presents an approach for the design and implementation of advanced digitalrncontrol systems that use a minimum-throttling algorithm for the feedwater control. The minimumthrottlingrnalgorithm for the feedwater control, I.e. for the control of steam generators' level andrnof the feedwater pumps speed, is applicable for NPPs with variable speed feedwater pumps. Itrnoperates in such a way that the feedwater control valve in the most loaded loop is wide open,rnsteam generator level in this loop being controlled by the feedwater pumps speed, while thernfeedwater control valves in the other loops are slightly throttling under the action of their controlrnsystem, to accommodate the slight loop imbalances. This has the advantage of minimizing thernvalve pressure losses hence minimizing the feedwater pumps power consumption and increasingrnthe net Mwe. The benefit has been evaluated for a specific plant as being roughly 2.4 MW. Thernminimum throttling mode has further advantages of lowering the actuator efforts with potentialrnpositive impact in actuator life and of minimizing the feedwater pipelines vibrations. Thernminimum throttling mode of operation has been developed by the Ukrainian companyrnLvivORGRES. It has been applied with great deal of success on several VVER-1000 NPPs,rnsix units of Zaporizhzha in Ukraine plus, with participation of Westinghouse, Kozloduy 5 and 6rnin Bulgaria and South Ukraine 1 to 3 in Ukraine. The concept operates with both ON-OFFrnvalves and true control valves. A study, jointly conducted by Westinghouse and LvivORGRES,rnhas just been completed to demonstrate the applicability of the concept to PWRs. Following thernpositive results of this study, the minimum throttling mode of operation can reliably bernimplemented on PWRs having variable speed feedwater pumps and having, or installing, digitalrnfeedwater control, standalone or as part of a global digital control system. The implementationrnof the algorithm at VVER-1000 plants provided both safety improvement and directrncommercial benefits. The minimum-throttling algorithm will similarly increase the performance of