Low-frequency Comprehensive Safety Risk Assessment And Coordination Control Research For Nuclear Power Plant Connected To The Grid

With a new generation of Nuclear Power Plant(NPP)completed and put into operation,the grid connection scale of NPP in China is increasing.NPP,which has high security requirements and large generating capacity,is vulnerable to the interference of low frequency fault in the grid.In the mean time,NPP and the grid easily interact with each other.A small disturbance from NPP or the grid could cause severe risk accidents.To analyze the interaction mechanism between NPP and the grid thoroughly,to evaluate the low frequency safety risk of station and grid comprehensively,to identify the high-risk or weak link accurately,and to formulate the coordination operation strategy effectively,are all the issues should be urgently gone into for the sake of guaranteeing nuclear safety and developing nuclear power orderly.This paper combined with the National Natural Science Fund Project and the practical electrical power project,carrying deep research on low-frequency comprehensive safety risk assessment and coordination control study for nuclear power plant connected to the grid.Synthetically considering the interaction between NPP and the grid,quantifiable risk indexes were proposed,comprehensive risk assessment model of station and grid was established,the risk values were calculated and analyzed afterwards,and coordination control strategy was put forward then.The main work and achievements are as follows.In this paper,the accident consequences on NPP and the grid caused by the low frequency fault were thoroughly analyzed.The working characteristics and parameters'setting scheme for both NPP's low frequency protectors related to the grid and the grid's automatic low-frequency load shedding device were explored.The the interaction mechanism between NPP's and the grid's low frequency safety devices was specially discussed.Combined with the Safety Risk Theory,the comprehensive low-frequency risk indexes for NPP and the grid were constructed,including low-frequency risk indexes,device risk indexes and the economic indexes.The indexes' mathematical method of calculation was proposed.The Safety Risk Assessment Theory was applied in the coordination control analysis of NPP's low frequency protectors related to the grid and the grid's automatic safety devices.A method for NPP and the grid's comprehensive low-frequency risk assessment was proposed.The method covered integrated quantitative risk calculation based on safety risk model,risk variation analysis to judge risk value's relative changing rate with protection device's parameter changes,safety risk value evaluation to comprehensively estimate risk indexes'order of magnitude and proportion,protectors' safety risk sensitivity evaluation which considered the risk parameters' effect on protectors.Combined with two economic parameters,nuclear power benchmark price and NPP's unit cost,considering the interaction and cooperation between NPP's and the grid's safety devices,the safety risk assessment model was established,including synthesized risk assessment models for NPP's cooling main pump low speed protector and steam turbine low frequency protector.Applying the method and the mathematical model proposed for NPP and the grid's comprehensive low-frequency risk assessment,comprehensive low-frequency risk assessment of an actual example on NPP connected to the grid was completed.The effectiveness and applicability of the method as well as the model were verified.The safety risk matrix was structured.The risk variation of steam turbine low frequency protector's frequency setting value changes,turbine low frequency protector's time setting value changes,Cooling Main Pump(CMP)low speed protector's frequency setting value changes was analyzed.And then,indexes' safety risk value evaluation and classification scoring,protectors' safety risk sensitivity evaluation were achieved.The results showed that these indexes,the abnormity level of core heat dissipation,the frequency setting value of turbine low frequency protector,the frequency setting value of CMP low speed protector,were high risk indexes.The following indexes,the load shedding amount of power system,the frequency setting value of automatic low frequency load shedding device,were less high risk indexes.The following indexes,the time setting value of turbine low frequency protector,the power vacancy in the power system,the power loss of the system,were common risk indexes.The results also indicated that CMP low speed protector was a safety risk sensitive device,while turbine low frequency protector was a frequency safety risk sensitive device,not a time delay safety risk sensitive device.Coordination control and optimization strategy for NPP and the grid were studied in view of the synthesis risk analysis.Based on the synthesis low-frequency safety risk modeling analysis for NPP and the grid,the safety and feasibility of protectors' current parameters setting scheme were explored.The cooperation degree of NPP's and the grid's protectors was discussed.The coordination and parameters optimization strategy were proposed.Optimization advices on the load shedding scheme,the turbine low frequency protector's segments and time delay were put forward.The safety risk assessment model for low-frequency protectors established in this paper realized quantitative comparison of both NPP's and the grid's risk factors.In addition,it intuitively reflected the relative strength degree of every risk index.Through risk guidance,the indexes and devices with higher risk,which called for special attention,could be identified.Furthermore,the regulation and control point could be focused on those indexes and devices.Thus,the NPP and the grid's coordination would be enhanced,which helped NPP's safe grid connection and stable operation.