Estimation of the frequency of occurrence for grid-related loss of offsite power (GR-LOOP) to a nuclear power plant

Abstract

According to the safety analyses, GR-LOOP holds the most significant contribution in the Core Damage Frequency of Nuclear Power Plants. Since this, in turn, depends on the GR-LOOP frequency of occurrence, an analytical method is presented to identify and evaluate the post-fault GR-LOOP scenarios. The probabilistic module of the hybrid method develops the post-fault sequences regarding the response of the line distance protection to 3-phase short circuit faults. Then, GR-LOOP consequences and frequency are identified by interpretation of the relevant parameters from the transient stability simulations of the post-fault (3Ph-SC) grid behavior. GR-LOOP frequency is estimated as 5.87E-04, 6.25E-04, and 8.60E-04/ (reactor-year) for three NPPs in New England Test System grid as a case study. To this end, 2482 probabilistic sequences and 408 dynamic post-fault scenarios were evaluated for each NPP. The results indicate that GR-LOOP occurrence depends on the real-time values of grid parameters, including the components specifications, pre-fault load flow quantities, and network configuration. Furthermore, the observed difference amongst the GR-LOOP sequences for differently-located NPPs in the grid questions the uncertainty of using mean generic frequencies. Regarding the inapplicability of the existing grid reliability assessment approaches, the hybrid method is recommended as an alternative for GR-LOOP evaluation. Besides, it can reveal the design weaknesses and prioritize the operational tasks via the competitive benefits of employing PSA techniques. Albeit, more precision needs more modifications in both modeling and calculation details, which, in turn, increases the complexity.

Keywords

• grid-related loop
• probabilistic risk assessment
• transient stability simulation
• post-fault