ENERCON recently completed a plant design modification that upgraded the existing electromechanical (analog) protection relays for two 9000 HP, 6.9 kV Circulating Water Pump (CWP) motors to new solid-state (digital) relays. The objective was to harden the CWP motor protection circuits by implementing modern solid-state electronic equipment and eliminating legacy electromechanical devices that can lead to a spurious CWP trip. The CWP relays that were upgraded are non-safety related.
In addition, ENERCON evaluated the trip setpoints for the CWP relays to ensure that they adequately protected the pump motors, pump switchgear, upstream switchgear and station personnel against a motor overload event or electrical fault. To determine the appropriate settings, ETAP computer software was used to perform the time-current characteristic analysis and analyze the device coordination in digital format.
ENERCON prepared a design change to evaluate acceptability and facilitate the replacement of the electromechanical relays with digital multifunction relays. The new relays are “all-in-one” devices that rely on microprocessor technology and fully programmable logic to provide the complete suite of protections that typically would have required multiple electromechanical relays. Therefore, one (1) SEL-710-5 digital relay was installed to replace all electromechanical relays for each pump motor while providing the same level of protection and the same trip functions as offered by all of the electromechanical relays combined.
Although the Circulating Water System is not credited in any analyzed accident response scenarios and is not required for any safety-related plant function, the design change was deemed adverse in the associated 10CFR50.59 Licensing screening due to the new relays having (1) different modes of failure and (2) the potential to increase the frequency of occurrence of an accident, specifically, the Loss of Condenser Vacuum event as described in the Updated Final Safety Analysis Report (UFSAR).
A detailed Failure Modes and Effects Analysis (FMEA) was prepared to identify and evaluate each of the potential failure modes associated with the SEL relays. The FMEA also provided a disposition of the impacts with regard to each failure mode. A qualitative assessment was prepared using the guidance found in NEI 01-01, Guideline on Licensing Digital Upgrades, and RIS 2002-22, Supplement 1, Clarification on Endorsement of Nuclear Energy Institute Guidance in Designing Digital Upgrades in Instrumentation and Control Systems. The qualitative assessment addressed the digital relay’s design attributes, the quality of the digital relay’s design process, and digital relay operating experience. The assessment concluded that the replacement digital relays exhibited a sufficiently low likelihood of failure as defined in RIS 2002-22, Supplement 1. The qualitative assessment, in conjunction with the FMEA, supported a conclusion that the modification did not create the possibility of an accident of a different type or create the possibility of a malfunction with a different result than previously evaluated in the UFSAR.
Prior to the electromechanical relay replacements with digital relays, ENERCON’s Engineering team performed a cost-benefit analysis for numerous digital relay replacement options, and recommended a selection that required minimal maintenance, was familiar to the operating plant, and that had been proven reliable in similar applications.
Work to complete the digital upgrade modification started on June 13, 2019 and completed through client approval on December 19, 2019. After approving the design, the station I&C Supervisor provided the following feedback:
“Thank you so much for your support. This is a milestone as this was the last SPV modification, so again, thank you.”