Abstract
On March 18, 2001, a 3A-level emergency accident occurred at Taipower’s ThirdNuclear Generation Plant, or 3rd Nuke Plant in brief. The investigation team organizedby Taiwan’s National Science Council pointed out that it is necessary to analyze the effect of three-phase unbalance of Taipower 345kV system on the 3rd Nuke Plant. Moreover, an accident on February 13, 1996, at the 3rd Nuke Plant triggered by negative sequence relay resulted in the tripping of a main generator of the plant. This research intends to ascertain whether the negative sequence current is the cause of theMarch 18 accident at the 3rd Nuke Plant.First, this thesis focuses on the negative sequence current due to the three-phase unbalance of Taipower 345kV system only, and calculates the ampere value of the negative sequence current flowing into each rotating machine and the voltage unbalance ratio of each bus at the 3rd Nuke Plant. For this purpose, the thesis proposes a methodology for quick evaluation of negative sequence current distribution within the plant network. Then the author utilizes a simple methodology to establish a whole negative sequence network model including submodels for Taipower 345kV system, 345kV transmission lines, and the 3rd Nuke Plant network. Comparing the simulation result with the record of the negative sequence voltage during the March 18 accident, the correction of model was then verified. The negative sequence voltage level was further compared with the IEC standard, to ascertain whether the negative sequence current was the cause of the March 18 accident at the 3rd Nuke Plant.Last, the research evaluates the improvement on phase-balance current relay setting to avoid the Reactor Cooling Pump (RCP) Motor caused by the insulator fault of 345kV transmission lines, and reviews whether the relay setting had kept enough margin for the unbalance phase current during the March 18 accident. Moreover, according to the voltage unbalance international standard, this thesis studies whether there are still some defects in the protection for the RCP Motor after changing the relay setting.