CCI or CCF incident at Forsmark NPP 25 of July 2006

摘要

On Tuesday the 25 of July a two phase short circuit occurred when a breaker was operated in the 400 kV switch gear that connects Forsmark units 1 and 2 with the outer grid. Unit 2 was at the occurrence shut down for annual maintenance. Unit 1 was operating on full power. Each unit has two turbines. As a consequence of the short circuit the unit 1 generator bus bar voltages dropped substantially whereupon the induced magnetization in the generator tried to compensate for this. At the same time the 400 kV unit breakers was opened due to under- voltage. This resulted in a voltage peek of about 120% during approximately 1 second on the generator bus bars. The voltage transient resulted in the failure of two out of four UPS, sub divisions A and B. Both the rectifier and the inverter in the UPS tripped because of over-voltage. Normally the rectifiers shall trip before the inverters but in this case the voltage changed in such an unfortunate way that transient was let through the rectifiers and caused also the inverters to trip. UPS for sub division C and D functioned as expected. Unit 1 then went into house turbine operation but both turbines tripped within approximately 30 seconds. As the turbine speed decreased the voltage and frequency of the generator fell. When the frequency reached 47 Hz the circuit breakers for the 500 V bus bars opened resulting in a loss of power for sub divisions A and B because of the failure of UPS. As a result of the power loss in two sub divisions the reactor protection system initiated a reactor scram and isolation of the containment. Two out of four electrically operated pressure relief valves opened and two out of four high pressure emergency core cooling pumps started. The diesel generators for all four sub divisions started but in sub divisions A and B the diesel generators were not connected to the 500 V bus bars because of loss of information about the motor speed. The information was missing because of the failure of the two UPS. In the control room many alarms and other information from trains A and B was missing because of the loss of power in these two trains. Approximately 22 minutes after the initial incident the power for the 500 V bus bars in all four sub divisions was restored manually by connecting the station to the 70 kV grid. Two protections that should have prevented/restricted the effects of the incident did not work as expected due to inappropriate parameter settings (UPS) and incorrect installations (under frequency relays) performed when the plant electrical systems was modernized in 2005. The incident has led to a number of changes and adjustments in order to prevent that a similar event has the same consequences in the future. A comprehensive corrective action plan was developed and approved by the management and the authority. The plan includes actions and improvements in the following areas: 1. Improvements in the management decision making process 2. Improvements in the plant modification/modernization process and in the maintenance process. 3. Improved safety culture 4. A sixty item hardware improvement action plan, including e.g. improvements in the Human-Machine interface in the main control room.