Inadequate protection relays and related setpoints

During periodic testing of the diesel generators automatic load sequence, it was discovered that the 380V safety related equipments were not energized as expected. The reason for that failure is that a protection relay had an unexpected high voltage trip value that was not appropriately adjusted. The setpoint was too low so that, when the diesel generator was supplying the 380 V board, the protection tripped on high voltage de-energizing power supply to safety related equipments. At the same time, it was discovered that the low voltage setpoint was not adjusted as by the design specification and that it was not possible to reach the right value due to construction features of the protection relay. Some of these safety related 380 V boards were replaced during the last plant outage in April 2005 and all the new boards had the same problem, having the same inadequate protection relay. The consequence was that some of the safety related equipments were not available in two out of three safety related trains in case of loss of off-site power supply. The same problem was also present in one train of the second level protection systems (plant protection against external events). After identification of the problem the License immediately re-adjusted the high voltage setpoint to a higher value. The adjustment of the low voltage setpoint is under investigation.

Location: TIHANGE-2
Event date: Mon, 04-07-2005
Nuclear event report
Legenda & explanation