 INES 3 |
UNIT SCRAM CAUSED BY GRID DISTURBANCES DUE TO TORNADO |
The unit was operating at 388 MWe power. As a result of tornado the 330 kV, 154 kV and 110 kV transmission lines in the "Kolenergo" power system were damaged. At 02:27 TG-4 was tripped by system emergency protection. The unit power was reduced to 47 MWe. Strong frequency oscillations in the system (from 44 to 52.5 Hz) at 03:42 led to power disbalance in the primary and secondary circuit causing TG-3 stop valve closure and AZ-1 protection (scram) actuation. DGs 3,4 and the standby diesel stated as designed and 1.5 mins later tripped on the "uncompleted start" signal (since the essential buses were under voltage). Personnel transferred DGs 3,4 and the standby diesel to manual control. After elimination of the primary cause of AZ1 signal DGs 3,4 and the standby diesel were brought to automatic control. Primary circuit cooldown was performed through steam extraction (bleeding) for unit "in-house consumption" using the BRU-SN lay-out steam dumping valves (controlled bleed to station service collector).
At 04:11 AZ1 signal was formed again. DGs 3,4 and standby diesel started for coast down. Control room personnel manually connected the 6 kV essential buses to DGs 3,4. Components involved in emergency shutdown cooling were periodically activated. At 04:26 the standby diesel was stopped by the lo-loi oil pressure protection. At 04:47 as a result of voltage drop in the 330 kV grid the normal feeders of the 6 kV bus were tripped by the minimum voltage protection and the standby bus breaker closed by automatic changeover action. Thus, DGs 3,4 turned out to be engaged for parallel operation with the 1TR standby transformer. At 06:15 as a result of system accident progression unit loss of power occurred. Due to 1TR transformer deenergization additional loads got transferred to DGs 3,4 causing their tripping by the maximum current protection. DGs 3,4 were taken out of service for maintenance to check the generator insulation. The buses of Category 1 reliability (vital buses) were energized via the RMG-4 from the batteries. Core cooling was performed in the mode of natural circulation with steam dumping through BRU-SN lay-out. Reactor parameters remained under control (instrumentation being fed from batteries). At 06:45-06:50 unit power supply from off-site source was restored.
Basis for rating:
1. No off-site impacts.
2. No on-site impacts.
3. This event is rated under the "degradation of defence in depth" criterion.
Real initiators:
Loss of off-site AC power sources including voltage and frequency fluctuations (initiator frequency - high/expected);
Reactor scram (initiator frequency - high/expected);
Safety function availability - adequate.
According to Section III-2.4.2 of the INES User's Manual (rev. 1992) and Table II (C1) Level 2/3 is assigned.
During the event the following violations of safe operation limits and conditions occurred:
- inadequate rate of primary circuit cooling and pressure in the interseal space of SGs 3,6,5 hot headers;
- DGs 3,4 transfer to manual control and their unavailability to perform safety function at unit total loss of power.
Taking into account additional factors (safety culture deficiencies) Level 3 has been chosen.
Event date: Tue, 02-02-1993
