INES-event
INES 0

UNIT OUTAGE ON CLASS-IV FAILURE

On 18.3.91, there was a grid disturbance due to tripping of 400 KV lines at Nagerjunnasagar Substation on bus bar differential protection. On sudden loss of Watt and VAR, the TNEB grid frequency and 230KV bus voltage at MAPS dipped to less than 47.5 HZ and 175 KV, respectively. Unit II tripped on under frequency protection. 6.6 KV auto transfer took place, but all loads transferred to SUT-2 tripped on U/V protection due to loss of grid supply. EMTR took place and both the DGs came on auto in MAPS I&II as per design intent.
BASIS FOR RATING: Unit 2 was operating at 150 MW(e) and Unit-1 was under shut down. When class IV supply failed to MAPS, both the DGs in MAPS I & II came on auto and core cooling was maintained with shut down cooling pumps in MAPS I and II. There was no interruption to Class II switch gear. Though the class IV supply was extended to MAPS from grid with a delay of 30 minutes no safety system was affected for want of power supply during class IV power failure. Hence level 0 was selected.
DIFFICULTIES IN RATING:

Location: KALPAKKAM-2
Event date: Mon, 18-03-1991
Nuclear event report
Legenda & explanation