Progress of Investigation of Power Supply Facilities Failure at

Progress of Investigation of Power Supply Facilities Failure at
Fukushima Daiichi Nuclear Power Station
< Reference >
March 25, 2013
Tokyo Electric Power Company
At around 6:57 PM on March 18, 2013, there was an incident where the power supply facilities in the Main Anti-earthquake
Building at Fukushima Daiichi Nuclear Power Station momentarily stopped. Upon investigation, the regular M/C (Metal-clad
switch gear) in the Process Building, common M/C4A and Units 3-4 temporary M/C (A) were found to be stopped due to the
incident.
Power supply system structure of the affected facilities
Okuma line 4L
Okuma line 3L
66kV Switchyard (GIS) 66kV Switchyard (GIS)
Common
M/C (1B)
Unit 1 SFP alternative
cooling system
(Secondary system)
Common
M/C (2B)
Backup M/C in the
Process Building
Recovered at 2:20 PM
on Mar. 19
Regular M/C in the
Process Building
(3A)
(3A)
(5A)
⑫ transformer
Common pool
temporary P/C
Legend
⑪ transformer
Receiving power
Affected and
suspended facilities
Load system switching
Circuit breaker
opened
Cesium adsorption
apparatus (Kurion)
(5B) Units 3-4 temporary M/C (A)
Recovered at 3:00 PM
on Mar. 19
Affected facilities which
have been recovered
Started at 12:01 PM on Mar. 19
Steady flow rate achieved at 12:17 PM.
(4B)
(4A)
Circuit breaker
opened
Nitrogen separator (B)
Power supply
suspended
Common
M/C (2A)
Backup M/C at
the substation
(10A)
⑥-1
transformer
⑥-2
transformer
Common
P/C 4A
Common
P/C 4C
Diesel
generator
Common pool
cooling system
Recovered at 12:12 AM
on Mar. 20
(4A)
Common M/C 4A
Unit 4 SFP
alternative cooling
system
(Secondary system)
Unit 3 SFP
alternative cooling
system (Primary and
secondary systems)
Temporarily recovered (by power supplied from a
Recovered at 10:43
diesel generator) at 4:13 PM on Mar. 19. Suspended
PM on Mar. 19
at 6:48 PM.
Recovered at 10:26 PM on Mar. 19 (with power
supplied from the regular M/C in the Process Building).
Unit 4 SFP alternative
cooling system
(Primary system)
Recovered at 1:20 PM
on Mar. 19
Unit 3 PCV gas
control system
(A system)
In-service at 5:00 PM
on Mar. 19
Analysis of the Power Supply Facilities Failure
[Power supply system structure]
Since power receiving cable renovation for the regular
M/C in the Process Building (as part of Tsunami
countermeasures) was ongoing at the time of the incident,
the regular M/C was temporarily receiving power from the
backup M/C in the Process Building.
[Analysis]
1. The ground relay in B system operated
: The failure is assumed to have occurred in B system
and high voltage power supply system.
2. The circuit breakers of the regular M/C (3A) and (4A) in
the Process Building and the backup M/C in the Process
Building (3A) tripped due to overcurrent.
: The failure is assumed to have occurred in the
downstream of the regular M/C in the Process Building
(4A).
3. The ground directional relays and the circuit breakers
of Units 3-4 temporary M/C (4A), (5A) and (10A) did not
operate.
: The failure is assumed to have occurred in the upstream
of the load systems of Units 3-4 temporary M/C.
4. Fault current was found in Units 3-4 temporary M/C (A)
(5B).
: The failure is assumed to have occurred in the
downstream of Units 3-4 temporary M/C (A) (5B).
Okuma line 3L
Legend
Receiving power
Suspended due to tripping
66kV switchyard in the south side
Suspended
1
Ground overvoltage
Common
M/C (2B)
relay confirmed
Backup
M/C at the
substation
Common
M/C (2A)
Vo＝6.45kV
Opened automatically during
power supply suspension in
order to prevent unnecessary
cutting of circuit breakers due
to magnetizing inrush current
upon restart
Ground overvoltage
relay confirmed
*
Vo＝6.75kV
Backup M/C in the
Process Building
Regular M/C in the
Process Building
(1B)
(3A)
2
(3A)
Circuit breaker
operated due to
overcurrent
Circuit breaker
operated due to
overcurrent
*
(4A)
(4B)
Circuit breaker
operated due to
overcurrent
Units 3-4 temporary
M/C (2A)
(5B)
(6A)
(4A)
(5A)
Circuit breaker
operated due to
undervoltage
Transformer No.10
Fault current
confirmed
4
(10A)
Protection relays and
circuit breakers did
not operate
Transformer
No.12
Common pool
temporary P/C
(1B)
Common M/C (4A)
(5A)
The failure is assumed to have occurred in
Units 3-4 temporary M/C (A).
Okuma line 4L
Transformer
No.11
Transformer
No.6-1
Transformer
No.6-2
3
Appearance of Units 3-4 Temporary M/C (A) (1)
Full view of Units 3-4 temporary M/C (A)
(Units 3-4 switchyard)
Power panel where
soot was found
Appearance of Units 3-4 temporary M/C
Inspection of Units 3-4 temporary M/C
Power panel
Power panel
door
Front side
Back side
Units 3-4 temporary M/C (A) power panels and their doors
Units 3-4 temporary M/C with the power
panel door opened (below the power panel
where soot was found)
Photos taken on March 20, 2013
Appearance of Units 3-4 Temporary M/C (A) (2)
Soot found on the instrument current transformer
(terminal) of Units 3-4 temporary M/C
Power panel where
soot was found
Soot found on the walls of Units 3-4 temporary M/C
Power panel
Front side
Back side
Location where a dead small animal (mouse) was found (on the
power panel floor underneath the power panel where soot was found)
Units 3-4 temporary M/C (A) power panels and their doors
A small animal (mouse) found dead (Photo
taken from above)
Inspection Results of Units 3-4 Temporary M/C (A) Compared to Sound Condition
Regular inspection
Visual inspection (check for abnormalities, strange sound/smell from the outside of the power panel) is
performed once a week during patrol. No problem was found at a visual inspection performed in the
morning of March 18, 2013.
Results of inspection of the inside of the power panels after the power supply facilities failure
No problem was found with units other than 5A where short circuit occurred.
Inside the power panel (5A) after
short circuit occurred
Sound condition of power panel (6A)
Soot
Trace of short circuit
Photo 1: Inside the power panel after short circuit occurred
Photo 2: Sound condition of power panel
Cause of Phase Fault Occurred in Units 3-4 Temporary M/C (A)
Back side of Units 3-4 temporary M/C (A)
and the power panel doors
Enlarged
image of
unit 5A
[Site investigation results]
- A trace of short circuit was found on the conductor in the back
of Units 3-4 temporary M/C (A) (5A). Also, a dead small animal
(mouse) with an electric mark was found on the floor of the same
unit.
- No trace of short circuit or foreign substance was found in other
units in Units 3-4 temporary M/C (A) and the main line.
Marks from fire damage
Conductor
15cm
15cm
Cable
Approx. 25cm
Dead small animal
(mouse)
A small animal (mouse, 25cm long) getting close to the conductor
generated an arc leading to a phase fault which developed into a
three-phase short circuit.
Discoloration of Units 3-4 M/C (A) Circuit Breakers
Vacuum valve
Unit 5A
Vacuum circuit breaker
Discoloration
Unit 5B
Side view (Transparent view)
Discoloration
Traces of Short Circuit Found in Units 3-4 Temporary M/C (A)
Traces of short circuit found near the terminal of unit 5A
Side surface
Terminal
Top surface
Traces of short circuit found on the top
surface of unit 5A
Traces of short circuit found on the side
surface of unit 5A
(Legend)
Trace of short circuit
Electric Mark Found on the Dead Small Animal
A dead small animal (mouse (photo 1)) was found on the floor of Units 3-4 temporary M/C (A)
unit 5A. An electric mark (photo 2) was found on its
Mark from electric shock
Photo 1: Dead small animal (mouse)
found in unit 5A
Photo 2: Enlarged image of the electric mark
Condition of the Opening of Units 3-4 Temporary M/C Container
Location where the cable is drawn in
(Side surface of the container)
The opening found in the location
where the cable is drawn in has
been closed with a sheet.
The sheet is removed for
inspection (which makes a large
opening). Normally, the opening
is closed with sheet.
Feces of Small Animal Found in Units 3-4 Temporary M/C (A)
Units 3-4 temporary M/C (A)
Inspection opening
of the container
Dead small animal
Side view (Transparent view)
Substances which are assumed to
be feces found in unit 5B
- A small animal (mouse) may have gotten into the unit from the opening generated by sheet partially coming off.
- Since the sheet was removed for investigation, trace of small animal (mouse), etc. could not be confirmed.
- Substances which are assumed to be feces of the deal small animal (mouse) were found on the floor of Units 3-4 temporary M/C
(A) unit 5B.
- No abnormality was found with the appearance of the sheet, etc. at the time of inspection performed in the morning of March 18.
Cause of the Power Supply Failure of Units 3-4 Temporary M/C (A)
A small animal (mouse) got into Units 3-4 temporary M/C (A)
Short circuit occurred due to the small animal (mouse) getting close to the current-carrying area
Overcurrent was generated among the backup M/C in the Process Building,
regular M/C in the Process Building and Units 3-4 temporary M/C (A)
Since the same overcurrent value is set for the M/C circuit breakers above,
the circuits were cut simultaneously and power supply failure occurred.
Cooling systems for Unit 3 SFP, Unit 4 SFP and the common pool were suspended.
M/Cs other than the above were affected by the potential changes accompanying the
momentary stop of power supply, Unit 1 SFP alternative cooling system
(secondary system) and nitrogen separator (B) were suspended.
End