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Vermont Electric Power Company, Inc.
Mitsubishi SF6 Gas Circuit BreakerType 100 SFMT-40E
Breaker Failure Report
Prepared by:
Jeffery M. WrightManager – Transmission Field Services
April 6, 2004 – 13:08
• VELCO experienced a breaker failure in the Essex FACTS switchyard. The 115 kV SF6 gas circuit breaker is a Mitsubishi 100SMFT-40E.
• Although there were no visible external damages, relay and protection engineers analyzed the relay information and all indications led to an internal failure of the K98 circuit breaker.
EXAMPLE ONLYMitsubishi 100SMFT- 40E Circuit Breaker
Location: Georgia C80 Cap Bank Breaker
DFR analysis indicates the fault lasted nearly 4 cycles with phase 3 volts dropping to .02 p.u.
IBM, the largest manufacturer in Vermont is electrically connected to redundant VELCO 115 kV sources. They are located .7 miles from the Essex substation.
IBM 115 kV phase 3 bus voltage dropped to approximately the same as what VELCO recorded at Essex. Approximately .02 p.u. for nearly 4 cycles.
The power quality interruption cost IBM in excess of $1.5M in lost tooling hours and production.
An internal inspection directed by Mitsubishi technical engineers verified
that an internal fault had indeed occurred.
The breaker was shipped back two days following the failure to Mitsubishi
(Pittsburgh, PA) for a thorough failure analysis.
SF6 arc byproducts
Formal Mitsubishi Response Friday, April 16, 2004 Conference Call
Failure Analysis Report1) Repair Efforts & Flashover Location
The tanks from Breaker SG006800004 were dis-assembled and inspected.The flashover was noted to have occurred at Bushing Assembly #4, center phase, at the moving end.The evidence indicates that the arc struck from the Bushing #4 conductor to the bushing adapter plate.
Fotos are attached.The quantity of arc byproduct powder in the center phase tank was moderate, indicating the flashover
was not severe.Arc byproduct powder was collected and the SF6 which came inside the breaker was also collected, for
possible future analysis.
Breaker components which were damaged or suspected of being damaged were replaced; major replacedcomponents included center phase interrupter, Bushing Assembly #4 (conductor & insulator), all 6
bushing adapter plates, the SF6 gas system, desiccant, and associated hardware, O-rings, etc.The other 2 phases were also disassembled; all major components from these 2 phases were re-used.
All 3 tanks were thoroughly cleaned; all re-used components were thoroughly cleaned.Breaker SG006800004 was re-assembled and re-tested following written instructions.
Breaker SG006800004 was shipped on 4/15/04.2) Investigation into Cause of Flashover
Our investigation indicated that randomly-occurring water was the likely cause of the flashover.Water was detected using our ultrasonic (UT) inspection technique, which can detect water in boltholes
without dis-assembling the breaker.Boltholes in every nozzle were evaluated for presence of water by the ultrasonic method, and every bolt was
removed for visual examination. This gave us an opportunity to visually confirm the ultrasonicindications.
This investigation indicated that water was present randomly, at different levels, in 3 of the 6 bushing nozzles,in boltholes
Bushing Nozzle # 2 (2 of 12 boltholes, very minor … <0.5 mm)Bushing Nozzle # 3 (2 of 12 boltholes, very minor … < 0.5 mm)Bushing Nozzle # 4 (6 of 12 boltholes, minor & moderate levels)
Small traces of a water-and-oil liquid were noted on the undersides of 4 of 6 bushing adapter plates, wherethey mate with the tank nozzles. Foto attached.
Visual observations of removed bolts & boltholes also indicate that some of the water in the boltholes doesevaporate, and is likely absorbed by the desiccant in each tank
The internal inspection conducted
by Mitsubishi uncovered evidence of the internal fault
between the bushing conductor and
bushing adapter plate inside the bushing throat.
Further investigation uncovered water droplets in the breaker bolt holes of four out of six bushing
adapter plates.
Following the failure analysis, the major components damaged were replaced, the breaker was rebuilt,
shipped back to the Essex substation and commissioned by VELCO
technicians on April 21st at 13:24.
Water Droplets Present
Formal Mitsubishi Response Friday April 16, 2004 Conference Call
• Mitsubishi admits knowledge of three other similar failures of the same type of breaker – one in Illinois and two in Nebraska.
• Mitsubishi admits to a known process control problem which was identified and corrected in June 2001, affecting breakers manufactured between 1999-2001.
•The manufacturing process began with pressure washing the cast breaker tanks with water. The tanks were set outside to dry and the failures are attributed to trapped water in internal bolt holes that eventually migrates into the tank providing a point of discharge, ultimately faulting the breaker internally at the point of the water droplets on the tank wall.
• Mitsubishi did not issue a service advisory to all of their customers notifying them of this problem.
• Mitsubishi has since developed a method for detecting the water in the bolt holes using an ultra-sonic method. They agreed to send a crew to Vermont on Monday April 19th to begin testing other subject breakers – yet to be identified.
Mitsubishi Response
Mitsubishi identifies the
subject breakers that VELCO owns.
Out of the thirty (30) 100SFMT-40E type breakers
VELCO owns, seventeen
(17) are potentially
subject to the same
problem. 2001100-SFMT-40SHESG009110001MITSUBISHIC82SANDBAR
2000100-SFMT-40HESG002770001MITSUBISHIK23WILLISTON
1996100-SFMT-40HE970360101MITSUBISHIK43WILLISTON
2002100-SFMT-40HESG013120006MITSUBISHIK34-40WEST RUTLAND
2002100-SFMT-40HESG013120002MITSUBISHIK37-40WEST RUTLAND
2002100-SFMT-40HESG013120003MITSUBISHIK39-37WEST RUTLAND
2002100-SFMT-40HESG013120005MITSUBISHIK34WEST RUTLAND
2002100-SFMT-40HESG013120004MITSUBISHIK30-39WEST RUTLAND
1998100-SFMT-40HE980360401MITSUBISHIC71NORTH RUTLAND
1999100-SFMT-40HE980360101MITSUBISHIC72MIDDLEBURY
2000100-SFMT-40HESG00564002MITSUBISHIC80GEORGIA
2000100-SFMT-40HESG006800002MITSUBISHIC36ESSEX
2000100-SFMT-40HESG006800001MITSUBISHIC35ESSEX
2000100-SFMT-40HESG006800004MITSUBISHIK98ESSEX
2000100-SFMT-40HESG006800003MITSUBISHIK97ESSEX
2000100-SFMT-40HESG007100001MITSUBISHIC39ESSEX
2002100-SFMT-40HESG008490001MITSUBISHIC38ESSEX
2002100-SFMT-40HESG013850002MITSUBISHIC37ESSEX
2000100-SFMT-40HESG013850001MITSUBISHIC34ESSEX
2000100-SFMT-40HESG007100002MITSUBISHIK25 ESSEX
2000100-SFMT-40HESG005640007MITSUBISHIK25-22ESSEX
2000100-SFMT-40HESG005640006MITSUBISHIK22-23ESSEX
2000100-SFMT-40HESG005640001MITSUBISHIK23ESSEX
2000100-SFMT-40HESG005640003MITSUBISHIK24ESSEX
2000100-SFMT-40HESG005640008MITSUBISHIK20-24ESSEX
2000100-SFMT-40HESG005640005MITSUBISHIK21-20ESSEX
2000100-SFMT-40HESG005640004MITSUBISHIK21ESSEX
2002100-SFMT-40HESG013120001MITSUBISHIC92COOLIDGE
2002100-SFMT-40HESG013120008MITSUBISHIC91COOLIDGE
2002100-SFMT-40HESG013120009MITSUBISHIK32COOLIDGE
YEARTYPESERIAL NUMBERMANUFACTURERPOINTSTATION
The Testing Process
•The breaker is removed from service and CT shrouds are removed, exposing
the bushing throats.
•The bushing throat is filed clean, providing a clean surface to apply the
ultra-sonic device to.
• By aligning the test probe directly with the bolt hole behind the test area, the test device indicates a
waveform. A non-distorted waveform indicates a dry
bolt hole, whereas a distorted waveform can be
an indication of a contaminated bolt hole.
Interpretation of test results is done on site by the
technician.
• An inspection can be completed within four hours.
•The bushing throat is filed clean, providing a clean surface to apply the ultra-sonic device to.
• By aligning the test probe directly with the bolt hole behind the test area, the test device indicates a waveform. A non-distorted waveform indicates a dry bolt hole, whereas a
distorted waveform can be an indication of a contaminated bolt hole. Interpretation of test results is done on site by the technician.
The Internal Inspection / Rebuild Process
• If water is suspected in the bushing adapter plate bolt holes,
the breaker is drained of gas, bushings are removed and all
mounting hardware is removed, cleaned and reassembled.
• The rebuild process typically consumes a twelve to fourteen
hour day.
• Four breakers were rebuilt.
• Once the bushing adapter plate is removed, the holes are inspected for water, blown dry and prepped for reassembly.
Mitsubishi Response
•Mitsubishi’s response was
timely.
•The failed breaker was repaired and
returned to VELCO in seven (7) days.
•Inspections followed
immediately afterward.
31302928272625
24232221201918
1715131211
108654
321
SFTWTMS
APRIL
BREAKER FAILS
BREAKER SHIPPED TO MEPPI
BREAKER SHIPPED
TO VELCO
FAILURE ANALYSIS
CONF CALL
INSPECTIONS START
Mitsubishi Response
•All work was wrapped up by the end of June.
•MEPPI was very cooperative and worked weekends to accommodate outage requirements.
•Of the seventeen (17) breakers that were ultrasonically inspected, four (4) required internal inspection and cleaning.
31302928272625
24232221201918
17161514131211
10987654
321
SFTWTMS
APRIL
29282726252423
3130
22212019181716
1514131211109
8765432
1
SFTWTMS
MAY
31302928272625
24232221201918
17161514131211
10987654
321
SFTWTMS
JULY
30292827
26252423222120
19181716151413
1211109876
54321
SFTWTMS
JUNE
FINISHED
VELCO Response
• VELCO has kept all of its owners and IBM informed of the results of the failure analysis report and the progress and results of the testing.
• VELCO has gone forth and publicized the failure on the Doble Engineering Support Group e-mail forum to inform other utilities of the facts related to theVELCO failure.
• VELCO is satisfied with the Mitsubishi inspection process and rebuilds.
• VELCO intends to hold MEPPI accountable for remedying any further problems identified and is currently negotiating the compensation for VELCO labor and materials. VELCO’s labor and material costs exceed $33,000.