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Root Cause Failure Analysis Current Transformer Ser. #811282

John Stead, Altalink Management Ltd.

Maintenance History Current transformer Ser.# 811282 TAU # 6369 is a Haefely type IOSK-245 installed in Jenner 275s in 1989. The unit did not actually fail violently in service; it was removed. when a switchman heard noise emitting from the unit while switching the line. Below are the power factor test results for the unit over its life. Table #1 shows all the tests related to the subject current transformer and sister units at the same site. As you can see the original power factor was .18% in 1989 with the reading in 2001 increasing to a corrected power factor of .29%. The limit for these current transformers applied to Doble Engineering is .5%. The power factor tests when compared to the limit files are acceptable. The power factors in red compare the 3 tests performed in 2001 across 3 current transformers on the same day and of the same type. While all the units tested were well under the .5% limit we can see a large difference when we compare the results between units tested .29% (unit removed from service) and .08% and .1% sister units. You can see the suspect unit is approximately 3 times higher than the other 2.

Table # 1 * shop test after unit removed from service

Location Manu. Type Serial No. mA Watts

Corr. Pf

Meas. Pf

Oil Temp

Air Temp Test Date

*FTSB (test) Haef IOSK-245 811282 4.556 0.211 0.29 0.45 30 30 12/1/2004

* FTSB (shop) Haef IOSK-245 811282 4.556 0.133 0.27 0.29 22 24 12/1/2004

275 S JENNER Haef

IOSK-245 811282 4.577 0.205 0.29 0.45 30 26 7/12/2001

275 S JENNER Haef

IOSK-245 811282 4.65 0.094 0.21 0.2 18 18 5/27/1992

275 S JENNER Haef

IOSK-245 811282 4.5 0.08 0.18 0.18 27 7/19/1989

275 S JENNER Haef

IOSK-245 811282 4.577 0.205 0.29 0.45 30 26 7/12/2001

275 S JENNER Haef

IOSK-245 811281 4.61 0.075 0.08 0.16 30 26 7/12/2001

275 S JENNER Haef

IOSK-245 811280 4.557 0.074 0.1 0.16 30 26 7/12/2001

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Graph # 1, below, shows the relationship of the power factor of the suspect unit to all type IOSK-245 current transformers (124 datasets) in service.

Graph # 1

.29% The suspect unit would fall into the upper level of bin #15, as you can see there are 5 units in bin 16-20 indicating that there are 5 units that have tested at a level higher than Ser. # 811282. As you can see we have 14 units in bins 15 to 20 or approximately or 11% of our sample. Since we have more than 1 test dataset for each current transformer some of the suspect tests are from the same device. When we eliminate this duplication we have 12 units in our suspect group, serial numbers: 820822, 811281, 811227, 821161, 890642, 811282, 811280, 830084, 821170, 811236, 820824, 811235. (Units in red indicate overlap with suspect watts readings)

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Graph # 2, below, shows the relationship of the watts loss of the suspect unit to all type IOSK-245 current transformers (124 datasets) in service.

Graph # 2

.205 The suspect unit would fall into bin # 20, no other unit had a higher watts loss than Ser. # 811282. As you can see we have 10 units in bins 12 to 20 or approximately or 8% of our sample. The spread of the data on the watts readings are more concentrated and defined with the majority of the readings falling in bins 8 and 9. Since we have more than 1 test dataset for each current transformer some of the suspect tests are from the same device. When we eliminate this duplication we have 7 units in our suspect group, serial numbers: 820824, 811226, 811259, 820823, 811282, 820822, 811227 (units in red indicate overlap with suspect power factor readings).

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The purpose of this exercise is to initiate a sample test program on the suspect units to determine if we have other units in danger of failure since the root cause was determined to be a design weakness.

Tear Down of Failed Unit The oil was sampled and tested for DGA in order to determine the extent of the deterioration of the unit. The results shown below Table # 2

Table # 2

Oil Inside secondary housing Oil from Primary Winding Insulation

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Complete Unit

Picture # 1 Upper Chamber with Secondary Windings and Bellows

Picture # 2 Primary Winding Insulation

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Picture # 3 Secondary Winding Housing

Picture # 4 Secondary Winding Insulation (carbon deposits bottom of housing)

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Picture # 5 Secondary Winding (carbon contamination)

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Picture # 6 Secondary Down Leads

Secondary Leads

Picture # 7 Down Lead Flange

Carbon Deposits

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Picture # 8 Welds to Bond Flange to Secondary Winding Housing

Broken weld spots

Picture # 9 Secondary Housing in Relationship to Down Lead Flange

Secondary Housing Secondary Leads Flange Plate

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Picture # 10 View of Secondary Housing from Inside (flange removed)

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Observations During Tear Down The solid primary insulation was in very good shape this explains the slight change in the power factors results as these are an average of all the insulation between the high voltage component and ground in addition to the secondary winding. See connection diagram below. The large amount of primary insulation in the head will mask any problems with the insulation inside the secondary winding housing. The illustration below is a hairpin design where the failed unit is a thru type current transformer. In the thru type the secondary winding is in the head not at the bottom.

The carbon contamination was primarily within the secondary winding housing (see picture # 4 above, the source of the carbon was from the ineffective bonding method used to bond the grounded flange on the down lead assembly to the secondary winding housing assembly (see pictures # 6, 7, 8, 9, 10). Picture # 8 shows 3 of the 6 spot welds used to bond the housing to the flange on the down lead assembly. Picture # 7 shows the extensive carbon build up between the housing and the flange.

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When we examine the 2 oil samples in Table # 2 page 4 using the Church Nomograph the ratios indicate: Sample #1 Inside Secondary Housing Hydrogen\Acetylene – arcing Acetylene\ethane – arcing Ethane\hydrogen – arcing Hydrogen\methane – arcing, discharge Methane\acetylene – arcing Acetylene\ethylene - heating

Sample #1 Primary Insulation Oil Hydrogen\Acetylene – heating Acetylene\ethane – arcing Ethane\hydrogen – arcing Hydrogen\methane – arcing Methane\acetylene – arcing Acetylene\ethylene - arcing

As you can see, the predominant failure mode is arcing, which is consistent with observations from teardown. You can also see from the DGA there is no indication of paper being involved in the fault, which is also consistent with the teardown.