600 Amp T-Body Connector Degradation Research

IMCORP Technology Users’ Group, October 21, 2019, Scottsdale, AZ by Jon Erickson San Diego Gas & Electric

© 2015 San Diego Gas & Electric Company. All copyright and trademark rights reserved.

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• San Diego Gas and Electric has identified legacy design overheating 600 amp T-body connectors inside manholes during routine maintenance and inspection

• In some documented cases, failures can occur shortly (within a few hours), after the removal of water from manholes where the separable connectors are submerged.

• This allows for the formation of two hypothesis:

– Hypothesis 1: The presence of water creates an environment conducive to material degradation that impedes normal performance of the T-bodies and connectors.

– Hypothesis 2: The symptoms of this material degradation are abated by the presence of water.

SDG&E Experience

Temperature Response Guidelines –Premolded Connectors

Utility response guidelines for premolded connectors

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T-Body and Connector Assembly

Semiconductive

Housing

Semiconductive

HousingConductive Band

Insulating Material

Continuity of outer stress control layer

maintained between T- bodies through

metal band on connector plug

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Connector Plug – Degraded Material

Category Picture Resistance of Metal Band at 0.5in

New 0.03 Ohm

Field Measured Hot Intermittent surface continuity

Faulted N/A

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Connector Plug – Electrical Activity

Oxidized and intermittently eroded metallic

band creates isolated pockets of poor or no

measurable conductivity between t-body

semiconductive housings. The compromised

edges of the band make poor contact to the

semiconductive housing.

Evidence of electrical activity across plug

interface between metallic band and

conductor connection inside T- body.

Failed T Body Connection – A Failure Postmortem

Electrical “treeing” pattern near thinned

T-body lip indicating surface erosion

activity.

Surface pitting of material due to PD/arcing activity at

interface.

T body lip has become thin due to

material erosion.

Contamination due to connector plug

interface failure7

Practical Test – Partial Discharge

• New and 2 SDG&E supplied, field degraded, connectors tested in dry and

tap water submerged conditions.

• Nominal operating voltage line to ground: U0 = 6.9kV

• Tests were performed at 5kV, 14.4kV, and 28.8kV

• Water shorts out intermittent conductivity to yield better PD performance

Sample ConditionPeak PD

@5kV [pC] Peak PD

@14.4kV [pC] Peak PD

28.8kV [pC] PD Incept. [kV]

<Uo Red

New

Dry 0 0 0 N/A

Wet 0 0 0 N/A

Dried Out 0 0 0 N/A

Field Measured HotM2593770427 -1

Dry 65 130 >500 4.0 kV

Wet 0 87 >500 9.2 kV

Dried Out 30 114 >500 6.2 kV

Field Measured HotM2593770427 -2

Dry 80 142 >500 4.3 kV

Wet 0 77 >500 10.5 kV

Dried Out 45 126 >500 6.4 kV

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• Intermittent conductivity on the plug surfaces between outer semiconducting T-body parts creates enough potential differences to induce arcing/partial discharge at <Uo

Practical Test – PD Activity

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• Test setup energized to U0 (6.9kV) in dry conditions. Temperature was recorded at hottest point at the connector in 5 minute intervals.

• Potential difference across connector surface with intermittent continuity yields heating due to arcing and PD activity.

Practical Test – IR Imaging

Time (min) IR Image Recorded Temp (oF) Temp. Delta (oF)

0 67.8 N/A

5 70.2 +2.4

10 72.9 +5.1

15 75.0 +7.2

20 77.2 +9.4

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• New connector did not yield observable PD up to 4Uo in dry or tap water submerged conditions

• PD performance of degraded connector plugs was improved by submerging under tap water and showed no PD at Uo.

• PD performance degraded as the connector dried out after submerged testing and showed PD at ≤Uo.

• PD was observable with relatively large magnitudes (approx. 50pC) at operating voltage with directly coupled and handheld sensors

• PD/Arcing on connector surface most likely damages EPDM T-body lip yielding additional PD and eventually plug interface failure.

• Elevated surface temperatures observed in the field most likely due to PD/Arcing activity observed – confirmed with lab observation.

Preliminary Conclusions

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Questions