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