200A Loadbreak Bushing Inserts are designed for assembly to a 200A universal bushing well and are used as a deadfront mating loadbreak connector for a 200A loadbreak elbow. The north american standard which governs 200A loadbreak bushing inserts is the latest IEEE 386. 200A loadbreak bushing inserts are typically found in:
- Deadfront Padmount Transformers
- Deadfront Switchgear
Partial Vacuum at 25kV and 35kV Class
In the late 1980's users of 25kV and 35kV (200A) loadbreak elbows and insulating caps began reporting incidents involving line-to-ground flashovers, where an arc was extending from an energized connector in the elbow or insulated cap to ground during switching operations. During a loadbreak switching operation two energized connectors are disconnected, where the male connector (e.g. elbow or insulated cap) is pulled from the female connector (e.g. bushing insert) using a hotstick. A loadbreak operation creates an open circuit, which was creating a flashover. The incidents were occuring amongst all manufacturers assemblies. Study of this flashover problem proved to be very difficult, the problem was intermittent in nature and at first the issue could not be reproduced in the laboratory. A number of common factors were discovered that contribute to the increased likelihood of these flashovers:
- Lightly loaded or unloaded circuits
- Higher voltages (25kV and higher)
- Cold temperatures (usually below 0 degrees celcius)
- Breaking of load, not during closing (i.e., removing at elbows from, not installation of elbows onto bushings)
- Sticking of elbows or caps during removal from bushings
Elastimold discovered that the problem was caused by the partial vacuum effect. Partial vacuum flashovers can be identified by tick marks at the back end of the probe. The flashovers occur along the 200A interface extending from the energized semiconductive insert and/or the back of the probe to the grounded semiconductive collar below the bushing shoulder. A partial vacuum is created along the mating interfaces when separating loadbreak connectors. The partial vacuum substantially reduces the withstand voltage along the 200A interface during the first one inch of travel of the connector. The reduction of dielectric strength of air as the pressure drops is governed by Paschen's Law. Both the loadbreak elbow and insulated protective cap experience the partial vacuum effect. A partial vacuum is created in both the cuff and nosepiece clearance regions at each end of the 200A interface. The partial vacuum developed in these regions is approximately 50% to 20% of an atmosphere. Information gathered from utilities indicates that the flashover rate due to the partial vacuum effect is greater at the higher system voltages, at colder ambient temperatures and when the connector is in a stuck condition. Separable connectors become more difficult to operate or stuck at lower temperatures and when the grease on the 200A interface has aged. Flashovers due to the partial vacuum effect typically occur under low or no load current conditions. A great majority of switching operations are performed at little or no load current during sectionalizing procedures. Data also indicates that insulated protective caps are more likely to flashover than elbows. The reason for the difference in the failure rate between caps and elbows is not fully understood. Analysis of field failures and high speed videos of flashovers have established that partial vacuum induced flashovers occur at the time when the elbow or cap has been withdrawn approximately 0.4 inches from the bushing. In summary, the flashover rate is primarily dependent on both the system voltage and the force required to separate the interfaces.
- Flashovers occurred with connectors of all manufacturers. - Flashovers occurred on unloaded or lightly loaded circuits. In the case of an insulated cap, the load current was zero. - The rate of flashover increased with increased voltage. - Flashover occurrences increased during cold weather.
A unique failure mode was common to most events: Flashover along the separating interfaces occurred early in the operation with an elbow or cap movement of less than an inch.
Flashovers: - Occur during break operation, not make - Occur before contact separation - Occur before cuff seal opens - Show telltale arc pits on probe
Conclusion: - Cause not related to circuit-interruption phenomena - Effect of decreased pressure reduces dielectric strength of air along opening connector interface and may result in flashover - Partial vacuum is root cause of flashover, which cannot be eliminated, only controlled - Affects all manufacturers and brands
Partial vacuum describes a decrease in internal air pressure that occurs when an elbow or insulated cap is separated from a bushing insert. It occurs in an assembled connector when the ambient temperature decreases. Furthermore, the operation of opening the elbow causes a decrease in pressure.
In four test cases, even with extreme parameters, the Elastimold vented bushing solution resulted in no flashovers. A new bushing insert, which vents the trapped air and prevents a partial vacuum, eliminated flashover.
The Elastimold (Thomas & Betts/ABB) vented solution to solve the partial vacuum issue in cold weather seems was the first solution developed. They put considerable research in this topic along with SaskPower to recreate, determine, and solve the problem, their patent is US5957712 Thomas & Betts and was assigned in 30 Jul 1997 and should expire on 30 July 2017.
It appears that Hubbell also uses a vented cuff ring on their insert like Elastimold (Thomas & Betts/ABB), it seems there is some sort of licensing agreement among the two companies.
The Cooper Power Systems (Cooper/Eaton) j-shaped groove solution on the insert to solve the partial vacuum issue in cold weather was the next solution developed after the Elastimold vented solution. It came to market about 9 years after the Elastimold solution. Their patent is US7083450 Cooper Power Systemsand was assigned 19 Jan 2006 and should expire on Jun 7 2025.
IEEE 386 Requirements of all 200A Loadbreak Inserts
|Ratings||15kV Class||25kV Class||35kV Class|
|Max. Phase-t0-Phase (kV)||14.4||26.3||36.6|
|Max. Phase-to-Ground (kV)||8.3||15.2||21.1|
|AC 60Hz 1 Minute Withstand (kV)||34||40||50|
|DC 15 Minute Withstand||53||78||103|
|BIL and Full Wave Crest||95||125||150|
|Min. Corona Voltage Level||11||19||26|
|Continuous Switching||10 Operations @ 200A RMS, 14.4kV||10 Operations @ 200A RMS, 26.3kV||10 Operations @ 200A RMS, 36.6kV|
|Fault Closure||10 kA rms symmetrical @ 14.4kV for 0.17s after 10 switching operations||10 kA rms symmetical @ 26.3kV for 0.17s after 10 switching operations||10 kA rms symmetical @ 36.6kV for 0.17s after 10 switching operations|
|Short Time||10 kA rms symmetical for 0.17s, 3.5 kA rms symmetical for 3.0s||10 kA rms symmetical for 0.172, 3.5 kA rms symmetical for 3.0s||10 kA rms symmetical for 0.172, 3.5 kA rms symmetical for 3.0s|
|Class (kV)||Length (inches/mm)||IEEE 386 Bushing Well Interface||IEEE 386 Loabreak Interface||Manufacturer||Catalog Number||Other Characteristics|
|15||7.18/182.3||Figure 3||Figure 5||Elastimold||1601A4||Red PBT Plastic Loadbreak Interface and Vent Ring|
|15||11.00/279.4||Figure 3||Figure 5||Elastimold||1601EA4||Red PBT Plastic Loadbreak Interface and Vent Ring, Extended Version|
|15||Figure 3||Figure 5||Cooper||LBI215|
|25||8.93/226.8||Figure 3||Figure 7||Elastimold||2701A4||Blue PBT Plastic Loadbreak Interface and Vent Ring|
|25||11.50/292.0||Figure 3||Figure 7||Elastimold||2701EA4||Blue PBT Plastic Loadbreak Interface and Vent Ring, Extended Version|
|25||Figure 3||Figure 7||Cooper||LBI225|
|25||Figure 3||Figure 7||Cooper||LBI225L||Extended Version|
|35||11.00/279.4||Figure 3||Figure 7||Elastimold||3701A3||Rated for 1 Phase and 3 Phase Switching, Black Vent Ring|
|35||8.93/226.8||Figure 3||Figure 7||Elastimold||3701A4||Rated for 1 Phase Only, Black Vent Ring|
Single Phase Version
|Three Phase Version
||Three-Phase Integral Loadbreak Bushing
Cooper and Elastimold have hex torque tools which go into the hex broach inside the insert to torque it into the 200A universal bushing well.
15-35kV 200 Amp Loadbreak Assembly Tool, internal torquing feature. 12.25 inch long x 3.00 inch wide. 5/16 inch Hex Drive. Has Eye instead of T-Handle.
15-25kV 200A Loadbreak Assembly Tool, 5/16" Hex.