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CID-00881: Bushing Insert, 200A, Loadbreak

Super-Class:
  200A, Loadbreak Seperable Cable Accessories
  Transformer Structure, 200A Wells, Loop Feed, Radial Connection, Single Phase, Deadfront, Padmount
  Primary Fuse Cabinet, 200A, Loop Feed, Structure
  Transformer Structure, 200A Wells, Radial Feed, Three Phase, Deadfront, Padmount
  Single Phase, Loop Feed, Open Point, Padmount Transformer, Structure
  Bushing, 200A, Medium Voltage, Transformer, Assembly
Class Name: Bushing Insert, 200A, Loadbreak
Sub-Classes:
Installation of a Bushing Insert, 200A Loadbreak
 
IEEE 386: Standard for Separable Insulated Connector Systems for Power Distribution Systems above 600 V
 
Summary: Bushing Insert, 200A, Loadbreak

Introduction

Typically used in a bushing well or reducing tap well for an elbow connection point.

Partial Vacuum at 25kV and 35kV Class

InsertConnection.png

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:

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 ˝ 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.

Elastimold Patented Solution - Vented Ring

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.

Cooper Patented Solution - J-Shaped Circumferential Groove

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 Systems and was assigned 19 Jan 2006 and should expire on Jun 7 2025.

Extended Inserts

If additional clearance between the transformer faceplate and the high voltage underground cables is required, the long insert version is recommended. Its longer design provides an additional three inches of clearance, which may be important if there is a problem with cable congestion due to the placement of the primary and secondary cables within the transformer cabinet. This design provides easier switching and cable movement where this issue may be a concern

Pictures

15kV

Elastimold Elastimold Extended Cooper Hubbell
elastimold200A15insertbushing.png elastimold200A15extinsertbushing.png cooper200A15insertbushing.png hubbell200A15insertbushing.png
       

25k/28kV

Elastimold Elastimold Extended Cooper Cooper Extended Hubbell
elastimold200A25insertbushing.png elastimold200A25extinsertbushing.png cooper200A25insertbushing.png cooper200A25extinsertbushing.png hubbell200A25bushinginsert.png
         

35kV

Elastimold Elastimold Extended Cooper
elastimold200A35insertbushing.png elastimold200A35extinsertbushing.png Picture N/A
     

[1]

Drawings

bushinginsert200aloadbreakpng.png

[2]

Manufacturers

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

References

  1. Elastimold Underground Cable Accessories, Thomas & Betts, Sept 2016
  2. Underground Construction Standards, San Diego Gas & Electric, 2016

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