IEEE C57 12 38IEEE C57.12.38

LV bushing cantilever strength analysis

Justin Pezzin, P.Eng, gCarlos Gaytan

March 2012

Overview

Problem description

LV bushing force diagram

Review of manufacturer specifications

Discussion / Next actions

Problem descriptionp

Oil egress past the LV bushings of 1 ph padOil egress past the LV bushings of 1-ph pad mounted transformers has been identified as a failure mode related to the torque applied to the q ppbushing by the attached wires.

LV bushing force diagramg g

Specifications summary

Cantilever strength

Manufacturer A5/8” stud = 250 ft-lb1” stud = 300 ft-lb

Manufacturer B 80 ft-lb

Manufacturer C Not available

Discussion / next actions

Is this a general issue?

Is standardization required?

Can we make a decision today?Can we make a decision today?

Standards on LV Bushing Cantileverg

IEC 60137 2008 Ins lated b shings for alternatingIEC 60137- 2008. Insulated bushings for alternating voltages above 1 000 VCantilever Load requirements:Cantilever Load requirements:

300 N (67.4 lb) for Ir ≤800 A1 000 N (224.8 lb) for Ir >800 A( )

IEEE Std C57.19.01. Performance CharacteristicsIEEE Std C57.19.01. Performance Characteristics and Dimensions for Outdoor Apparatus Bushings

Does not cover LV Bushings (Scope: 34.5 kV and above)

IEC 60137- 2008 Test Method

bushing shall be completely assembled and filled with thebushing shall be completely assembled and filled with the insulating medium specifiedA pressure equal to 1 bar ± 0,1 bar (14.7 psig ± 1.47 p q , ( p gpsig), above the maximum operating pressure shall be applied inside the bushingLoad shall be applied perpendicular to the axis of theLoad shall be applied perpendicular to the axis of the bushing at the mid-point of the terminal for 60 sBushing passes if there is no evidence of damage g p g(deformation, rupture or leakage) and if it has withstood a repetition of all routine tests without significant change from previous resultsfrom previous results

Force Applied for Cantilever Testpp

Cantile er Load = Torq e AppliedCantilever Load = Torque Applied

Cantilever Load = Force * Distance

Si L d i fi d th tSince Load is fixed, the torque applied varies with the length of the LV bushing and its terminal

ForceDistanceDistance

in/ftLoad

lbTorque

ft-lb6 / 0.5 224.8 112.46 / 0.5 224.8 112.4

8 / 0.75 224.8 168.610 / 0.833 224.8 187.2

Standards on LV Bushing Cantileverg

Most LV Bushing Manufacturers specify the cantileverMost LV Bushing Manufacturers specify the cantilever torque used on their certification testingThe following is a sample of the cantilever torque values that g p qare published for LV Bushings:

Stud diam. Max. Current Bush. Torque Max. load distance(in) (Amp) ft-lb in / ft5/8” 830 250 13.34 / 1.111” 1500 280 to 375 14.9 / 1.24 (*)

1 ¼” 2200 320 17.1 / 1.42

(*) Worst case

Standards on LV Bushing Cantileverg

Conclusions:Conclusions:No current IEEE Std. Covers LV Bushing requirementsIEC 60137 Standard provides cantilever requirements for LV BushingsMost LV Bushing manufacturers provide cantilever torque test values that would meet IEC 60137 requirements for std. qdimensions of bushing-terminal assembliesConcerns would raise on larger terminals that would cause torque distances above 12” where mechanical support may be requireddistances above 12 , where mechanical support may be required Findings may suggest to review any reported issues, to determine whether a LV bushing standardization is needed