S12-lvbushingcantilever-presentation

IEEE C57.12.38 C57 12 38 LV bushing cantilever strength analysis

Justin Pezzin,, P.Eng g Carlos Gaytan March 2012

Overview

„ Problem description „ LV bushing force diagram „ Review of manufacturer specifications „ Discussion / Next actions

Problem description p

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

LV bushing g force diagram g

Specifications summary Cantilever strength Manufacturer A

5/8” stud = 250 ft-lb 1” 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?

Standards on LV Bushing g Cantilever „ IEC 6013760137 2008. 2008 Ins Insulated lated b bushings shings for alternating voltages above 1 000 V Cantilever Load requirements: „ 300 N (67.4 lb) for Ir ≤800 A „ 1 000 N ((224.8 lb)) for Ir >800 A

„ IEEE 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 the insulating medium specified „ Ap pressure equal q to 1 bar ± 0,1 , bar (14.7 ( psig p g ± 1.47 psig), above the maximum operating pressure shall be applied inside the bushing „ Load shall be applied perpendicular to the axis of the bushing at the mid-point of the terminal for 60 s „ Bushing gp passes if there is no evidence of damage g (deformation, rupture or leakage) and if it has withstood a repetition of all routine tests without significant change from previous results

Force Applied pp for Cantilever Test Cantile er Load = Torq Cantilever Torque e Applied Cantilever Load = Force * Distance

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

Distance

Force

Distance in/ft

Load lb

Torque ft-lb

6 / 0.5

224.8

112.4

8 / 0.75

224.8

168.6

10 / 0.833

224.8

187.2

Standards on LV Bushing g Cantilever „ Most LV Bushing Manufacturers specify the cantilever torque used on their certification testing „ The following g is a sample p of the cantilever torque q values that are published for LV Bushings: Stud diam. (in)

Max. Current (Amp)

Bush. Torque ft-lb

Max. load distance in / ft

5/8”

830

250

13.34 / 1.11

1”

1500

280 to 375

14.9 / 1.24 (*)

1 ¼”

2200

320

17.1 / 1.42

(*) Worst case

Standards on LV Bushing g Cantilever Conclusions: „ No current IEEE Std. Covers LV Bushing requirements „ IEC 60137 Standard provides cantilever requirements for LV Bushings „ Most LV Bushing manufacturers provide cantilever torque test values that would meet IEC 60137 requirements q for std. dimensions of bushing-terminal assemblies „ Concerns would raise on larger terminals that would cause torque distances above 12” 12 , where mechanical support may be required „ Findings may suggest to review any reported issues, to determine whether a LV bushing standardization is needed