Cantilever Test Load Specifications & Considerations for Limiting Cantilever Loads During Service on Transformer Bushings.

Cantilever Loads on Bushings 

Several factors induce cantilever loads on the transformer bushing when it is in service: 

• Conductor connections: Overhead lines and rigid busbars can exert significant forces on bushings. 

• Environmental and operational factors: Wind forces, ice accumulation, seismic events or short-circuit forces. 

Cantilever Test Load Specifications 

Cantilever test is specified as a Type Test to verify the withstand capability of the bushings. 

• IEC 60137:2017 specifies minimum withstand values of cantilever load as level I (normal load) and level II (heavy load) for different rated currents and for different highest voltage of the bushing. 

• Also given in IEC 60137 are the "Cantilever Loads" which have to be limited during operation.

Highest voltage for equipment  Um kV	Rated current (A)
	≤ 800		1000		2000		≥ 3150
			1600		2500
	Cantilever Test load (N)
≤ 36	1000	1000	1250	1250	2000	2000	3150	3150
52	1000	1600	1250	1600	2000	2500	3150	3150
72.5 to 100	1000	2000	1250	2000	2000	3150	4000	4000
123 to 145	1250	3150	1600	3150	2500	4000	4000	4000
170 to 245	1250	4000	1600	4000	2500	5000	4000	5000
≥ 300	2500	4000	2500	4000	3150	5000	5000	5000
≤ 36	1000	1000	1250	1250	2000	2000	3150	3150
	Cantilever Operating load (N)
	Bushing installed ≤ 30° from the vertical
	I	II	I	II	I	II	I	II
≤ 36	500	500	625	625	1000	1000	1575	1575
52	500	800	625	800	1000	1250	1575	1575
72.5 to 100	500	1000	625	1000	1000	1575	2000	2000
123 to 145	625	1575	800	1575	1250	2000	2000	2000
170 to 245	625	2000	800	2000	1250	2500	2000	2000
≥ 300	1250	2000	1250	2000	1575	2500	2500	2000

• Cantilever test method: The load shall be applied perpendicular to the axis of the bushing at the mid-point of the air-end HV terminal for 60 seconds.

• Acceptance criteria: No evidence of damage (deformation, rupture or leakage) and must withstand a repetition of all routine tests (as per IEC 60137:2017) without significant change before the cantilever load test.

Considerations for Limiting Cantilever Operating Loads During Service on Transformer Bushings

To minimize the risk of bushing failure due to excessive cantilever loads during service, the following points need to be taken care of:

• Use of flexible connectors to reduce stress from differential thermal expansion.

• Connection between bushing and switchyard: Using flexible connections, like rope conductor / ACSR conductor / AAC conductor helps prevent these issues.

Back