Protective means for distribution transformer

Claim 2

In a distribution transformer for use on high voltage power distribution system of high fault current capability and having a core and coil assembly including a magnetic core linked by primary and secondary transformer windings immersed in a dielectric insulating fluid within a casing closed by a metallic cover having an aperture therein, an elongated primary insulating bushing having an axial opening mounted on said cover and extending through said aperture, an elongated current limiting fuse having a fusible element and terminals at its upper and lower ends disposed within said axial opening so that said lower lower end carrying said terminal depends below said bushing, a protective link immersed in said dielectric fluid and connected in series with said current limiting fuse and said primary transformer winding, and a conductive line shield coating on the inner surface of said bushing forming said axial opening for reducing the voltage gradient around said fusible element to said metallic cover, the clearance between said conductive line shield and said lower terminal of said fuse being sufficient to prevent flashover therebetween immediately after said fuse has ruptured and when the voltage of said power system subsequently appears across said ruptured fuse.

Claim 3

In a distribution transformer for use on high voltage power distribution system of high fault current capability and having a core and coil assembly including a magnetic core linked by primary and secondary transformer windings immersed in a dielectric insulating fluid within a casing closed by a metallic cover having an aperture therein, an elongated primary insulating bushing having an axial opening mounted on said cover and extending through said aperture, an elongated current limiting fuse having a fusible element and terminals at its upper and lower ends disposed within said axial opening so that said lower end carrying said lower terminal depends below said bushing, a protective link immersed in said dielectric fluid and connected in series with said current limiting fuse and said primary transformer winding, and a conductive line shield coating on the inner surface of said bushing forming said axial opening for reducing the voltage gradient around said fusible element to said metallic cover, the clearance between said conductive line shield and said lower terminal of said fuse being sufficient to prevent flashover therebetween immediately after said fuse has ruptured and when the voltage of said power system subsequently appears across said ruptured fuse, the lowest portion of said inner surface of said bushing defining said axial opening not being covered by said line shield conductive coating and the clearance between said line shield conductive coating and said lower terminal being sufficient to provide high enough dielectric strength to prevent restrike of said fuse or flashing from said lower terminal to said line shield by the transient switching surge voltage occurring at the instant of interruption.

Claim 4

In a distribution transformer for use on high voltage power distribution system of high fault current capability and having a core and coil assembly including a magnetic core linked by primary and secondary transformer windings immersed in a dielectric insulating fluid within a casing closed by a metallic cover having an aperture therein, an elongated primary insulating bushing having an axial opening mounted on said cover and extending through said aperture, an elongated current limiting fuse having a fusible element and terminals at its upper and lower ends disposed within said axial opening so that said lower end carrying said lower terminal depends below said bushing, a protective link immersed in said dielectric fluid and connected in series with said current limiting fuse and said primary transformer winding, and a conductive line shield coating on the inner surface of said bushing forming said axial opening for reducing the voltage gradient around said fusible element to said metallic cover, the clearance between said conductive line shield and said lower terminal of said fuse being sufficient to prevent flashover therebetween immediately after said fuse has ruptured and when the voltage of said power system subsequently appears across said ruptured fuse, the lowest portion of said inner surface of said bushing defining said axial opening not being covered by said line shield conductive coating and the clearance between said line shield conductive coating and said lower terminal being sufficient to provide high enough dielectric strength to prevent restrike of said fuse or flashing from said lower terminal to said line shield by the transient switching surge voltage occurring at the instant of interruption.

Claim 5

In the combination defined by claim 2 and including metallic means engaging the periphery of said bushing adjacent said cover for clamping said bushing to said cover, and a conductive ground shield coating on the outer periphery of said bushing extending above and below said clamping means.

Claim 6

In the combination defined by claim 2 wherein said current limiting fuse is coordinated with said protective link so that said protective link will rupture before said fuse on currents in the lower portion of the primary fault current range.

Claim 7

In the combination defined by claim 4 and including a low voltage circuit breaker immersed in said insulating fluid within said casing and having a pair of contacts and a bimetallic element in series with said secondary transformer winding and being adapted to open said pair of contacts in response to flow of secondary fault current through said bimetallic element and also in response to heating of said bimetallic element to a predetermined temperature by said insulating fluid, and wherein said protective link is coordinated with said circuit breaker so that said circuit breaker opens said pair of contacts before said protective link melts in the lower range of fault currents to which said transformer is subjected.

Claim 8

In the combination defined by claim 5 wherein said primary bushing has a terminal electrically connected to said upper terminal of said current limiting fuse and also including a lightning arrester mounted on the exterior of said casing and having one terminal grounded and the other terminal thereof electrically coupled to said primary bushing terminal and wherein said lightning arrester is coordinated wiTh said current limiting fuse so that it does not sparkover on the switching surge transient voltage developed across said fuse when it operates.

Claim 9

In the combination defined by claim 2 wherein the phase-to-ground voltage of said power system is approximately 19.9 kilovolts and said current limiting fuse is rated to interrupt up to approximately 40,000 amperes at this voltage and the clearance between said line shield conductive coating and said lower terminal of said current limiting fuse is approximately 5 inches.

Claim 10

In the combination defined by claim 2 wherein the phase-to-ground voltage of said power system is approximately 14.4 kilovolts and said current limiting fuse is rated to interrupt up to approximately 40,000 amperes at this voltage and the electrical clearance between said line shield conductive coating and said lower terminal of said current limiting fuse is approximately 4 inches.

Claim 11

In the combination defined by claim 1 wherein said line shield conductive coating extends down to the lower end of said inner surface of said bushing defining said axial opening and the clearance between said line shield conductive coating and said lower terminal on said fuse is sufficient to prevent restrike of said fuse or flashing from said lower terminal to said line shield by the transient switching surge voltage occurring at the instant of fuse interruption.

Claim 12

In the combination defined by claim 9 and including metallic means engaging the periphery of said bushing adjacent said cover for clamping said bushing to said cover, and a conductive ground shield coating on the outer periphery of said bushing extending above and below said clamping means.