Novel Approach To Medium Voltage Recloser Insulation Design

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This article has been extracted from a technical paper entitled "Novel Approach For Insulating Medium Voltage Reclosers" by Dr Alexey Chaly and Sergey Benzoruk of Tavrida Electric.

Modern pole top, medium voltage, switchgear is generally either SF6 or solidly insulated. Both systems have advantages and disadvantages. SF6 provides more design flexibility particularly when designing dead tank type switchgear. Dead tanks are attractive for utilities because they allow voltage sensing to be incorporated into all six bushings.

In recent years the use of SF6 in switchgear design has become the subject of serious environmental concern. In particular, SF6 insulated switchgear requires special handling and disposal methods which creates additional expenses for utilities when considered from a total lifecycle standpoint. It is also necessary to continually monitor SF6 levels within the device. This requirement contradicts the modern approach of reducing service costs by designing maintenance free switchgear. Finally, SF6 switchgear must have a tank design capable of withstanding pressure differences caused by temperature variations. This can substantially increase the weight of switchgear.

Solidly insulated reclosers were developed during the 1990s as a competitive response to the problems affecting SF6 switchgear. These reclosers are based upon the application of epoxy material with good track resistance, being used simultaneously as both a mechanical support structure as well as insulation. This method avoids the disadvantages related to SF6 switchgear.

One problem related to this approach is the limited design flexibility. In particular, implementing voltage sensing from both recloser sides becomes impossible. This shortcoming creates extra expense for customers using this equipment.

Another problem is related to a lower track erosion resistance when compared with dead tank designs utilising bushings covered with silicon rubber. This feature prohibits the use of solidly insulated switchgear in areas where high pollution is an issue.

Finally, the solid dielectric material is subjected to permanent electrical stresses that may cause ageing damage if the insulating material has minor porous imperfections after manufacturing. The manufacturing technology must be very precise and stable to avoid electrical puncture problems during the product's lifetime.

Tavrida Electric has spent considerable effort developing an insulation system that combines the advantages of SF6 and solid insulation systems, while simultaneously avoiding their shortcomings. The solution uses a combined insulation system housed inside a tank. This allows the use of voltage sensing on all six bushings without the need for heavy tank designs.

Another feature is the low level of electrical stresses the insulation is subjected to. This reduces the ageing effect on the insulation medium leading to an increased reliability and long lifetime. To eliminate porous imperfections during manufacture, Tavrida uses high pressure injection moulding for polymer components and hot pressed silicone rubber for joints and external insulation.

To get an idea of the OSM recloser's combined insulation, imagine a dielectric barrier between all live parts and between live parts and dead tank, extended and shaped in a way to avoid flashover. This insulation system is protected by international patent No 402904.

Although at first glance this system seems simple, it has consumed a lot of R&D effort by Tavrida Electric scientists and engineers, simulating electrical fields and stresses in the air, solid insulation and rubber joints, and optimising the insulation design to achieve the highest reliability.

The end result is a novel combined insulation system for medium voltage reclosers. It presents an effective alternative to SF6 and solidly insulated switchgear designs. The positive aspects of both systems are combined to provide the design flexibility offered by SF6 insulated switchgear with the environmental advantages of solidly insulated switchgear while eliminating their shortcomings.