Corona Shield in transformer dry type termination

[ravindranathan]

Why do we need corona shield in a EHV transformer dry type termination?The same is not applicable in a GIS dry type termination.

 

[cuky2000]

Grading rings help to reduce the potential gradient along the insulator/bushing. The potential gradient (electric field) across an insulator is not uniform, but is highest at the end next to the high voltage electrode. See in the following Link indicating the effect of the grading ring particularly at the end of the insulator/bushing.
_{It should be noted that porcelain insulators are less sensitive to fail by corona or flashover than the polymer type insulators. However, weight and fragility in heavy seismic zones or hight polluted environment, favour the use of polymer insulator.}

For EHV (>345 kV) application, most of the SF6 to OH air termination bushing that I see are provided with some type of grading ring as shown in the following link₁ & Link₂

 

[ravindranathan]

Hello Cuky2000,

Many thanks for the input and the links.They are absolutely helpful.
I was referring to cable being terminated to GIS(220kV) inside a indoor substation.Why those don't have corona shield.

 

[cuky2000]

Probably you are referring to "Plug-In Termination for GIS" similar to the one shown in the following Link
The electric field stress of Plug-In does not need a corona ring because space constrains and the termination is designed with an Integrated prefabricated stress control system made with a solid dielectric polymer such as silicon rubber taking into consideration the cable capacitance and the shield created by the metallic cover and other components.
Notice that the plug-in connector could be used for GIS and transformer equipped with cable connectors.

Often, transformer bushings are directly exposed to air connected to an underground or overhead cable. In this case, the electric stresses need to be controlled either with extra-large bushings or graded corona ring similar to the following Link

 

[prc]

Corona starting voltage gradient is low in air compared to SF-6. Hence in SF-6 instead of corona ring, rounding of live electrodes and avoiding sharp edges may serve the same purpose as a corona ring.In oil to air EHV Transformer bushings also, the corona ring is avoided by removing sharp edges and corners.