Bushing Earthing Cap in Power Transformer

Hi bluestone

Thank u very much.I got it.

1)what is the type of test tap used in 230KV and 400KV bushings?

2)Is this 5KV enough to burst the bushing or spoil the insulation ?

KInd regards

gova

230 and 400 kV bushings will normally have a potential tap (or capacitance tap). They will typically use small porcelain bushings with a spring-loaded internal contact to the capacitance tap condenser foil layer. They will also probably have a filler hole in the upper surface of the housing, sealed with a removable pipe plug for insertion of insulating compound in the cavity around the bushing when the tap is to be used with a cable connection to a potential device.

This arrangement provides a useful source of potential for operation of instruments, relays and other devices that do not require metered power. Another advantage is that it offers a practically constant voltage ratio and phase relationship to the high voltage transmission system.

Continuous open circuit operation with the grounding cap removed could cause flashover and damage to the outer portion of the tap bushing.

Hi bluestone

Thank u very much.

And I wishes all our members to have peace and happiest New year 2009.

Kind regards

gova

Hi

I could not understand the concept actually.

How does the voltage builds up to burst the bushing ??

Could you please help me to understand?

The paper/foil layers in a HV bushing form a capacitor voltage divider that controls the voltage gradient from the center conductor (HV side) to the mounting flange (grounded side). The capacitance tap is connected to one of the last outer capacitor layers, so that the voltage is not much above ground (as mentioned in other posts, up to 5kV).

Those last layers are only used to test the insulation of the bushing by measuring the capacitance, or to provide a known lower voltage ratio from the HV conductor for metering, protection, etc. If not grounded the voltage can rise to levels that could flash over or puncture the capacitor insulation. Damage to any capacitor layer in the bushing puts electrical stress on the other layers due to the change in voltage across the dividers of the layers. See that links below for some diagrams and additional info:

Fundamentals of Condenser Bushings

Testing & Maintenance of HV Bushings