Isolating 69-138kV potential instrument transformers

[yms]

What is a common engineering practice of fusing or isolating 69-138kV potential instrument transformers connected directly to the substation bus in order to Doble test or perform other required maintenance without de-energizing the bus?

 

[rcw retired EE]

In my experience, the VT's are treated as a part of the bus or line with no high voltage disconnect switch or fuse. It is difficult to find a fuse at that voltage that will protect the VT. (Not that it is any easier to find a fuse at lower voltages.)

To Doble test the VT, the bus should be dead and the connections unbolted to isolate the VT.

A disconnect switch would be useful for maintenance, but it adds another failure point and requires some room.

 

[scottf]

I agree with rcwilson.

VTs at 69 kV and above are generally not provided with isolation on the primary side.

Also, tryinfto Doble test a 138 kV VT with the line energized isn't really a good idea, from a safety and testing perspective.

Because of the capacitive nature of the grading in oil/paper insulated VTs, the line being energized will affect the Doble readings, even if the VT is isolated from the line.

Easiest way to perform Doble testing with the line deenrgized is just to simple unbolt the primary terminal connection. Normally for VTs, the conductor size is small enough to make removal and replacement fairly simple.