Core Ground Bushings on Top of Transformer?
Core Ground Bushings on Top of Transformer?
(OP)
•I came across these bushings on a Delta/Wye 2 winding transformer recently. They are separate from the H1-H3 and X0-X3 bushings. Are these core grounds? Is there any reason why there are two brought out to the top of the tank? One was grounded and one was ungrounded as shown.
http://i.imgur.com/EfJViYT.jpg
http://i.imgur.com/4bSOqUF.jpg
•Also, when shorting low side bushings for various tests (jumpered X1-X2-X3), it is often recommended that an additional jumper is included from X3 back to X1. Is there a reason for this? Is it just for redundancy?
•When performing TTR, winding resistance, and bushing CT tests on three phase Dyn two-winding and auto transformers, is it recommended to leave the neutral bushing connected to ground or left floating?
Thank you.
http://i.imgur.com/EfJViYT.jpg
http://i.imgur.com/4bSOqUF.jpg
•Also, when shorting low side bushings for various tests (jumpered X1-X2-X3), it is often recommended that an additional jumper is included from X3 back to X1. Is there a reason for this? Is it just for redundancy?
•When performing TTR, winding resistance, and bushing CT tests on three phase Dyn two-winding and auto transformers, is it recommended to leave the neutral bushing connected to ground or left floating?
Thank you.






RE: Core Ground Bushings on Top of Transformer?
I dont think X1-X3 shorting is additionally required.
RE: Core Ground Bushings on Top of Transformer?
RE: Core Ground Bushings on Top of Transformer?
With the exception of the LV Excitation Test, I can't think of any other standard tests (TTR, Megger, Power Factor) that require X0 to be grounded during the test.
RE: Core Ground Bushings on Top of Transformer?
Any Core ground or Frame (clamp) ground that is brought out should go to earth.
If it was left open due to an inadvertent ground then that bushing should still go to ground thru a large resistor because if the inadvertent ground clears the Core or Frame will be floating.
I would also take a close look at the DGA of that transformer to see if an issue exists.
2. The jumper from X3 back to X1 just ensures you have all three phases properly shorted together.
Think of it as creating a "Delta" connection out of the Wye terminals vs. a "Open Delta" connection with just shorting X1-X2-X3.
3. I agree with DTR2011. LV excitation on a Dyn is the only test I can think of where the X0 should be grounded. Be sure to state "X0 Grounded" in the notes section of that test.
The LV lead will be on the H0X0 of an autotransformer during LV excitation.
When testing winding resistance, CTs and TTR the X0 will often be in the test circuit or floating when not.
RE: Core Ground Bushings on Top of Transformer?
Section 7.1
http://www.testguy.net/library.php?viewdoc=2981153...
Since a wye winding with neutral will have all measurements taken between the phase terminals of each winding and neutral, I assumed the neutral ground should stay put to prevent it from floating to a high voltage.