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Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
The company i work for is designing a 20KV test stand for transmission line dead-brakes. After looking at the stand today i happened to notice something! We do have an electrical engineer on the project but i think he may have missed something. The stand is made out of an aluminum frame, holds a steel tank that has dielectric oil in it. The tank houses a smaller tank that is driven by an pneumatic cylinder. When the cylinder extends, it forces the smaller tank into the larger one, in turn, raising the oil level in the larger tank. This is how the oil level is raised above the dead-brakes during the test and lowered below them after the test is complete. All of the control electronics are isolated (or insulated) from the stand itself. The stand is sitting on casters with rubber tires. Here is where i have to ask the question!! Should the tank and frame have a dedicated ground connection, earth, but not at the same point where the control electronics are grounded? The reason i raise the question is that everything on the top of this system is rounded off to avoid corona. Should the steel tank be treated like the topload on a large tesla coil? Could the tank hold residual charge that should be bleed off to earth before the operator goes to change out the dead-brakes? The operator does this by hand. Sorry for the novel! Any input would be greatly appreciated!!! Thanks in advance guys, i know this is the place to ask! Mike..
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Every piece of metalwork in a setup like this that isn't connected to high voltage should be solidly connected to ground. It's a safety issue, as floating metalwork can pick up a large voltage by capacitive coupling.
It's also a measurement issue, if there is any kind of partial discharge detector in the apparatus then any floating metalwork may arc to ground and give false readings, causing parts to be rejected as bad when they're actually OK.
The setup must have some sort of high voltage power supply that plugs into the wall. Even though it sits on rubber wheels, it may be grounded to the wall through the wires that go to the power supply. This would be OK for safety purposes, but they should still make sure that all the metal parts are connected to the ground. Another big hefty, clearly visible green wire to back up the power supply's own would be nice.
Grounding of the control electronics: If you get it wrong they will go crazy or get trashed if the thing ever arcs over. It's a problem for whoever designed those electronics.
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
Thanks Steve! Thats pretty much what i had thought! I did talk to the engineer about it today and he kind of surprised me! There was a frame ground, inside the frame on the bottom where you couldnt easily see it. He told me that that was the ground for the HV. He then told me that the tank and the inner tank were not grounded! The reason i guess, according to him, was that grounding the tank would increase the tolerance between the parts too much to allow the machine to fit in the area the customer has to place it! I guess the tank is also sitting on an insulating pad but you cant see it. Sure surprised me to learn that! According to the customer, there will be a separate control circuit mounted in the wall near the stand that closes a contactor that will short the tank to ground after the test is complete. Wowwie! I appreciate the input Steve!! Thanks for the reply, thought i would post the details after the engineer showed up. He is never in the building.
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20KV test station, Need input!
