Testing of Iso-phase bus enclosure 2

[Alwaysexciter]

One of my customers who is a fairly large generating facility is have trouble with one phase of the LV side of his step up transformer (Showing hot with an IR scan).

This unit is under warranty so the manufacture is involved. We have performed several resistance tests of the connecting lead and windings but no obvious loose connection found. An internal visual inspection was made. Again, no obvious problem. The manufactures' engineer said that it could be due to circulating currents in the iso-phase bus and suggested that it be tested. The encloser of this iso-phase bus is grounded at many locations. Other similar units on site do not have this problem.

What could possible be wrong with a grounded iso-phase bus and what test should be conducted?

 

[jghrist]

I'm no expert, but I'd look at all of the joints in the bus enclosure. There are high circulating currents in the enclosure of iso-phase bus duct. The IR scan should pinpoint any hot spots in the enclosure, however, and make any joint resistance problem obvious.

 

[ScottyUK]

A phase-isolated busbar relies on image currents flowing in the shield or outer casing to reduce the external magnetic field to near-zero. To facilitate this, the three phases usually have massive bonding bars connecting them at their extreme ends. If you have multiple grounds, the image current will follow a path that designer probably didn't consider, and the magnetic field cancellation will be imperfect. This could lead to eddy current heating of the supporting structure.

Where is the hotspot you have identified?

When checking resistance, you will need a heavy current test set. T&R Instruments make one which delivers 200A; I think Programma make units up to 600A or so. Resistance checks with lower current units will be bordering on useless. What did you use?




-----------------------------------

Start each new day with a smile.

Get it over with.

 

[rentubs]

Perform dc withstand test but consult the manufacturer first for recommendation. Ensure the CT and PT are isolated, GCB open and the bus isolated from GSU. NETA MTS Table 10.17 gives you the maximum test voltages.
Insulation resist

 

[rentubs]

Perform insulation resistance test (megger)

 

[rentubs]

By the way there are heaters or dessicants on the iso-phase bus enclosures, ensure they are working. You may find water accumulation on several sections and check for the low point drain valve maybe they are plugged.

 

[electricpete]

There are very large currents within isophase with the potential to create large magnetic fields which create circulating currents and heating in adjacent equipment.

As Scotty mentioned, the duct creates a self-shielding effect which nearly cancels the fields along the lengths of the duct. However at the ends of the ducts the symmetry breaks down and there can be problems.

You mentioned heating at the transformer, apparently blamed on the bus.

The LV bushing enclosures are usually insulated from the bus. There is usually some kind of shorting/crossover bar at the ends of the enclosures between the three phases. This is one source of high current unshielded which can create circulating current in the transformer enclosure/tank.

More important to look at is the lv bushing enclosure itself ("doghouse"). Unlike the bus duct it is not grounded at both ends, and does not have as much symmetry. This creates a lot of potential for circulating current within the enclosure and possibly between enclosures via the tank. Often it is seen B phase is the hottest.

One problem we had is the enclosure has a gasket between it and the tank. This leaves only a relatively higher resistance path through the moutning hardware. The mounting hardware shows up hottest on IR for us and the whole area is hot raising concerns about extra thermal stress on LV bushings.

We developed but not implemented a solution. Insulating washers/sleeves on the lv bushing enclosure, and grounding strap between lv bushing enclosure and main tank, to control the path of that circulating current.

One thing I know many plants have fought this type of problem and it is usually not an easy or cookbook one to solve. Each installation needs to be considered since combination of different vendors of isophase and transformer results in many possible problems at the termination of the isophase. My recommendation is to hire a good consulant with experience in this area. We had good advice from Canadian GE.



=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

"The LV bushing enclosures are usually insulated from the bus".

Of course. What I meant to say was
"The LV bushing enclosures are usually insulated from the bus duct".

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Back to your original question: what tests.

Obviously it starts with a thermography and try to identify what is the path and source of the circulating current.

Magnetic field measurements may be helpful as one tool to identify the source.

I would do continuity check to make sure that components which are supposed to be insulated are in fact insulated. Particularly between isophase duct and transformer tank.

Can you describe more specifically where is the hot spot?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[jbartos]

Suggestion: Since only one phase exhibits the problem, the corrective focus should be on that phase installation workmanship, power quality that may reveal the phase voltage and current waveforms including harmonics, and other deficiencies that develop in time (e.g. due to vibrations or short circuits). Incidentally, how old is the bus installation?