Power Transformers

[foxhoundme]

Can core steel burr levels cause localized hot spots on power transformers? The burrs are on the edges of the core steel lamination what is the best tool to use to read the burr level? Please acknowledge.

 

[Skogsgurra]

Yes, shorting plates is a good way of creating local heat. Grinders are used, perpendicular to the laminations, to break the current paths.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[prc]

Earlier days(25 years back),core laminations were cut using high carbon steel cutters. It resulted very high burr heights of the order >50 microns on the cut edges. In service these can cause an inter laminar short between sheets leading to hot spots in core. Hence each lamination used to be ground at edges by sending sheets through edge grinders. This operation resulted in excess stress in sheet causing additional core losses. To avoid this laminations used to be annealed again at 800 C.

Today cutting is done using silicon carbide cutters giving burr of the order of 10-20 microns. This requires no more grinding and annealing.

Burr height is measured in accordance with IEC 60404-9 or using handheld micrometers.

 

[foxhoundme]

Thank you Skogsgurra, so if I have these "shorting plates" as you have called them are you saying I could use grinders perpendicular to the lamination to break these current paths?

The individual laminations are M-6 (0.014in)(0.35mm) thick (total stack build is 4in or 102mm), I can feel the prickliness of the burrs and I am looking for a plausible solution to resolve these burrs. What size grinder would you suggest I use to smooth out these burrs? Do you have a specific tool known that would solve this issue? After grinding the surface will I need to apply a coating to protect the expose steel? Will I create more damage than good using a grinder to smooth out the burrs?

If this works I will test the core and conduct an excitation test to check the health of the core electrically and mechanically. I am hoping the grinding process will work so that I can avoid tearing down the unit and replacing core laminations.

Thank you for your responses and I ask others to please provide comments on this subject.

 

[waross]

If this is an oil filled transformer, it may be impossible to grind the laminations in situ without leaving behind some conductive contamination that will contaminate the oil.
I once witnessed a dirty wrench used to change the connections in some oil filled transformers. Two transformers failed within a few hours due to contaminated oil.
What capacity and voltage is the transformer?
New or in service?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[foxhoundme]

Hi Bill,

The unit is new and has the following parameters:
15MVA oil filled unit, HV = 24940V GrdY, 125kV BIL; LV = 4160V GrdY, 60kV BIL
STD. Taps +/-2 x 2.5%

I will remove unit from tank and dismantle.


James

 

[Skogsgurra]

"shorting plates" does not men plates that short something: I meant "to short plates" i.e. shorting them.

If these transformers are brand new, shouldn't you talk to the manufacturer before doing anything at all? And also make very certain that the burr is the cause of any problem you may have. It may turn out to be very costly if you do anything without involving the manufacturer.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Let's start over and check our facts and assumptions.
The transformer has hot spots.
The transformer has burrs.
The hot spots may not be caused by the burrs.
First, what is the nature of the hot spots? How has it been determined that the transformer has hot spots?
Burrs will cause hot spots when they make electrical connection between adjacent laminations in more than one location. that is usually on opposite edges of the laminations.
The highest resistance in the eddy current circuit will generally be through the burrs. However, the burrs have the best chance of dissipating the heat produced by eddy currents. The area of highest temperature may not be the area of highest heat generation.
If the laminations have been stressed in the manufacturing process and not properly annealed then hot spots may arise due to excess hysteresis.
The burrs may be causing hot spots.
HOWEVER the burrs may be an indication that the steps of de-burring and annealing were skipped in the manufacturing process.
You may try checking for continuity between adjacent lamination plates with an ohm meter. There should be no continuity however this is not a conclusive test.
No continuity is good.
Continuity between plates does not mean that the stack is bad.
Laminations may be shorted together in one location without developing hot spots. They must be shorted in such a way as to form a circuit for eddy currents to flow.


Bill
--------------------
"Why not the best?"
Jimmy Carter