Temperature of LV transformer terminal connection is high 1

[JonoJakarta]

Dear All,

I have case in our transformer 20/0.4 kV,2500 kVA, 2002 year, DYn5, Ip, when we did IR thermograph today, 19 July 2022, we found temperature of S phase is 99 deg.C
This case is repeateable case before on April 20, 2022. At that time, temperature is 102.5 deg C at same location (S phase terminal). We are tightening on 20 April 2022 with 70 Nm.
As information, as per data on PM5330 display, the pf is leading. I attached some potos here.

My question:
1. What is caused the temperature become high?
2. What is caused the bolt is loose?
3. Is this related with pf leading?
4. is it right apply 70 Nm on that bolt?

Your prompt reply will be highly apprecisated.

Best Reagards,

Jono

 

[TugboatEng]

Is the bolt not tight now? If it is tight, making it tighter isn't going to make all that much difference. Is 70Nm ok? I don't know, you haven't given us any information about the fastener.

If the bolt is right, the wire size/lugs sufficiently large for the current, I would look at cleaning the contact surfaces between lugs.

 

[stevenal]

2300 A split four ways? The heating is by (I^2)Rt, and improving the pf would reduce the current some. I would recommend the use of two hole pads, going back to back on the 4 hole pad. Assuming aluminum, make sure you remove any surface oxide, and use a non-gritted anti-oxidant compound. Also use good Belville washers.

 

[che12345]

Dear Mr. JonoJakarta (Electrical)(OP)19 Jul 22 06:13
"..#A I have case in our transformer.... when we did IR thermograph today, 19 July 2022, we found temperature of S phase is 99 deg.C. This case is repeateable case before on April 20, 2022. At that time, temperature is 102.5 deg C at same location (S phase terminal)....
My question:
1. What is caused the temperature become high?
2. What is caused the bolt is loose?
3. Is this related with pf leading?
4. is it right apply 70 Nm on that bolt?..."
#A. I noticed that all three-phases current are near balance but only S-phase terminal connection shows high temperature.. Conduct a contact resistance test (by using 4-point method) with current has high as possible >100A dc. Compare the contact resistance with the other cool (R,T) phases. Note: contact resistance is a direct measurement for i2 R heat generation across the contact. Tightening torque is an indirect useful and easily done alternative but no substitution to contact resistance test.
1. High contact resistance. Dirt, bur around the punch hole, thin and small diameter washers etc.
2. Loading in the day is usually much higher than at night. The heat generated is proportional to I2. Heating (expansion) and cooling (contraction) caused the bolt to loosen.
3. Not related to Leading pf. But why Leading pf? Wrong meter CT polarity?
4. As an indication: use grade 8.8 bolt and grad 8 nut: M8 (17Nm), M10 (28Nm), M12 (45Nm) , M16 (91Nm). If the bolt is M12, 70Nm is adequate. Use thick and big washers or Belville washers are strongly recemented.
Che Kuan Yau (Singapore)

 

[electricuwe]

For such connection I generally prefer Belleville washers:

https://en.wikipedia.org/wiki/Belleville_washer

As such kind of washer acts like a spring, it is more resilient against the thermal aging of the connection as described by che12345.

For further analysis some details on the cable and the used terminations would be helpful.

 

[EdStainless]

We never used steel hardware for high current connections.
We wanted something closer in thermal expansion to either Cu or Al.
We nearly always used Si bronze hardware.
Here is one guide

https://usbr.gov/power/data/fist/fist3_3/vol3-3.pdf

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[che12345]

Dear Mr. JonoJakarta (Electrical)
When I go through the photos again, I noticed in both TR-3 photos show the palm temperature are Green but the cable entry to the lug compression point are Red (same as the areas around the bolting holes). Another area to investigate, why Hot?
Che Kuan Yau (Singapore)