burned motor bushing 1

[Emadshaaban1987]

Hi Folks
4 electric motors,each is driving a centrifugal pump in an oil field . motor size 150kw /0.4KV /star (see picture) .we have an frequent issue with each of them in which,the cable terminal at down side of motor bushing often get melted and Leeds the motors to trip by earth fault or phase over current protection .Although, The bushing were replace by a very good quality types ,torque wrench used to ensure correct tightening force ,the problem kept to appear time to time in each motor
the questions are :
what is the main cause ? weather starting currents or bushing qualities ?
What is the best set point of current unbalance protection (the actual relay setting 20%In )

https://files.engineering.com/getfi...d1d-43bc-a957-97baa13866ad&file=IMG_3318.jpeg

https://files.engineering.com/getfi...4dcf-9918-716154cf77ac&file=IMG_3319_(1).jpeg

 

[waross]

Are these motors on VFDs?
If Yes, do you have adequate filtering?
Are these motors DOL?
If Yes, do you have lightning arrestors on the incoming primaries?
If Yes, do you have surge arrestors on the service or on the motor circuits?
If Yes, are any of your arrestors faulty?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Emadshaaban1987]

Thank you Bill.
This motor is a direct online start.
There are no lightening arrests, the climate in North Africa is very clear most of the year .

 

[TugboatEng]

That's interesting to see the bushings in the open like that. In my experience the bushings are potted on epoxy. Bill mentioned lightning doing damage. Dry conditions can absolutely cause very high static electricity charges and even lead to a dry lightning phenomenon so a lack of cloud cover will actually exacerbate your situation.

Way above pay grade so I'm not suggesting this as a cause but I found this article and it mentions mechanical stress can degrade dielectric strength. I assume that when using the term charging they're talking voltages much higher than were used to. Electricians love to run oversized wires which puts a lot of strain on connections and without potting that can put a lot of mechanical stress on insulation.

https://www.google.com/url?sa=t&sou...FjALegQIHxAB&usg=AOvVaw2GUQyiAbRvUmz4MaBu7d5c

I do suggest doing a power condition study using a recorder that can detect transients of very short duration.

 

[RRaghunath]

I too think you should first rule out the possibility of the cable putting strain on the bushing. The problem could be inadequate external support to the cable.
This will most likely be the case if the same phase bushing is failing repeatedly.
It could also be that the cable box is not sized for that cable size and hence not enough bending radius etc.
I suppose the Power contactor is not vacuum type. If it is Vacuum, switching overvoltage could be a reason and this could be mitigated by installing Surge Arresters.