Electrical Flash over in 415V LV Switchgears 2

[raj151857]

Following document is collective information regarding flashover occurred in Power Control Centre Panel.
Brief of PCC Flashover

No External element was inside the panels.
1] Both Incomer (3200 A) were charged,Buscoupler off & in service.
2] Feeder 3FA 220KW Motor feeder
3] Feeder 3FB Spare 800A Outgoing feeder (ACB Rack Out & No Cable connected on links).Both Feeder 3FA & 3FB are in same vertical Panel.
4] Insulation Value was checked at Motor feeder & found OK (>500 Mega ohms)
5] 220KW Motor was started & flashover Occurred.

Observation after Flashover

A] No Damage was found on Motor feeder.
1] Most affected Part is Contacts of Bottom feeder 3FB connected to vertical bus dropper located on back (Incoming contact of ACB) & Outgoing links connected to Outgoing links. Pics cannot be attached.I can send by mail on request.
2] Entire busbar on Bottom feeder 3FB got blackened & carbonized.
3] Fault was cleared by Motor feeder protection 50/2.
4] CT Ratio 400:1 I>> setting 8.97 A Time 0.01 sec.
5] Motor started from LCS Starting current was 2.16KA, 2.05 KA, 2.16 KA. For 283 milli second after that current in R & Y phase rise to 11.7 KA & tripped on 50/2 protection.
6] No voltage dip was observed.
7] Both the feeders were covered with shrouds, so No possibility of foreign material fall on 3FB feeder.
8] Motor was meggered after flash over and found ok.


I want to know the reason why Flashover occured in bottom spare non energised feeder.

 

[rbulsara]

Assuming that you have discounted the obvious, the follwing are only conjectures, as this type of things are nearly impossible to diagnose without being at the site.

1. Electromechanical forces due to high starting current could make the conductors swing considerably, if not properly braced. The moving cables could have caused the loose "links" to move around too close to other phase terminal to cause a flash over.

2. Less likely but worth mentioning, flash over could occur due to overvoltages or rapid break down of insulation. If this motor or PCC is connected to power factor correction capacitors. I would look into possiblity of ferroresonance that could cause severe overvoltages. Look for the signs of any gouged metal on the bus bar or enclosure. I say less likely as this would have affected the motor feeder terminals more than the other. Unless the motor feeder terminals are too close to the PCC bus.

 

[Zogzog]

I think something is being missed by the testing, you mentioned the motor was meggered, but was the switchgear meggered? Did everything get disconnected? Was phase to ohase checked along with phase to ground? (Cant megger the motor feeder phase to phsae without disconnecting the motor.)

 

[alehman]

I'd say rbulsara's #1 is most likely.

A photo might be very helpful. You can upload an image file by clicking on the "upload your file" link right of the attachment box.

 

[burnt2x]

At 415 Volts, I can't imagine phase-phase flashover just by being "close"! Except when the conductors snapped from bracings and "touched". The large starting current should have initiated the conductors to jump out of their respective braces and short,IMO.
I agree with alehman, pictures speak a thousand words!

 

[raj151857]

Thanks Sir,
I would like to mention that Switchgear was meggered that why both Incomers were charged before the motor starting.No tracing mark was found,No damage found on Motor feeder.

Please find pics of bottom spare feeder ( ACB was not in service,No DC control supply was ON & no wire were connected).

 

[raj151857]

PICS

 

[raj151857]

PIC1 bottom feeder

 

[raj151857]

pic2

 

[alehman]

Based on the unburned areas, it appears there were cables terminated on the burned bus bars. Were these bars from the "spare" breaker? What were the cables for? Do you have a "before" photo of the burned bus bars? Were there burn marks on the enclosure?

 

[burnt2x]

"no voltage dip was observed"

This is a sign that what happened was not a "bolted"fault. Something must have bridged the bus bars, with some ohmmic value and developed into a flashover when it broke apart! Did you inspect the carbonized materials? Any sign of stray animal parts?

 

[raj151857]

Sir,
Please find pics before the flashover. Yes the pics is from spare feeders,Secondly as per burnt2x,we had checked & analysed the carbonised materials there is no sign of any animal part .

 

[raj151857]

Am still waiting for some clue/hint, Any Experts comments.

 

[Elmir]

What were the events that took place before the fault occured? Was there scheduled maintenance performed?

You reported an insulation resistance reading of >500MOhm, what test voltage did you use? In my experience I have seen situations where the megger will not expose a fault in insulation until a greater test voltage is applied.

 

[waross]

I had a similar flashover years ago. The priority was to restore service ASAP, not analyze the fault. We worked through the night and got the MCC back on line with minutes to spare.
Our conclusions where:
First event, fine sawdust built up on the porcelain insulators that supported the vertical bus bars in each MCC section. It was normally a dry climate but the night was more humid than usual. We think that the sawdust absorbed moisture and started a flashover.
Second event, We pulled an MCC cell out and inspected the bus bars and insulators. The damage was below the stabs of the lowest cell. The MCC meggered good. By this time the utility had refused the primary and we re energized. All was well. The operator commenced to start his motors. There were some large contactors that shook the MCC a little when they closed. Suddenly there was an explosion and the lights when out. It was a great light show until the fuses finally cleared. The second arc burned through one bus bar and then jumped above the next insulator and burned through another bus bar. The fuses cleared before the arc climbed high enough to damage the top mounted horizontal bus bars.
We think that the first arc left a splatter of molten aluminum on the back of one of the cells. We believe that the shock of the contactor(s) closing jarred it loose and it fell across the buses.
We had to remove the bus bars with a hack saw. We fed each cell in that section with cable drops from the main horizontal bus.
Will either of these scenarios fit your event?

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