VCB EXPLOSION

[ajlimuran]

To All Experts,

Specs of the VCB are below;
VD4 ABB - 17.5 KV/630 Amperes/ 25 KA

Our plant is diesel/bunker C power plant supply the small island. The unit is status is on going commissioning and it is synchronize to the grid.
The diesel generator rated 13.8 KV and 2.1 MW capacity connected to generator breaker and bus then, taking off to the feeder line to utility network.
It were happened 4 times explosions, below are the sequence of events;
1. First , the DG was synchronized and run 2-3 hours and switch gear explosion and DG tripped with the indication of 51V and 87G.
After explosion , my team checked and it was found out the the PT fuse blown and flash over evidence observe in the PT and VCB.
We neglect to check the internal of VCB and replaced it nee VCB and PT fuses.
2. Second , again the unit is synchronized and and run 2-3 hours and switch gear explosion and DG tripped with the indication of 51V and 87G.
After explosion , my team checked and it was found out the the PT fuse blown and flash over evidence observe in the PT and VCB.
We neglect to check the internal of VCB and replaced it nee VCB and new PT ans fuses again.
3. Third , again the unit is synchronized and and run 2-3 hours and switch gear explosion and DG tripped with the indication of 51V and 87G.
After explosion , my team checked and it was found out the the PT fuse blown and flash over evidence observe in the PT and VCB.
We check the VCB all replaced then we found out the the caused of flashover and switchgear explosion is from the VCB. And, why that the only damage and failed bottom contact of the VCB is only phase B among the 3 and Phase A & C is is not damages.

My questions now, is what would be the cause of series of failure of the VCB.
Before we instaled was the insulation resistance in the bottom is 200 Mega ohms respectively while the top terminal is in 3 giga ohms.

Please help.

 

[7anoter4]

The problem is not so simply. If the VCB in Switchgear is supplying an induction motor the capacity of a long -usually three-core three phases] cable -and the inductance of the circuit [the motor windings, mainly] could create a resonance circuit for a certain harmonics. These harmonics are produced in VCB due to reigniting at motor starting.
See [for instance]:
file:///C:/Users/win7/Downloads/302233.pdf
It is not exactly your case but it may draw an image.

 

[ajlimuran]

Sir the generator is connected directly to the grid.

 

[Marmite]

There are too many potential failure modes to hypothesise on the root cause. Lucky no one was killed. Not many people would be lucky enough to survive 4 attempts at blowing up a switchboard.
Regards
Marmite

 

[ajlimuran]

What would be the root cause of 3 time explosion of the VCB?

 

[Marmite]

Is this a withdrawable circuit breaker, ie rackable, or is it fixed pattern? The VD4 is available as either. Do you have any photos you could post?

Regards
Marmite

 

[ajlimuran]

Sir it is withdrawble VCB. Please see VCB insulating rod evidence of damage.

 

[collies99]

I guess there is never a good excuse from doing a proper commissioning job.

Have these failed breakers and the switchgear gone through the required AC dielectric tests ?

There should be a dielectric test voltage level for a 1 minute or 10 sec test recommended by the manufacturers of both the equipment, else you can use voltage levels at 75% of specified design test levels in IEC or IEEE, whichever standard it was manufactured to. In IEEE world, we can refer to NETA in absence of manufacturers data.

Insulation resistance test (most at 10kv DC max) does not stress the dielectric above the nominal operating level, so equipment can still fail once subjected to normal voltage stress and accompanying spikes on the system.

It is odd that the same phase failed / flashover. There should be some physical oddity with regards to Phase-B compared to the other phases, like smaller ground clearance ... anything, else there is an even chance that if these are substandard breakers they could have failed on other phases. Lucky that it was not a phase-to-phase fault, else damage would be much more.

Flash over are easily cleaned by blasting frozen CO2, if anyone qualified over there.

 

[ScottyUK]

I'm pretty sure that the VD4 manual recommends that you do not lift the breaker by the primary disconnects. It has slinging points for a lift truck, and your photo shows the yellow label next to one. I wouldn't be surprised if you've damaged the resin insulation.

 

[Zogzog]

ScottyUK, this is from the VD4 book. Either all 3 breakers were damaged from improper handling or perhaps the cell side bottles were damaged.

Before carrying out any operations, always make sure that the
operating mechanism spring is discharged and that the apparatus is
in the open position.
To lift and handle the circuit-breaker, proceed as follows (fig. 2):
– use a special lifting tool (1) (not supplied) fitted with ropes with
safety hooks (2);
– insert the hooks (2) in the supports (3) fixed to the frame of the
circuit-breaker and lift. Put the hooks (2) into the support holes (3)
according to the type of apparatus (see table);
– on completion of the operation (and in any case before putting into
service) unhook the lifting tool (1) and dismantle the supports (3)
from the frame.
During handling, take great care not to stress the insulating parts and
the terminals of the circuit-breaker.

 

[ajlimuran]

Sir,

The VCB is properly handling in lifting, the main issue on this , why the VCB explode when synchronize after 3 hours?

 

[davidbeach]

You're really going to need to find someone locally that can figure it out. There's not been enough information definitely narrow it down and everything that's been suggested has gotten a reply of "no, that's not the issue, why did .... happen?" First it was the VT, now the breaker. I don't know what happened, but I'm pretty certain that the answer is at the site, and the "No we didn't do anything wrong" attitude isn't going to help. Maybe nobody there did anything wrong, maybe they did. The only way to get the bottom of something like this is to cards fall exactly as and where they may.

Relays are my thing; whenever something goes wrong on our system that might involve relays I always go after everything that might possibly have gone in the relays and let other influence the investigation in other directions. Then when I get to the final report and the principal cause turns out not to be relays I've got a certain additional credibility simply because I didn't try to point the blame elsewhere in the beginning.

The working theory of what went wrong always has to be that which best explains everything that happens and doesn't require any "ifs, ands, or buts". A beautiful theory that covers everything except for one fact is of no use at all, you have to have the a working theory that explains everything, regardless of where it might point. In the initial stages of an investigation that working theory can change frequently. You can have a great theory of what happened and discover one new fact that makes you start all over again. You can't ever be defensive about what might have happened, it is what it is.

 

[ScottyUK]

"The VCB is properly handling in lifting, the main issue on this , why the VCB explode when synchronize after 3 hours?"

Your photograph does not support this - someone has tilted the breaker using the primary disconnect. This is exactly what the OEM says to avoid.

 

[ajlimuran]

Sir, we opened checked the VI as we found out that there is current leaks between contact force spring conductor insulted coupling rod due to possible insulation failure.


Please see pics.

 

[ScottyUK]

Given the amount of metallic and/or carbonised contamination on the surface it would be surpising if there wasn't a leakage path. Has the interrupter itself failed, or was there a flashover elsewhere?

 

[ajlimuran]

This is the photo after explosion.

 

[krisys]

I suspect a ferroresonance which might have caused the voltage rise and explosion. Or VT grounding issue like VT floating neutral.