Flash-over in drive. Second opinion needed. 5

[Skogsgurra]

I have this problem:

A customer has four drives installed in one section of his plant. They are made by a well-known manufacturer and identical drives perform flawlessly in other parts of the plant.

The power range is 390, 545, 615 and 765 kW and they are fed from a 690 V, 50 Hz IT grid with a 3.15 MVA 5.73 % DY11 transformer. Input rectifiers are thyristor controlled and keep the DC link at 930 V constant.

The problem: The fuses of the two larger drives blow infrequently. The "MTBB" seems to be about one month, but has been as short as two days.

Observations:
*The two drives never blow their fuses at the same time.
*There are no visible transients on the mains voltage (recorded with 16 kS/s and 12 bit resolution, no filter).
*Current rises from normal load current (around 300 A) to 5400 A in 3 milliseconds (same recorder) when the Silized fuses blow and current drops to zero in about 2 milliseconds.
*The arc guard (ABB) reacts about 4 milliseconds after the current starts to rise.
*What I said about "no transients" is not quite true; there is a 6 kHz ringing started by the thyristors in the input rectifier of the inverters. The amplitude of this ringing is about 100 Vp-p. But there are no "killer transients" on the three phases.
*The fault current has a 100 000 kA/s maximum rate of rise and that corresponds well to what can be expected with 3% line reactors.
*Traces of flash-overs can be seen on the thyristor heat sinks (disc thyristors, marks on bolt and heat sink). The flash length is about 8 mm. Which (in my thinking) corresponds to more than 10 kV.
*Other equipment on the same grid are not influenced.

My analysis:
I have excluded false triggering because the DC link voltage is already high and premature triggering cannot produce this kind of overcurrent.
There is no overvoltage on the line. Not recorded and not influencing other equipment.
The high voltage must be generated by something. What?

I would like to bounce this idea in the forum:
The holding current (Ih) of a normal thyristor in this size is something between 100 and 500 mA and the snubbers are sized to take care of the "snap-off voltage" when the current goes below Ih.

Now, if the 6 kHz ringing snaps the thyristor off prematurely - remember that these thyristors work against the DC link, which is a counter-EMF. What will happen? Say that it is turned off while line current is at 5 A. The magnetic energy in the line reactor will then be one hundred times the normal energy at turn off and that will take the voltage on the snubbers up to more than ten times the normal voltage (energy in capacitor plus voltage drop in series resistor).

If the thyristors can take this high voltage without damage, then there would certainly be a flash-over. Wouldn't it? The question is: Can a thyristor withstand 10 kV or more for the time needed to ignite an arc?* I have not been able to find any data on this.

I need an informed second opinion. Or even better: Have you had this problem? Is it described anywhere in literature?



* The snubbers are connected with rather long wires and I do not think that the inductance is optimal. The fast snap-off voltage is probably not absorbed by the snubbers.

 

[jraef]

skogsgurra,
Flash News!
Related but not necessarily applicable to this thread is some information I recieved on my earlier post. To avoid sidetracking I started a new post for it, thread237-96679.

"Venditori de oleum-vipera non vigere excordis populi"

 

[light1]

Questions:
Are SCRs naturally commutated or is there a forced commutation applied?
Are they true SCRs or other types of SCRs, e.g. LASCRs, GTOs, MOS-SCRs, FET-SCRs, etc.?

 

[Skogsgurra]

Light,

They are mains commutated, i.e. naturally. Normal thyristors. Normal six pulse fully controlled bridge.

 

[ScottyUK]

skogsgurra,

A curveball thought, as I prepare to go home for the evening. I am tired and my brain is freewheeling.

A high voltage mysteriously occurs due to high dI/dt. The drive itself doesn't seem to be able to produce the high dI/dt. So if that is true, consider the following question: What happens to the voltage on the drive side of the reactor if the fuse on the drive blows? I am assuming that the fuses are between the reactors and the rectifier, or are on the individual thyristors.

I can't model a blowing fuse with the tools I have available in order to predict what the voltage on the rectifier side of the line reactor would do if a fuse blew while carrying load current, but if the fuse has a fast cut-off it would seem possible that the voltage at that point would rise to fairly high levels. Once an arc is established, a path for the fault current you're observing would be established. Is there anything to suggest that the fuse blows first and the voltage transient is a result of this, rather than the fuse being a victim of the transient?

What are the fuse ratings relative to load current? Are they in areas of high vibration? Transient load changes? Is it hot where they are located? A totally stupid question, but are the fuses the correct rating?


This might all be total nonsense and can be discarded if it is - in fact red flag it so it goes away! Hopefully it will stir up a thought or two.


regards,


Scotty.

------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[Skogsgurra]

Scotty,

Why should fresh thinking be red-flagged? I never thought of it that way. It is a possibility. I will try and figure out if and how that could happen. The fuses start melting after 3 ms overcurrent (current still rising) and the arc is extinct in 2 ms. So a 3 % line reactor should produce a rather high voltage when the current goes from 5,4 kA to 0 A in that short time. I will have to do some homework before I know what voltage it will produce.

The fuses are correct. Both amps and characteristics ("Silized" thyristor fuses). The load is only around 50 % when I read the inverters in normal operation and the operators say that the operating point is the same all the time. So they shouldn't blow. But there could be some hitherto unknown reason.


I finally got the unit with the arc traces. It was sent the day before yesterday using a "fast" carrier. I didn't get it yesterday as expected and just after lunch today, I still did not have it. I phoned the regional office and asked for my packet. It had been sent to a completely different location and now, they didn't want to send a car so I could have it. I had to go there myself, 100 km in total, to get something to work with.

I have just started on it. Results will be published on a computer near you.

 

[Skogsgurra]

It pays to ask for a second opinion!

I have finally got the faulty thyristor unit and I have also tested it with DC and AC before I took it apart and inspected its internal insulating parts.

It is a little bit early to say definitely, but one thing is for sure: I do not need 10+ kV to get a flash-over. I get it continuously at 1,5 kV RMS and sporadically at lower voltages. I think that the solution is near now.

Thanks a lot to all of you that has contributed with fresh ideas and good knowledge. You put me on the right track!

 

[Skogsgurra]

FLASH NEWS!

The problem is solved. And as always, the problem turned out to be far from what we thought when we started on this.

I have now been told that the sprinkler system on the floor above the electrical room went berserk almost two years ago. The floor was flooded and the cable ducts from that floor down to the inverters right beneath had not been sealed off. So the dirt on the floor above was spooled into the inverters "Niagara style".

The inveters were cleaned on the surface and dried out with hot air. But there are still lots of dirt that has amassed in places where you cannot see it.

Lesson learned: It is never what you think. And there is always a natural cause for seemingly improbable phenomena. Thanks to all for guiding me through this one.

 

[jraef]

Skogsgurra,
Thanks for the update.
I forgot what this axiom is called or exactly how it goes, but it is something like "Given all the possible choices of solutions, it is most likely the simplest one rather than the complex".

I once chased down the cause of frequent SCR failures AND contactor chattering for 3 months (back and forth to a jobsite 400 miles away) and came to find out by accident one day that when this brand new equipment had arrived on site the first day, everyone was standing around admiring it with the doors open when a water truck went by and sprayed them as a prank. They said they "forgot" to mention that detail when they called for warranty service (a sympathetic electrician spilled the beans).


"Venditori de oleum-vipera non vigere excordis populi"

 

[ScottyUK]

Hi Skogsgurra,

All the hypothesising and postulating in the world would not have picked that up! As you rightly say, sometimes the obvious things are the ones that you miss - meanwhile all us 'experts' - ha-ha! - are looking for increasingly convoluted ways to explain your problem! I'm pleased you found the solution to your problem - an oblique lesson to us all I think.

------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[busbar]

skogs, I hope you did not have to give up any “billable hours” given the long-after-the-fact contamination discovery.

 

[Skogsgurra]

That remains to be seen, busbar...

 

[Skogsgurra]

Epilogue

All hours paid for now. Even the extra hours for picking up that parcel that Schenker didn't deliver. The two inverters will be replaced by new ones. Thanks again for all support!

 

[mc5w]

Skogsgurra,

There is also another thread in the electric power forum under the heading of H2S Certification that has some links to the phenomenon of silver whiskers shorting out electrical equipment.

Mike Cole, mc5w@earthlink.net

 

[Skogsgurra]

Thanks mc5w,

I have some trouble finding that one. Do you have the thread number?

When you mention it; I had a similar problem with ceramic capacitors in a thyristor drive. The capacitors were connected in parallel to the burden resistor for the actual current measurement. A new cooling air system had been installed and as temperature went down, relative humidity went up. RH got so high that silver migrated through the pores in the ceramic so the whiskers shorted the capacitors out so that actual value was zero. Current rised, of course. The increased current blew the fuses, but also burnt away the whiskers so nothing was there to tell what had happened. It was a tough one. There are some pictures on

http://www.gke.org/rapporter/Stroemriktare Bel Bond.pdf

The text is in Swedish, but the pictures showing the capacitors with the typical white "fungus" are illustrative. There is also an old (1979) article from Philips describing the phenomenon.

 

[mc5w]

The thread number is 238-103502.

Mike Cole, mc5w@earthlink.net

 

[Skogsgurra]

Thanks for the thread! This phenomenon was unknown to me. The pictures of the silver whiskers are really thought-provoking. There was an accident in an old 500 V distribution in a paper mill where one guy got badly burnt. Could have been whiskers like this that started the arc. It took the HV breaker about 40 seconds to clear the fault and the poor guy was exposed to the arc during all this time.