Lincoln Street Substation Transformer Explosion Burbank, CA - 04/10/2020 8

[Mbrooke]

Transformer explosion and fire:

https://www.youtube.com/watch?v=IIMlHjfhb6U

Looks like there was sustained arcing on the secondary before the explosion:

https://www.youtube.com/watch?v=i82WIATHXNY

I'm guessing a short circuit occurred on the 12kv (or 16kv) side with relaying failing to catch/clear it. The transformer remained severally overloaded overheating to the point the oil inside it ignited.

I don't take credit for these picture but to give you an idea that power transformers typically contain anywhere between 7,000 to very well over 25,000 gallons of oil.

https://imgur.com/a/KzJVNKp

https://imgur.com/1wdbAnf

Can I make the claim that my practice of 200E fuses on the primary would have prevented all this?

 

[waross]

Thanks prc.

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

 

[Mbrooke]

You also could have included me in this reply, as well as posting earlier as a rebuttal to my question-

1) As per IEC 60076-5, transformers are designed to withstand thermally 2 sec of over currents. Copper temperature from the heating at the end of 2 sec shall be less than 250 C. So designer calculate maximum short circuit current and calculate copper temperature from higher current, using the formula in standard. This formula is assuming all the heat from I2R is stored in copper ie time is not enough for the heat to come out of copper and start transferring to oil. Suppose the current flow continues > 2 sec. Copper temperature will reach much above 250C and interturn paper insulation gets charred and interturn fault results. Tripping either by differential or by back up at far end or by gas operated relay occurs. Winding time constant is 6-10 minutes. Fault cannot sustain that much time as the arcing may open out winding or tank ruptures leading to fire. So before the oil temperature rises, winding fails and lead to scenarios as above. I had couple of experiences of this type of relay failures .I think I explained those incidents some time back. Fire did not occur in those cases, may be we were lucky. All tank ruptures I had seen were from arcing inside tank.

How is the winding time constant defined?

I feel like I'm being left out of this conversation due to a simple difference of opinion / research conclusion which is inevitable on an online forum.

 

[crshears]

You also could have included me in this reply...

I'm confused; this is an on line forum, so unless ones submissions and others' responses are somehow routed through an e-mail program, how can anyone be "left out"?

Just trying to understand...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

I feel like PRC has been ignoring me since our disagreement on just how much the oil heated during the incident. And doesn't want to answer my questions there on.

waross&Solar prestige :

In no way am I denying his experience and knowledge in regards to transformers. I merely want to learn from his experience- even if at first we agree to disagree.

I genuinely would like to know many BTUs can be emitted by the core and winding into oil for a sustained short circuit lasting several minutes before the transformer fails internally and explodes.

Its ok to not agree- my difference of technical opinion is no way meant to be a sign of disrespect, or my failure to acknowledge his decades of dedication to the subject matter.

 

[cuky2000]

Mbrooke, while Wating response from PRC, check the winding time constant formula on the IEC Srd 60076-7 and te info on IEEE STD C57.91 & 119.

Beware that for simplification purposes, the winning time is considered as a constant but strictly speaking is a variable that change with the loading and winding temperature. The accepted margin of error may be around 20% compare with the exact solution.
[sub]NOTE: Unless you are in the transformer design business, ím not sure if this info I important for a power engineer.[/sub]

 

[Mbrooke]

Thank you- I will read up on this.

BTW, this info is important to me (I think) as I'm evaluating 51 protection of a transformer.

 

[SolarPrestige]

Interesting to read Cigre 537 , 2.2 Fire Scenarios.

It is recognised in this document that over current can lead to oil overflowing and oil boiling, which is something I had not realised, the boiling part. But I do think that the issue with this Burbank unit is related to exceeding short circuit capability of the unit since the winding cannot carry that very high current for very long (as prc says, only for maybe 2 seconds).

In my experience with transformer designs, unless the designer has to prove the S/C ability of their unit, they won't do it. The proof is a technical assessment because there is no testing of this capability, we don't do withstand short circuit tests on new transformers. I don't know whether it is even possible to do this test.

 

[argotier]

For power transformers the short-circuit verification its usually done by default, at least with a calculation report according to the corresponding standard.
The thermal capability its rarely a problem, but the dynamic short-circuit withstand can be tricky, specially with low impedance units (like autotranformers).

And this CAN be tested! But there are only a few laboratories in the world with such capacity.
CESI in Italy is one of them Link. Fun fact: they perform this test late at night, to minimize disturbances to the rest of the grid consumers.

 

[waross]

In the first clip, the transformer explodes at about 57 seconds in.
That is after the news chopper got into position to start recording,
Is the light brown smoke in the first clips NOX generated by the heat of the arc?
While the withstand for a bolted fault may be 2 seconds, the current drawn by an external arcing fault will be less than for a bolted fault.
The fault current on the primary will be causing voltage drops which will further reduce the fault current.
Given the I[sup]2[/sup]R relationship we have a time to failure in minutes rather than seconds.
The same thing happens but it takes a little longer with less than bolted fault current.

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

 

[Mbrooke]

@Waross- I'll agree with this. But I'm really curious just how much heat is emitted by the core and windings during those several minutes. From various Youtube videos this roughly the time I'm getting and it could be more considering the time it took for the chopper to mobilize and people to start recording.

The oil might actually have been warm during the explosion, or it might have been at its smoke point already

 

[prc]

Solar prestige- It is good that you have gone through that report prepared by experts from around the world. They have clearly explained how tank rupture occur due to internal arcing and not by oil boiling. But unfortunately they have mentioned oil boiling due to over currents at one place . Even experts can be carried away by myths. Let us see by an actual case:

1) 100 MVA 220/33 kV Transformer- Typical Losses 40/250kW Impedance 10 %. With a dead short circuit on secondary terminals, copper losses will shoot up by 100 times by the 10 times over current. Typical oil content for such a transformer will be 45,000 KG. Specific heat of oil at 40C= 1.9 kJ/Kg/C . This will go up with higher temperature. Let us see the increase in oil temperature due to current flow for 10 seconds. In reality this will never happen. As I mentioned design is done for a copper temp of 250C after 2 sec. So after 2 sec, paper will burn out and the interturn fault will trip out unit by differential and Buchholz. Otherwise copper will melt and open out winding. I had such an experience in a 450 MVA 400/220/33 kV bank. A dead SC on 33 kV terminals was cleared by a relay 200 kms down of 220 KV line. Differential did not trip as CT wires were interchanged! 400 KV cleared by over current relay. On internal disassembly copper globules weighing 250-300 gram were obtained from tertiary winding. Still tank did not rupture as it was not a low impedance fault. After 48 hours of tripping out, low voltage diagnostic tests were done on failed unit. Pressure release valve opened out and oil splashed from the vent. Inflammable gases were still inside tank and 230 v voltage application caused an arc inside and gases caught fire !

Energy release within 10 seconds= 25000 x10 Kw seconds (kJ)
Oil temperature rise = 25000x10/(45000x 1.9) = 2.9 C .Here we neglected extra cooling by convection and assumed all the heat is stored in oil. So in reality oil rise will be nothing. Please make sure I am not wrong, by rechecking the calculation.

Oil temperature at full load will be 20+50 =70C; oil flash point >140C .Oil will not locally boil as due to high viscosity and convection , oil will move fast around when heated up.

2) When a short circuit occurs on secondary terminals, during the first cycle the current peak will be 2.69 times the RMS value of SC current ( 10 times in above example)Forces will be square of the times current. So transformers will failing during the first couple of cycles when the forces are maximum. If it stands these cycles, there is no chance of failure later except by copper heating. As waross correctly said, the over current will sharply come down due to voltage dip at primary due to regulation.

 

[prc]

For those interested to know more about transformer cooling, this link may be useful:

https://www.brown.edu/Departments/Engineering/Courses/ENGN1931F/TransformerCoolingOil.pdf

 

[prc]

solar& argotier: Short circuit testing of Transformer is not rare, at least in some markets. During the last 20 years, my factory has done short circuit tests on 33 transformers. Other manufacturers also have done similar tests. Most of them pretty large Eg 1000 MVA 765 KV GSU bank (2016)780MVA 420 KV GSU bank 315 MVA 400 kV Auto etc, etc. We had to take these units to KEMA for testing as they have upgraded facilities to 15,000 MVA. They are not using on line testing but dedicated special Generators. Now we have both on line and dedicated power labs in India to test such large units. Since grid capacity is nearly 370 GW, we have no limitations or issues of voltage dip.

During the last CIGRE A2 colloquium, (A2-Transformer Division) I was a key-note speaker and made a short presentation on Short circuit Capability of Transformers- Mainly history and evolution of this test during the past 120 years. May be interesting to some of you.

In India, most of the utilities insist for a SC test (though extremely costly ) for any new order or new supplier or new design. They claim this gives an assurance on the quality and reliability of transformer. Manufacturers try to dissuade buyers from tests and persuade to accept calculations in lieu of.Irony is I was the first to volunteer to do SC test on a Power Transformer in India,way back in 1983 for Indian Railways as they want to get assurance of reliability for track side supply transformers.

 

[prc]

1) I forgot to post my presentation on short circuit testing Please see.

2) waross, my submission is we should not venture out (guessing game) to find out the root cause of any transformer fire based on some TV clips, google maps inadequate data and inappropriate comments. Transformer experts can easily find out the reason based on DR data, maintenance history esp of primary bushings and of line end OLTC + inspection of carcass. Many times we could find out the start of fire from such inspection.

3) Some times, fire may not be due to any fault in transformer. Once a 20 MVA transformer had oil leaking out from radiator. Oil was falling on dry grass in yard. On a hot summer, fire started from the oil soaked grass and the transformer was gutted. A 50 MVA 220 KV unit had ACSR jumper connection from bushing to a catenary. Connection had no slack and made to the middle of the catenary. When wind blows or during SC current the catenary swung violently and broke the upper porcelain of bushing. Oil went out exposing condenser core . An arc in air caused paper condenser core to catch fire. As bushing clamping pressure released, oil from tank fed to fire.

 

[Mbrooke]

@PalletJack: I'm not sure what you are trying to get at. I'm asking a legit question: how many BTUs of heat were released into the oil during this incident. Instead I'm being ignored. Acting like a 13 year old doesn't add anything of value to this thread- in the adult world exchanging ideas instead of insults is what actually makes you popular.

 

[cuky2000]

Pallet jack,

I do concur with Mbrooke. Your comments do not add any value to this discussion.
I suggest to voluntary remove this post an apology tu a valuable member could be a better way to keep the forum professional values.
Your call.

 

[waross]

I am not guessing when I see an arc established when a video starts and then persist until an explosion at the 57 second mark.
Much more than 2 seconds.
I am not guessing when I suggest that the arc is not a bolted fault.
Many transformers do not have enough air internally to support internal combustion.
Consider conservators or nitrogen blankets.
A transformer with an internal air space will only support combustion or an explosion until the air has been reacted, and then only if the mixture is within the explosive limits.
Compare to an internal combustion engine. One bang per stroke and if the mixture ratio is not within limits, no bang.
Electrical explosions caused by the sudden release of large amounts of energy are a well known electrical event, not restricted to transformers.
We will may never have the information as to the exact cause of the transformer failure, other than the failure of the protection to clear a fault.
That said, I feel that speculation as to possible causes is within the scope of this forum.
Zero sequence current and stray flux may have contributed to the heating.
Possibly a bushing failed, possibly the paper insulation was degraded and flashed over.
Possibly a winding or lead melted and was the origin of an internal arc.
But let's all take a deep breath and lighten up.
This self isolation may be making us all a little touchy.


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

 

[argotier]

You could say that the thread is heating up... adiabatically!

Beyond the probable cause for the transformer explosion, still remains the absurd amount of time for this kind of fault to remain uncleared.

Not only the protective relays failed, various other alarm signals should have warned the operators: loss of downstream voltages, overcurrents, transformer pressure (at some point) and perhaps even transformer temperature (OTI would be too slow to react, but WTI could have alarmed at least if the current increased enough?).

The time was probably long enough to open the breaker manually.

For the looks of it the substation is unmanned, but someone is always "watching" remotely, right?

Maybe the one that failed was the breaker trip unit?

 

[Palletjack]

Palletjack,

I do concur with Mbrooke. Your comments do not add any value to this discussion.
I suggest to voluntary remove this post an apology tu a valuable member could be a better way to keep the forum professional values.
Your call.

What did I write that was insulting or not the truth?

I even paid a compliment and said he was smart. Everything else is truth.

 

[Mbrooke]

Where have I upset you? I'm willing to apologize if I've legitimately offended you.

Actually I'm rather intrigued why the concept of winding's dissipating heat into mineral oil is such a hot potato subject.