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Old 07-19-2009, 12:14 PM   #16
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failure analysis


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Originally Posted by frenchelectrican View Post
WFO and Inphase277.,

The last major fault I did work on was 14.4KV this afternoon that was a 400 amp fuse btw man that sucker is loud when it blew and yes I can feel the ground shake a little but not much but nice shockwave my ears were ringing on that one for while

Merci,Marc
...doesn't do much for your underwear, either!

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Old 07-19-2009, 04:40 PM   #17
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I used an unused dryer outlet to measure the voltage at the panel, so I didn't need to remove the panel cover.

The kitchen wall oven is the load, marked as 5.1 kw at 240v.

21.25A for the oven = 11.29 ohms

~247v dropped to ~246.5 when broil is selected and oven is switched on.

246.5/11.29 = 21.83A

0.5v/21.83A = 0.0229 ohms

Isc = 247/.0229 = 10,800A, for resi. service but the pole 'former is right out front.

So the Thevenin equivalent circuit of my PoCo is a 247v source in series with a 23 milliohm impedance.

Last edited by Yoyizit; 07-19-2009 at 04:45 PM.
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Old 07-19-2009, 11:15 PM   #18
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Interesting that we're talking about fault current, etc. My project for tomorrow is to go to a geothermal power plant currently under construction, the electrical is being done by a contractor I don't work for. I've done the control house and medium and high voltage terminations on 6 plants for the company that owns the plants, and know the electrical engineer well.

I've been invited to participate in a 'fault recovery incident'. It seems that a 120 KV SF-6 breaker was closed in to a 45 MVA transformer (120 KV primary, 12.470 KV secondary) that fed a breaker bus via 8-750 MCM 15 KV copper conductors where the ground clamps had not been removed.

Spectacular, indeed!

I really don't know what I'll find when I get there, but I'd bet it'll be less than pretty.

The company I work for has been hired (by both the owner and the engineer) to send me out as a consultant. Apparently, I get to decide what steps must be taken to insure that when the breaker is closed in next time, the outcome is a bit more desirable.

Closing a 20 amp 120 volt breaker in to a bolted fault is not such a big deal, closing a 120,000 volt breaker in to a bolted fault (even through a transformer) is a bit more involved.

I'll post more later.

Rob
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Old 07-20-2009, 03:56 AM   #19
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Rob.,

Hate to steal your thumper show here for a min but you mention SF-6 breakers

There one thing I hate about SF-6 breakers is timming to open or close.

The last SF-6 I done was in France and yes we do have them there in France we have double SF-6 in series and one did fail on us it was pretty instering event to see it flashover

The cuprit was the timming mechsim it supposed to be in sycronous with both breaker but it was not so got it fix and work good after that { yes we have to replace that one before we can go more on that one }

Merci.Marc
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Old 07-20-2009, 03:58 AM   #20
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...doesn't do much for your underwear, either!

MDR { LOL }

naw it did not affect my underwear but one other guy yeah he have to change it I kinda expected to happend like that due we are only 1 KM from the substation.

Merci,Marc
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Old 07-20-2009, 11:23 PM   #21
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Originally Posted by micromind View Post

I've been invited to participate in a 'fault recovery incident'. It seems that a 120 KV SF-6 breaker was closed in to a 45 MVA transformer (120 KV primary, 12.470 KV secondary) that fed a breaker bus via 8-750 MCM 15 KV copper conductors where the ground clamps had not been removed.



Rob
I trust you are familiar with the hazardous effects of SF6 residue after a fault and will take appropriate precautions. It is highly hydroscopic and will totally mess with your lungs if you breath it.
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Old 07-21-2009, 11:03 PM   #22
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Yes indeed! SF-6 is extremely dielectric, as well as extremely poisonous. Fortunately, the breaker survived without damage. The maximum fault current on the primary side of the transformer was less than the interrupting capacity of the breaker. It was tested by both high-potential and fall-of-potential. Both tests were OK.

This isn't exactly DIY, but it might help explain just what your local utility goes through to supply your house with power.

The system consists of a 120 KV 3 phase 3 wire overhead line that originates from a medium-sized substation, about 15 miles away. It then connects to an air-switch. This is alot like a simple knife switch, except the blades are about 3' long. Next is the SF-6 breaker. This breaker is nothing more than a switch. The difference between it and a light switch is it can turn 120,000 volts on and off, and it is remotely controlled. It's about 12" in diameter, and about 36" long. It's filled with SF-6 gas, under about 80 PSI. The contacts only open up a couple of inches. Without the SF-6 gas, it'd arc over. Because of this, if the pressure gets too low, it'll lock itself closed. If it is opened with low pressure, it'll blow up.

There are 3 of these breakers, about 3' apart. One for each phase. Next is another air-switch. These air-switches exist so that when they're both open, the breaker is isolated from any voltage, and can be serviced safely. Next, is the transformer. It's about 10' X 10', and 12' high. The current on the primary side (120 KV) is about 210 amps. The secondary side is 12,470 volts, 3 phase. The current here is about 2,000 amps. This transformer would supply (I'm not kidding) about 30,000 houses. You'd think that a transformer this size would send millions of amps into a short-circuit, but in reality the absolute maximum it can supply is about 17,000 amps. Based on the system, my guess is about 8-10,000.

Next, there are 8 sets of 3-750 MCM 15 KV copper wire going from the transformer secondary, underground, to a utility meter cabinet, then underground to the back of a 15 KV switchboard. Each of these wires is about 2" in diameter. The actual wire inside the insulation is about 1" in diameter. These are connected to busses, about 6" wide, and 1/2" thick, This is where the ground calmps were connected as well. I say 'were connected' here because there was enough energy present to basically vaporize the ground clamps.

The ground clamps are 4-4/0 copper wires, very flexible, joined together at one end. The other end of each wire has a clamp on it. One of these goes to the ground bus, the others to each of the 3 phase busses.

Whenever work is being done on a system of this voltage (12,470) after the power is turned off, ground clamps are installed. This serves two purposes, the first is to ground out any stray voltages (these can be lethal), and to give you a couple of seconds to get away if the line is accidentally energized.

The breaker (which is actually a remote-controlled switch) is controlled by relays that detect various faults. If a serious enough fault occurs, the relay will trip the breaker. In this case, the relays were not programmed correctly, and there were some control issues that prevented one of the relays from tripping the breaker at all. Why this was not tested before closing the breaker, I have absolutely no clue!

I corrected the control issues yesterday, and the relays were re-programmed today. Later today, both myself and the test guy (whom I've worked with a lot, and highly respect) looked the system over, were satisfied that everything was OK, and closed the breaker again. This time, no explosions, fires, smoke, or any other undesirable stuff!

Safety on a simple 120 volt circuit is very serious, but if something does go wrong, damage is usually minimal, because of the limited energy available. On a distribution/transmission system like this one, the potentail for damage is MUCH greater.

Rob
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Old 07-22-2009, 01:47 AM   #23
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Rob.,

I have a Youtube link that I found with double SF-6 breaker/ isolaiter switch unit



One of the phase have arc flashover on it and the OP mention this is 500 KV L-L system

Merci,Marc
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Old 07-22-2009, 10:14 AM   #24
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That video shows why just about everything in a switchyard is done remotely.

Despite our best efforts, high voltage gets out of control occasionally. When it does, it's best not to be nearby!

Rob
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Old 07-22-2009, 08:21 PM   #25
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"Vacuum circuit breakers (VCBs) are displacing SF6 breakers in industry as they are safer and require less maintenance. Although most of the decomposition products tend to quickly re-form SF6, arcing or corona can produce disulfur decafluoride (S2F10), a highly toxic gas, with toxicity similar to phosgene. S2F10 was considered a potential chemical warfare agent in World War II because it does not produce lacrimation or skin irritation, thus providing little warning of exposure."
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Old 07-22-2009, 08:36 PM   #26
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Originally Posted by micromind View Post
That video shows why just about everything in a switchyard is done remotely.

Despite our best efforts, high voltage gets out of control occasionally. When it does, it's best not to be nearby!

Rob
I'm no lineman, but this doesn't look like anything was out of control. This just looks like a circuit being opened under load. The switch opened, a plasma arc formed, and hot air rises so the arc rose with it. Eventually, it was too long to sustain and self extinguished. The distance between the contacts was too great for it to re-strike in the normal air.

Is this not what happened? It looked completely normal.
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Old 07-22-2009, 09:21 PM   #27
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I'm no lineman, but this doesn't look like anything was out of control. This just looks like a circuit being opened under load. The switch opened, a plasma arc formed, and hot air rises so the arc rose with it. Eventually, it was too long to sustain and self extinguished. The distance between the contacts was too great for it to re-strike in the normal air.

Is this not what happened? It looked completely normal.

No.,
It is not normal open sequince due the one of the concats did not exitished the arc before the main knife blade start to swing open and the SF-6 gas should be in there and if that the case then that breaker you see on the youtube video that is double SF-6 breakers in series and it must work together to disconnect the current simitulsly { at the same time } and I think what happend that time one of the concats did not break in proper sequince as other two did work fine but not the third one as you see.

Rob can fill you in little more details if he have time with it.

Merci,Marc
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Old 07-22-2009, 10:25 PM   #28
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Originally Posted by frenchelectrican View Post
No.,
It is not normal open sequince due the one of the concats did not exitished the arc before the main knife blade start to swing open and the SF-6 gas should be in there and if that the case then that breaker you see on the youtube video that is double SF-6 breakers in series and it must work together to disconnect the current simitulsly { at the same time } and I think what happend that time one of the concats did not break in proper sequince as other two did work fine but not the third one as you see.

Rob can fill you in little more details if he have time with it.

Merci,Marc
I see it now. The other two phases didn't do it. Gotcha. I wonder why they were video taping this unless they expected it to fail? Also, look at the far right side when it zooms out. At the front of the truck one of the guys is hiding out
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Old 07-22-2009, 11:38 PM   #29
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You can see the arc established across the SF6 bottle before the switch opens. If memory serves, this was intentionally shorted out for the demo (hence the camera in the right place at the right time).
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Old 07-23-2009, 08:00 PM   #30
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You can see the arc established across the SF6 bottle before the switch opens. If memory serves, this was intentionally shorted out for the demo (hence the camera in the right place at the right time).

Not excatally intentinally shorted out and this is a reactor capaitor and they draw little current IIRC I think about 10 to 20 amp the most when first engerized up then it tapered down once you get more load on it.

( the reactor capaitor is used on end of the transmisson lines to control the open circuit voltage )

That test I am not sure excat number of amparage it was running thru the SF-6 breakers but one thing for sure it is not much current there.

If it was much higher current the arc flash will be much brighter and louder as well.

Merci,Marc

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