|
TasogareNoKagi posted:Stupid question! Why didn't pulling fuses on a faulted circuit cause another arc fault in your face? Well, the fuses were 30' above me, I was pulling them with an extendo stick. Also, the fuse holders are loadbreak devices. Sorry, I wasn't clear this was a pole mounted platform bank.
|
#
?
Dec 30, 2013 06:10
|
|
|
|
| # ? May 10, 2024 14:49 |
|
|
I am researching/considering a career change to working as a lineman, is this the right thread to ask about it or should I create a new Tell Me instead?
|
|
#
?
Jan 8, 2014 18:09
|
|
|
Crimpanzee posted:I am researching/considering a career change to working as a lineman, is this the right thread to ask about it or should I create a new Tell Me instead? I don't know to be honest!
|
|
#
?
Jan 8, 2014 23:28
|
|
|
Crimpanzee posted:I am researching/considering a career change to working as a lineman, is this the right thread to ask about it or should I create a new Tell Me instead? I work for a transmission contractor but not as a lineman, you should contact or lookup the IBEW. There is a ton of transmission work going on right now and should be going on strong for awhile, hopefully. Linemen make a lot of money, but can be dangerous and you will very likely have to travel, unless you work for a utility possibly.
|
|
#
?
Jan 9, 2014 03:42
|
|
|
Don't forget being up in a bucket around 13000V lines in freezing rain. Or in the blazing sun covered in PPE.
|
|
#
?
Jan 10, 2014 01:33
|
|
|
I'm a lineman for a distribution utility. Sometimes it's fun but a lot of the time I'd like a job that wasn't so demanding of my time; I'd like being more free to spend more time with my family. And I worry about getting injured with kids I'm responsible for now. Not things I anticipated as a single 22 year old, almost 7 years ago. Anyhow if it's something you're interested in, it will be a lot easier if you're young (I'd say 25 or under really), very fit and healthy, and don't mind working your balls off in all weather. Don't underestimate that last part.
|
|
#
?
Jan 10, 2014 03:48
|
|
|
Last night, around 3:00 AM, my broken UPS/makeshift power conditioner strip thing cut power to my computer and started squealing. Its battery is completely dead, and usually when the power goes out it lets out a brief blip and dies, but apparently this was a pretty severe brownout. Luckily, this thread taught me that I should go around the house and turn off/unplug motorized stuff to keep it from burning out; the heaters were still heating, but the fans were barely spinning. My laptop and phone kept charging, and the LED light bulbs emitted a normal amount of light just fine, but nothing else worked. When I got up this morning, the power was out entirely. Apparently there was a tree on fire a few streets away around the same time last night. What's the likely cause and effect here: transformer fails, lighting tree on fire and causing voltage sag, or PG&E's system has a brownout, voltage sag causes increased current draw in various appliances, which causes lines to heat up and sag into a poorly maintained tree?
|
|
#
?
Jan 16, 2014 22:29
|
|
|
atomicthumbs posted:Last night, around 3:00 AM, my broken UPS/makeshift power conditioner strip thing cut power to my computer and started squealing. Its battery is completely dead, and usually when the power goes out it lets out a brief blip and dies, but apparently this was a pretty severe brownout. I'm not sure about the cause/effect, but yeah with an AC induction motor if you decrease the voltage, the current will dramatically increase and that can damage the motor.
|
|
#
?
Jan 16, 2014 23:08
|
|
|
I found an interesting video of a nuclear power plant simulator training scenario. This is a large-break loss of coolant accident at a PWR, with a reactor trip and safety injection. https://www.youtube.com/watch?v=swr74_CDyLk
|
|
#
?
Jan 17, 2014 00:05
|
|
|
atomicthumbs posted:What's the likely cause and effect here: transformer fails, lighting tree on fire and causing voltage sag, or PG&E's system has a brownout, voltage sag causes increased current draw in various appliances, which causes lines to heat up and sag into a poorly maintained tree? 1. Tree gets into primary line on 3 phase circuit. It could have fallen from decay, wind, weather, take your pick but it got into one of the phases. Tree catches fire during arc fault. 2. Circuit protection device opens on single affected phase; utility has 3 individual devices that do not operate simultaneously on all phases. Your home is connected to this phase. 3. The newly open phase is backfed from numerous 3 phase banks connected to the affected circuit. This backfeed does not successfully energize the open phase to full line voltage under load, giving you an undervoltage condition at your home.
|
|
#
?
Jan 17, 2014 00:35
|
|
|
angryrobots posted:Lots of possibilities, but my money would be on This is a residential neighborhood; I don't think there's 3-phase running through there (just the standard three line 120/120/ground, whatever you call it). Found a news article: quote:A wayward power line triggered a stubborn tree fire in Woodacre early Thursday.
|
|
#
?
Jan 17, 2014 01:38
|
|
|
Gisnep posted:I found an interesting video of a nuclear power plant simulator training scenario. This is a large-break loss of coolant accident at a PWR, with a reactor trip and safety injection. One thing to explain - you'll notice the annunciation panels at the top of the control boards. The way those work is that when a new alarm comes onto the board, the window flashes. The way I've seen these typically work is you have two buttons/switches: ACKNOWLEDGE and RESET. Hitting ACKNOWLEDGE makes the lights on the window stop flashing. However, if something new comes up, the new window will flash until the operator hits ACKNOWLEDGE. That helps the operator keep track of what is happening. Hitting RESET makes the acknowledged lights turn off if the alarm condition is clear. Like if you had a window that said "WATER LEVEL LOW" with a limit switch, if the water level went back up and the operator hit RESET, the window would turn off. You can get/design annunicators to automatically reset when a problem clears, however with this it's possible that there could be a transient condition that the operator would be unaware of if it came and went. You can connect annunciators to audible alarms as well, and these also have a SILENCE button. (Usually the alarm sounds again if a new alarm comes in even after the operator hits the button to silence the horn.) Notice too the books - they have "playbooks" to follow for different emergency and alarm scenarios. atomicthumbs posted:This is a residential neighborhood; I don't think there's 3-phase running through there (just the standard three line 120/120/ground, whatever you call it). If you see overhead lines, it's high voltage, and very likely three phase. What they do is have each house tap off one of the phases (or a pair of the phases). Then the transformer gives you 120V/240V (Edison - for North American users). Three-Phase fucked around with this message at 02:13 on Jan 17, 2014 |
|
#
?
Jan 17, 2014 02:03
|
|
|
Although I do not work within the field of industrial electricity, the subject of electricity, how it works and the theory has always held a fascination for me. Have you guy's and recommendations concerning possible books that a "layman" could learn from or perhaps websites / good youtube channels that have a good selection of tutorial videos? I learnt basic electrical theory at school, but that was a long time ago, lol. most of what I learnt in those days has disappeared into a haze of memories.
|
|
#
?
Jan 17, 2014 03:15
|
|
|
Best layman's explanation I've seen:
|
|
#
?
Jan 17, 2014 03:21
|
|
|
woppy71 posted:Although I do not work within the field of industrial electricity, the subject of electricity, how it works and the theory has always held a fascination for me. Have you guy's and recommendations concerning possible books that a "layman" could learn from or perhaps websites / good youtube channels that have a good selection of tutorial videos? I would recommend "Electrical Trade Principles" by Hampson or "Electrical Principles" By Jim Jenneson. Both books are highly recommended to apprentices. Both books I believe are Australian but the principles should be universal Big Steveo fucked around with this message at 21:49 on Jan 17, 2014 |
|
#
?
Jan 17, 2014 21:47
|
|
|
Is there an easy way to calculate available fault current in a place where no study has been done? I ask because at my new job, they're having me re-design some of the controls for a test station. I wanted to go the route of using MPCBs in a group installation to save panel space, but the current panel is "rated" at 100kA SCCR, and the best I can do with MPCBs is 30kA. I say "rated" because the panel has been monkeyed with over the years, so I don't know if that rating is valid anymore, but I don't want to downgrade from that if I can avoid it. To be sure, this is all a moot point anyhow since cost dictates that I stick with spare parts that I can scrounge from the shop, which will be fuses and contactors, so I'm not worried about the SCCR anymore. But, this kind of spurred me to dig deeper and now I'm curious about how to get a rough calculation so I can at least get a worst-case scenario to start from when no other data is available. I've never done this sort of calculation before, so I did some googling and found a document from Bussmann (located here). I trust them in this arena, so I looked it over. I know that the supply transformer is the limiting factor, so I used Bussmann's document to gather the parameters I needed and run the numbers: it's a 500kVA transformer with a 480Y/277V secondary and 2.7% impedance. With this, I calculate that the available fault current at the transformer is about 27,500A (assuming a 10% voltage sag and a 10% lower impedance to account for tolerance). Now without accounting for motors adding fault current (which I could find if necessary), would this be a reasonable worst-case scenario for the entire building? Or are there other factors that might increase the fault current? It seems to me that the length of wires will just add impedance, which would result in a lower fault current at any given point beyond the transformer. What I'm trying to avoid here is doing a full on analysis that includes cable sizes and lengths, since I don't have any of that information. If I can get a worst case estimate from just the supply transformer and any motor loads, I can at least get a quick ballpark that will tell me if I need to beef up my panel's SCCR or not. edit: Just checked the main 800A breaker to the building and it has a 35kA IR. I sure hope my calculations are right, or else we're in trouble... DaveSauce fucked around with this message at 20:07 on Jan 23, 2014 |
|
#
?
Jan 23, 2014 19:58
|
|
|
Are there any generators at this site? If so, are they open or closed transition? If your napkin calculations are coming out that close to the MPCB rating, you may have to bite the bullet and call in a pro do to a full fault current analysis & coordination study. Honestly, arc flash analysis is code required now anyway.
|
|
#
?
Jan 23, 2014 22:51
|
|
|
Big Steveo posted:I would recommend "Electrical Trade Principles" by Hampson or "Electrical Principles" By Jim Jenneson. Both books are highly recommended to apprentices. I used both of these at tafe during my electrical apprenticeship. I would recommend Jenneson over hampson as hampson is more of a straight up reference book where as jenneson is much better for learning the material in the 1st place. Also how many other motor winders are here/what countries? I used to do motor winding in Australia.
|
|
#
?
Jan 24, 2014 00:25
|
|
|
Aliass posted:I used both of these at tafe during my electrical apprenticeship. I would recommend Jenneson over hampson as hampson is more of a straight up reference book where as jenneson is much better for learning the material in the 1st place. Yeah I was issued Jenneson when I was a first year. I still use it at times. I'm haven't done any motor winding, but I did witness one time a transformer winding being assembled, which was interesting. I work as a substation technician.
|
|
#
?
Jan 24, 2014 01:04
|
|
|
My understanding is that the First Energy grid is under a fairly heavy load right now due to the cold weather and electrical heating demands.
|
|
#
?
Jan 24, 2014 11:47
|
|
|
grover posted:Are there any generators at this site? If so, are they open or closed transition? If your napkin calculations are coming out that close to the MPCB rating, you may have to bite the bullet and call in a pro do to a full fault current analysis & coordination study. Honestly, arc flash analysis is code required now anyway. None that I know of. Just the transformer and a few motors. Really, I'm just trying to educate myself on how to get a basic calculation done to get a ballpark for my own reference. I've already scrapped the idea of MPCBs for this panel, but the whole thing just got me thinking is all. Since most places don't have any sort of fault current study done, I just want to be able to run a quick number to see if a potential worst case scenario is worth further investigation. My original question is a perfect example. Since my quick calculation puts me a hair under 30kA, I know that a panel with a 30kA rating is pushing it and would require further study to see if it would work. But, since I know there aren't a significant number of motors or other sources, a 65 or 100 kA rated panel will be more than adequate. So, all that said, how does one get in to doing short circuit and coordination studies? I just do controls so I'm usually given a SCCR number to design towards. I don't ever get in to calculating the available fault current, but recently I've found it interesting. Since it affects my design, I'm interested in what goes in to this calculation so I have a better idea of the hows and whys of it all.
|
|
#
?
Jan 24, 2014 15:09
|
|
|
Your estimate of 10% tolerance on the transformer is quite high. Is the impedance from the nameplate or a design value? If the latter I'd take 5% tolerance but none otherwise. You'd be looking at 25 kA on the transformer terminals and much lower for subpanels because of cable impedance. For a proper verification of short circuit withstand you need to compare the peak withstand of breakers with the half-cycle peak as well though and this is where motor contribution matters. Cables will affect those pretty significantly, just adding the maximum contributions doesn't tell you much unless it's lower than the withstand. If you want to know about short circuit calculations I'd recommend IEEE 551 / Violet Book as a practical guide. Doing it by hand is good for understanding but software can do it so much faster that it's not something that will save you time or money if you do it on a scale larger than a few calculations. On the other hand most software is $10k for a license, getting an engineer involved would be cheapest in most cases. As someone who does arc flash studies on a lot of old industrial sites (though not US) let me tell you that a vast majority never had a proper short circuit study done, but it tends to work out OK unless a contractor cheaps out on breakers.
|
|
#
?
Jan 24, 2014 15:44
|
|
|
DaveSauce posted:Is there an easy way to calculate available fault current in a place where no study has been done? Place a non-contact ammeter around a spud bar. Place the spud bar across the terminals, bus bars, or cables with the most warnings near them. Document result.
|
|
#
?
Jan 28, 2014 08:24
|
|
|
I like this video I found: https://www.youtube.com/watch?v=C1-kOE9Yr04 Non-seamless transfer from utility to generator.
|
|
#
?
Feb 1, 2014 06:14
|
|
|
I love the lamps and synchroscope  What's that noise in the background when they try to do the transfer? Generator being loaded down, or something else? atomicthumbs fucked around with this message at 10:50 on Feb 1, 2014 |
|
#
?
Feb 1, 2014 10:48
|
|
|
atomicthumbs posted:I love the lamps and synchroscope If you listen very hard, you can just about make out the sounds of poo poo hitting cotton when the lights go out. (Incidently, there are batteries that keep the switchgear going- that's why the meters and lights on the switchgear are still lit and breakers able to recharge, even when the power was lost.) This was the successful test of the same system: note that the tie switch closes before the generator breaker(s) open. https://www.youtube.com/watch?v=Qo2PqtSwWtk grover fucked around with this message at 15:56 on Feb 1, 2014 |
|
#
?
Feb 1, 2014 15:42
|
|
|
Aliass posted:Also how many other motor winders are here/what countries? Winder here (some of my work is 2 pages back), good 'ol US of A.
|
|
#
?
Feb 2, 2014 02:08
|
|
|
One of the questions I'd like to figure out is how to gracefully "un-parallel" a generator. If you have a power plant with, say, three 100MW generators, and you need to take one offline without impacting the power grid, how would that be accomplished? I think the way to do that with a synchronous generator connected to a turbine would be to slowly throttle back the turbine until you get to a point where there is zero current flowing out of the generator or you actually go into the opposite quadrant and begin to run the generator as a motor, and then you can cut excitation and open the circuit breaker. I was also thinking if you have a very large drive with the right topology, you could switch over so that the generator provides power through the VFD, slow down the turbine's speed and make the drive sort-of act in reverse, you go from some frequency to 60Hz out until you slow the machine down to a low speed and the power output is small enough to be able to open the breakers and disconnect the generator. (The problem here is that a variable speed drive in the 100MW range is really, really super big.) The problem with just opening the circuit breaker (assuming you have a generator breaker and aren't utilizing a higher-voltage breaker upstream) would be:
Three-Phase fucked around with this message at 02:46 on Feb 4, 2014 |
|
#
?
Feb 4, 2014 02:41
|
|
|
Can't you increase load on one Generator set, while decreasing load on the others, or separate one, parallel the two to the point where you can drop the turbine generator in question, then reparalel the last two?
|
|
#
?
Feb 4, 2014 02:50
|
|
|
Three-Phase posted:One of the questions I'd like to figure out is how to gracefully "un-parallel" a generator. If you have a power plant with, say, three 100MW generators, and you need to take one offline without impacting the power grid, how would that be accomplished? We don't always reduce power that slowly though, it's just preferred for the sake of the rest of our plant equipment rather than concern for the grid. We might have a situation that warrants a 4% per minute load reduction. We also have automatic load reductions that will shed 320MW in about 9 seconds. Or we might just trip offline from 100% power due to some fault. quote:The problem with just opening the circuit breaker (assuming you have a generator breaker and aren't utilizing a higher-voltage breaker upstream) would be: quote:The turbine would need to respond quickly to avoid an overspeed (you've lost what was absorbing power from the turbine, effectively you've lost the brake) quote:You'd have to kill excitation quickly to ensure you don't excite an open-circuit stator and damage it from generating a very high voltage Gisnep fucked around with this message at 15:44 on Feb 4, 2014 |
|
#
?
Feb 4, 2014 15:37
|
|
|
Gisnep posted:We have a circuit that continuously monitors generator electrical output and compares it to the turbine steam inlet pressure. If it senses a significant mismatch between the two, it will automatically throttle the turbine inlet valves to prevent an overspeed. Power/load unbalance is quite the trip. We compare first stage pressure and output current. When the breaker opens a modern static exciter can respond pretty drat fast to keep terminal voltage steady. Opening the exciter contactor with any field on the generator can be really bad as the reverse voltage generated by the collapsing magnetic field can be very high so we have a deex and crowbar circuit. The sequence of events for our exciter is stop on running with the 52g open, stop bridges, then deex/crowbar to kill the field, and finally the exciter contactor opens.
|
|
#
?
Feb 5, 2014 00:30
|
|
|
Lets talk about what it takes to avoid rolling blackouts. I know ERCOT will ask industrial customers to drop loads in the early stages of an emergency. How does this work? I can't quite picture someone at ERCOT picking up the phone to call every plant in the state saying "hey you guys, we're like, out of this electricity thing, can you shut everything off for a bit so grandma can cook her waffles? THANKS!". I also can't quite picture an entire facility suddenly being shut off with no warning; that seems like a fantastic way to destroy equipment (and cause injuries). The only personal experience I have at home with power emergencies is rolling blackouts. And those don't get implemented until "oh poo poo, guys we're so out of electricity that the entire state grid is about to go into meltdown!". In a commercial setting, I've seen about 3/4 of the lighting get shut off on a college campus out of nowhere, along with a good chunk of the HVAC at the exact same moment (as a student). How's this  stuff work? It can't be all manual, and I doubt it's 100% automatic.
|
|
#
?
Feb 5, 2014 10:58
|
|
|
some texas redneck posted:Lets talk about what it takes to avoid rolling blackouts. Some sites will enter into agreements with the power company that they agree to run off generator during times of emergency in exchange for paying a reduced electrical utility rate the rest of the time; in these cases, it really is a matter of the power company calling these customers and asking them to come up on gen, because they've entered a contract and are legally obligated to. The customer walks to the gen controls, pushes a couple buttons, and they're on gen a few seconds later. [Win-win for most of these customers, because they'd be up on gen during the brownout or rolling blackout anyhow.] In some cases, where they're configured right, companies can come in parallel with the grid and sell energy back; otherwise, they just operate in island mode, taking their own load off the grid and easing issues. Residential grover fucked around with this message at 11:58 on Feb 5, 2014 |
|
#
?
Feb 5, 2014 11:52
|
|
|
Further stupid questions. Is any kind of sync required when switching between power sources? Or is the delay from whatever breakers/relays interrupting power to switch between gen/poco enough? (I know you need to synchronize if you're generating while still taking poco power, but...) I know an emergency backup generator only kicks in after the power has failed, but I have no idea how even those switch back to poco beyond a few switches. I know the generators we had at the grocery store I worked at took about 15 seconds to start providing power to lights/registers after a poco failure, but it literally powered about 1/50 of the store (handful of lights + all registers + electric doors + a couple of outlets in each dept - no refrigeration). randomidiot fucked around with this message at 12:05 on Feb 5, 2014 |
|
#
?
Feb 5, 2014 12:03
|
|
|
some texas redneck posted:Further stupid questions. No these are good questions. If you have a switch that breaks before makes, you can just switch from one source to another. So you are on one source, the system disconnects from power supply one, and connects to power supply two. If you have a setup that makes before breaks, you need to make sure both sources are paralleled (voltage aplitude, frequency, and phase all match), you close both breakers, then open the breaker you want to disconnect from. One thing to consider with generators and switching over is that if you, say, start a 20HP motor up, you'll see very minimal voltage dip. For the electrical system up to the utility (assuming nothing is undersized), that one motor starting is just a very small drop in the bucket. However if you are on a comparatively very, very small local generator, that single motor starting up can potentially cause the voltage and frequency to drop. So I had a bus with a generator powering several motor loads, and it is set up so after the generator starts there is a several second delay between each motor coming on line. The generator would trip if all those motor loads were connected simultaneously, but if they are turned on one at a time it can run them all.
|
|
#
?
Feb 5, 2014 22:39
|
|
|
some texas redneck posted:Further stupid questions.  The automatic transfer switch (ATS) is the brains; it monitors utility power and makes sure it's in tolerance. If power fails, it waits a few seconds to make sure the power is REALLY out and not just a blink, then tells the generator to start. Usually a small emergency diesel will be on and up to speed within 1-2 seconds. The ATS monitors the generator voltage/frequency, and as soon as it's in tolerance, switches power over to it. (TVSS is transient voltage surge suppressor) The whole process can be as quick as 5 seconds total after power failed to bring the lights back on, but it depends how it's programmed. There is often an intentional delay of 10-15 seconds to allow time for motors to spin to a stop before power is restored. The most inexpensive form of ATS is "open transition", or "make before break" which basically means the generator will disconnect before utility power is reconnected. This happens in the blink of an eye- typical transfer time is about 100ms. To do it successfully without blowing every running motor, though, the generator has to be sychronized with the utility power- in other words, the 60Hz peaks of both waveforms must align. The old-school method was to use a syncscope and manually pull the lever, but modern ATSs do this automatically. They don't usually control the generator at all, just watch and wait for them to drift fairly close in phase with each other. More sophisticated ATSs will be "closed transition", or "make before break", which places the generator in-line with the utility power. In the simplest form, they synchronise as above, but make the 2nd connection before breaking the first, and will be in parallel for about 100ms. This allows the return to utility power to be absolutely seamless. This also allows a site to transfer to generator if an outage is likely (storm anticipation mode). The most sophisticated ATSs are "soft transition", which bring the generator in full parallel with the utility to slowly transfer power back and forth by carefully regulating generator voltage and phase angle; this eliminates the dip in voltage often associated when generators have full load quickly dumped upon them like Three Phase described, and is less stressful for the attached equipment. It's also expensive, complicated, and require a lot of sophisticated protective relays and controls. It's more common on very large systems because the controls involved scale up well, but don't scale down well; it's easier to absorb a $300k controller into a $5M project than a $100k project. This is what you're seeing in that switchgear video with the generator failure. grover fucked around with this message at 02:55 on Feb 6, 2014 |
|
#
?
Feb 6, 2014 00:40
|
|
|
This video demonstrates what happens when you try to parallel a generator that is out-of-phase. Several times in a row.
|
|
#
?
Feb 6, 2014 01:31
|
|
|
some texas redneck posted:Lets talk about what it takes to avoid rolling blackouts. Oddly enough, this is exactly what they do. At least, it's what I've seen from an end user perspective. Several years ago I worked in the IBM building in Rochester, MN doing a 2 semester co-op. It's a pretty big campus and probably uses a good deal of power for all the servers and shock/vibration test equipment they have. Several times that summer there were announcements over the PA that told everyone to shut off lights and unneeded computers/equipment because the power company called and asked nicely. I don't know how much good it did, but I'm pretty sure they were the largest power customer in the area, so they were probably first on the "shut off your lights" phone tree.
|
|
#
?
Feb 6, 2014 02:21
|
|
|
some texas redneck posted:Lets talk about what it takes to avoid rolling blackouts. I know at least at one steel mill in CA, they do get a phone call that is their 30 min notice to be under X% of their normal draw (basically, "shut down the mill, but you can keep the lights and AC on") within 30 minutes. They also schedule their heavy maintenance at the power company's request, so they're shut down at least 2 days a week in the summer, but for all this trouble, they pay about what plants in the south do for power instead of the normal CA prices. As for the automation, it's really not very automated, from what I gather. The down days are scheduled via email (though they have a typical schedule), and the high demand shutdown is literally a phone call. They do consume something like 200MW though, so that phone call is pretty productive from a load shedding POV.
|
|
#
?
Feb 6, 2014 04:38
|
|
|
|
| # ? May 10, 2024 14:49 |
|
|
Three-Phase posted:This video demonstrates what happens when you try to parallel a generator that is out-of-phase. Several times in a row. Christ I wish I wasn't on ship internet. Regarding what you guys were talking about with frequency/voltage drops on large loads, I can personally testify that on departure, when starting our thrusters (around 2MW each, x3) with about 19MW on the board, we very often get a frequency drop of up to 3HZ.
|
|
#
?
Feb 6, 2014 11:28
|
|
