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Three-Phase posted:Hell, I've worked around motors that were like 3 megawatts. Just one motor. That's nothing; we're putting in some 47MW motors later this year. VFDs for them too.
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Aug 13, 2014 05:05
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KaiserBen posted:That's nothing; we're putting in some 47MW motors later this year. VFDs for them too.  I'm going to a job site the next couple weeks where they'll have a couple of multi-MW Solar generators just as temporary stand-in feeds for doing functional tests.
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Aug 13, 2014 05:23
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My friend and I are having a disagreement that I hope you can help resolve. He claims that in an AC circuit, there must always be a closed loop for current to flow. I believe that an earth ground would have enough "free" electrons for a current to flow, even if there was no connection between grounds. To that end, we made a thought experiment: Imagine a superconducting transmission line between the earth and mars with a single conductor. Now imagine an AC generator placed halfway on this line. If you ran the generator, would there be a current through the line?
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Aug 16, 2014 04:00
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No.
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Aug 16, 2014 04:07
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KaiserBen posted:That's nothing; we're putting in some 47MW motors later this year. VFDs for them too. Nice. What company are you looking at for the drive? (I think ABB has water cooled Megadrives in that range.)
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Aug 16, 2014 10:33
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KillHour posted:My friend and I are having a disagreement that I hope you can help resolve. He claims that in an AC circuit, there must always be a closed loop for current to flow. I believe that an earth ground would have enough "free" electrons for a current to flow, even if there was no connection between grounds. No, there wouldn't. If there was, it would only be to balance the charge of the two planets, would be DC, and would stop when they had the same potential. Look up kirchoffs current law - you must have a return path, basically. Currents at any node sum to zero and voltages around any loop in a circuit sum to zero as well.
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Aug 16, 2014 13:43
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kastein posted:No, there wouldn't. If there was, it would only be to balance the charge of the two planets, would be DC, and would stop when they had the same potential. I looked this up, and now I'm even more confused. Wikipedia posted:They are accurate for DC circuits, and for AC circuits at frequencies where the wavelengths of electromagnetic radiation are very large compared to the circuits. Wouldn't this imply that those laws don't apply or are inaccurate for my example? Edit: And another problem I can't wrap my head around is that the wire would be so long that it would take a significant amount of time (minutes) for light to traverse the distance. Since information can't travel faster than the speed of light, there would be no way to tell from the perspective of the generator if the ends are connected or not. Would there be a current for a few minutes until [something] goes "Oh poo poo, there's no connection!" and just dies? Or conversely, would there be no current in an extremely large loop until [something] could traverse the entire thing, and then the current would magically start? Otherwise, you could test to see if a switch is open on the other end of the universe by trying to put a current through a giant loop connected to the switch at the other end - and that would violate relativity.  Double Edit: What if you had an infinitely long wire stretching in each direction? KillHour fucked around with this message at 16:36 on Aug 16, 2014 |
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Aug 16, 2014 16:16
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Suppose you have a wire that goes from one side of your generator to the other side. As expected, current flows. I come along and cut the wire, now no current flows. If it then attach each end of the cut to a pair of big flat plates that are separated from each other by a small gap, then current flows until the voltage cannot build up more charge on the plates. It is a capacitor. If the generator is producing AC at a frequency where the voltage reverses before the capacitor is fully charged, current will flow as though the wire were unbroken (there is some difference concerning the relative timing of current and voltage changes that are being deliberately ignored, but current does flow). If the wires are long enough, you don't need plates because the wires themselves have enough capacitance.
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Aug 16, 2014 21:30
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Frozen Horse posted:Suppose you have a wire that goes from one side of your generator to the other side. As expected, current flows. I come along and cut the wire, now no current flows. If it then attach each end of the cut to a pair of big flat plates that are separated from each other by a small gap, then current flows until the voltage cannot build up more charge on the plates. It is a capacitor. If the generator is producing AC at a frequency where the voltage reverses before the capacitor is fully charged, current will flow as though the wire were unbroken (there is some difference concerning the relative timing of current and voltage changes that are being deliberately ignored, but current does flow). If the wires are long enough, you don't need plates because the wires themselves have enough capacitance. So the answer is current would flow, but only because of how long the wires are? Does the size of the mass at the end of the wires have anything to do with it - i.e., would the Earth and Mars act as the plates in the capacitor? Edit: Doesn't this assume the plates are near each other? If you had two wires going off in opposite directions, they wouldn't be near enough to have the same effect as a capacitor, right? Maybe this is more of a question for the physics thread. KillHour fucked around with this message at 21:57 on Aug 16, 2014 |
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Aug 16, 2014 21:55
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Yes you would have current. Let's simply the question. You have an AC source in the middle of 2 lines at neutral potential. When you apply potential to the line it takes time for that potential to reach from the source to the end of each line (electricity can only move at the speed of light). Since the potential is not the same across the entire line, charge will move to balance and you will have current on the line. Now if we had an ideal DC source, the potential would eventually be the same and once the transient currents died off, you would have no current on the line. In this problem we have a constantly changing AC source, so there will constantly be a potential difference as the new potential moves across the line, hits the end and reflects back. This is the basis of how an antenna works. Kastein: Kirchhoff's current law is a huge simplification intended for use in network theory. It breaks down pretty easily. The junk collector fucked around with this message at 01:33 on Aug 17, 2014 |
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Aug 17, 2014 01:26
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This is a really great question.
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Aug 17, 2014 02:32
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I have an expensive dinner riding on this, guys.
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Aug 17, 2014 02:51
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Yes current would flow, at those length the wires would basically act as antennas.
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Aug 17, 2014 02:54
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You're right, he's wrong because you can only rely on Kirchoff for steady state problems. No need to bother with generators halfway to Mars though, as mentioned a simple antenna is all you need.
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Aug 17, 2014 02:56
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Here is a very thorough answer to the question.
Platystemon fucked around with this message at 07:44 on Aug 17, 2014 |
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Aug 17, 2014 03:27
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The solar wind is a massive outpouring of charged particles. If you had a high enough equal and opposite potential on Earth and Mars, would the interplanetary medium complete the circuit?
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Aug 17, 2014 10:36
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It depends. The current would be dc, not ac, from that, and it really depends on whether the moon or mars has a higher potential. I certainly wouldn't want to be in between to find out for myself because I wouldn't be surprised if the charge was significantly more than a lightning strike... like orders of magnitude higher. We're talking about a nuclear powered van de graaf generator the size of the solar system, basically, and that is more of a physics question than EE/electrician stuff for sure. It's been a decade since I took those classes  This is actually pretty interesting, and I (re)learned something. I forgot kirchoff was steady state only. Whether it acts as an antenna or not depends on how closely matched it is to 60hz resonance, which is a factor of the electrical length of the wires and also how much inductance and/or capacitance is involved. You can make an antenna shorter than it should be resonant by putting capacitive loading hats on the ends of the elements, I imagine mars and earth would make pretty good loading hats. There are a lot of details I am sure I am forgetting here and basically the answer is "it depends". Assuming you aren't vaporized by the interplanetary solar wind powered lightning bolt when the circuit is completed, that is. It would get even worse during a geomagnetic storm because we on earth and mars would almost certainly not see the same surge of charged particles, or even best case it would come hours apart, so in the meantime you would be looking at charge transfers on the order of terawatts (terajoules per second), iirc. Been a while since I read up on sunspots/coronal mass ejection however.
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Aug 17, 2014 15:20
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Three-Phase posted:Nice. What company are you looking at for the drive? (I think ABB has water cooled Megadrives in that range.) It's our drive/motor, we're selling to a compressor OEM for a natgas compressor. ABB does offer the Megdrive in that range, but it's considerably older tech (being an LCI drive, using thyristors vs ours which is a voltage source drive using GTOs). I got to see factory testing on a 25MW version of the same drive, the thing is absolutely massive.
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Aug 17, 2014 23:27
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Yeah, and I think LCI (load commutated inversion) only works with synchronous motors where you can overexcite the rotor. I take it the harmonics thrown back on the line are a lot less with a voltage source drive than a current source drive? Do you still need a 12 or 18 pulse rectifier? I've also found that water cooling scares some people.
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Aug 19, 2014 01:26
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Three-Phase posted:Yeah, and I think LCI (load commutated inversion) only works with synchronous motors where you can overexcite the rotor. You're correct on the LCI, and it's often used with a "6-phase" motor, where you essentially have two 3-phase motors at 60deg electrical angle mounted on a common shaft, using them as a master-follower pair, to smooth the torque pulsations that type of drive is known for. The voltage source drives can put a lot fewer harmonics back; they still need 18/24 pulse rectifiers in the larger sizes to maintain IEEE519 compliance. We have one MV drive with an active front end, using LV IGBTs in series (5-10 cell modules per phase), but I've not seen one "in the wild" yet. Water cooling does irrationally scare a lot of people. OTOH, we did have a customer disable all the interlocks and manage to run a drive lineup iwth the water cooling system shut off. Nice $1m or so mistake there.
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Aug 19, 2014 06:25
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Would it be a good idea to take the groundsman bootcamp class at my local CC or applying to the JATC for apprenticeship in DC if I wanted to get into this field?
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Aug 27, 2014 00:22
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What electrical career are you looking to get in to? Do you have any field experience, even just general construction? There are several commercial and industrial electricians on here, and I'm a lineman for a co-op power utility.
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Aug 27, 2014 02:47
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angryrobots posted:What electrical career are you looking to get in to? Do you have any field experience, even just general construction? Lineman mostly, however I dont have any construction experience at all other than around the house DIY's which is why I was wondering about the groundman class. But I figured everyones gotta start somewhere. my location is in NOVA. Tenchrono fucked around with this message at 05:31 on Aug 27, 2014 |
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Aug 27, 2014 05:29
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Are you looking to get hired by a utility company or by one of the contracting companies? I cant speak for other utility companies but the one I work for does not care whether or not you have any formal training since no matter what you have to go through theirs. They usually prefer you dont because they would rather you do things the way they want.
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Aug 27, 2014 12:09
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I can tell you too that the linemen message board I follow (mostly for cool pictures) usually tells guys looking to start a career, to not bother with the line schools. If the union is in your area, they usually tell them to "sign the books!" whatever that entails. And I agree with the previous post, my utility doesn't hire 'experienced' workers. It would be better to have any kind of construction experience, something to show you are technical and willing to work in the weather.
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Aug 27, 2014 13:15
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TheFargate posted:Are you looking to get hired by a utility company or by one of the contracting companies? I cant speak for other utility companies but the one I work for does not care whether or not you have any formal training since no matter what you have to go through theirs. They usually prefer you dont because they would rather you do things the way they want. I would like to be at a utility company itself. angryrobots posted:I can tell you too that the linemen message board I follow (mostly for cool pictures) usually tells guys looking to start a career, to not bother with the line schools. If the union is in your area, they usually tell them to "sign the books!" whatever that entails. So should I bother with applying to the groundman position at the local power? https://careers-dominion.icims.com/jobs/5036/groundman/job Or should I somehow find a way to get some construction experience in first. I really have no idea where to start.
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Aug 27, 2014 16:35
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I'd def try to get an interview. Play up whatever technical experience you have, and that you understand the job is outdoors in all weather, and very demanding. I'm assuming you're in pretty good shape, they probably require a physical prior to employment. They spell out pretty clearly what they want in an applicant: quote:Previous experience in working with mechanical or electrical concepts and applications
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Aug 27, 2014 16:57
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Definitely try to get an interview. Write up your resume best you can to match the criteria they are looking for. Also its pretty easy to move around inside of the utility company I work for. We have our own "in house" IBEW local. I started basically as a laborer and after a few months I was able to bid on a job as a relay tech. If that company has a similar setup just bid on any position to get your foot in the door. I know if definitely helped i used to do jobsite safety for them and i pushed safety like crazy at the interview. What previous jobs have you worked before that you could list? E: looked at that job posting and it sounds a lot like the first job I had. I had a CAST test and it was pretty simple. Mostly simple mechanical concepts. Definitely apply. TheFargate fucked around with this message at 18:38 on Aug 27, 2014 |
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Aug 27, 2014 18:34
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Thanks for the advice  . I'love be writing up a resume later though the only other job I've had was being a manager at a pizza Hut.
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Aug 28, 2014 01:39
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So is there anyone on here thats good with doble protest? Im having some issues writing macros for testing SEL 311Ls and GE L90s. Most of my coworkers only really know how to use state simulation and ramp macros. Im trying to write a binary search current macro and as it stands the relays are actually picking up at a lower value than intended and Im not 100% on whats going on. There seems to be a complete lack of info online about protest macros so any advice would be great.
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Aug 28, 2014 12:53
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I'm looking at the user's guide right now and there's a fairly thorough explanation of the binary search macro in Appendix C. Do you not have that document? Also which function are you trying to test?
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Aug 28, 2014 15:12
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Yes I have the user guide and I understand exactly what the binary search is doing. I am testing phase to ground fault. The problem I keep running into is that with the binary search, at least with the L90, picks up an amp lower than what its supposed to. Usually that wouldnt bother me, however, when I run a state simulation to check that pickup value the L90 will not pickup at any value lower than that specified. The only thing I can come up with is in my macro the ramp at the end is too long on each pulse. I havnt had a chance verify this and wanted to see if anyone here has any advice since I am still fairly new to this.
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Aug 28, 2014 16:07
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Might be on youtube, not sure, saw it on Facebook. three phase set of pole pigs cooking off. https://www.facebook.com/video.php?v=853253108018855&permPage=1
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Sep 1, 2014 23:22
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Dumb this down for me (it was in the video comments) I assume by cutouts, they physically cut the lines somewhere, but what's a load break tool?
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Sep 2, 2014 06:39
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some texas redneck posted:Dumb this down for me (it was in the video comments) Recloser is a device that isolates a fault and then attempts to close the section of a circuit back in to see if the fault on the lines have cleares. If the fault is still there it should reopen and isolate the faulted section. I believe by lost he means it failed to open. Cutouts are switches manually opened by a lineman with a fiberglass stick to energize or de-engergize a section of a circuit. I assume by load break tool he means load break disconnect which is a big switch in a substation designed to break load on a circuit. E: I can elaborate further if you like. TheFargate fucked around with this message at 10:21 on Sep 2, 2014 |
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Sep 2, 2014 10:02
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No a load break tool is a separate attachment that goes on a fiberglass stick to open up a switch (of various types - bladed, fiberglass cutout, whatever) that is carrying load. On our system, if there was no upstream recloser beyond the failed recloser (so the failed device was at the substation, basically), the high side breaker at the station would have opened on overcurrent, because we'd rather it open up than burn the world down. I can only imagine that the protection scheme is different if you have critical loads (hospital, industry) that the engineers are trying to keep on at all cost. angryrobots fucked around with this message at 17:52 on Sep 2, 2014 |
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Sep 2, 2014 17:20
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Our schemes get fairly complicated on our 13kv systems. My experience with 13kv is fairly limited since I do the construction side. The group Im in typically works on 69-500kv. I do know that our 13kv system is really flexible when it comes to sectionalizing and feeding from different stations. Thanks for clarifying on the load break tool.
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Sep 2, 2014 18:23
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Another example of failed fault protection from the OSHA thread : http://www.youtube.com/watch?v=mvNA2kawKVE Couple minutes in, you see a house that is obviously getting fed primary voltage over its service. Description says a 30 kV transmission line fell onto the 13 kV distribution line, which may be but I suspect that one of those primary lines fell onto the neutral.
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Sep 4, 2014 23:27
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Right now mother nature decided that September is an awesome time to dump a huge amount of snow on southern Alberta where the leaves are still on the trees. Broken branches and trees are falling on lines and I know they're fixing it. However, I don't really know what's going on in a more technical sense. Does the power company have a controller somewhere that can see which lines are down? Or which breakers have tripped? Is there anything special when you're trying to fix a whole city while the whole province is hosed up? There's no trees close to anything high power in the substations but would high power lines be affected? I almost regret not doing much power engineering when going through school.
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Sep 10, 2014 14:46
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Jyrraeth posted:Right now mother nature decided that September is an awesome time to dump a huge amount of snow on southern Alberta where the leaves are still on the trees. Broken branches and trees are falling on lines and I know they're fixing it. On some of our reclosers, we can view the event that tripped it out in the control panel. The fault current data can tell us about how far out the fault is. But generally in a storm we just ride to find lines down. I suppose utilities that use SCADA can probably retrieve that data as well, and get an overall map of outages and potential fault locations, maybe someone else has experience with that. Anything special, not sure what you're asking about but obviously the first step in a large outage is to get main feeders back up and on, sort of triage our work to get the most back on the quickest. Long taps through the woods will be waiting longer, that's the price of having private property way off the road. The exception is of course when company executives or the mayor or who the gently caress ever is out, and we will get pulled to get so-and-so's lights back on, because that's the way the world is. High voltage transmission lines are usually not as affected because they usually have much wider right-of-way, but they hit the dirt sometimes too. Everything in line work is temporary.
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Sep 10, 2014 19:03
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