|
rainwulf posted:Question again for anyone: does anyone know/work with the high current low voltage DC stuff used in aluminium refineries? I have heard of stories revolving around the current being so high that its magnetic fields can actually bend stuff, but i would like to hear from an expert. I've worked with high-current low AC voltage before during some special testing. As far as magnetic fields being so high it can bend stuff, it's possible, but it's much more likely during faults. I've seen switchgear rated for, say, 60,000A faults, which means it has to handle both the heat generated from the short-duration fault as well as the tremendous forces involved. It is possible to do things like warp metal busbar sections in a serious fault, or whip cables. I've heard of cables being ejected from cable trays that were tie-wrapped every few feet with really serious tie-wraps that would take over a hundred pounds of force to break. Usually busses are fed from a step down transformer, and when you buy that transformer you get a nameplate on it: 2000kVA PRIMARY 4160V DELTA SECONDARY 480/277 WYE 4% Z The "4% Z" is the transformer impedance, and that's used to determine the maximum fault current that can occur in a short circuit on a bus. The silicon-controlled rectifiers I've used are as big as dinner plates, and they had to be cooled with water blocks on either side of them. While a normal little diode or SCR can handle milliamps or an amp or so, these could handle 3000A. Three-Phase fucked around with this message at 11:36 on Oct 5, 2011 |
#
¿
Oct 5, 2011 11:26
|
|
|
|
| # ¿ May 17, 2024 20:22 |
|
|
What voltage do you generate at your plant? I'm assuming you're using medium-sized natural gas turbines, right? (Like generating at 13.8kV and moving that around to unit substations around the plat at either 13.8kV or 4160?) Are you doing something like electroplating? Biggest deal with a chemical plant (especially petrochemical) is that you're gonna need to be really careful in areas that have potentially explosive atmospheres - using explosionproof equipment (the case will contain an exploding gas inside of it and not vent hot gasses) or intrinsically safe instruments (it's impossible to form a spark that can set off a certain type of explosive atmosphere). - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Let's talk contactors really quick! (I like this guy's videos): http://www.youtube.com/watch?v=RjYyAPcr_7k&feature=related A few things that aren't mentioned: 1. The contacts are sometimes rated differently, like 250V 10A resistive - if you hook an inductive load like a motor up, you cannot interrupt as much current on the contacts and it has to have the current derated 2. For high voltages and currents, the contacts are contained within a vaccum bottle (the difference between this and a vacuum circuit breaker is that the breaker is typically controlled via a protective relay where a vacuum contactor may or may not have this feature or the ability/rating to interrupt faults) 3. The "circuit breaker" inside the medium-size contractor he showed is commonly called an "overload", when connected to a contactor it's called a "starter". The overload ONLY provides protection against overloads! It does not protect against a short-circuit fault. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Looks like this guy is testing a Magne-Blast breaker? http://www.youtube.com/watch?v=f3aHMLZZlkU&feature=related If the breaker was really installed in a cubicle and operating, you'd never see the blades separating because each of the three poles would be covered up by an arc chute to spread out and cool the big electrical arc produced by switching. On a breaker this big, you have a close and trip coil, you energize the close coil to close the breaker, and energize the trip coil to trip it. Three-Phase fucked around with this message at 04:07 on Oct 29, 2011 |
|
#
¿
Oct 29, 2011 03:47
|
|
|
Vanagoon posted:A bit off topic, but have you ever heard of Aluminum referred to as being "congealed electricity" because it takes so much power to smelt it? I had heard about people trying to see if there was a way to somehow "reverse" the smelting to store and regenerate electricity. Sounds odd though.
|
|
#
¿
Dec 29, 2011 22:16
|
|
|
Groda posted:You aren't of proposals (there was something on Slashdot a while ago) to use aluminum-oxidizer batteries as a way of storing surplus electricity via aluminum production? Yeah, that's what I was talking about. The way I initially described it does, as the other poster said, sound like utter nonsense. I heard something on Quirks and Quarks about it awhile back when they were talking about massive energy storage systems for power grids.
|
|
#
¿
Dec 30, 2011 01:38
|
|
|
What is the system voltage there helno? 24kV?
|
|
#
¿
Jan 7, 2012 05:25
|
|
|
grover posted:(Some equipment even has "battle short" which bypasses fuses and circuit breakers in critical systems during battle, and eliminates the risk of nuisance trips, but at the tradeoff of repair costs cost, since the fuses no longer limit damage during equipment faults. Better to replace a few circuit boards than lose a ship and its crew, though!) Now that's a neat feature. I believe that on fire protection equipment, you don't install circuit protective equipment. The idea is that if there's a fire, you don't want anything to interrupt critical pumps that are being used to fight the fire. Also, on some motor protection relays like the GE Multilins, there is an emergency start feature. Big motors are usually limited to so many restarts per hour because this causes too much thermal stress/wear on the motor. If you short out the emergency restart terminals, the Multilin will "forget" the current thermal status and let you start the motor (serious problems like ground faults, failure to start/stall, phase loss, will still trip out the motor).
|
|
#
¿
Jan 11, 2012 00:14
|
|
|
One thing I remembered about 69/120: I believe that on a lot of PT circuits, you typically get that ratio on the secondary. Like on an open-delta PT. So 4160/2400 is stepped down to 120/69, except it's a delta so you really should never see the 69 volts.
|
|
#
¿
Jan 13, 2012 14:07
|
|
|
I've got to print that label and show it to my co-workers. I have access to SKM Power Tools as well. Is that a real piece of equipment you did calculations on? I'm guessing that's the immediate secondary terminals before any protective systems or cables/busbars of a HV to MV transformer maybe 10MW or larger. Actually, if you get burned badly enough, you won't feel any pain, but that's usually a really bad sign. Criminally stylish posted:My uncle used to work alot with power and he was shocked pretty bad one day at work. Has this ever happened to you on some scale, or has it happened to any of your co-workers? I was hit with 70Vrms at a previous job across my fingers, a painful zing, and that was the highest. I was hit with more at home. Where I work, it's getting harder and harder to do live work even at voltages as low as 120VAC. "Peeking" a live panel (removing or opening the access door from an energized 208/120 panel) is now effectively prohibited without a work permit and arc flash gear. Anything over 600VAC requires two LO/TO'ed breaks for safe work. Depending on the situation, opening and racking out a MV breaker may or may not count as two breaks. Everything is checked using a live-dead-live checking technique before anything is worked on. Buddy systems are used and there are safety guidelines that have to be followed at all times. One of my co-workers was hit with 480VAC (not 277, it was line-to-line across his chest) at a previous job. He woke some time later up several feet from the panel with tremendous pain in his chest (broken ribs) and workers over top of him - frantically giving him CPR (hence the broken ribs) since his heart stopped. I think he was clinically dead for about a minute. Fortunately there weren't any lasting problems besides almost dying. A good colleague for another company suffered a massive heart attack at place where he was doing field work. That wasn't electrical, however. Fortunately everyone he was around (electrical folks) were trained in CPR, and had an AED on him in less than a minute. Fortunately, the facility I work at has a very stringent safety culture, one of the best I've come across. They definitely foster a "if you don't think what you're doing is safe, put your tools down and tell someone immediately, kick it up the chain of command if necessary." Lots of people are also CPR/AED trained and understand the electrical hazards present at a large industrial facility. (The high-voltage workmen especially are masters at their craft, but everyone who works on electrical systems is very competent.) We have had a few close shaves, but fortunately I don't think we've ever had an electrical fatality where I work. We intend to keep it that way if at all possible. The biggest thing that scares me is miscommunication: like someone having an order to isolate breaker B012 but they isolate B021, and people open up a panel assuming everything is dead (when it really isn't). That's what double-checking for voltage is for, as well as multiple isolation sources, and making sure that when you build a system, you always put isolation means (like clearly labeled disconnect switches, both low and high voltage) near the loads that may be worked on. Three-Phase fucked around with this message at 21:23 on Jan 16, 2012 |
|
#
¿
Jan 16, 2012 21:12
|
|
|
Cheesemaster200 posted:Close. Its actually two networked 5000 KVA transformers in an outdoor 33kV/4160V substation. I only show the utility above it with around 10,000 MVA of fault duty (to simulate an infinite source), which is why SKM freaks out. According to the arc flash report, the bus has 140kA of bolted fault current.... at 4160V. The arc-flash boundary is somewhere around 75 feet, haha. Transformer secondaries scare me. Truly, legitimately, scare me. Do you have a vacuum or SF6 breaker on the secondary that's rated for 1GVA of interruption or have some kind of fault limiters installed? One thing I heard about awhile back (and I am very green with arc flash calculations, so correct me if I'm wrong) is that sometimes if your source has a higher fault capability, it can result to lower arc flash levels and vice versa. The idea being that a much more serious fault with tremendous amounts of current will trigger the instantaneous trip on the breakers in the path of the fault, where a source that limits the fault current will (obviously) have a smaller amount of fault current, but that would move up the trip curve on protective devices, so you may have a smaller amount of current, but be exposed to more cycles. I need to check, but where I work I think with our connections we could easily have >1GVA of fault. Can the utility company tell you what the line-line and line-ground fault current levels are? (Oh wait, maybe you ARE the utility company...) Three-Phase fucked around with this message at 23:37 on Jan 16, 2012 |
|
#
¿
Jan 16, 2012 23:28
|
|
|
Frozen Horse posted:That's up there with the "Do not stare into beam with remaining eye." laser warning sign I saw outside one of the labs when I was an undergrad. On the other hand, if it can set your shirt on fire, you probably don't want to have your cornea focusing it on your retina. Not only can it set your shirt on fire, it can probably do it from more than ten feet away.
|
|
#
¿
Jan 18, 2012 00:35
|
|
|
Frozen Horse posted:How does a ground rod ground? Is dirt more conductive than I give it credit for? Does the answer depend on the dirt? I suspect that driving a ground rod into a dry sand dune won't do much. Dirt can be conductive, it's got moisture and minerals in it. Now in some parts of the world, you need to do different things to get a good ground, like adding salt to the ground, or installing a "Ufer ground". Concrete also conducts electricity, something I didn't know until I was in college.
|
|
#
¿
Jan 18, 2012 11:51
|
|
|
My opinion on the dust collectors? It almost sounds like it's a little too big for LV, and a little too small for MV. Does the plant already have a medium voltage source like 2400 or 4160V? Or does it have a higher MV source like 13.8kV? I also take it that those are induction motors, not synchronous. He's gonna get dinged for power factor unless he installs some form of compensation. What is that at 0.8pf, around 100kVAR? Yeah, 3x350, the bend radius has to be in the multiple foot range. I've seen Okonite 15kV shielded triplex at work and that's even worse, you're talking a 5-10 foot bend radius.
|
|
#
¿
Jan 19, 2012 11:58
|
|
|
Cheesemaster200 posted:Medium voltage is expensive, gigantic and a bigger pain in the rear end to work with. It is also potentially more dangerous, which tends to shun people away. Where I work, there are separate electricians that do HV work (over 600V) and LV (less than 600V). That might be one major complication. Has anyone here done electrical stuff in mining? I see these circuit breakers that are for mining applications and they're rated at 1000VAC. (I think that's 1000/690.) What's up with that strange voltage? Three-Phase fucked around with this message at 03:54 on Jan 20, 2012 |
|
#
¿
Jan 20, 2012 01:08
|
|
|
McJuicy posted:I got shocked with 347 lighting. That poo poo makes you stick. 347 lighting? You in Canada?
|
|
#
¿
Jan 21, 2012 23:21
|
|
|
ruro posted:This is a great thread, I love reading about things like this that most people aren't aware of but are essential to our daily lives. A couple of videos I found interesting: 800kvDC They test some of the apparatus at 1,000,000V AC for a minute. Three-Phase fucked around with this message at 00:52 on Feb 3, 2012 |
|
#
¿
Feb 3, 2012 00:50
|
|
|
Stonelegs posted:When you're demoing old duct bank or conduit, how do you tell if a line is live without putting someone in danger by accessing it? This assumes you have no drawings and no idea what's in the ground. You'd visually trace the line or bus back to where it originates. It takes skill and understanding of the installation to do this, particularly in crowded conduit or cable tray. Bigger equipment (including HV) is much easier to trace because at higher voltages special cables/bus are used, smaller stuff can be more of a hazard/pain in the rear end. In a badly crowded cabinet, you can tug or wiggle on a single wire and use that to trace where it goes, or use a fox/hound tracer. As far as safety, there are items like the Fluke VoltAlert pens that glow and beep if there is a hazardous AC voltage present. The problem is when you have 125VDC that the VoltAlert cannot detect. 4160VAC in an insulated but unshielded cable makes the VoltAlert beep from about four inches away. (Yes, I cringe at that a little too.) Frozen Horse posted:The bad-movie option: sever the conduit with a linear shaped charge. If the explosion is followed by arcing, it contained an energized circuit. On the other hand, you're at a safe distance provided that you didn't use wires to trigger the charge. We need a "BAD ELECTRICITY" feature here- like "Bad Astronomy" except where movies and video games get electrical stuff right and wrong. Three-Phase fucked around with this message at 01:08 on Feb 4, 2012 |
|
#
¿
Feb 4, 2012 01:02
|
|
|
Bad Electricity: Jurassic Park 1 http://www.youtube.com/watch?v=vWmIA5GGeFQ GOOD: You can't just throw the breaker. This is correct for larger low and medium voltage breakers. You have to charge internal springs, this is usually done by an electric motor (sometimes it's a modified electric drill motor!), but if you have no control power available, you can get a breaker with a manual charging device, like a lever or a crank. GOOD: The circuit breaker looks very accurate, I cannot see what make/model it actually is but I'm suspecting a Siemens low voltage breaker. I'm not sure if they make medium-voltage breakers (2400V or 4160V) that have a charging handle like that. GOOD: The operation of the breaker is accurate. BAD: The panel with all the different sections and the controls for that is a bit hokey. GOOD: The color coding for the different subsections panel where red indicates ON and green indicates OFF. Some industries use red (danger) for ON, and green (safe) for OFF. BAD: The raptor pushing through a curtain of large cables. Not neat or workmanlike for an installation- those would likely be inside a cable tray/riser. Overall that scene had surprisingly good attention to detail I wouldn't expect in a movie nowadays.
|
|
#
¿
Feb 4, 2012 01:19
|
|
|
Little video from Ferraz Shawmut (BUY OUR FUSES!) that show the effects of magnetic fields generated on cables during a fault. Fuses have a lot of drawbacks, but to my knowledge they offer the best current-limiting protection in a power system. Bigger fuses aren't cheap, I saw where three fuses blew out that cost over $1000 each. However, them blowing protected a massive (multi-million dollar) >10,000HP drive. Three-Phase fucked around with this message at 17:28 on Feb 11, 2012 |
|
#
¿
Feb 11, 2012 17:25
|
|
|
Guy Axlerod posted:Simply Perfect! The only thing I could think of, in the future, if we are able to create solid-state circuit breakers that have a high interruption rating. Then you have the near-instantaneous action of a fuse, with the adjustability and reusability of a circuit breaker.
|
|
#
¿
Feb 11, 2012 18:46
|
|
|
Jonny 290 posted:The spring pumping thing on that Jurassic Park breaker, that's real, eh? That's right. Small hand-operated molded case circuit breakers (low voltage, usually 600V or less) have handles that can be thrown up or down, and when you push the handle into the CLOSED position, it automatically should tension the mechanism for tripping the breaker. Those you don't pump, and I think those range from a fraction of an ampere up to 1200 amperes. On a molded case circuit breaker, there are three positions for the operating mechanism: -CLOSED (usually up, circuit is completed and electricity can flow) -TRIPPED (middle, breaker detected or told there's a problem and opened the circuit) -OPEN (down, circuit is broken on purpose) When you reset a tripped breaker, you need to typically move the lever from TRIPPED to OPEN and then to CLOSED so you prime the spring that can trip the breaker. --- Usually in a larger circuit breaker it's done with internal electric motors that prime the close and open springs. If you don't have any control power (like in Jurassic Park, the park only had some emergency power), you may have an override lever to pump the springs (like in the movie) or a crank to crank the springs. (You need to be careful, sometimes there are also cranks that rack the breaker out of the cubicle, and that can be extremely dangerous to do.) It's important that (if they're separate) BOTH the close and open springs are primed so that if you close the breaker into a fault, it can immediately trip open. Most molded-case breakers have a trip-free design, so even if the handle is pushed (or held/locked) into the CLOSED position, the breaker can still trip. For large circuit breakers, you have a TRIP and CLOSE coil. You energize (or de-energize for failsafe operation) the TRIP coil to trip the breakers, and you energize the CLOSE coil to close the breaker. The breaker has no way to tell if there's a problem in the electrical system, you need to install a protective relay to sense a problem and command the circuit breaker to open. In a lot of installations, the circuit breaker and relay control power runs off 120VDC. This is so you can have a bank of batteries to run the breakers even if there's a power outage, and if need be, you can get car batteries out of ten people's cars and operate the breakers in a pinch. In real life, the circuit breakers and critical power systems in Jurassic Park would probably have been run off 120VDC, so that whole priming thing would have been unnecessary. The 120VDC system allows a facility to "pull itself up by its bootstraps" in the event of a power failure. Three-Phase fucked around with this message at 14:15 on Feb 18, 2012 |
|
#
¿
Feb 18, 2012 14:12
|
|
|
One other interesting tidbit about big breakers: Where I work, the large medium-voltage breakers are racked into and out of cubicles, this is so the breakers can be maintained, and additional isolation can be provided for safety. But you can also rack the breakers into a test position. This allows the breaker to be controlled, closed and opened, but it's not actually plugged into the bus - so no electricity flows. Pretty Cool Name posted:Just wanted to say I really appreciate this thread. Being from Västerås, Sweden where the Asea part of ABB got its start it's kind of cool to hear about the sort things they do (although obviously this thread isn't ABB exclusive in the slightest). ABB was a huge part of the town as I as growing up and my and several friends dads worked in various departments there. As an aside: I was joking around with co-workers about how cool it would be to have an ordering option to include an ABB engineer with the larger drives.  "So there's the new Megadrive... wait, who the hell's that guy?" "Oh, he's Paul. You add an '-ENG' to the end of the part number, and they also ship a engineer in that other crate." "Hallo! Ich kam mit dem Hochspannungs-Laufwerk!" "I can't understand a word he says, but he showed me how to replace the IGBTs."
Three-Phase fucked around with this message at 14:44 on Feb 18, 2012 |
|
#
¿
Feb 18, 2012 14:19
|
|
|
One thing I also see on drives and exciters are cable mechanisms - where there's a circuit breaker inside a cabinet, but it's connected via a cable or lever to an operating handle on the outside of the cabinet. The handle is also interlocked so you cannot open the cabinet without throwing the handle to the OFF position. On most doors, that interlock is easily defeated with a small screwdriver so an engineer can enter the cabinet while it's energized. Same thing with motor control center bucket doors. Three-Phase fucked around with this message at 14:46 on Feb 18, 2012 |
|
#
¿
Feb 18, 2012 14:40
|
|
|
Stonelegs posted:We've got a few larger orders with ABB and all the boxes say "DO NOT OPEN UNLESS ABB REPRESENTATIVE IS PRESENT" or somesuch. What's so special about their poo poo? I don't know, but lot of the equipment isn't cheap. You want four galvanic isolated 4-20mA analog input channels with 12 bit resolution? That's about a thousand dollars for an AI825 S800 I/O card, or $250 per channel. I'm not going to go into details for several reasons, but you can easily fill a cubic-foot cardboard box with nearly a hundred thousand dollars worth of ABB's higher-end stuff - things that are the brains and communication hubs for their automation equipment. It's odd too, you have a little cardboard box smaller than a half a loaf of bread containing a device that costs over $10,000. If I ran ABB, those would ship in a ruggedized attache case, not a $1 cardboard box. But that's just me. Three-Phase fucked around with this message at 04:21 on Feb 19, 2012 |
|
#
¿
Feb 19, 2012 04:15
|
|
|
Frozen Horse posted:Why not just drive the controls off of the line feeding the breaker (appropriately stepped-down)? If it's dead, then the breaker controls are a moot issue. One important thing to consider is that larger breakers require electrical power to both close and open. (ABB does have a new medium-voltage AMVAC breaker that stores electricity in a capacitor and can be set to automatically open the breaker in a power failure, or remain closed, or trip after a small time delay.) Usually on low-voltage equipment, you have a circuit breaker and a contactor to control a load. The contactor acts like a switch, and the breaker provides isolation and protection from shorts (and sometimes overloads, ground faults, etc.). On medium-voltage equipment, the circuit breaker is remotely controlled and performs double duty. Let's have a possible scenario here: code:So lets all the protective relays are run off the bus power. You have a power failure when a squirrel (let's call him Todd) is violently electrocuted across two phases somewhere else in the power system, and the upstream breaker opens. Suddenly all your motors stall. After three unsuccessful retries, the upstream recloser faults out. Your motors coast to a stop and an eerie quiet fills your plant. A half-hour later eventually the charred and burning body of Todd is removed from where he initially was, clearing the fault path. The upstream power is restored. Now your "A" transformer is trying to start four induction motors at once because all the breakers on the bus are still closed. Instead of pulling about 900A, your four motors trying to start are instantaneously trying to pull over 5000A. This is also ignoring any loads connected to the A-5 and A-6 transformer, as well as the synchronous condenser attached. A few things could happen: The Multilins detect that both the bus voltage has now dramatically dropped since the transformer is saturating, and the motors are having a really hard time starting, and trip out all the motors. The 1200A bus main breaker may also trip out before the Multilins even have a chance to act. Remember, in this scenario the breaker cannot open if it has no control power. The only option would be for one of the workers to suit up and manually trip breakers A1-7 on the bus, then close breaker A-MAIN, and then start the motors and loads up again one by one. They also might be delayed because the Multilins may have a time delay in them to prevent the motors from restarting immediately after having a really hard, failed start. (Unless you jumper the emergency restart pins in the back, in which case you'll want management to sign something that says if the motors get damaged, it's not your fault.) So now things are a mess, and Todd is joking around in squirrel heaven about how he stopped production at the plant. - Grover, if you could, please check/critique my scenario here, I want to make sure what I'm describing is accurate. Three-Phase fucked around with this message at 15:07 on Feb 20, 2012 |
|
#
¿
Feb 20, 2012 14:44
|
|
|
^ - I should create a one-line for a hypothetical industrial plant we can use for explaining and looking at different scenarios. I'm thinking some interesting stuff like generators, PF correction, metering, bus ties and backfeeding, etc. One other problem is opening the breaker of a synchronous (not an induction) motor. My understanding is that if you have a running synchronous motor where: 1. The breaker is CLOSED and the motor is running at synchronous speed 2. The field discharge resistor contact is OPEN 3. The exciter is ACTIVE And you suddenly open the breaker, you must immediately CLOSE the field discharge resistor contractor and DEACTIVATE/FAULT the exciter. Otherwise the synchronous motor will be acting as an open-circuited generator, and that's extremely bad on the windings and other equipment. Basically you can induce an insanely high voltage on the stator windings that can potentially blow through and short the stator windings. Then your $x0,000 to $x00,000 motor is effectively trashed and needs to be replaced or rewound. An additional problem is if you OPEN the breaker for a large synchronous generator. Even if you kill the excitation on the rotor, you've suddenly lost a load. It's like if you are cruising down the road at 70 miles per hour when suddenly you shift into neutral while still pushing down on the gas pedal. There's nothing to limit the speed of the engine, all that energy goes into increasing the speed that the shaft rotates. So that means if you don't stop the prime mover, like blowing off steam going through a turbine or bypassing the turbine, the thing will accelerate until it gets damaged or destroyed. If a turbine flies apart, it can kill people too. I've heard of metal parts being thrown through ceilings and floors and walls like a howitzer shell. I think the only real difference between a synchronous motor and a synchronous generator is that the synchronous motor will have some starting means (such as a pony motor or armitissour windings (an induction motor within the synchronous motor, basically) where the generator doesn't need that. (Sorry, it's an old habit to capitalize verbs/statuses/actions when talking about this stuff.) Three-Phase fucked around with this message at 18:25 on Feb 20, 2012 |
|
#
¿
Feb 20, 2012 18:16
|
|
|
helno posted:Here are a few things in regards to excitation of syncronous generators. Ahh, I didn't realize the time constant was so large on the generators. But of course you still don't want to inadvertently have the FDR out of the circuit or the exciter on at that point, right? I believe the exciters I've worked on have both a mechanical two-pole contactor (to the FDR) as well as a set of SCRs in parallel hooked up to a firing board. Those SCRs discharge the field if there's a really rapid boost in voltage, I think it's set to about half of where the field would flash over at an arc gap. If that fails, there's an arc gap that can flash over across the field. The ones I've worked on typically have an FDR from about 1-10 Ohms. Forgot how many kW they're rated at, they're probably really overbuilt. Three-Phase fucked around with this message at 21:20 on Feb 20, 2012 |
|
#
¿
Feb 20, 2012 21:17
|
|
|
Cute as heck posted:Hey 3P, I have a question Oh poo poo, you're Daniel Day Lewis, right? I didn't recognize that name even after checking your post log. So I sort of sent a moderator report saying that you were being a "creepy motherfucker". So hopefully a mod reads this, and understands it was Dan, and you were not, in fact, being a "creepy motherfucker". Sorry about that! What can I say? I'm an idiot. Three-Phase fucked around with this message at 00:40 on Feb 21, 2012 |
|
#
¿
Feb 21, 2012 00:01
|
|
|
So wait, when you take a clamp ammeter and move it to different locations along a power cable, you get varying readings? Is this cable somehow shielded or triplexed? I'm wondering this - is your clamp ammeter true-RMS? Have you looked at the output waveforms from the UPS on a scope?
|
|
#
¿
Feb 21, 2012 21:34
|
|
|
You have to be REALLY CAREFUL if this thing is already online and powering mission-critical loads, but can you bypass the UPS? I believe with some Powerware systems, there is an option that allows you to bypass the UPS so that you can do maintenance safely while your critical load is still energized. Just to make sure the problem is directly related to the UPS and not your power line. You'll see these circuit breakers marked "MBP and MIS". Be sure to read Eaton's instructions extremely carefully if you are running a critical load like a production server, an iron lung, nuclear reactor control rod servo motors, etceteras so you don't get in trouble/kill people. Grover, isn't there also a way in software to bypass the UPS, or is that to just shut it down? (The last time I shutdown an Eaton Powerware 93XX at work, despite running all the required paperwork, resulted in some "unexpected fun".) Three-Phase fucked around with this message at 03:53 on Feb 22, 2012 |
|
#
¿
Feb 22, 2012 03:41
|
|
|
Can you get a "real-deal" power quality meter like a Dranetz PowerVisa? That way you can see the actual output waveform for voltage, and you can also get current clamps up to 3kA. (For about, maybe $10-$15k for the meter, clamps, and software license.)
|
|
#
¿
Feb 22, 2012 03:55
|
|
|
The coolest thing I found in the Dranetz is that it can give you a FFT for: -Voltage -Current -Power What's amazing is that you can get a positive or negative value for harmonic power in watts. I think it looks at the phasors for current and voltage versus the fundamental. So you can see a load generating harmonic power and sending it back into the electrical power system. Not sure how accurate this is, but I think it gives you some idea of what's going on in the power system. The Dranetz will also give you a K-factor number. Last time I saw that was on a 2400V system where I was seeing a minimal amount of harmonic power being generated by the load, like 500W of 5th harmonic versus about 5MW of fundamental. Three-Phase fucked around with this message at 01:33 on Feb 25, 2012 |
|
#
¿
Feb 25, 2012 01:28
|
|
|
Ah, perfect. I just found the settings for doing split-phase monitoring. There's a system I'm working on where we have A, B, and C phase current transformers, but only phase A and B PTs.
|
|
#
¿
Mar 8, 2012 02:35
|
|
|
We've got 480V loads all over the place. Still gotta treat the low voltage stuff with the utmost respect because it's so common. What's scary is when you're inside a cabinet where you have a 600VDC source, and on the busbars are bolted two 800A fuses in parallel with one another. If you're interested in big loads, here's a sort of cable I've been using: 15kV Triplex 2AWG to 750kcmil I think that the bend radius for the 500kcmil triplex is over five feet. They bring it in on a spool that's about 10 feet in diameter and must weigh a ton. Three-Phase fucked around with this message at 05:10 on Mar 9, 2012 |
|
#
¿
Mar 9, 2012 02:14
|
|
|
grover posted:The attitude I've seen is one of widespread and chronic complacency. They'll wear PPE if you call them them out on it, but get pretty bitchy about it. I agree. There are technicians and engineers I've run into who are over 50 and are pretty lax with a "well, we never did it that way and we never got shocked/killed/blown up." But even these guys are starting to see that they need to get with the times. Arc flash protection especially is becoming the name of the game. Customers are starting to ask for/demand products that can mitigate arc flash and provide additional protection to both their workers and power systems. I personally think stuff like 480 is the most dangerous. There's a sizable voltage there, but it's not quite high enough to turn heads. Still lots of electrocution and arc-flash potential, and the arc flash isn't always easy to nail down accurately. As far as NFPA 70E, I'm pretty by-the-book. I had to tell people "No, I can't/won't do this without higher level PPE" for monitoring a 120/208V panel that was directly downstream from a large transformer, where the arc flash was pretty high even for 120/208. They were cool with that. What scares me are places where people would pressure other people to do blatantly dangerous things. To be honest, my biggest fear in this industry is not being shocked, killed, or ending up in the hospital with the poo poo burned out of me in an oxygen tent. My biggest fear someday is lying in my bed at night thinking "I told John it was OK to work on that system live. His wife and little kids will never see him again." Three-Phase fucked around with this message at 02:39 on Mar 25, 2012 |
|
#
¿
Mar 25, 2012 02:26
|
|
|
This might also be relevant to the current discussion: Jones is dead! Thank god we don't see many open-knife switches anymore. I've seen old rooms that had BIG knife switches that looked (and probably were) very dangerous. Three-Phase fucked around with this message at 02:38 on Mar 25, 2012 |
|
#
¿
Mar 25, 2012 02:35
|
|
|
some texas redneck posted:The union, for example, has a 100 amp service, if the bigass transformer is to be believed (it's about 4-5 ft tall). It's a massive 4 story building with a hydraulic elevator, has its own chiller, etc. Kinda curious what kind of voltage would be going in and out of those transformers, I'm assuming there's another transformer in the building to knock it down to 480. There are a lot of different configurations possible. Typically the incoming voltage to service a building like that would be 13.8kV or 7200V. In the industry, that range (I believe from 600V to about 32kV is called "medium voltage"). The transformer might have multiple windings, so you have a configuration where you put 13.8kV in, and you get 277/480 out as well as 120/208 out on separate windings. In a system like that, you'll probably have really fat cables or busses coming out of the secondary going to a distribution center that then feeds power to panels throughout the building. In industrial facilities, you may feed a building with its own substation. On the extreme high end I've seen massive buildings fed with 138kV right from the power company's distribution system. In a large building or facility, you may use medium voltage both to feed power to different sections of a large building (2400, 4160, or 7200V) as well as running large motors (500HP to 10,000+ HP). In some systems you have what's called a "unit sub". On the far left side, you have a high-voltage load-breaking (typically load breaking?) disconnect switch that connects to the HV lines. That feeds an air-cooled transformer in the middle, and that feeds a distribution center to the right. The voltage you step down to can range from 120/208V to 5kV. mrmacomouto posted:Back in physics class we had a transformer arranged as two pipes, inner windings and outer windings, the further you moved the inner winding inside the outer winding the higher the voltage goes. I don't think that's an autotransformer. I think what you have is simply a transformer with a lot of losses by being able to reduce the area where the magnetic fields can effectively couple. Typically transformer are build with massive iron cores that can easily and efficiently accept the magnetic field. As a little factoid: autotransformers are "legal" for making small steps, typically called buck or boost transformers. (Not to be confused with buck or boost power supplies.) So if you have a two-pole, 208V breaker but need 240V, you can boost that. But it's forbidden to use an autotransformer to, say, go from 480V down to 120V. The reason is that if there's a break somewhere in the winding, the load can see the full voltage on the "primary". (This apparently doesn't apply to HV systems, I saw where three autotransformers the size of a small house were used to drop I think 800kVAC down to 500kVAC.) Three-Phase fucked around with this message at 12:41 on Mar 25, 2012 |
|
#
¿
Mar 25, 2012 12:31
|
|
|
If you wanted to (potentially) dramatically lower PPE as well as increase protection dramatically, install a fused disconnect switch with current limiting fuses before you feed into the panel (like a drive, etc.) This would be ideal for cabinets you expect will need to be checked/modified/maintained periodically. For a modern drive, on a dual-element fuse you should never have an overload trip since the drive would stop working (overload alarm/fault) before the fuse blew (if properly coordinated). If the instantaneous, current-limiting section blows, then you've got something really wrong downstream of the fuse. You get the added benefit that you can dramatically limit damage within a cabinet. I had a line-to-line 480V short in a drive cabinet. I heard a quiet "pop" sound, smelt burning plastic death, and noticed the drive died. Without the fast-acting fuses we selected, we might've blown the cabinet apart inside and had a major incident. Be advised that adding fuses or other changes to the power system is not a silver bullet, you still need to have an experienced electrical engineer look at the system and analyze the potential exposure levels. I'm not yet experienced enough to do that, and I don't pretend that I am comfortable doing arc flash equations. (Not yet at least.) Grover - are there any things like Lexan shields that could be installed in a cabinet to mitigate arc flash? (I doubt it, I've never seen anything like that, except for "touch-proof" shielding.) Three-Phase fucked around with this message at 23:14 on Mar 25, 2012 |
|
#
¿
Mar 25, 2012 23:11
|
|
|
^ GEEZ. Now that's scary. Are those two lower ones near the middle hand-operated rheostats or switches with arc chutes?  Cheesemaster200 posted:The only problem is that if you have a partial electrical system, using an infinite bus on the transformer is not necessarily a good idea. You need to model more of the system. I've read that for looking at short circuit fault calculations, using an infinite bus will give you the worst-case fault currents. However, there are situations where using an infinite bus may actually (inaccurately) lower the arc flash levels because the extremely high fault levels may cause equipment to trip faster than at lower currents. Not an Anthem posted:I was just browsing this thread because I'm going to ask some electricity questions when I get my bandsaw revamped, but I just bought an old 40's or 50's bandsaw that features that exact SquareD switch and my friend was joking that it will somehow kill me. Forgive the utter ignorance but is this switch a bad thing or a better thing than a previous bad thing? You'd be surprised how well old equipment can work. It seems like stuff back then was really overbuilt too. I wish I could find a big version of the Andry-Farcy advertisement poster for Merlin-Gerin. Anybody here work at Schneider Electric? Three-Phase fucked around with this message at 01:29 on Mar 27, 2012 |
|
#
¿
Mar 27, 2012 01:17
|
|
|
Now as far as arc flash mitigation goes, I've seen three technologies out there that might help: 1. Breakers with an arc-flash reduction setting: when someone's working downstream and live, you set the breaker to basically have a "hair trigger" trip setting 2. Fiber-optic sensors that detect the brilliant light from an arc and tells the upstream breaker to trip immediately 3. GE Arc-Vault system, where an arc is detected by light and a sudden dI/dt and basically "shunted" into a special canister upstream that generates/contains the fault until the breaker opens Siemens is also selling medium-voltage GIS for 4160V to I think 38kV, and since everything is contained in gas-insulated ducts, there's no live equipment to interact with or test, so there's inherently no real risk of arc flash within the switchgear itself. As far as fault levels, where I've worked it's not uncommon to have a >100MVA fault potential. (100MVA probably is enough to run a large town.) Three-Phase fucked around with this message at 10:46 on Mar 28, 2012 |
|
#
¿
Mar 28, 2012 10:44
|
|
|
|
| # ¿ May 17, 2024 20:22 |
|
|
Drheat posted:It appears to be about the size of a 1 liter bottle. They seem to attach to the existing fuse holders. Sounds like a single-phase recloser. Basically, when you have a fuse, in a short circuit or overload condition, it'll blow to protect equipment in the power system from damage. With a recloser, you have a circuit breaker (probably a vacuum interrupter or SF6 breaker) that detects a fault, opens the circuit, waits, and then tries to close. Here's a little scenario where a recloser is useful. You have a power line that services a section of a town. The line is 13kV, and the circuit breaker feeding that section has a recloser. Let's assume for simplicity that the system is referenced to ground. You have a squirrel, let's call him "Fluffy". Little Fluffy climbs the power pole, and while standing on the crossarms, tries to climb up on an insulator and grabs the line, completing a path from phase A to ground. (I will spare the gory details about Fluffy's fur catching on fire and various other unpleasantness that 13kV going into a small mammal causes.) Eventually there is a good enough current path that the line arcs over to ground with a brilliant flash of light and a loud bang. The relaying at the substation sees a fault on one of the lines, and waits until it reaches the correct point in the time-current curve to trip the circuit breaker. Once the line flashes over, it immediately decides the breaker must open NOW. The fault is interrupted within a cycle or so, the lightshow ends, and Fluffy's cremated body falls from the line. The recloser waits three seconds, then commands the breaker to close. The breaker closes, and since there's no fault, everything ends fairly well. (Except for Fluffy.) The Proc posted:If you've ever wondered how long it takes to boil the inside of a tree at 33kV, wonder no more! It looks like you can see recloser action in this very video! 4:02 - Severe fault (brilliant flash) 4:03 - Fault interrupted (probably between 10-30 cycles) 4:13 - System attempts to reclose (ten seconds after initial interruption) 4:18 - Another flash, system interrupts again, goes into lockout (recloser does not attempt to close breaker again) Three-Phase fucked around with this message at 00:44 on Mar 30, 2012 |
|
#
¿
Mar 30, 2012 00:36
|
|