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Did I really make the title "As me" instead of "Ask me"? Dammit, I'm so dumb as hell.  ABOUT ME I've been working for several years in the electrical power and controls field. It's been a blast (figuratively), I really enjoy my field. The job can be challenging and frustrating at times, but it's still rewarding to see something you designed come to life. INDUSTRIAL ELECTRICITY Large industrial establishments (chemical plants, steel mills, factories) use tremendous amounts of electricity. A power company typically supplies electricity to these users at a high voltage, from 6900V to 138000V. In a plant, that voltage is stepped down and transferred around the facility to where it's needed. There are three key components, transformers, breakers, and busses. Transformers - these step down (or up) a voltage. Typical ratios at a plant would be: 13.8kV to 4160V - this takes the electricity supplied by the power company and drops it to a lower (but still high) voltage for distribution around the plant 4160V to 480V (or 480/277V) - lots of equipment in a plant runs at 480V/277V, such as motors, heaters, and some high-bay lighting 4160V to 120/208V - this is for things like your computer, microwave oven, and some lighting Breakers - breakers allow parts of the power system to be "turned off" and isolated from one another. They also are designed to open in fault conditions, such as a short circuit, or an overload. Small breakers have a built-in trip circuit that says "hey, something's wrong, trip!" Large breakers need to be connected to a protective relay - something that looks at the electrical system and tells the big breaker "hey, something's f'ed up, trip!" Busses - these are like a power strip, you can plug different loads into them. A bus can be as simple as the metal strips in the back of a circuit breaker panel, or as complicated as metal-clad switchgear for moving thousands of amperes at thousands of volts. I've worked with machines that are much larger than most people deal with. Your garage door opener is 2 horsepower. In a large plant, there are motors bigger than 10,000 horsepower. SAFETY Humans and electricity don't mix. A voltage as low as 120V can, under the right conditions, kill. At higher voltages more current can flow through a person, and insulation can even be punctured. A pinhole in a rubber glove can be lethal at the right voltages. Special safety measures are taken at higher voltages, such as grounding equipment so that if it's turned on by accident, nobody is hurt. Even scarier is arc flash, where a short-circuit occurs in a piece of equipment, and suddenly there's a fireball that's hotter than the surface of the sun, and you're inside of it. Arc flash is more dependent on fault current than voltage. You can get a dangerous arc flash on large equipment operating at low voltage such as 120/208V or 277/480V. Particularly on the secondary (low voltage) side of a large step-down transformer. This video has a person involved in a relatively minor arc flash incident. He still may have suffered burns, eye damage, ear damage, etc. There are videos on YouTube of much, much more serious accidents where people didn't survive. - So if you want some funny stories, horror stories, or just are curious what's inside those big grey boxes with the warning signs on them, this is the thread for you! - Some additional videos: Closing 4160V breaker (PPE on improperly. Not good.) Stupid kid demonstrating fault-current availability Bringing a transformer online Bringing another transformer online - note that the red light means the circuit is energized (danger) and green light means the circuit is off (safe). Some places use the opposite color code. ELECTRIC ARC FURNACE - Turn your speakers DOWN. You need to phone the power company before you start one of these. I'm not joking. Three-Phase fucked around with this message at 17:01 on Sep 4, 2011 |
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Sep 3, 2011 14:15
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Naffer posted:Do the big industrial customers still get 60Hz AC? SHORT ANSWER In North America, yes. We operate at 60hz from 120V (your household outlets) all the way to the highest AC transmission voltages (800kV-ish). Many other places in the world operate at 50hz. LONG ANSWER The frequency you see at your outlets is based on the frequency that it's generated at, and that all depends on how the generator is built and how fast it spins. A four-pole AC generator, spinning at 1800RPM, will generate 60hz output. Likewise a 32-pole generator spinning at a much slower 225 RPM (like at a hydroelectric plant) will also generate AC at 60hz. So that you can have a power grid, where hundreds of generators are dumping energy into a huge bus, all the generators must provide electricity at the exact same frequency. When I've done power quality monitoring, the line frequency is usually between 59.99hz and 60.01hz. Typically the larger the bus, the more stable the frequency is. In other countries, especially islands, the frequency could me much more variable. The most important things about 60HZ AC (versus other frequencies) is that it impacts how transformers are sized/built, and the speed of AC motors. Three-Phase fucked around with this message at 21:34 on Sep 3, 2011 |
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Sep 3, 2011 21:25
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hermand posted:During your tests, how much does the voltage vary? I'm under the impression that over here in the UK everything works on 220+/- 10. Does that sound right / normal? 10% of nominal is generally regarded as OK (per the EN5160 power quality standard). I'm not familiar with UK power systems, but that does sound acceptable. Some of the worst swings I've seen when testing (we were experiencing issues with some small 480V motors) were from about 475V to 520V. That was a swing over a 24-hour period. I left the recorder on it (the motor terminals) for a week. Late at night the voltage would slowly creep up to around 520V, and during the day it would drop down to about 475V. This was the sort of power system in question: 32kV bus --(Transformer and breakers)-- 4160V bus --(Unit sub transformer and switchgear)-- 480V bus That sort of swing could be fixed by adjusting down the 4160V transformer secondary tap. (Electricity comes in at 32kV, comes out at 4160V , but you can change the output voltage plus or minus a percent or so.) Some facilities have automatic tap changers that detect the voltage being too high or too low, and mechanically switch to a different tap automatically. Because of the economic downturn, some major loads on the power company's grid started to go away. As they did, it looks like the system voltage creeped higher, and so that trickled-down through the entire power system. Three-Phase fucked around with this message at 02:07 on Dec 22, 2012 |
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Sep 3, 2011 23:15
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Anti-Hero posted:Have you performed many short-circuit and coordination studies? I'm an electrical engineer in the power industry, though I work more on the utility side of things. I've done quite a lot of protective relay coordination studies for both utilities and industrial clients (typically petrochemical). Some design work, as well. Not yet. All I know is you want to assume an infinite bus for short circuit studies, but you don't want to do that for arc flash studies. That and the equation for using the %Z for a transformer to calculate secondary short-circuit capability, stuff like that. As far as coordination, I've done some basic things looking at time-current curves using Power Tools, mainly to look at protecting small (100kVA) exciter/drive isolation transformers and similar equipment. I have the most basic knowledge of protective device coordination on low voltage systems. As far as protective relaying, I've only done work with the GE Multilins for large motor protection, I'd like to do more with equipment like the SEL relays. I'm assuming you (AH) are also doing things like distance protection relaying and more complicated reliability analysis - looking at what happens to the power system if any specific piece (or pieces) of equipment fail at any time. Feel absolutely free to add or discuss anything you'd like, AH! You're not stealing my thunder in the least. Anti-Hero posted:Did the transformer have an automatic load-tap changer on the secondary? No, on that bus we did not have one. I've seen some plants where they have busses that are specifically regulated medium-voltage busses (4160V) that use automatic tap changers. Sucks because we'd need an outage across several areas to change the taps. Have you ever seen a system where a tap changer introduces electrical transients into the system during operation? I'm talking transients that are significant enough to cause flicker, make UPS units go online, crash electronics, etc? Three-Phase fucked around with this message at 23:31 on Sep 3, 2011 |
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Sep 3, 2011 23:20
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bear shark posted:Does anyone still get DC power transmitted to them or does everyone rectify AC nowadays? I really doubt it, especially since rectifying AC to DC is so much easier using power diodes and SRCs than, say, a mercury arc rectifier like the used to, or a motor-generator set. With the SCRs and some filtering, you can create DC at varying voltages as well. This is good for applications like electroplating, welding, etc. There are some limited HVDC applications, but those are mainly for transmission of huge amounts of power across very long lines. The efficiency savings in HVDC is offset by needing to build a thyristor hall for converting it back to AC, so you need to look at the costs carefully. Here's some cool HVDC videos: ABB HVDC Light - England/Wales Connection Siemens Ultra-High Voltage DC If you're ever driving and see a large power transmission line that only has two wires (instead of three or even multiples of three) it's likely a HVDC line! Three-Phase fucked around with this message at 00:44 on Sep 4, 2011 |
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Sep 4, 2011 00:37
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^ - I read about those superconducting wires too! Neat stuff, if we can get better superconductors (ie: room temperature) it will be an absolute godsend to the power industry. Ender.uNF posted:Can you talk more about how the heck you can turn circuits on and off under these sorts of currents and voltages? Isn't ionization of air related to voltage so with high voltages, air is no longer enough of an insulator (eg: if you just try to mechanically withdraw a metal contact it will form a spark and ionize the air)? That's right. Even flipping a lightswitch will generate a small arc. In a big circuit breaker, the effect is much more intense and can be catastrophic. There are a few tricks: 0. Opening the contacts fast enough that an arc cannot easily sustain itself 1. (Smaller/medium breakers) Have arc chutes - when the breaker opens, the arc flows up into these dividers, gets split up and cools, until the arc cannot sustain itself 2. Use the magnetic field generated by the flowing current slam into the magnetic field generated by the arc and blow it into an arc chute 3. Use compressed air to blow the arc out 4. Flood the contracts in oil 5. Flood the contacts in Sulfur Hexaflouride, an insulator 6. Keep the contacts in a vacuum bottle 4, 5, and 6 are generally used more in high-voltage circuit breakers (over 600V). Also, breakers have an AIC - ampere interruption capability. So you buy a 120V, 20A breaker, it may have a 10,000 AIC. What that means is that for a fault under 10,000A, the breaker should be able to interrupt the arc. If it's over 10,000A, it may not be able to stop the arc (and will probably blow up in the process.) You need to look at the makeup of the power system and say "gee, this breaker is only rated at 10,000A. If I short out what this is connected to, will more or less than 10,000A flow through it? Will something else break the circuit in time?" Another fun thing about big breakers is they need electricity to operate, typically 125 volts DC. You energize the close coil to close the breaker, you energize the trip coil to trip the breaker. Or you have power constantly applied to close and hold the breaker closed, and when you remove power the breaker trips. Tidbit - disconnect switches Companies also make disconnect switches, these are generally used for safety, so you can "unplug" a large device. Sometimes the circuit breaker powering a device may be in another area of a building, and you don't want someone turning a medium-voltage motor on when you're working on it. These disconnect switches are either load interrupting or non-load interrupting. Load interrupting is like a household light switch. It interrupts the load. A non-load interrupting switch must NEVER EVER EVER be closed or opened with the potential for current to flow through it, or with current flowing through it! It the mechanism doesn't operate fast enough, it can cause a violent explosion within the disconnect switch. This is what happens when you open a large disconnect under load! Three-Phase fucked around with this message at 01:09 on Sep 4, 2011 |
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Sep 4, 2011 00:54
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Ender.uNF posted:Doesn't the arc in these sort of situations degrade the contacts? I presume there would be a limited number of disconnects before it is no good... actually I think the cheap non-silvered home light switches have the same issue. I know I've had to replace one or two that would audibly arc when switched on/off and I presume that was due to oxides and crap building up on the contacts. I believe that in breakers, there are sometimes "flicker blades" that are where the arc starts/goes during operation. Those don't conduct electricity when the breaker is closed. The idea is that when the breaker operates, the arc is specifically directed to those blades. For a light switch, you're only talking about interrupting a few amperes as well, so the problems are likely not as severe. The problem is when you're a circuit breaker (let's say 480V, 100A) trying to interrupt a 15,000A short circuit. quote:Awesome. I also like the video of the oil-cooled transformer exploding as it superheats the oil. Not sure what would cause that though, given the oil they use is non-conducting. Non-conducting != really, really flammable quote:I would presume most people who try to flip a non-load interrupting switch aren't around to talk about it. Do they usually have safety sensors to prevent you from operating it while under load? Even better. There's a thing called interlock keying. What you have is a special lock and key on the circuit breaker compartment, as well as the disconnect switch. These are special keys and locks that can mechanically hold a key in place. It's set up so that you need a key to mechanically operate the disconnect switch. To get the key, you need to have the circuit breaker open. Once it's open, you can remove the key, take it to the disconnect switch, and operate it. That also prevents someone from closing the breaker while someone's messing with the disconnect switch. The trade name for this is commonly called "Kirk Keying", at least around Ohio. My understanding is it was developed in the early part of the 20th century as a lot of people kept being accidentally killed by this sort of problem. Here's another video on trapped key interlocks Also, there's operator competence - people working on these systems are trained to understand how they work, what the risks are, and how to safely do their jobs. quote:For that matter, it makes me wonder how industrial equipment can be switched on and off... or even if it can be switched off internally. I have no idea how you'd make a regulator that could control the motor speed of a 10,000 HP motor. For a large synchronous motor, 10,000 HP, you don't really need a regulator. If it's properly loaded, a four-pole synchrnous machine will rotate at very close to 1800 RPM on a 60hz line. Now if you overload the motor mechanically and it pulls out, that's another story. On my systems if there's a pull-out, the exciter for the motor (it generates the DC electromagnet inside the rotor, the rotating bit) will detect it, fault the exciter, and trip the motor out. The protection relay can also alarm to warn the operators if the motor is overloading. (That relay will also tell the SCADA (supervisory control and data aquisition system) what's going on. So the operator will see screen like this: code:As far as turing on and off, companies do make vacuum contactors to turn on and off very high voltages and current, but are not designed to interrupt faults like a breaker can. The easiest way to do this is to have a soft start/soft stop drive that can gently decrease the voltage/current until it's zero, and then the breaker or contact opens. Three-Phase fucked around with this message at 02:01 on Sep 4, 2011 |
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Sep 4, 2011 01:29
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helno posted:The station I work at has been switching over to vacuum breakers on all of the 13.8 switchgear. The older arc chute style breakers moved the main contacts about 6 inches to break the contact. The new vacuum breakers move the contacts just over an inch and require no pre arcing contact. One of the things I really like on ABB's new AMVAC medium-voltage vacuum breakers is that there's under 10 moving parts. That's basically no-maintainance compared to other, older arc chute breakers that have tons of moving parts. I've seen only vacuum on 13.8 equipment, highest I've seen air-blast or magnetic-blast on is 7200V. I was looking at a model of one and literally said "That's it?" Plus there's no need for anti-pumping circuitry or any of that other bullshit, just a command to close, a command to open, control power, and status outputs from the breaker aux contacts. Plus you can also program the breaker to automatically trip on loss of control power, or after so many seconds without control power, if you so desire. One interesting thing about the vacuum bottles (the container holding the contacts) - I read that you need to be careful if you perform hi-pot testing on them. If there's a small gap between the contacts and you apply HV, you can start to generate X-rays. helno posted:I maintain the generator excitation system at work so I can answer some questions about that aspect of power generation. What's your opinion on brushless exciters? Where I work I've only heard bad things about them, they want slip-rings all the way. Also, when you have multiple generators on a bus, do you need to match the power factor (via excitation) on each of the generators? I think you'd have to. Do you have any synchronous condensers, or know any company that uses synchronous condensers? Three-Phase fucked around with this message at 03:13 on Sep 4, 2011 |
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Sep 4, 2011 03:00
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helno posted:It's a good thing it is simple because the german to english translation of the manual was really really bad. Siemens or ABB?  One of my professors in college did something neat once - we were given the datasheet to a component, I think it was a memory chip, but it was in german. We had to pick through it and figure it out. That was a clever man, and it was a very good lesson. Just keep an eye out for "LEBENSGEFAHR!" and "HOCHSPANNUNG!" ABB really just needs a high-end option on their larger drives to deliver a polite german man named Jörg with the drive. (For maintaining the drive as well as conversation.) Three-Phase fucked around with this message at 03:26 on Sep 4, 2011 |
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Sep 4, 2011 03:18
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helno posted:Tons and tons of really obsolete but reliable protective relays. Oh yeah, built like a rock. I still like the newer relays with the little electronic screens (which sometimes don't flip out and display junk characters like on the Multilins). helno posted:Regrding exciters. I have never worked with brushless exciters but the PSS people seem to hate them because it is hard to verify the computer modeling of them because you cant directly measure the rotor potential. I am quite happy to change generator brushes on a weekly routine but it is a bit freaky knowing each brush holder carries about 120A (2700A/22holders). Wow, 900MW is nothing to sneeze at. Are those air, water, or H2 cooled? Do you have high-pressure "lift oil" systems on those generators to push up the rotor to make them easier to start by the prime mover? Also, are those generators connected via phase-isolated bus ducts? Three-Phase fucked around with this message at 03:33 on Sep 4, 2011 |
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Sep 4, 2011 03:27
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Anti-Hero posted:fake edit: Have you guys seen GE's new 480V arc-dome product? It's a standalone cabinet that is integrated with existing switchgear to reduce arc flash incident energy levels. It has a dome that is made up of 3 electrodes separated by air. During a fault, photo-sensors detect the light from the event and trigger the electrodes which ionize the air and create a lower resistance arc than the fault, "stealing" all the energy and exhausting it in the dome. Pretty neat stuff, we had some GE sales reps come by the office a couple weeks ago and show us some promotional materials. It's a neat idea, better safety and protection of surrounding gear. Does the dome need to be replaced or inspected after intercepting a serious fault? I've seen similar products that use a light/current detection scheme, but just shunt-trip a breaker rather than use the dome. However, the trend I'm seeing is more of a "no live work, period" attitude where I'm at. That and more and more people are using simple remote-racking for large circuit breakers, since that's one of the most dangerous operations. Bummer about the Multilins, but you're not the first person to tell me they're junk. Oh well, we always make sure to put the NO "service" contact in series with the NC trip contact. If the relay goes south, it should trip the breaker. One of the most nerve wracking moments was once during testing when we had a motor trip unexpectedly. I check the Multilin and it's indicating a trip condition and over 300 degrees Celcius on one (just one) of the motor RTDs, I nearly poo poo myself! It turned out to be a bad connection to the RTD, thank god. The discussion between me and my boss was basically "if it was that high, we'd smell it, and the windings would practically be on fire". The one cool thing about the Multilins is the emergency restart contacts if you short them together, it makes the Multilin "forget" the stored thermal data, so you can immediately restart a stopped motor without waiting X minutes for a cooldown due to the starts per hour setting. Of course, this is only for emergencies (like if you need to restart a ventilation fan supplying air to a mine shaft) and testing without a load. It's kinda like in Star Trek where the captain just says "override" when the computer wants to stop him from doing something. You CAN seriously damage or reduce the life of a million-dollar motor with those contacts, so you need to be very, very careful. I've only ever shorted them with very explicit permission from a higher-up (literally unlock and open the cabinet, take a strand of wire, and short the contacts together until it indicates a restart on the screen). Anyone on the shop floor should NOT have access to this. Or access to anything on the Multilin for that matter, or the cabinet containing the Multilin, etc. Some of the people I've talked to act like the fact that you can do this is a trade secret, and I don't blame them. Three-Phase fucked around with this message at 04:55 on Sep 4, 2011 |
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Sep 4, 2011 04:37
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grover posted:Speaking of which, have either of you (or anyone else) seen or used and solid state breakers yet? I've heard they exist, and see big advantages in reducing arc flash, but I'm still a bit wary. Drives have current limiting and fault detection, so sometimes they can act to interrupt a fault before a circuit breaker will. The only issue is that if the fault is severe enough, a current-limiting or silicon protection fuse on the drive will probably blow as well (within a half-cycle). (The fuses in a big air-cooled MegaDrive LCI are about $1000 each - not cheap to replace.) I would only trust a solid-state breaker if it was coupled with high interrupting capacity fuses as well as a fail-safe should the electronic section fail in any way. Maybe we'll see SSB's as more of a contactor replacement in the near future, where you have applications where the open/close cycle is high enough to reduce the life of normal breakers and contactors? (Do not confuse the ABB Megadrive with the Sega Megadrive. This is important.) Three-Phase fucked around with this message at 12:47 on Sep 4, 2011 |
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Sep 4, 2011 12:42
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Crackpipe posted:What parts of these would be safe to bump into? I've always viewed substations as death mazes where bumping into absolutely anything will kill you instantly. You want to be very careful, it's not even as simple as what not to bump into. If you're close to an energized conductor at, say, 500kV, you don't need to touch it. If you get close enough, and you're grounded (even wearing rubber sneakers) the electricity can jump from the line and slam into you. There are minimum working distances that specify "a worker (even with PPE - personal protective equipment) shall NOT be closer than A feet on a B voltage system." Even if you're very well insulated, you may create a capacitive path to ground, and can still get fried. Some of these systems can be incredibly dangerous. I've seen photos of someone who trespassed into a substation and made contact with an energized line. You might not want to read this next part. The "charred corpse" portrayal of someone who died under similar circumstances in the game Half Life 2 were pretty similar. When loading the body onto a gurney, one of the man's legs just broke off and had to be set aside. His skin was charred pitch black, almost down to the skeleton. He was effectively carbonized by the electricity. Also, if a wire that's very high voltage falls onto the ground, it's possible that a voltage gradient can be created across the ground. So let's say you take a big step with 3 feet between each of your shoes. That could be ten thousand volts or more, and you can get electrocuted. In the rare event you are ever standing near where a power line falls to the ground, you're supposed to hop with both feet (like you're in a potato sack race) away, or shuffle with very, very small steps until you're at least like 30 feet from the exposed line. (In a substation, a properly designed grounding grid may limit the size of the gradient and improve safety.) With all that said, the conductors are usually supported by insulators, which look like stacks of dinner plates. The higher voltage equipment also have those metal "halos" on them - that's to reduce the effect of corona discharge. You put a really high voltage on a sharp point, like the head of a pin or the edge of a metal connector, you can develop corona. Ender.uNF posted:Indeed, it looks like a death trap to walk around in there while energized. I presume there are designated foot paths that have a somewhat lower chance of electrocution. Typically equipment is installed high enough to prevent someone from walking under it from being electrocuted. However, that's not a bulletproof assumption. There are still tons of dangers. If you're elevated somehow, or moving something like a pole or a long tool, it can quickly get much more deadly. There was an unfortuante death of someone putting up siding on a house awhile back near Columbus, where his little ladder swayed in a gust of wind over into an overhead powerline, and it killed him. There are a lot of similar deaths that occur each year where a piece of equpment, a radio antenna, or a tool makes contact with a line. That line was probably only 7200V, where this stuff can be as high as 500000kV. . As far as safety goes, to work inside a substation, you need to: A. Understand the system, how it works, and the dangers involved B. Have a clear plan of what you're going to be doing (troubleshooting a breaker that isn't closing, checking for hotspots with a thermal imager, repairing or replacing an insulator or pothead, etc.) C. Have the protective gear, rubber insulating mats, hot sticks (insulating poles for holding tools), high-voltage detectors (you hold them near a line, and they beep and light up if there's AC on it, up to hundreds of thousands of volts), and all the other tools you need. Your gear also has to be routinely tested, like making sure there are no pinholes on gloves or rubber sheets. D. Have radio communication to the control room for the substation E. Have a buddy with you at all times, preferably away from you, who knows how to respond in an emergency, how to safely free you if you make contact with HV, etc. Those factors combined can make work much more safe, probably safer than, say, crossing a busy street. Some Guy From NY posted:Here is a Transformer which is dropping 345KV to TWO 138KV feeders. SEXY. How many MVA is that? 500? 1000? Three-Phase fucked around with this message at 23:58 on Sep 4, 2011 |
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Sep 4, 2011 23:37
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See, there's three parts of the "power triangle": -Apparent power -Real power -Reactive power This all has to do with AC circuits. -Apparent power is just what it says on the label, the apparent power consumed by a system. -Real power is actual power that does real work, like heating a resistor, turning a motor, lighting a bulb, etc. -Reactive power is power that gets "bounced back" to the source. This doesn't do any real work, but consumes current anyways. A capacitor or inductor is a purely reactive load. The relationship is like a right triangle: Apparent Power ^2 = Real Power ^2 + Reactive Power ^2 Reactive power is classified as "leading" or "lagging" by a certain amount, or the power factor of a system. PF at 1.0 is neither leading or lagging, or unity. It then goes to 0 either in the leading or lagging direction. So let's say you have two motors, both are 10 horsepower. One operates at a 0.95 power factor, and another operates at a 0.8 power factor. The one with the 0.8 power factor is going to consume more current to do the same amount of mechanical work as the one with the better 0.95 power factor. - A small customer is billed based on volt-amperes (correct me if I'm wrong on this and it's just watts - I'd assume they'd measure VAs to account for reactive power). If you're a large to huge customer, like a steel mill, you are billed (and can even be fined) based on your power factor - how much reactive power versus real power you consume. Some factories have things like big motors that have a lagging power factor, so they install either power factor correction capacitors (which lead and cancel out the lag), or they install synchronous condensors - basically unloaded synchronous motors that just convert real power to leading reactive power. (All you do is decouple the motor, get it spinning, and overexcite the rotor!) Power factor correction is pretty important. Say you have a big mill and you start a dozen 10,000 HP motors in the morning without your PF correction capacitors in. The power company might see this and say: "Hey, you pulled 100 million volt-amps and 40 million VARs (volt-amps reactive) this morning for five minutes. We're now going to fine you $x0,000 for doing that in addition to the extra power costs. It's in the agreement your company signed. Sucks to be you." Yeah, some facilities (steel mills, etc.) are so massive that they need to talk to the power company on a daily basis to go over what loads they're running. I've seen situations where we were told by the power company to please not run large loads during ultra-hot days in the summer due to concerns about the stability of the power system, where an extra 200MVA load would not be well received. In some facilities where the motors can be decoupled, the tables can turn on the power companies on hot days. The poco may call up and say "Hey, could you please decouple several of those 10,000 HP motors and supply us VARs? We'll pay for it! Please! The million air conditioners running today is killing us, we're down to 130kV on our 138kV lines and we don't want rolling blackouts!" (That sort of thing needs to be arranged ahead of time, of course, but I've heard of some industrial customers doing just that - offering to create extra VARs for the power company by decoupling and overexciting their big synchronous motors.) Three-Phase fucked around with this message at 01:53 on Sep 5, 2011 |
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Sep 5, 2011 01:38
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Vanagoon posted:Regarding power factor, I've noticed recently that there is a lot of bitching going on about PF in regards to Compact Fluorescent light bulbs. If you have a large building, or a complex of buildings, the power factor could add up if you have mega-VAs worth of lighting.
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Sep 5, 2011 02:47
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Cheese and Kaiser did a nice job explaining reactive power. In my opinion it's one of the hardest to explain topics. (Phasors come in second.) Kaiser, you using SCRs, IGBTs, or IGCTs? (For those wondering what the hell those are, they basically act as big switches to turn current on and off. A "normal" transistor in a circuit can be as small as a grain of sand down to microscopic size. These look like hockey pucks up to the size of dinner plates.) How much THD you get out of those drives? 20%? Three-Phase fucked around with this message at 02:23 on Sep 7, 2011 |
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Sep 7, 2011 02:17
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Electricity can be even scarier: http://www.youtube.com/watch?v=tVJzwG4Ee4c
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Sep 7, 2011 02:38
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IratelyBlank posted:I'm an EE student (although early in the curriculum) and is there a reason that it is always voltage being described when people talk about power lines and etc and not current? I have a hard time putting a "danger value" on a piece of equipment or whatever without both a current and a voltage, but that may be something I haven't been exposed to yet? I've seen larger equipment rated in MVAs, where they combine the voltage and current together. I believe that 50-100mA AC at 60hz can kill a person from either suffocation or fibrillation. Boy, did anyone hear about the massive blackout in the SouthWest?!?
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Sep 9, 2011 01:51
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slorb posted:Equipment voltage ratings are standardised across the whole power distribution system. Current ratings aren't. At the transmission and distribution levels there are a range of different conductors used with widely varying current capacities but usually* the same voltage rating. Yup, so you want as high a voltage as you can go so you can move the same amount of power (volts * amperes) with less amperes - that means smaller cables. Of course, at higher voltages, you need bigger insulators, and it gets harder to build circuit breakers, transformers, etc. Big HVDC systems need gigantic insulators, like 25 feet long.
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Sep 9, 2011 23:18
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Jows posted:I hope by "start one" you mean commissioning. Sure, any mill running an EAF will be working with the local power company to make sure that they'll have enough juice while the thing is being built, but the operators don't call them up every time they start a heat. What about for power purchace/billing? I thought that was handled on a day-by-day basis at the largest plants. (Maybe that's more plant-wide than just a single furnace.) Aliass posted:Motors are never rated in MVA's however transformers and other such machinery are. Forgot to mention fault ratings on breakers in MVAs. But yeah, I've never seen a motor rated in MVAs, just HP. Aliass, how big does your shop go? Anything bigger than 10,000 HP (synchronous or induction)? Three-Phase fucked around with this message at 01:50 on Sep 10, 2011 |
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Sep 10, 2011 01:46
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ANIME AKBAR posted:A common rumor I've heard from some engineers is that much of the grid is synchronized using GPS signals, and if enough satellites were jammed or disabled, it would wreak havoc. Bullshit, right? I believe that GPS signals are used for synchronizing timing across a wide area. That might have to do with carefully synchronizing the frequency and phase offset of many different generating stations. Or for coordinating events so you know exactly when an event in the power system happened. I'll have to ask my co-workers about that. (I'm not sure of the difference between using GPS and, say, using an atomic clock signal.) EDIT: Aha, here's a site that talks about phasor measurement and using GPS signals. It's basically so you can very accurately measure the phasors at different locations on the power grid at the exact same moment. Frozen Horse posted:75 or 400? 400hz is used in aircraft, because you can make much lighter transformers than at 60 or 50hz. An additional nice thing about higher frequencies is that when you have a simple rectifier (converting from AC to DC), you need a filter circuit to smooth out the ripple voltage. If you have a much higher frequency, it gets easier to smooth out the ripple with smaller filtering components like capacitors. (Three-phase rectification has even less ripple than single phase if you have a three-phase full-wave bridge.) Three-Phase fucked around with this message at 04:26 on Sep 10, 2011 |
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Sep 10, 2011 04:20
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GreenTrench posted:Does anyone have a good book or lesson plan for learning about medium voltage switchgears? To be completely honest, I'd just go to Eaton's web site and find their online catalogs. I know this isn't perfect, but it does have some good informative information. My guess is this is what you'd want to cover: 1. Theory and applications 2. Internal design (indoor vs. outdoor, etc.) 3. Circuit breakers and breaker controls 4. Current transformers (CT) 5. Potential (voltage) transformers 6. Relaying and bus protection, safety, arc flash detection, etc.
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Sep 10, 2011 05:15
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squeakygeek posted:GPS is a pretty convenient atomic clock signal I guess. I know people have clocks and watches that use a central atomic clock signal, but doesn't that just fire a "reset" signal once a day at a very specific time?
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Sep 10, 2011 05:48
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RDevz posted:Metering is done in the UK based on 30 minute settlement periods for sites over 100kW. These readings are collected on a daily basis, which effectively means that we get 17520 meter readings per site, per day. Multiply by the appropriate tariff that the customer has signed on to, and bill them. All of Europe meters on a 15 to 60 minute settlement period length. I'd be surprised if the US didn't do something similar. That sounds similar to US large customers, but the stuff I'm more familiar with is a lot larger than 100kW+, more like 10MW-250MW range. But yeah, there's complicated billing based on peak watts/VARs (I think it's in a sliding 15-minute range) along with total consumption and making sure you don't lag more than, say, 0.95 power factor at any time unless you want to be fined extra. I've seen some really hot days when major loads cannot be operated, the power company phones up and basically says "we really can't sell you power to do this today, it's 100F out there and we're getting clobbered by the air conditioner loads." I've gotta look back in the SCADA system and see if there are yearly cycles on the voltage that we get provided. I've seen a system where we had daily varitions on an unregulated bus - 480V nominal during the day creeping to 525 at night. I suggested they adjust the taps on the 2400V-480V secondary down maybe 2.5% to help a little bit. Three-Phase fucked around with this message at 15:55 on Sep 10, 2011 |
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Sep 10, 2011 15:50
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Frozen Horse posted:Interesting. This is making me wonder about the usefulness of designing sound, lighting, and power-generation equipment for portable-generator driven outdoor festivals (or raves, etc.) to work at 400 Hz. Lighter amp-stacks to load and unload, less of the expensive copper wiring and permalloy in the amps, easier-filtered mains hum, and one can always run it off of some sort of inverter when there is mains power available. Is this complete crack-pottery? I'm not sure how much the 400hz would impact things like cabling to transmit power. Plus the big problem is that almost nothing runs natively at 400hz.
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Sep 10, 2011 20:51
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Frozen Horse posted:Could we do that in a way that produces less light pollution? I've seen where some people are pushing for modern streetlights (including LED lights) that generate less light pollution. I think there are some additional environmental benefits, like not messing with migrating birds at night. Bearings - I've heard several stories about where a power plant experienced a severe power failure and completely shut-down. One thing that was lost in this power failure was lubrication oil to the bearings. So as the massive turbines spun down, they generated tremendous amounts of friction in the bearings, getting so hot they welded in place and had to be jackhammered apart. Not fun when you're talking about a 1000MW turbine.
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Sep 17, 2011 02:12
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modig posted:2. Tell me about 3 phase power? With three phase power, you have three power lines, and on each the voltage is a sinewave. Each wave is 120 degrees out of phase. So one phase is at 0 degrees, another at 120 degrees, and another at 240 degrees. There are Delta and Wye systems. With a delta, you can connect loads between each of the phases. With Wye, you connect from one of the phases to a neutral. With Wye systems you'll see two voltages, line to line and line to neutral, such as: 120/208 277/480 347/600 quote:3. Bonus question! What kind of connectors do you use, are there some standard quick release/connect that work for your loads? We don't really use connectors for that combination of voltage and power. There are twist-lock and specialized plugs for up to around 100A and 600V. At that point it gets dangerous to disconnect them from the load. Some have locking mechanisms where the plug cannot be pulled unless the outlet is switched off. At high currents and voltages, a cable that could be released would result in an arc flash, and probably result in the fiery death of the person who pulled the plug (if they're lucky.) Let's say I have a 5000HP synchronous motor that needs 300A at 7200V. What I'll do is run a massive triplex cable (three phases in one package) from the motor to the switchgear. The cable bundle is about 4" in diameter. You need to plan this carefully since you cannot turn this cable on-a-dime, a 90 degree turn may require a six foot bend radius. After shutting down the switchgear, and lockout/tagout of course, you open up the back. There are copper or aluminum busbars back there from the breaker serving the load. You use a hydraulic crimper to attach lugs to each of the three phases on the cable after stripping back the outer insulation and inner insulation for the cables, and bolt them onto the appropriate busbars. (You need to watch phasing, which cable is hooked to which busbar, or the motor will spin in the wrong direction, very bad for some loads!) You also may typically apply some kind of electrical grease to the connections to prevent corrosion and overheating. The bolts are torqued using a torque wrench to a specific foot-pound rating. Then, you take a special type of electrical tape, it's like normal electrical tape but spongier and thicker. You wrap this tightly around the entire connection. Then you take another type of electrical tape and cover it again. You do this for each of the splices. Doing this well is a bit of an art, but it's sometimes crucial for protecting the connection. Depending on the location/application, they may omit the taping. Sometimes you don't want to tape if you plan on reversing the motor direction (by swapping two of the phases) in the future. Then, you need to make sure the ground cables inside the main triplex cable are properly grounded at the switchgear. The biggest reason for this is safety, in case the cable is cut. Don't laugh, I've heard of an incident where someone sawzalled into a live 2400V cable, the grounding sheath and our circuit breaker relaying saved his life. Then you do the same thing at the motor. For a big motor, there's a connection compartment. (On really small 1-100HP motors, they're called "peckerheads" but that term is falling out of favor.) Once you're connected, you may want to megger the cable to make sure there isn't any leakage to ground. modig posted:a coworker is currently designing an experiment that will use like 12kV and 200A. That's still a hell of a lot of power, and very dangerous. They're taking a lot of safety precautions, right? Three-Phase fucked around with this message at 01:56 on Sep 19, 2011 |
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Sep 19, 2011 01:45
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Talking about water - I saw a video of how bird crap is cleaned off large insulators - they have a pressure washer on a helicopter, and the washer uses pure, distilled water. . Some of the large motors I've worked with (>5000HP) either have a large internal fan, or a "top hat" assembly with one or two blowers. If the blowers fail or there's a problem with the motor, the protective relay keeps an eye on the RTDs (thermometers) embedded within the stator. The relay also sends the hottest temperature value as a 4-20mA DC signal (such as 4mA = 0C and 20mA = 300C) to a SCADA system. If the motor's temperature begins to approach the thermal damage curve of the motor, the relay trips the motor. Sometimes the SCADA system can also send a signal to trip the motor if it doesn't like what it's seeing. In a well designed system, the SCADA system should give the operators multiple levels of alarms. Such as: (these temperatures are approximations) BAD VALUE - signal is over or under the 4-20mA range, usually means a wire is broken LOW TRIP - temperature is too low to operate the motor (this is probably rare) LOW ALARM - highest stator temperature is unexpectedly low, like under 10C HIGH ALARM LEVEL 1 (High) - highest stator temperature is over 125C, the operators should look into this or at least keep an eye on it HIGH ALARM LEVEL 2 (High-High) - highest stator temperature is over 150C the operator must ACT RIGHT NOW to find why the temperature is going up and correct it, or gracefully bring the machine offline HIGH TRIP- highest temperature is over 175C, the SCADA system has caused the motor to trip, assuming the protective relay didn't already bring it down. This is the point where people start getting really angry phone calls from engineers at the plant. Possibly with shouting matches and getting in people's faces, depending on the plant and if the device that went down is costing the company significant money for each minute it's offline. (I've been told this high-stress bad behavior is a fairly common phenomena in the automotive industry.) My understanding is that tripping poses additional dangers when you trip a generator. Those are: 1. The excitation of a synchronous generator must immediately be stopped and the field discharge resistor inserted into the field circuit to prevent a catastrophic buildup of excessive voltage on the stator 2. The turbine must also immediately be tripped. When the circuit breaker opens, it's like having the driveshaft from an engine snap. Now there's nothing to absorb the energy, and if corrective measures are not taken, the turbine could accelerate until it blows apart. Then you may need to do something with the excess superheated steam, like blow it off. Not sure how they safely do that at a boiling-water reactor. Three-Phase fucked around with this message at 01:49 on Sep 20, 2011 |
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Sep 20, 2011 01:35
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Is it correct that the reactor "physics" changes over the life of the fuel? So a reactor with fresh fuel that has more pure uranium will behave differently than a reactor where half of it's uranium has been depleted and replaced with other isotopes and compounds? I'm also assuming the pump ramp-down over five seconds is to reduce the effect of water hammer, right? Three-Phase fucked around with this message at 01:13 on Sep 21, 2011 |
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Sep 21, 2011 01:10
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helno posted:^^^^ True. See http://en.wikipedia.org/wiki/Burnup Does this depend on the exciter? The older ones I've seen have two routes through the FDR: 1. Through a normally-closed electromechanical contactor 2. Through a crowbar circuit If you don't correctly apply the crowbar circuit (a set of zener diodes that will fire an SCR that's in parallel with the discharge contactor at some voltage between, say, 200VDC and 800VDC), you may still risk flashing the rings. I'll need to check with one of my coworkers for details on the FDR and flashing the slip-rings. (On the slip-rings is an adjustable arc-gap, so if something really bad happens at the exciter, there's a set point for the arc to occur at.)
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Sep 21, 2011 02:13
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Is it just me, or are BWRs a little "scarier" than PWRs? I just don't like the fact of maintaining a turbine/generator set that has radioactive steam blowing through the turbine, that and the fact that you have bona-fide boiling going on inside the reactor vessel.
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Sep 22, 2011 11:22
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Cool video I found: showing the magnetic forces involved in a serious electrical fault. From Ferraz-Shawmut. http://www.youtube.com/watch?v=pSxqVh2RYGU Also, Kaboom! (I don't think he was hurt. Much.) Pulling fuse while current was flowing perhaps? http://www.youtube.com/watch?v=ZjXK8Vhm4Po There are some amazingly dangerous videos on YouTube of kids playing with pole transformers they wired up backwards, too. Speaking of danger... http://www.youtube.com/watch?v=uDB0t5ZL_p0 Shouldn't this guy: 1. Be wearing the outer protective gloves and not wearing just the inner portion 2. Be wearing arc-flash gear This looks to me like an electrical accident waiting to happen. Three-Phase fucked around with this message at 01:41 on Sep 24, 2011 |
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Sep 24, 2011 01:27
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The Proc posted:They just work faster. All "current limiting" means, in the context of breakers and fuses, is that they interrupt the circuit and extinguish the arc within 1/2 of a cycle from the beginning of the fault. One point - some fuses are "dual element" fuses. They have two separate parts - they have a mechanism that will blow during a sustained overload, and they also have a series of links that will blow in a very serious high-current fault. As far as whipping, I've heard stories about situations where there was a "triplex" bundle of wires in a cable tray, just three wires bundled together every few feet with those really thick tie-wraps. There was a fault and over a half-mile of cable every tie wrap was broken. Also, compared to circuit breakers, fuses have very specific trip curves so you can accurately predict when they'll blow, and that also makes coordination easier. Three-Phase fucked around with this message at 12:11 on Sep 24, 2011 |
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Sep 24, 2011 12:03
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freeforumuser posted:What is difference between 3-phase 415V vs residential 1-phase 230V mains? What are the reasons they are used as it is? 415? You're from Europe, right? With a three-phase system you have three wires that have a potential between each other (A-B, B-C, C-A) and optionally a neutral (A-N, B-N, C-N) as a fourth wire. With a (US) single-phase system, you have two 120V lines and a neutral, 120V from either line to neutral, and 240V between BOTH lines. Three phase is generally used for larger applications - industrial, commercial, and power transmission. KaiserBen posted:Yes, he should. Speaking of electrical accidents, we've had a good one here lately. A guy was checking voltage on a new transformer (4160V->460V stepdown transformer), and he put a rotation meter (to check phase rotation) on the output, and then closed the breaker feeding it. Meter blows up. Then he puts his voltmeter on the output terminals (while holding it, not wearing all the gear, but had gloves and safety glasses, IIRC) and found out the hard way that it was a 4160->4160 isolation transformer, not a stepdown. Meter was a total loss. He was fine, but a bit shaken up. Oops! He didn't check the nameplate?
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Sep 28, 2011 00:00
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SeaBass posted:Honestly, for all of the reports I have reviewed, less than half are done correctly and that is because the systems are so small there is little room for error. The larger the system, the more room for something to get overlooked or not considered as a fault current contribution. On our equipment we have bus protection relaying, so if there's a fault detected, not only does the bus' circuit breaker trip, but every large machine also connected to the bus simultaneously trips off as well. (I think that's an 86 relay.)
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Sep 28, 2011 00:17
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rainwulf posted:A question to ask of the Op, or relevant members, what are the typical specs of station batteries? The ones I've seen are set up in arrays that provide about 120VDC for breaker control. They aren't really that large, I've seen them stored in a cabinet about the size of a photocopier.
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Sep 30, 2011 11:22
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grover posted:Typical VRLA cabinet (40 batteries/cabinet): That cabinet looks like it's for an Eaton PowerWare 9xxx double-conversion UPS? quote:Typical wet cell: One of the drawbacks of these wet cell banks is safety/maintenance. When you install a bank of battery "jars" like this, you've really got to: 1. Install them in a locked separate area or a cage and limit access to only authorized personnel with the proper PPE 2. Install an eyewash station or emergency shower/deluge near the batteries 3. Make sure there are sufficient air exchanges/hour to ensure hydrogen doesn't accumulate (my understanding is that these don't generate a ton of H2, but it has a tendency to scare safety people) 4. Make sure the racks are rated for the battieries (they're HEAVY AS HELL) 5. Make sure you've got diking and cleanup/neutralization materials on hand 6. Hire someone to check the chemistry on a regular basis, add distilled water to the cells, visually inspect the lead plates, etc. 7. Dispose of hazardous wastes when you need to replace jars One other quirk is circuit breakers for DC applications - it's harder to interrupt DC than AC, so you still need a multiple-pole breaker for higher DC voltages. I saw some Siemens breakers where if you want to interrupt, say, 500VDC, you needed to use all the poles of the breaker in a certain arrangement depending on if the voltage source was grounded on one end. Some applications required a four-pole breaker. grover posted:
What device is that? It looks like a Fluke. Can you get a fast Fourier transform on that little guy? Three-Phase fucked around with this message at 00:36 on Oct 1, 2011 |
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Oct 1, 2011 00:20
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grover posted:Yes, and yes. The source image title was 9390, but they sell the same cabinet for 9395s as well. And the PQ analyzer is a Fluke 345, a full-function (and very compact) clamp-on PQ analyzer. There's another screen where it plots/logs all the harmonic levels. I'm a bit disappointed in the software, though; wasn't compatible at all with Win7, and didn't let me add notes to any of the screen snapshots or export any of them as jpgs. I had to actually do a screen capture on my laptop and transfer it to my desktop to post it. The FLIR i7, however, is great- everything is saved right to a microSD card. That sucks if it doesn't have Windows 7 compatible software. Looks good that it can do non-sinusoidal current (including DC) though. Got a couple questions on that unit: 1. Can you set up scaling factors on the device? So let's say I clamp it on a phase-B CT and run the voltage leads to a phase-B PT, where the CT ratio is 2000:5A and the PT ratio is 7200:120V? (With the 10mA resolution on the 0-40A scale, it would be more of a ballpark for current.) EDIT: it looks like in the specs it can only go up to about 1600kVA. Bummer. Plus its accuracy isn't that hot at the higher ranges in current/voltage. 2. Can you capture or report on transient phenomena, such as variations/distortion in waveforms when you have a device start, a capacitor bank close, arcing, etc? 3. Can you get current/voltage harmonic FFTs for: -Quantity of voltage/current harmonic -Phase of voltage/current harmonic -Power of harmonic (including direction, so you can see if the load is generating harmonics or absorbing them) 4. Can the unit generate an EN50160 report including a magnitude/duration chart? The device I'm using at work can do this, but it's a bit more expensive than the Fluke. Three-Phase fucked around with this message at 14:25 on Oct 1, 2011 |
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Oct 1, 2011 14:16
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115% THD. One-hundred-fifteen f'ing percent THD.  I think on the PowerWare 9000 I was monitoring, the THD was easily under 4%, I think the output on that was less than what the power company was supplying us. And that's an entire active-conversion UPS system. Three-Phase fucked around with this message at 03:38 on Oct 2, 2011 |
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Oct 2, 2011 03:34
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cycleback posted:Any recommendations for brushed DC machine design books? I have a book from college that goes over the DC design a bit. Let me get the title...
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Oct 4, 2011 21:51
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