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Flash18 posted:Been reading through this, I'm an EE student, and admittedly I have trouble wrapping my head around the concept of reactive power. The actual effect is that it makes generators and cabling see higher currents, with less power actually being used to do whatever the load is intended to do. Much of the VAR power is simply reflected to the source, so 1A reactive is 1A you can't use for load power. 1A reactive going down the line with 19A real = 20A apparent, and if that's capacity, well, you get 19A of work instead of 20, despite doing the work of generating 20A. Reactive power takes up capacity in all equipment in the circuit (genset, cabling, motor) and does not contribute to the work being done. Specifically in motors, a low power factor can cause excessive motor heating, since the motor ends up carrying much more current to do the same amount of work. This image is a pretty fun analogy: http://madamenrg.files.wordpress.com/2011/04/beer.jpg?w=435&h=426 KaiserBen fucked around with this message at 16:14 on Sep 6, 2011 |
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Sep 6, 2011 16:07
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Ender.uNF posted: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. Ooh, a question for me. I'm a VFD commissioning engineer working in the steel industry. First, the AC power is rectified into DC. For a 10k HP motor (7.5MW), we'd typically have 3.3kV incoming power, rectified to a 3 level DC bus, +3400V, 0V, -3400V, then we use an inverter (with enormous transistors, capable of handling up to 6000A) to turn the DC into AC again, but with variable voltage, current waveforms, and frequency. Most VFDs can do at least 0-120hz, some up to 400hz, so you don't have to use 60hz motors (and the limited speeds that restricts you to). I typically see lower frequencies in bigger motors, due to the slow speed and high torque needed. Main drives (7.5-10MW) are typically in the 11-30hz range, 100-300RPM max.
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Sep 6, 2011 16:13
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Cheesemaster200 posted:It makes everything see higher currents. This equates to more losses in transmission and requires beefier alternators, cables, etc. The reactive power doesn't create higher loads by itself. True, fixed.
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Sep 6, 2011 16:14
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Three-Phase posted: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.) All of the above. We're using SCRs for rectifiers (our drives can have one large rectifier with several smaller inverters), and IGBTs, IEGTs, and IGCTs for inverters, in increasing order of size. THD is pretty low, a bunch of our new products are IEEE 519 compliant (some need inductors on the output or DC link). Edit: I've been thinking about doing an "ask me about working in a steel mill" thread, if anyone has any interest? Work has me pretty busy right now (12hr x 6 days), but I figured I'd ask. KaiserBen fucked around with this message at 15:06 on Sep 7, 2011 |
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Sep 7, 2011 13:24
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Crackpipe posted:The last two explinations made a ton of sense, thanks guys. A disconnect switch is just a way to lock a disconnect in the system for maintenance, and cannot be opened under load (eg: to stop a runaway machine); you must first turn the load off by another means. A load-interrupting switch can be opened under load, like a circuit breaker, and is much more heavily built to be able to do that.
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Sep 8, 2011 13:39
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ANIME AKBAR posted:So has anyone here actually worked on or in thyristor halls? They're the one part of super-high power electronics that really blows my mind. How the hell can you trigger hundreds of devices in a balanced, consistent manner and not just blow everything up? What the hell happens when a thyristor stack fails, anyways? I've worked near a thyristor power factor correction setup, with 72 cells per stack, 6 stacks. As for how they're synchronized, it's not all that difficult, a good timing board + pulse transformers (at least that's how ABB did it on this one). In our thyristor drives, we use a specialized timing IC to make sure that we don't trigger both sides of a bridge at the same time, but in that application, timing is a bit less critical. When one fails, it's a bit of a mess. Tends to spray the guts of the thyristor all over the cabinet, magic smoke, etc. The main issue that I've seen is that the fuses take longer to interrupt the current than the thyristors do, so you're pretty much guaranteed a spectacular failure.
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Sep 13, 2011 15:48
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Guy Axlerod posted:I'm looking for a second opinion here. I'm working on a repair technician course for a piece of equipment that is dual-fed 480VAC and 230VAC, each three phase. Short answer: No. Long answer: Hell no. NFPA deals with max fault currents, which are determined by voltage, transformer impedance and size, and the clearing time of the breaker. Fuses change none of this; the new GE arc-flash dome thing might, but it's not been evaluated and classified yet. IIRC, 480V with any sort of power = Cat. II minimum. I'd get them to explain it fully (and do some hot work themselves, if that's what they're asking you to do).
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Sep 14, 2011 01:55
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Guy Axlerod posted:That's what I suspected. The only real explanation I had was that the clearing time of these fuses was fast enough that it all didn't matter. Basically, "It's cool bro." Yeah, I'd double check their math. And then get them to do it first. Check the clearing time on the fuses, and run the numbers yourself. It should still be Cat. I or II, depending on transformer sizing.
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Sep 14, 2011 02:25
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Three-Phase posted:I'm also assuming the pump ramp-down over five seconds is to reduce the effect of water hammer, right? That's one reason to do it, though pumps and fans usually have pretty long ramp times anyway due to high inertia relative to motor power needed. Fans can be especially sensitive, due to the lighter-built ductwork being far easier to damage with sudden changes in flow/direction. One of the fans I'm working on here is ramped at 45 seconds to accel/decel.
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Sep 21, 2011 13:53
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Three-Phase posted:Speaking of danger... 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.
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Sep 27, 2011 03:27
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rainwulf posted:MORE IME, you use IGBT/IEGT/IGCTs (in rough order of power level) instead. That's what our inverters use, up into the 75MW range, no experience higher than that (we use the same tech up to 120MW though).
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Oct 27, 2011 03:31
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PPoison posted:Nice thread. I have been pondering on some DC motor theory and perhaps you can help me throw some light on an issue. IIRC it's due to residual magnetization in the armature/field windings. So flux is not actually 0, but rather just very small, allowing an unloaded motor to accelerate freely until it tears itself apart. ETA: Flux limits the speed of the motor by providing counter-EMF, proportional to field current. This is why you have to weaken the field to exceed base speed, and consequently end up with less torque at >base speeds. KaiserBen fucked around with this message at 21:40 on Dec 5, 2011 |
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Dec 5, 2011 21:33
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Cheesemaster200 posted:I found the label making tool in SKM today.... I've actually seen this kind of label, calculated flash was 200+ cal/cm2 and boundary was 50+ feet. "No safe PPE exists" was the last line on the label. It was on a drive cabinet for a MV drive that was installed without an isolation transformer, directly onto a 10kA+ bus network at 4160V. I didn't like being in that room, much less opening that panel (fortunately, I was just there for the LV stuff). quote:Im sorry if this question has come up and feel free just to quote the answer, and I would be a happy man Yeah, I got hit with ~250VDC from a big breaker's trip coil. Hurt like hell, arm tingled for the rest of the day.
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Jan 22, 2012 06:31
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Meow Meow Meow posted:I've been doing electrical for mining for almost a year, I haven't seen anything like that...yet. Most sites I've dealt with do their distribution at 13.8kV (4160V for older mines), and step down to 600V (Canada). I've seen a few 900VAC devices before, and we have a lot of equipment in the 900-1800V class. Several of these drives are installed in mine hoist systems, so I can see 1000V breakers being useful there (though usually we put the breaker on the medium voltage side).
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Feb 2, 2012 07:32
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Three-Phase posted:I'd be interested in hearing about what you have. At 5000HP you could have synchronous or induction motors. I'm guessing no less than 2400V (for 5000HP 2.4kV may mean parallel cables), maybe 4160V or 7200V? In most of the mills I've worked at (steel or aluminum), the main motors (5-10MW range) were either 3300V or 4160V, depending on whose drives were being used. Of course, I've never seen a motor that big on a starter, since everything I've seen has required fairly precise speed control for process reasons. Parallel cables are almost always used on a motor of this size, IME.
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Jun 7, 2012 14:12
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Three-Phase posted:10MW at 4160V is a hell of a lot of current. 1300A would definitely require parallel cable connections like 4 per phase. What was the inrush for a motor like that starting across the line? 10kA? Yeah, they usually used 6 cables, not 100% sure, but they were 500kcm range. The inrush was limited by the drive, we try not to start such things across the line, for obvious reasons. Even on a VFD, it routinely pulled 300% current (for <one minute, as per the overload capability) as a slab went through the mill. I'd honestly have to check for the currents, we almost always deal in terms of per unit power/percentages, so raw amps generally have to be calculated. I'm on a much less exciting (power-wise) project now, automating container cranes; lots of cool automation equipment (laser measurement scanners especially), but the power requirements are pretty small (1MW per crane or so, 5-6 for the ship-to-shore cranes). Each crane is fed from an 11kV cable trailing behind it that gets wound up on a reel as it moves down the track. Interesting stuff, but I prefer the steel industry overall.
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Jun 8, 2012 08:14
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Meow Meow Meow posted:Ahh, thanks for clearing the synchronous motor starting, I'll have to look into these mills a bit more and see what method they use to start. If it's anything like the grinding mills we've done (granted, we always used VFDs on both motors), they are solidly mechanically linked, and one motor is run with a speed regulator (the "master"), the other with a torque regulator (the "slave"), so that both motors are running approximately the same load. I've also heard of a dual-winding motor with a 60deg phase shift between the two windings, to improve the "linearity" of the torque produced (in essence, a 6 phase motor). They use two VFDs, each one driving one winding), generally on separate transformers. To me, that diagram shows a system similar to what I described, looks like two SCR drives (cycloconverters? not really familiar with SCR based AC devices) and redundant transformers. I'd certainly suspect them to be mechanically linked (though perhaps with a way of free-wheeling one if you lose a motor). Three-Phase's explanation of the delta-wye reasoning makes sense to me (though I've always seen SCR devices on individual transformers).
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Jun 28, 2012 10:45
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Three-Phase posted:Speaking of fire, a long time ago I read something from a switchgear manufacturers where you can install smoke detectors inside the switchgear. I guess the idea is that you could hook that into a SCADA system so that the workers can respond and gracefully bring down the system and find out what's going on. Not sure which vendor did that, or if all vendors offer that for low voltage gear. On the project I'm working on right now, we have a fire suppression system (CO2 based, mobile equipment), that attaches to the control system. The PLC can trigger the CO2 dump, or can get notified when the fire system detects a fire and do things like trip the upstream MV breaker. I have seen switchgear with fire detection in it, but I'm not sure whether it was original or added later (but it was tied into the SCADA system, even showed on the HMI).
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Jul 14, 2012 05:58
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Three-Phase posted:Here's my question - if you have equipment that's purpose is to protect equipment and the facility, should you have a different safety PLC or tie it into a fire protection system? If there's a fire at a facility, I can see where being able (or unable) to trip the MV upstream breaker would be a possible life safety issue for firefighters and personnel depending on the situation. Unless tripping the breaker might create more dangerous conditions like if you had ventilation blowers that would suddenly stop. The equipment room the CO2 protects is only ~8x14', and is normally unmanned during operation. Water would be impossible without mounting a tank, there's no permanent water lines to the cranes. The breaker is controlled by the control PLC, the safety PLC is not involved in the fire system operation (thankfully, else we'd have CO2 dumps every 5 minutes when it becomes unhappy). There can't be a general "facility fire" here; it's a container terminal where the entire yard is paved in stone and the e-rooms are on rail-mounted cranes (powered by a trailing cable that gets reeled in/out as they move down the track). If there is a fire, the firefighters will arrive after the CO2 dump (and thus MV breaker should be off). If the MV breaker somehow does not trip, there's a shutoff just upstream of it in the substation, ~300m away. As for the risk of the CO2 system, it has a 15sec holdoff, and there's a button to prevent CO2 release in the room it protects (but again, nobody should be in there during operation). For equipment needed to protect/evacuate teh facility, I'm not entirely sure on the protocol. I'll ask one of my friends in the mining division of our company what they do for mine hoists/blowers. Pretty sure all that stuff is switched off manually only though, no automated trip.
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Jul 14, 2012 14:49
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Three-Phase posted:They drat well better be! And if everything that "drat well better be" was, I wouldn't have all the fun safety videos I just watched. I also wouldn't be trying to convince people that unshielded instrument cable next to VFD motor cables (unshielded too, of course) was an issue.
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Oct 26, 2012 02:49
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Three-Phase posted:Well i didn't notice this until today Photonic Induction's killer screwdriver. Those are insanely common in the developing world; all the electricians I worked with in India and China had one. Since most of our plants use 24vdc or 120vac control power, it made a reasonable amount of sense, since it wouldn't electrocute you even when accidentally used on 240VAC. If you magically poke it into 4160V, I'd assume it'll hurt (if briefly), but I've seen them used up to 240V with no ill effects (not that you'd catch me trying it). They're largely banned (by law or regulation) in 1st world countries; the Aussie jobsite I was at threatened to kick a guy offsite for having one. Apparently the bulb can fall off the contacts, leaving the user as the line->ground resistor.
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Sep 13, 2013 17:17
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M_Gargantua posted:We started putting florescent red adhesive strips next to the entry terminal board for any external sources of power. Mainline, Standby, Control. The only exception is the florescent green we use for <28VDC which is only ever used for indicator lights. Everything has a sharpie pen in for the designator of whatever source that is. We're working on getting nice professional label plates made for everything which has a full in depth list of every source. Gotta have that aesthetics before anyone will approve any alternations. Granted, the tape alone is unapproved. But out of sight out of mind and all that. I was at a plant in Ohio where they had a florescent orange label on each cabinet detailing the incoming power sources (voltage, current capability, arc flash hazard, and a lockout point) on every cabinet. Struck me as a decent idea, but nobody else seems to want to implement it.
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Sep 13, 2013 17:20
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grover posted:I'm a big fan of Extech. Every Extech meter I've used has been every bit as good as a Fluke, but without the Fluke tax attached. If it is, I can name at least 10 major plants in the US that are out of compliance. Arc flash data is, but lockout points aren't (at least that I can find in the code books).
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Sep 13, 2013 17:39
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It's legally required now. Well, most of that at least. "Nobody else" really don't have much choice in the matter.
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kastein posted:I'd bet on those resistor banks being for braking actually, not speed control. Something like the gigantic resistor banks and cooling fans in a diesel locomotive. It really looks like a wound rotor motor; they're still used quite commonly in mining equipment, especially ore crushers (with VFDs now used in place of the speed control resistors). The three slip rings are a dead giveaway, synchronous motors would only have 2. The big switch looks like it could be for switching resistances, and thus speeds. It appears to be wired to various combinations of resistors, and that resistor bank is quite oversized for braking.
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Oct 15, 2013 03:35
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some texas redneck posted:Lets talk about what it takes to avoid rolling blackouts. I know at least at one steel mill in CA, they do get a phone call that is their 30 min notice to be under X% of their normal draw (basically, "shut down the mill, but you can keep the lights and AC on") within 30 minutes. They also schedule their heavy maintenance at the power company's request, so they're shut down at least 2 days a week in the summer, but for all this trouble, they pay about what plants in the south do for power instead of the normal CA prices. As for the automation, it's really not very automated, from what I gather. The down days are scheduled via email (though they have a typical schedule), and the high demand shutdown is literally a phone call. They do consume something like 200MW though, so that phone call is pretty productive from a load shedding POV.
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Feb 6, 2014 04:38
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Three-Phase posted:Hell, I've worked around motors that were like 3 megawatts. Just one motor. That's nothing; we're putting in some 47MW motors later this year. VFDs for them too.
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Aug 13, 2014 05:05
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Three-Phase posted:Nice. What company are you looking at for the drive? (I think ABB has water cooled Megadrives in that range.) It's our drive/motor, we're selling to a compressor OEM for a natgas compressor. ABB does offer the Megdrive in that range, but it's considerably older tech (being an LCI drive, using thyristors vs ours which is a voltage source drive using GTOs). I got to see factory testing on a 25MW version of the same drive, the thing is absolutely massive.
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Aug 17, 2014 23:27
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Three-Phase posted:Yeah, and I think LCI (load commutated inversion) only works with synchronous motors where you can overexcite the rotor. You're correct on the LCI, and it's often used with a "6-phase" motor, where you essentially have two 3-phase motors at 60deg electrical angle mounted on a common shaft, using them as a master-follower pair, to smooth the torque pulsations that type of drive is known for. The voltage source drives can put a lot fewer harmonics back; they still need 18/24 pulse rectifiers in the larger sizes to maintain IEEE519 compliance. We have one MV drive with an active front end, using LV IGBTs in series (5-10 cell modules per phase), but I've not seen one "in the wild" yet. Water cooling does irrationally scare a lot of people. OTOH, we did have a customer disable all the interlocks and manage to run a drive lineup iwth the water cooling system shut off. Nice $1m or so mistake there.
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Aug 19, 2014 06:25
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