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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 best way to put it is that reactive power represents power which is not consumed. If you have a reactive load, it will pull current due to the inductive (for lagging) load, but it will not have a mechanical force applied to the alternator to feed that load. Its like a car with its rear wheels off the ground and the engine being floored. The wheels are turning extremely fast (current), but it represent little load on the engine. Look at it from the perspective of vector analysis. For a motor you will have a fixed vector in the positive j axis that does not change that represents your reactive load (kvar) On the real axis you will have the actual power (kw) consumed, which does change based on load. At zero real load, you will have a resultant vector that equates to only that reactive component and your current will be based upon only that. As you ramp up the mechanical load on the motor, the real vector will increase (somewhat) proportionally and the resultant vector of the triangle will represent your apparent power. Current will always be based on the resultant vector. In physical terms, the reactive power in a motor represents the core losses of the inductor, eddy currents, etc. These losses will generally be constant and unrelated to the loading of the motor. quote:I may have overstated the issue of residential harmonics; yes, CFLs and TVs and computers have cheap power supplies and have an impact, but the sum total is still rather small (a few hundred watts total). All the big loads in a typical home are resistive or motor loads- air conditioners, ovens, refrigerators, hot water heaters, coffee pots, etc. A single coffee pot can draw more power than all the lights and typically running cord & plug stuff in rest of your house combined. Cheesemaster200 fucked around with this message at 16:07 on Sep 6, 2011 |
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Sep 6, 2011 15:59
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quote:The actual effect is that it makes generators and cabling see higher loads, with less power actually being used.
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Sep 6, 2011 16:09
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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? The resistance of a person (or whatever) will stay generally constant. The higher the voltage, the more current can potentially run through you. Current kills, but in most situations current is almost always a function of voltage. Higher voltages are also more likely to arc. They are almost always always closer to the generation source and have higher available incident energy. Transmission also functions a lot different than building distribution in regards to how faults are handled. quote:Why would a power bill increase monthly by 10% in less than 6 months?
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Sep 8, 2011 20:00
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Speaking of arc furnaces: http://www.youtube.com/watch?v=gmXzNMoea5E&feature=related .. and yes, they do have to call the power company when they turn these things on. They are such a large load that they can potentially gently caress with generation and transmission capacities. The same is true of very, very large motors. They can only start them at certain times of day because the inrush is so high. They need to coordinate the start up with the loading of the surrounding transmission grid.
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Sep 11, 2011 18:06
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SeaBass posted:It is not as common as it should be, but with OSHA being more assertive about assessing arc flash hazards, design engineers will be more diligent about protective device evaluation and coordination. Or they can pay me to do it for them The problem is that you can't coordinate most molded case circuit breakers below around 100A, especially for arc flash. If you have low fault currents in your system you are going to have a lot of incident energy at your downstream breakers and coordination is next to impossible without using fuses. quote:I've often heard this could happen and have seen it in video, but had never seen it first-hand until one of the contractors I work with mis-wired a load bank- essentially feeding the full output of a 480V 400kVA transformer into a pair of 30' long #12 control wires. Damned things whipped like crazy! The 2A fuse blew almost instantly, but the magnetic forces from that much instantaneous current was amazing. I also talked to a guy who apparently shorted some control wiring in a generator in a power plant, tripping the output breaker and turbine. This in turn tripped out the other generators (as the load was spread to them), which tripped out the plant, which tripped out all of Guatemala City, Guatemala. Don't know if it was for real, but I found it funny.. Cheesemaster200 fucked around with this message at 17:23 on Sep 29, 2011 |
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Sep 29, 2011 17:06
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I found the label making tool in SKM today.... I know its an old picture joke thing, but I always still get a chuckle out of it:
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Jan 16, 2012 19:11
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quote: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. 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.
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Jan 16, 2012 21:43
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quote: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.
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Jan 17, 2012 15:36
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One often overlooked function of a ground is that it sets a reference for all other voltages in the system, making it much easier to analyze and protect. Once you get into ungrounded systems it gets really  , really quickly.
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Jan 18, 2012 18:02
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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.
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Jan 19, 2012 21:23
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Three-Phase posted: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.) Please give me work...  I also haven't done an actual arc flash calculation since studying for my PE exam. I just plug that poo poo into SKM and it poops out the incident energy levels for me. Its actually pretty simple. 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. On another topic, does anyone know how to put ground fault protection on a DC supply feed from a PV system? CTs obviously won't work so how do you actually build a residual current device to trip a ground fault? Eaton tells me that they do not build a dc GFP device and I am a bit perplexed as NEC requires it. Cheesemaster200 fucked around with this message at 04:38 on Mar 26, 2012 |
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Mar 26, 2012 04:24
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Three-Phase posted: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.
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Mar 27, 2012 22:31
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http://www.ecowattplug.com/p/63/ecopac Someone asked me today on whether or not this was a good deal. How people can get away with making such statements is beyond me. Meters don't even bill kVA, they only bill kW... Cheesemaster200 fucked around with this message at 18:11 on Apr 16, 2012 |
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Apr 16, 2012 18:08
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Who can spot the state which has had massive government subsidies for solar installations! http://www.pjm.com/about-pjm/renewable-dashboard/solar-power.aspx
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Jul 27, 2012 19:37
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Three-Phase posted:I'm not going to lie, I'm a fan of fuses, or at least fuses used in careful conjunction with other protective devices. It's great for preventing nasty catastrophic "blown apart equipment" failures that cost lots of money and time to fix, and reducing arc flash hazards. I am more concerned about single phasing with fuses. However if you are using them in a MV distribution setting for something which isn't a wastewater treatment plant, they are usually much easier to coordinate and provide better protection, as well as cheaper.
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Oct 24, 2012 15:53
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Three-Phase posted:With single phasing, wouldn't either: In most cases this would be correct, especially in a MV setting where you would set every protection available for large motors. However, I still work on a lot of older projects with straight magnetic controllers which are not equipped with such relays. Every now and then we get a call about motors burning up because a fuse blew or the utility was being useless again.
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Oct 26, 2012 19:01
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peepsalot posted:This is neat, I always wondered how car inverters generate a nice sine wave. The answer is they don't! I don't know what kind of alternator is used for those plots, but if you use a synchronous generator and the prime mover is providing a constant rotation, your output will be a pretty perfect sine wave. That plot was using a crappy home generator that most likely uses brushes or something to output a single phase residential voltage. A car alternator is going to be three phase to provide better DC rectification through a diode bridge. That output will not be as jagged, though also not a constant frequency. Grover: For a home generator, you can just plug into a 6-30R (or whatever) mounted on the bottom of the panel and wire a neutral/ground wire from the genset to panel neutral/ground bus, right? Those portable generators are bonded, and the utility service bond will be outside the house, correct? Also, if a portable home generator puts out 240V, is it a true split single phase in that you will get 240V from each lead and 120V to the neutral? A few people were asking me about this and I wasn't too familiar with those home depot specials.
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Nov 12, 2012 03:15
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Three-Phase posted:ABB's game-changing HVDC breaker. This is going to be big for all those theoretical wind farms in the middle of nowhere or offshore.
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Nov 18, 2012 23:32
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http://www.baltimoresun.com/business/bs-bz-jonathan-libber-smart-meters-20130317,0,4437036.story I want to hit these people, repetitively.
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Mar 25, 2013 14:42
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So essentially this was a time delay relay with the caveat of phase protection? - If the current exceeded the trip setting it would initiate a time delay before tripping? - If one phase then went to under 3.5A after exceeding the trip setting it would assume phase loss and trip as well? What was the functionality of the blocking function?
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Mar 29, 2013 16:32
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Did they use this thing to avoid having to put in PTs and phase voltage sensing monitor? I am assuming so since it was a fused switch and not a (more expensive) circuit breaker as the primary OCP. I guess the sequence of operations make sense. If you had a phase imbalance relay, you could potentially nuisance trip it depending on how the stadium start up was done. Likewise if you tripped it at any time it went below 3.5A you would get a lot of nuisance trips when the stadium is not in use. Therefore they put in this fancy thing that only trips the motorized switch if the phase current exceeds the fuse rating and then drops below 3.5A. The blocking function just keeps the OCP function in the hands of the fuses. I kind of wonder what the time delay is between an overcurrent situation and a phase loss initiating the trip circuit. In other words, how long does it stay in "waiting" mode per grover's sequence. E: ah, I guess that is the time delay on the relay from what I can deduce. Cheesemaster200 fucked around with this message at 21:44 on Mar 29, 2013 |
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Mar 29, 2013 21:38
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Three-Phase posted:If it was transformer inrush, I'd expect that to be dissipated within, what, no more than ten cycles? Two seconds is an eternity for current flow. This poo poo kills your arc flash levels though and it slows the instantaneous opening time on the trip curves.
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Mar 29, 2013 21:45
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Anyone proficient in SKM here? I have a five section 4160V ring bus fed from five paralleled transformers of equal impedance and size. Loads are out of balance to a degree on some busses versus others, but nothing that major. The bus itself can handle the full load amps of four of the transformers. I put this all into SKM, but for some reason the section 4 transformer will not accept any load and dumps it all onto section five, overloading it. If I isolate section 4 it works peachy, but refuses to accept load when I close the tie. In fact, when I do that the another transformer on the ring bus just disappears. I want to do a load flow analysis, but this crap is making me bonkers. It just does what it wants!
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Apr 11, 2013 22:26
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Three-Phase posted:Just remembered this - do you have some impedance between the busses? Circuit breakers by themselves have no impedance, and SKM gets cranky in you do things like connect busses together with no impedance (typically a cable or bus). It won't let you connect them together if there is no impedance. Everything *should* be working from the perspective of mechanics. I think there is a bug in the programming or I am missing some setting somewhere. I sent the model to SKM to have them troubleshoot it.
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Apr 13, 2013 16:18
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Your European one-line symbols confuse and perplex me...
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Apr 18, 2013 20:35
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Gradient rings allow you to use smaller insulators. The small electrical gradient on the conductor makes it less likely to breakdown, requiring less material.
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Nov 11, 2013 03:59
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