|
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.
|
#
?
Sep 10, 2011 05:15
|
|
|
|
| # ? Apr 26, 2024 16:56 |
|
|
Three-Phase posted:(I'm not sure of the difference between using GPS and, say, using an atomic clock signal.) GPS is a pretty convenient atomic clock signal I guess.
|
|
#
?
Sep 10, 2011 05:42
|
|
|
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?
|
|
#
?
Sep 10, 2011 05:48
|
|
|
Three-Phase posted: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? Yeah it does something like that.
|
|
#
?
Sep 10, 2011 06:05
|
|
|
Three-Phase posted: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.) I used to work for a company that supplies electricity to a number of steel mills in the UK. The supplier's demand forecasting team receives updates of what the plant is planning on doing on a daily basis. This then gets aggregated up into our net forecast of demand, and fired across to our within-day electricity traders, to balance the positions. I've spent the past two years trading electricity, and scheduling power stations based on our trading. I now get to deal with the demand forecasters wandering over on the most expensive day of the year, and saying "This customer is doing another melt at about 5pm, so you'll lose 30 MW over the peak of the day." 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.
|
|
#
?
Sep 10, 2011 11:20
|
|
|
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 |
|
#
?
Sep 10, 2011 15:50
|
|
|
Three-Phase posted: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 don't know a whole lot about the billing situation at the mill I worked at. I was an engineer there and didn't get into the business side of anything during my time there. But based on what some other posters have said, I think it far more likely that the guy in charge of energy purchasing would be more apt to call up the power company and tell them if they weren't going to be melting for a night vs. when they were. For some more information, the mill I worked at is the last one in Chicago proper, right smack in the middle of the city. They're the 2nd largest consumer of electricity in the city second only to the Sears Tower. They have two EAFs--one with a 12 MVA transformer and one with an 18 MVA transformer. In exchange for a much better rate on electricity, ComEd wouldn't let them melt during the day due to power availability and on hot days could tell them to delay melting or shut down entirely for the day. More about the Cubs night game thing: Considering the plant is only ~2 miles from Wrigley Field, they were also told to shut down for the night of the first night game at Wrigley because they weren't sure if there was enough juice available to power both the EAFs and the new lights.
|
|
#
?
Sep 10, 2011 17:07
|
|
|
Three-Phase posted:400hz is used in aircraft, because you can make much lighter transformers than at 60 or 50hz. 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?
|
|
#
?
Sep 10, 2011 19:34
|
|
|
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.
|
|
#
?
Sep 10, 2011 20:51
|
|
|
Three-Phase posted: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. True, I've just read http://en.wikipedia.org/wiki/Utility_frequency, and the rabbit hole seems even deeper. It seems like reactive power loads scale with the frequency, so 400 hz loses for long-distance transmission. Then again, for this application, the longest cabling run I can conceive of would be a few hundred yards from generators to stage lights or amps at a large festival. Would operating at 400 hz make it easier to simultaneously standardize on ~200 V vs ~100? Could the reactive load issue be avoided by using cabling with the correct characteristic impedance (disclaimer: the closest I come to power transmission professionally is getting weak RF signals through coax from spectrometer to pre-amp)? Compared to 60 hz, there is almost nothing at 400 hz, but there is some MIL-STD-704 and other rather weird-beard gear out there like this concrete-saw company: http://www.pentruder.com/en/400hz-technology__78. Audio also has its own weird world of incompatible gear, witness the PowerCon connector, this would just add another layer of weirdness. Sorry for the derail. More topically, have you ever worked on/with/around aluminium smelting equipment? I've heard of aluminium metal referred to as "solidified electricity" due to the amount of energy required to reduce it from ore.
|
|
#
?
Sep 10, 2011 22:43
|
|
|
Frozen Horse posted:True, I've just read http://en.wikipedia.org/wiki/Utility_frequency, and the rabbit hole seems even deeper. It seems like reactive power loads scale with the frequency, so 400 hz loses for long-distance transmission. Then again, for this application, the longest cabling run I can conceive of would be a few hundred yards from generators to stage lights or amps at a large festival. Would operating at 400 hz make it easier to simultaneously standardize on ~200 V vs ~100? Could the reactive load issue be avoided by using cabling with the correct characteristic impedance (disclaimer: the closest I come to power transmission professionally is getting weak RF signals through coax from spectrometer to pre-amp)?
|
|
#
?
Sep 11, 2011 00:29
|
|
|
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.
|
|
#
?
Sep 11, 2011 18:06
|
|
|
Cheesemaster200 posted:Speaking of arc furnaces: I spent a few months working in the same campus as one of north america's larger high power test labs. Supposedly they were capable of putting up to 10% of British Columbia's power output through the DUT, although they generally didn't since as I recall they didn't have a generator-motor style isolation setup and so maxing it out could cause blips as far as california.
|
|
#
?
Sep 11, 2011 19:48
|
|
|
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?
|
|
#
?
Sep 13, 2011 03:25
|
|
|
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.
|
|
#
?
Sep 13, 2011 15:48
|
|
|
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? It'a a decent thought, and applied on aircraft where weight savings are important, but as grover already covered, you'd have to compensate in your cabling and you would see a more pronounced skin-effect through the conductors. Thanks for this thread Three-Phase, I only took two high-power classes back at school, I'm mostly high-speed signals / digital electronics these days. And whoever posted that beer analogy for S/P/Q, that is amazing, I stole it immediately.
|
|
#
?
Sep 13, 2011 16:12
|
|
|
230 delta vs 208 wye: What are the pros and cons of each and how do you decide which is better for a particular installation?
|
|
#
?
Sep 13, 2011 18:22
|
|
|
The Proc posted:230 delta vs 208 wye: What are the pros and cons of each and how do you decide which is better for a particular installation? Also, what is the difference? I've heard of it but never got a good explanation.
|
|
#
?
Sep 13, 2011 18:28
|
|
|
The Proc posted:230 delta vs 208 wye: What are the pros and cons of each and how do you decide which is better for a particular installation? Bit rusty... Wye (208V) 208V between any two legs 120V between any leg and neutral Delta (230V) 230V between any two legs 115V between two legs and ground** some higher voltage for the last leg to ground (180V?) ** - some Delta systems are not grounded So Wye delivers three 120V rails for your goodies, Delta can only deliver two (obviously). Delta would deliver a higher voltage. Note that Wye provides two voltages: 208V between phases, and 120V between a phase and the neutral. Useful for a large installation, I would imagine. Delta only* provides 230V between phase-to-phase. *I am not an electrician, I am an EE, but the stories I've heard about some guy picking the "wrong" leg from a delta to get some 115V scare the poo poo out of me, so I'd leave it to an expert to say something about getting phase-to-ground/neutral voltages out of Delta. squeakgeek: Wye vs Delta referring to the circuit topology of each and their resemblance to the Greek letter delta and the letter Y, respectively. References [1] [2] movax fucked around with this message at 18:50 on Sep 13, 2011 |
|
#
?
Sep 13, 2011 18:46
|
|
|
Awesome thread! I have always loved industrial scale electricity since I was a kid. My dad was an instrumentation tech at a coal power plant, and he would take me though the plant looking at things. When I got older, I eventually worked there for contractors during shutdowns as a way to make really good money. It was convenient to quit school for about two semesters to every other year or so to stock-pile cash. I had no formal training, so I had to learn drat quick. My brother is a journeyman electrician so he tried to make sure I didn't end up a greasy spot on the floor from messing around with the switchgear. I was in the next room once when we heard something like a bomb going off. He felt like an earthquake. When we investigated the power distribution room, we found that one of the 480 breakers shorted out. Well, that is what we assumed since we never actually found it. The drat thing vaporized. It's things like that which make me respect electricity. Another interesting anecdote was the day some goob in the control room engaged the turbine before it had spun up to the correct RPMs to match the load on the lines. The jolt went down the lines and shook every power plant in the state drat near. Plants over a 100 miles away felt it. I would love to hear other people's stories.
|
|
#
?
Sep 13, 2011 19:20
|
|
|
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. Initially, there were all over me to make sure the students knew all about arc flash hazards. Then when I pushed them hard enough on getting the right PPE in, they changed their tune and decided to install some magic fuses on the 480 line that bring the hazard down to class zero. Now they claim it's all business as usual. I just need to review Lock-Out Tag-Out. Can these fuses really reduce the hazard? I'm more a mechanical guy than electrical, I have trouble making sense of the NFPA standard.
|
|
#
?
Sep 14, 2011 00:40
|
|
|
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).
|
|
#
?
Sep 14, 2011 01:55
|
|
|
movax posted:Bit rusty...  Delta and wye are both 3-phase systems, with 3 phase conductors. Delta uses 3 wires, Wye uses 4. "Delta" does not have a neutral, it's just "hot" phase conductors, with line voltage measured phase-to-phase. Wye is when the load is placed between the phase conductors and neutral. 120Y/208V 60Hz is a common voltage in the US. 230Y/400V 50Hz is a common voltage in europe. For 120Y/208, you get 120V phase-to-neuteral, and 208V phase-to-phase. In Europe, it's 230V phase-to-ground and 400V phase-to-phase. The Proc posted:230 delta vs 208 wye: What are the pros and cons of each and how do you decide which is better for a particular installation? I think you might be referring to 115/230V (120/240V), which is a common residential feed. It's used because it's cheap to distribute small amounts of power over long distances; the power companies only have to run one phase conductor down your street. In this case, a single 240V phase is tapped in the middle with two phase conductors on either end and a neutral in the middle, so that when you measure it, you get 120V phase-to-neutral and 240V phase-to-ground. grover fucked around with this message at 02:12 on Sep 14, 2011 |
|
#
?
Sep 14, 2011 01:55
|
|
|
KaiserBen posted:Short answer: No. Long answer: Hell no. 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."
|
|
#
?
Sep 14, 2011 02:18
|
|
|
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.
|
|
#
?
Sep 14, 2011 02:25
|
|
|
Sweet, thanks movax.
|
|
#
?
Sep 14, 2011 04:03
|
|
|
grover posted:I think you're confusing delta with center-tap delta, which is a special case and not all that common. Delta and Wye are a lot simpler than that: Whoops, yeah that's it, thanks grover. 
|
|
#
?
Sep 14, 2011 04:33
|
|
|
Take the Honda Portable Generator and all of the other listed equiptment and go out and hunt for a green base. Make sure it is one on the ground or hanging at head level from a pole, not the huge ones at the top of telephone poles. Open it up with anything convienent, if you are two feeble that gently caress don't try this. Take a look inside... you are hunting for color-coordinating lines of green and red. Now, take out your radio shack cord and rip the meter thing off. Replace it with the voltage meter about. A good level to set the voltage to is about 1000 volts. Now, attach the voltage meter to the cord and set the limit for one thousand. Plug the other end of the cord into the generator. Take the phone jack and splice the jack part off. Open it up and match the red and green wires with the other red and green wires. NOTE: If you just had the generator on and have done this in the correct order, you will be a crispy critter. Keep the generator off until you plan to start it up. Now, sauder those lines together carefully. Wrap duck tape or insultation tape around all of the wires. Now, place the remote control right on to the startup of the generator. If you have the long pole, make sure it is very long and stand back as far away as you can get and reach the pole over. NOTICE: If you are going right along with this without reading the file first, you sill realize now tHat your area code is about to become null! Then, getting back, twitch the pole/remote control and run for your drat life. Anywhere, just get away from it. It will be generating so much electricity that if you stand to close you will kill yourself. The generator will smoke, etc. but will not stop. You are now killing your area code, because all of that energy is spreading through all of the phone lines around you in every direction. From: http://www.phreak.de/infos/english/blotobox.txt - Were these guys crazy or what?
|
|
#
?
Sep 14, 2011 04:54
|
|
|
Ibsen posted:Take the Honda Portable Generator and all of the other listed equiptment and go out and hunt for a green base. Make sure it is one on the ground or hanging at head level from a pole, not the huge ones at the top of telephone poles. Open it up with anything convienent, if you are two feeble that gently caress don't try this. Take a look inside... you are hunting for color-coordinating lines of green and red. Now, take out your radio shack cord and rip the meter thing off. Replace it with the voltage meter about. A good level to set the voltage to is about 1000 volts. Now, attach the voltage meter to the cord and set the limit for one thousand. Plug the other end of the cord into the generator. Take the phone jack and splice the jack part Eight year olds, dude.
|
|
#
?
Sep 14, 2011 05:29
|
|
|
Ibsen posted:Complete stupidity This is likely more expensive, more dangerous, and less effective than cutting your phone cord and sticking it in a power outlet. With that much current your soldered joints are going to melt/catch fire/blow in short order, leaving you with a small fire, a torn open equipment box, and a running generator you need to get rid of in a hurry. If the joints and wires somehow hold up, you'll probably trip whatever overvoltage protections are on the other end. Yes, they have those: the telcos know drat well phone lines aren't immune to power surges or lightning strikes. You can definitely inconvenience a few customers this way. You already have wrecked some equipment, and may have damaged some more on the other end of the wires. Under no circumstances will this shut down an entire area code, even in the old days when all the phones had to be physically wired up.
|
|
#
?
Sep 14, 2011 05:40
|
|
|
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. Read #1: http://www.lewellyn.com/pdf/os_pdf/Arc%20Flash%20Myths%20and%20Misconceptions.pdf Sums it up better than I can.
|
|
#
?
Sep 14, 2011 20:48
|
|
|
Three-Phase posted:Aliass, how big does your shop go? Anything bigger than 10,000 HP (synchronous or induction)? My Current shop doesnt do anything over 2MW. But my previous overhauled/rewound up to 10MW. However i have done HV testing on larger salient pole motors. (Tan delta testing is an awesome bludge). As for arc furnaces and whatnot, extremely large loads are either run at nightime to benefit from the lower tarifs to save money. Or if its nessicary for production then its usually run 24/7 no matter the size. The state i live in (queensland australia) has 1/3 of its power used alone by one aluminum smelter. Let alone the masses of industry and any mines that are connected to the grid. The only time people need to talk to the supply authority is if they have to start a shitload of large inductive devices up. Mainly beacuse of the power factor correction required. I asked my farther (he works for the supply authority in distrobution management) if they had ever asked people responsible for larger loads to switch off during peak demand. But they never had to thanks to the "smart grid technology" making the grid that much more efficient. However what i do find interesting is often times at night they have to make the power factor of the grid worse using switchgear and whatnot. Just so they don't have the shut the base loads down. AFAIK it takes about 2 weeks to restart a coal turbine.
|
|
#
?
Sep 14, 2011 22:23
|
|
|
Aliass posted:AFAIK it takes about 2 weeks to restart a coal turbine. That sounds like bull to me. The coal plants in these parts are used for load foallowing and are up and down on a daily basis. Our nuclear units only take about 2 days from cold shutdown to sync to the grid.
|
|
#
?
Sep 14, 2011 22:44
|
|
|
helno posted:That sounds like bull to me. The coal plants in these parts are used for load foallowing and are up and down on a daily basis. I was always under the impression that it took a long time to restart a coal plant from cold shutdown. Obviously they can vary the load day to day, but that's not the same as being turned off.
|
|
#
?
Sep 14, 2011 23:24
|
|
|
Streetlights, how do they work? Are they wired into the local circuit for the street that they're on? Do they mainly exist to keep enough load on the grid at night? Given that most are either sodium vapor or metal halide lamps, what kind of load are they? Could we do that in a way that produces less light pollution?
|
|
#
?
Sep 15, 2011 06:52
|
|
|
helno posted:That sounds like bull to me. The coal plants in these parts are used for load foallowing and are up and down on a daily basis. Im talking from complete cold start. That was the figure i was told but most of ours are pretty old. We normally use gas turbines to deal with the excess demand and only use coal fired power stations to provide the baseload power. My understand is that you can get much faster starting coal power plants but they arnt nearly as efficient, for some reason (ill have to ask my electrical engineer friend about that). Unfortunatly my state is retarded and wont allow nuclear power for some reason. Hence coal as the baseline.
|
|
#
?
Sep 15, 2011 10:00
|
|
|
Frozen Horse posted:Streetlights, how do they work? I'm pretty sure they exist to keep enough light on the street at night.
|
|
#
?
Sep 15, 2011 19:29
|
|
|
grover posted:I think you're confusing delta with center-tap delta, which is a special case and not all that common. Delta and Wye are a lot simpler than that: I'm in Chicago and a significant portion of our services here are center tap delta. by significant I mean 90% of anything more than 20 years old. this gives you 2 legs of 120 volts for your everyday 120V stuff and one leg of WTF for letting the smoke out of your everyday 120 volt stuff. But between any two legs you get 230 volts, great for running large single or 3-phase motors. The slightly higher voltage makes motors start easier and run cooler. It seems like the majority of new buildings are using 120Y/208 service now. Drheat fucked around with this message at 02:56 on Sep 16, 2011 |
|
#
?
Sep 16, 2011 02:54
|
|
|
movax posted:
Better yet, some industrial Delta systems are Corner Grounded. I've only seen one, and it will really throw you for a loop. Its a 3 phase system, but your main panel will only have 2 bus bars, and will use 2-pole breakers rather than 3-pole. you connect two phases from your load to the breaker, and the 3rd phase connects to the grounded "neutral bar" you cannot get multiple voltages from this system and its more dangerous due to the higher phase to ground voltages versus 120Y/208 As for ungrounded delta it has an interesting advantage that is also a hazard. In the event that a piece of equipment or wire "shorts" to ground, nothing will happen. Everything continues operating as normal. however if a second short to ground occurs on a second phase, all hell breaks loose. in a facility using this type of power there will normally be a ground detector, which is just some light bulbs hooked up in a fashion that will alert the facility operators that a ground fault has occurred and the equipment needs attention. This makes ungrounded delta nice for critical equipment because it can continue to run with a single ground fault. Drheat fucked around with this message at 03:16 on Sep 16, 2011 |
|
#
?
Sep 16, 2011 03:10
|
|
|
|
| # ? Apr 26, 2024 16:56 |
|
|
Drheat posted:Better yet, some industrial Delta systems are Corner Grounded. You do have to be careful; some delta equipment uses the ground for reference or has internal surge protection that can cause issues if you hook it up to a center-tap delta or corner-grounded delta.
|
|
#
?
Sep 16, 2011 22:51
|
|