Power distribution leads to big numbers - Page 1
Forum section: electronics / general

author: IanB
author_rank: Super Contributor
timestamp: April 21, 2014, 01:14:28 am
content: Really big. Any familiar references for scale get blown out of the water.For instance, while browsing around the net I came across a reference to a 400 kV/132 kV substation transformer rated at 240 MVA. How many amps is that, I wondered?On the low voltage side, it would seem to give a phase current of:I = 240 x 106/ 132 x 103/ sqrt(3) = 1000 AHoly ampacity, Batman!What size distribution cable do they need for that? I'm not an EE so I don't readily know how you would size it, but I did find reference to distribution cables of 630 mm2in another context. This would seem to be in the ball park for 1000 A.Another amusing snippet I found is that there is a current tendency to locate new buried power cables deep underground in bored tunnels since digging up city streets to create trenches is becoming unfeasible. But since this is a new approach, the EE's don't have a lot of data to go on about temperatures and cooling requirements. So they are attaching temperature sensors along the cables to find out how hot they get and determine whether they need to install ventilation fans for cooling.This whole area seem so much fun. Why would you want to mess about with electronics when you could have fun with megawatts?

author: Fsck
author_rank: Super Contributor
timestamp: April 21, 2014, 01:20:00 am
content: Quote from: IanB on April 21, 2014, 01:14:28 ambecause you can't do it in an air conditioned, climate controlled lab.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 21, 2014, 02:12:38 am
content: The numbers are cool, but the industry also moves incredibly slowly, so it's not very exciting. Keep in mind their last big innovation was AC current, and that was over a century ago!Tim

author: AlfBaz
author_rank: Super Contributor
timestamp: April 21, 2014, 02:20:48 am
content: Quote from: IanB on April 21, 2014, 01:14:28 amAny fun would soon be killed off by the paper work requiredAs an industrial electrician the largest 3 phase cabling I've ever run were 2 parallel runs of 3 XLPE 650mm^2 single core in trefoil formation where the phase layouts were important. Special rubber sheathed triangular metal (aluminium? - cant remember) brackets were used to bolt them down to the cable trays.It was a feed from a 17MW gas turbine alternator

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: April 21, 2014, 03:01:49 am
content: Quote from: IanB on April 21, 2014, 01:14:28 amI also am nots' an EE, but my retired father worked nearly his whole career for an electric utility. As a young boy I had many on site visits to dams and AC - DC converter stations. Occaisionally my father or another engineer would hand me a fun 'artifact' of the job to play with. I can remember handling a narrow slice of cable maybe 15 cm diameter. It really could only be described as a cable of cables. In the center core was a steel (for tensile strength) multi-strand multi-bundle about 3cm diameter and around that there were 7 or 9 similar sized aluminum cables for current carrying. I can't remember the ampacity. That kind of thing was only modestly interesting to me at the time. More in the range of what fancies young boys was the january midnight trip to witness the live testing of a circuit breaker which had a compressed air tank for blasting away any arcing that formed. It needed its cold weather commissioning test and at those temperatures materials behave in strange ways. My father was disappointed the ambient was only -40 as he was hoping for -45.Also impressive to me at the time was walking in an inspection/access tunnel that runs along the very base of a dam. You can feel the whole river thundering over you and get a sense of the total power, very similar to the tunnels under the Niagara falls if you have ever visited those. A bit scary and clausterphobic especially since the tunnel in the dam had dingy lighting and the walls all had hairline cracks which were weeping.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: April 21, 2014, 03:06:05 am
content: I think Dr. Diesel is our resident expert on all things megawatt. IIRC he works at or runs or oversees a large modern coal fired power plant.

author: IanB
author_rank: Super Contributor
timestamp: April 21, 2014, 03:14:09 am
content: Quote from: T3sl4co1l on April 21, 2014, 02:12:38 amCertainly there are innovations.XLPE (cross-linked polyethylene, mentioned above) is an innovation. Before that they used liquid insulated or gas insulated cables, which had much higher maintenance demands.Deep bore tunnels to replace trenches are a recent innovation. In those tunnels they found cable trays were a pain to install the cables in. So next time they just laid the cables on the tunnel floor and covered them with a sand/cement mix to make a level roadway above for inspection vehicles. The sand mix conducts heat from the cables through the tunnel walls and reduces the cooling requirement.With overhead lines they are introducing carbon fiber reinforced aluminum cables to replace the more traditional steel reinforced cables or the more recent all aluminum cables. The carbon fiber is lighter and stronger than the steel and it conducts electricity better too.Then there are the plastic composite poles to be used instead of wooden poles. Resists both rot and woodpeckers!When you want your new house hooked up to the mains you may think the connection fee is outrageous. But imagine what happens when a new skyscraper is built, and it has a projected demand in the MW! Suddenly the electric utility has to consider adding massive infrastructure improvements to cater for your needs.Maybe it's just me, but I do think big stuff is more fun than small stuff.

author: Zero999
author_rank: Super Contributor
timestamp: April 21, 2014, 10:19:12 am
content: There's been a lot of innovation.50 years ago, mercury arc rectifiers were used for rectification, now silicon diodes are. Rotary converters used to convert DC to AC and now they've been replaced with SCR based inverters.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 23, 2014, 10:22:18 am
content: Quote from: AlfBaz on April 21, 2014, 02:20:48 amMakes the C280 (Cat 3600 I've seen) look like a baby

author: Psi
author_rank: Super Contributor
timestamp: April 23, 2014, 11:12:59 am
content: Some of photonicinductions videos on youtube have super high current cables.As thick as your wrist!He used a few turns of it in a low voltage/CRAZY high current transformer which was then shorted with a crowbar that glowed red hot after a while

author: dr.diesel
author_rank: Super Contributor
timestamp: April 23, 2014, 11:41:26 am
content: Quote from: IanB on April 21, 2014, 01:14:28 amThe power industry is quite unique! Everything is huge, massive, big and bigger, not to mention usually really really old! (except the control system and networking, which is TOP notch)As an example, a 550WM generator outputs around 18KV@ almost 19K amps per phase. There are no "wires" from the generator to the main power transformer, its all copper bus bar, 3"x8" solid bar if memory serves. From there the voltage is stepped up to 230KV/345KV/500KV and even some 750KV where it hits the grid.The main power transformers are huge and designed for the lift of the plant. Physical size is around 20'x20'x30' in height. They are so heavy (I'll see if I can find out the weight) they are brought in on special train cars, tracks are laid right up to the point of installation. In-fact one was brought in via train faced the wrong direction when my plant was built, had to be shipped all the way back to the manufacture to be turned around!I had a power plant related thread 6 or so months ago, gave a few more details, I'd be happy to further explain if anyone is interested.

author: dr.diesel
author_rank: Super Contributor
timestamp: April 23, 2014, 12:06:13 pm
content: For sizing comparisons, here is a pict of a MPT, main power transformer. Note not my plant (I've actually retired from the plant) and probably couldn't get away with posting picts anyhow.

author: dexters_lab
author_rank: Unknown
timestamp: April 23, 2014, 12:22:03 pm
content: i have always had a fascination about national grid scale power distribution, probably how i found RODALCO2007's channel he gets to play with some big bits of iron and copper at work.i remember that video from photon, one of his classics!speaking of which, you seen his new variac?

author: madires
author_rank: Super Contributor
timestamp: April 23, 2014, 12:44:08 pm
content: Quote from: IanB on April 21, 2014, 01:14:28 amDrilling tunnels isn't feasable in all cases, especially in large cities. If you have to care about historical remains, underground tram, non documented surprises, unsuited geological environment and other things digging becomes inexpensive quite fast.

author: richard.cs
author_rank: Super Contributor
timestamp: April 23, 2014, 02:28:20 pm
content: One of the interesting things about power distribution is that over the whole range of power levels (a few tens of kW to GW) the range of current stays comparatively narrow. In most parts of the system the current is between 100 and 10,000 Amps, but at greatly varying voltages - 100:1 variation in current but perhaps 100,000:1 range of power. It's mostly a consequence of the limited range of physically feasible conductor sizes. Much less than 16 sq mm is fragile, much more than 630 sq mm gets unwieldy. Obviously there are exceptions but they tend to be in fairly localised areas such as inside substations or generation plants.

author: IanB
author_rank: Super Contributor
timestamp: April 23, 2014, 02:38:07 pm
content: Quote from: madires on April 23, 2014, 12:44:08 pmThey've got quite good at handling that in London. There are more tunnels under London than there are holes in Swiss cheese

author: dr.diesel
author_rank: Super Contributor
timestamp: April 23, 2014, 11:51:37 pm
content: Quote from: dr.diesel on April 23, 2014, 11:41:26 amFull of oil, about 800,000lbs.

author: IanB
author_rank: Super Contributor
timestamp: April 24, 2014, 12:27:19 am
content: Here's an interesting article I found about transporting one such transformer:180 tonne transformer’s incredible journey

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 24, 2014, 01:38:15 am
content: Quote from: T3sl4co1l on April 21, 2014, 02:12:38 amIronically, DC current is a large (and much more recent) "innovation".For interconnection between two grids of different voltages or frequencies, or for long distances, DC is more efficient than AC. We (the US Pacific Northwest) take large amounts of power from our hydro-dams on the Columbia river, convert it into DC in Ceililo, then ship it down the "Pacific DC Intertie" into a giant inverter in Sylmar, California (just north of Los Angeles). The mass of the earth is used as the negative current return path.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 24, 2014, 01:52:30 am
content: As you say, it being an innovation is technically arguable...Allow me another observation then: until relatively recently (80s or 90s, I forget?), HVDC plants were entirely vacuum tube technology -- they still call 'em "valve houses", even though they're stacks of thyristors now.If they were truly current technology, they'd have piles of MOSFETs or IGBTs or SiC in there, but that wouldn't be a good idea. Reliability and longevity are priority. Especially since you don't want to be replacing a million transistors every five years.I wouldn't be surprised if just one of those installations could dry up the world supply of SiC devices for several years, if they had reason to push for it. Another good reason: only the most mature technology is suitable, being available in sufficient quantity, having well-established reliability, and reasonable price.Induction heating is a somewhat parallel business too. There are still plenty of customers out there with tube oscillators. Why change it if it's still making good parts after fifty years' service? They're generally even less concerned about efficiency than the power company -- who obviously have some interest in that, being that sheer power is their bread and butter.Tim

author: nihilism
author_rank: Regular Contributor
timestamp: April 24, 2014, 06:10:56 pm
content: Quote from: madires on April 23, 2014, 12:44:08 pmOr like in the case of where i live, all the ground is sand so half the time when they drill a tunnel it turns into a trench anyway.

author: SeanB
author_rank: Super Contributor
timestamp: April 24, 2014, 06:54:17 pm
content: They dug a tunnel recently under the harbour here, in what basically is 50-100m of sea sand. The tunnel borer came out with the cutting face still containing paint on it, which is not common. Digging through basically sand and mud underwater is quite easy. There are plans afoot to dig a vehicle tunnel near it as well, to cut down travel time, though this has been a plan in the making for at least the last 100 years.

author: AlfBaz
author_rank: Super Contributor
timestamp: April 24, 2014, 11:13:39 pm
content: Quote from: Richard Crowley on April 24, 2014, 01:38:15 amI remember being told about dc links in Japan, connecting 50/60Hz grids during my apprenticeship 30 years ago.A quick search revealed this doc and it seem the Swede's were first back in 1954www.elect.mrt.ac.lk/HV_Chap11.pdf?

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 24, 2014, 11:53:50 pm
content: What would be interesting is if entire neighborhoods could be V/Hz throttled to better handle peak demand. How much that could be done would be limited by the loads, but 5/6 the nominal value (100V/50Hz out of a nominal 120V/60Hz outlet) would probably be the limit since that's the voltage/frequency used in some parts of Japan and modern electronics would be designed to be able to operate there. (In reality, that extreme would only be used in a severe overload or when recovering from an outage.) It's certainly much more preferable than having to power down neighborhoods when there's too much load. (Of course, that would only work when the load is mostly motors and/or resistive, which is almost always the case for peak residential loads. When the load is mostly electronic, it would actually increase power use slightly.)There's actually an off grid inverter out there that lets you change the output to 100V/50Hz (from a nominal 120V/60Hz) and it's claimed to get 20% more runtime out of a battery when operating motor loads. A lot of the cheap inverters can also do that by tweaking some internal trimpots.