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This is a long read, yet quite instructive of where China has been and where it plans to be when it comes to Ultra High Voltage (UHV) transmission lines, where and why it uses AC vs DC, how it converts between the two, and what some of the challenges are that lay ahead.
China's Ambitious Plan to Build the World's Biggest Supergrid
Wind rips across an isolated utility station in northwestern China's desolate Gansu Corridor. More than 2,000 years ago, Silk Road traders from Central Asia and Europe crossed this arid, narrow plain, threading between forbidding mountains to the south and the Gobi Desert to the north, bearing precious cargo bound for Imperial Beijing. Today the corridor carries a distinctly modern commodity: gigawatts of electricity destined for the megacities of eastern China. One waypoint on that journey is this ultrahigh-voltage (UHV) converter station outside the city of Jiuquan, in Gansu province.
Electricity from the region's wind turbines, solar farms, and coal-fired power plants arrives at the station as alternating current. Two dozen 500-metric-ton transformers feed the AC into a cavernous hall, where AC-DC converter circuits hang from the 28-meter-high ceiling, emitting a penetrating, incessant buzz. Within each circuit, solid-state switches known as thyristors chew up the AC and spit it out as DC flowing at 800 kilovolts.
From here, the transmission line traverses three more provinces before terminating at a sister station in Hunan province, more than 2,300 kilometers away. There, the DC is converted back to AC, to be fed onto the regional power grid. Since it opened in mid-2017, the 26.2 billion yuan (US $3.9 billion) Gansu–Hunan transmission line has moved about 24 terawatt-hours.
The sheer scale of the new line and the advanced grid technology that's been developed to support it dwarf anything going on in pretty much any other country. And yet, here in China, it's just one of 22 such ultrahigh-voltage megaprojects that grid operators have built over the past decade. In the northwestern region of Xinjiang, China recently switched on its largest UHV link: a 1,100-kV DC circuit that cost over 40.7 billion yuan. The new line's taller transmission towers and beefier wires parallel the Gansu–Hunan line through the Gansu Corridor, before diverting to Anhui province in the east.
The result of all this effort is an emerging nationwide supergrid that will interconnect China's six regional grids and rectify the huge geographic mismatch between where China produces its cleanest power (in the north and west) and where power is consumed (in the densely populated east). By using higher voltages of direct current, which flows through conductors more uniformly than does alternating current, the new transmission lines dramatically reduce the amount of power that's lost along the way.
(Score: 2) by deimtee on Wednesday March 06 2019, @07:59AM (1 child)
Those big old welders are nice to use, they seem smoother somehow. I'm not sure running it on 110V drops the current, it should drop the voltage. I'm really not sure about that though, USA wiring is odd. The 110V is effectively a centre-tap on 220V. I would say doing nice welds with a 10A or 20A stick is pretty skilled.
Here in AU we have 3-phase 415V running down the street, the household 240V is one phase. For a fairly small fee you can get your house/shed connected to the 3-phase for high power stuff.
I think some of the rotary ones put an insulator between the contacts, the snap-thunk is to stop people turning them slowly and creating an arc. If you try to turn one slowly nothing moves as the pressure builds up until it suddenly lets go and switches. I do know there are some switches that are oil filled.
Regarding the light bulb story, it is possibly an urban legend, but it makes a good story and is quite believable. Thinking back, it might have been in an electronics magazine rather than on the web. It was a long time ago.
Maybe for the lighting circuit. To boil a standard 2KW kettle and make a cup of tea you would be pulling 180 amps. Nope. No thanks. I like my wiring in the solid phase of matter.
If you cough while drinking cheap red wine it really cleans out your sinuses.
(Score: 3, Informative) by RS3 on Wednesday March 06 2019, @06:20PM
> Those big old welders are nice to use, they seem smoother somehow.
Like you pointed out, AC breaks the current flow, so you need energy storage to keep the electrons flowing (smooth arc). The big old transformer has more L. The heavy iron core stores more energy before magnetic saturation (a very complex topic...) The more iron and bigger the L, the more energy available to fill in the gaps in the current. Much like power supply filter capacitors, really. In fact I added some big capacitors to my DC MIG welder. Many have them, mine didn't. I added a switch so I could try it both ways. It's very interesting.
Our (US's) 240 center-tapped system is very good IMHO. There are many advantages- too many for me to type up but I can if anyone is interested. Sorry for the pedantry, but we abandoned 110 a long time ago, but many people still say 110 and 220. It went to 113, then 115, then 117, then 120, and I've measured 125 at times (I think the power companies want to sell more power!). I guess the biggest advantage is you have 240 for big stuff, and somewhat safer 120 for smaller normal stuff.
I am an EE and would not advocate 12V for heating! Just for many appliances and lighting. I'm too lazy to look it up, but a super-smart expert electrician I know told me that most fires attributed to wiring / electricity are caused by people using light-duty extension cords to run space heaters and other high-power devices. I would rather reset our whole system and use different plugs and voltages for heaters and such. In the US, a typical portable "space heater" can draw 13 A, but we have plug-compatible extension cords only good for 6 or 8 amps, and the problem is: people don't know any better.
Lowering the voltage into the welder will lower the current pretty much by definition (inductor core permeability curve, saturation, etc., aside). Inductance "L" (Henrys) is used to calculate the inductive "reactance", or impedance, which for simple circuit analysis looks like resistance but dependent on frequency. Complex impedance Z = 2 * pi * f * L, and for steady-state sinusoidal signal I = V / Z. It's obviously very complex in a welder, but the end result is you can get a fairly steady predictable current based on the welder's setting.
Oh- a cheating trick when stick welding (I didn't do it) is to use the rod coating as your spacer. You essentially lay the rod on the work after you start the arc. Of course you have to move it, etc. Oh, and I used to cut the sticks in half- a 1/16" rod is much too wiggly at full length to control the arc and feed. Glad those days are over!
3-phase 415- very interesting- I didn't know. We have 3-phase here too. I know this will sound weird to you, but it works for us: most big-stuff (commercial / industrial) 3-phase panels run at 480 phase-to-phase. You get 277 phase-to-neutral, and that's used for large building lighting, parking lots, and some more normal indoor lighting- commercial / industrial only- not residential. And generally only hard-wired stuff. There are 277V outlets but I've never seen one in use. Then we drop it down to 208 phase-to-phase which gives us 120 phase-to-neutral. Most single-phase 240V stuff is designed to run at 208, and the nameplate will say "208/240", but if not, you install an "auto-transformer" (voltage booster). I've never seen 3-phase in a residence, and I don't know if it's allowed.
I'm aware of the snap-action of larger switches, relays, and circuit breakers. Those 1000+ amp ones scare me! They go BOOM! when things are working correctly!