DC used in power distribution

Yes that's a new one... Typo?? Always though power transmission was best done the Tesla way....AC.

I'll take a guess...maybe DC to avoid 53 miles of skin effect from

"With financing in place as expected by the end of the month, the

This 400kV one is the longest with 290 km under water.

....... Phil

I think it's mainly to avoid corona. Corona depends on the peak voltage on the cable, so with DC you can put twice the power through the same line.

Cheers,

Phil Hobbs

Most of our new EHV lines are DC. MHD conversion on each end. I was involved in the research (as a student technician in MIT Building 20) from 1959-1962, under Professors Woodson and Jackson (and PhD candidate Jim Melcher, later head of the MIT EE Dept.), but it has been really slow getting it to a practical level.

...Jim Thompson

Although shorter, the UK - France HVDC interconnect at 70km long can transfer up to 2GW either way between the two countries.

"Icky Thwacket"

Says that in the first like I provided - piss head

....... Phil

No, it's not a typo - it has been done for years, especially for long distances and under water.

It allows long transmission paths without relative phase shift problems with approaching the wavelength of the current. Current limitations for AC (no pun intended!) are about 1000 km or miles in air, and about 1/10 - 1/20 that in under water transmission due to dielectric effects. There is a big problem with transmitting power under water with AC from large wind turbines and to islands. It's a similar problem with circuit board design where pulses have not arrived to the end of a trace before the next pulse is sent. It is actually possible to get a much higher voltage (up to double) out of the transmission lines then what was put in, due to the phase relationships.

DC also maximizes conductor efficiency, unlike AC. Conductors are very expensive to stretch and maintain across many km of land. Making them significantly fatter is not an viable option. Ever wonder what say a 3" diameter chunk of metal weighs? Now stretch 3 to 5 similar chucks of metal across about 1/2 km spans. Imagine what that tower would have to support, now imagine the loads placed on it during high winds or in an ice storm. Then there's skin effect, which is about 6 mm at 60 Hz.

DC links also allow 50 and 60 Hz power to be shared. Likely a big problem in Japan, and I know of one HVDC link that this is done across Europe. Also included in this is mixing different grids together with phases slightly off. Large rectifiers and inverters are used on the ends. Siemens make some.

Long distance power transmission is often done in DC. If it has to go from point A to point B without being tapped off too many places in between, if it has to send lots of power and if it doesn't need to get turned on and off too many times, then DC transmission is a good way to go.

With AC power transmission you get dielectric, induced conduction and radiation losses from the AC; these are significant enough on a dedicated transmission line that it's worth while to go DC.

What you _don't_ get with DC is the ability to slap the lines onto a transformer and get easy step-up or step-down, and if you don't design for it sudden interruptions in the current can be hazardous because of inductive discharge. When they cost out one of these lines they have to balance the cost of the extra equipment to do the AC-DC transform and back against the cost of the power lost.

"from the East Bay to San Francisco through an underwater cable"

A DC line has a Peak-Average voltage ratio of 1.00; as compared to 60 Hz AC's . . . [Now, calss... let's not always see the same hands...]

That means less insulation dollars. It also means no capacitance reactance loses...but on this length run I can't see that mattering as much.

And wouldn't the alternating magnetic field induce currents in the surrounding salt water? Would that be "leakage inductance"? Is that a pun? ;-)

Thanks, Rich

More due to the capacitive losses in cables. 53 miles over land (on big towers) on voltages like 500 KV isn't quite economical yet, considering the cost of the conversion equipment. But for cables, its well worth it.

I hadn't though of L di/dt under these circumstances, but I bet it would be a hell of a kick.

Thinking about HV distribution, I don't recall ever seeing a transformer on a tower, so I guess DC is the norm.

go.

dedicated

for

back

consulting

With HV, the power levels are high - you won't see a high power transformer on a pole - they put those in substations.

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

MHD? Is that magetohydrodynamics? Can anyone provide a reference to this, the ones I've found stop way short of megawatts and make no mention of power conversion for transmission.

Well yes, I know that, but I don't believe I've seen a substation all by it's lonesome. That is, towers, then down to the substation, then more towers. I always thought the substation was a distribution point.

The other thing that is useful in DC transmission, is if the two grids being linked, are not operating synchronised. This makes the economics of using a DC-AC converter system far easier to justify.

Best Wishes

snipped-for-privacy@sushi.com wrote in news: snipped-for-privacy@x40g2000prg.googlegroups.com:

They exist.

The convert the 500KV from distant generating stations to 230KV for distribution in a region. I know the Toronto area has four; Milton, Cherrywood, Markham, and Clearview. There is about a dozen or more stations that convert 230KV to the local ditribution (115 or 66KV), to neighborhood substations or direct to industry.

Paul Hovnanian P.E. snipped-for-privacy@seanet.com posted to sci.electronics.design:

Hmmm, there is an about 500 km 1 MV DC line from Washington to mid California. Been there for almost 20 years now. Originally it was

snipped-for-privacy@sushi.com snipped-for-privacy@sushi.com posted to sci.electronics.design:

Elsevier/Newnes,

Usually with a voltage change from (many) 100's of kV to (few or fractional) 10's of kV. That is where the twenty foot on a side transformers are, lots of MVA.