DC used in power distribution

[[email protected]]

"With financing in place as expected by the end of the month, the
approximately 53-mile-long high-voltage, direct-current cable is to be
laid beneath the Bay starting in June 2009."

http://www.contracostatimes.com/news/ci_6647828

 

[D from BC]

"With financing in place as expected by the end of the month, the
approximately 53-mile-long high-voltage, direct-current cable is to be
laid beneath the Bay starting in June 2009."

http://www.contracostatimes.com/news/ci_6647828

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
60hz AC??
D from BC

 

[Phil Allison]

<[email protected]

"With financing in place as expected by the end of the month, the
approximately 53-mile-long high-voltage, direct-current cable is to be
laid beneath the Bay starting in June 2009."

http://www.contracostatimes.com/news/ci_6647828

** Been done since the 1960s.

http://en.wikipedia.org/wiki/Submarine_power_cable


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

http://en.wikipedia.org/wiki/Basslink



........ Phil

 

[Phil Hobbs]

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
60hz AC??
D from BC

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

 

[Jim Thompson]

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

 

[Icky Thwacket]

** Been done since the 1960s.

http://en.wikipedia.org/wiki/Submarine_power_cable


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

http://en.wikipedia.org/wiki/Basslink

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

 

[Phil Allison]

"Icky Thwacket"

"Phil Allison"

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

** It ain't one link - but is made up of 8 x 46 km DC cables.

97.5% of the juice goes one way.

Says that in the first like I provided - piss head





........ Phil

 

[Jeff L]

Yes that's a new one... Typo??

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.

 

[Tim Wescott]

"With financing in place as expected by the end of the month, the
approximately 53-mile-long high-voltage, direct-current cable is to be
laid beneath the Bay starting in June 2009."

http://www.contracostatimes.com/news/ci_6647828

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.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[David Lesher]

http://www.contracostatimes.com/news/ci_6647828

I find it odd this power distribution scheme uses high voltage DC.

"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.

 

[Rich Grise]

http://www.contracostatimes.com/news/ci_6647828

I find it odd this power distribution scheme uses high voltage DC.

"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

 

[Paul Hovnanian P.E.]

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
60hz AC??
D from BC

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.

 

[[email protected]]

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.

--
Tim Wescott
Control systems and communications consultinghttp://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes,http://www.wescottdesign.com/actfes/actfes.html

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.

 

[Jeff L]

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.

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

 

[Bruce Varley]

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.

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.

 

[[email protected]]

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

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.

 

[Roger Hamlett]

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.

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

 

[Gary Tait]

[email protected] wrote in

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.

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.

 

[JosephKK]

Paul Hovnanian P.E. [email protected] posted to
sci.electronics.design:

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.

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
500 kV.

 

[JosephKK]

[email protected] [email protected] posted to sci.electronics.design:

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.

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.