On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

On 27/07/2014 22:24, newshound wrote:
On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, which is
why our links to the continent are HVDC. Even then, the cables are very
specialised and there are IIRC only three manufacturers of HVDC
underwater cables in the world. That is a limiting factor on how many
offshore wind farms can be built, as they need HVDC underwater cable,
the manufacturers have a limited capacity and well filled order books,
while increasing capacity is not something that can be done overnight.

--
Colin Bignell

On 27/07/14 22:33, Nightjar "cpb"@ insert my surname here wrote:
On 27/07/2014 22:24, newshound wrote:
On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, which is
why our links to the continent are HVDC.

I thought it was more the issue that you cannot phase lock France and
the UK? So DC is a natural choice if you have to re-invert it. And if
you have to do that, might as well transmit in DC too.

Not saying your reason is not a good reason - I just thought it was a
secondary reason to a fairly immutable primary problem.

Even then, the cables are very
specialised and there are IIRC only three manufacturers of HVDC
underwater cables in the world. That is a limiting factor on how many
offshore wind farms can be built, as they need HVDC underwater cable,
the manufacturers have a limited capacity and well filled order books,
while increasing capacity is not something that can be done overnight.

On 27/07/2014 23:49, Tim Watts wrote:
On 27/07/14 22:33, Nightjar "cpb"@ insert my surname here wrote:
On 27/07/2014 22:24, newshound wrote:
On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, which is
why our links to the continent are HVDC.

I thought it was more the issue that you cannot phase lock France and
the UK? So DC is a natural choice if you have to re-invert it. And if
you have to do that, might as well transmit in DC too.

Not saying your reason is not a good reason - I just thought it was a
secondary reason to a fairly immutable primary problem.

I think the electricity actually needs to reach the other end of the
cable before phase locking becomes a problem :-)


--
Colin Bignell

On 27/07/14 23:49, Tim Watts wrote:
I thought it was more the issue that you cannot phase lock France and
the UK? So DC is a natural choice if you have to re-invert it. And if
you have to do that, might as well transmit in DC too.

Not saying your reason is not a good reason - I just thought it was a
secondary reason to a fairly immutable primary problem.

No. I visited the first link to france at the UK end and the issue is
primarily one of losses

'we can draw an arc for 30 minutes off the capacitance in that cable'

To drive that capacitance takes a LOT of out of phase current and that
suffers resistive losses.

Big ones



--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On 28/07/2014 02:06, The Natural Philosopher wrote:
On 27/07/14 23:49, Tim Watts wrote:
I thought it was more the issue that you cannot phase lock France and
the UK? So DC is a natural choice if you have to re-invert it. And if
you have to do that, might as well transmit in DC too.

Not saying your reason is not a good reason - I just thought it was a
secondary reason to a fairly immutable primary problem.

No. I visited the first link to france at the UK end and the issue is
primarily one of losses

'we can draw an arc for 30 minutes off the capacitance in that cable'

To drive that capacitance takes a LOT of out of phase current and that
suffers resistive losses.

Big ones

The main AC losses are inductive and leakage by capacitance to ground,
not resistive.

That's what they told me when I took a group of trainee electrical
engineers to the site a few decades ago. The guy was actually quite
surprised when none of them could answer the question he asked of why
they did it the way they do, but the coach driver could.

Assuming the power *could* be usefully transmitted that far through an
underground/ undersea cable, it wouldn't be much harder to lock the
English and French grids than it is to lock the French, German, Italian
and so on grids.

--
Tciao for Now!

John.

On 28/07/2014 09:21, Huge wrote:
On 2014-07-28, John Williamson wrote:
On 28/07/2014 02:06, The Natural Philosopher wrote:
On 27/07/14 23:49, Tim Watts wrote:
I thought it was more the issue that you cannot phase lock France and
the UK? So DC is a natural choice if you have to re-invert it. And if
you have to do that, might as well transmit in DC too.

Not saying your reason is not a good reason - I just thought it was a
secondary reason to a fairly immutable primary problem.

No. I visited the first link to france at the UK end and the issue is
primarily one of losses

'we can draw an arc for 30 minutes off the capacitance in that cable'

To drive that capacitance takes a LOT of out of phase current and that
suffers resistive losses.

Big ones

The main AC losses are inductive and leakage by capacitance to ground,
not resistive.

That's what they told me when I took a group of trainee electrical
engineers to the site a few decades ago. The guy was actually quite
surprised when none of them could answer the question he asked of why
they did it the way they do, but the coach driver could.

The coach driver had done the tour before ...


No, the coach driver is the son of an electrical engineer who
specialised in the heavy stuff, and had been playing with electronics
since he was about 11 years old.

--
Tciao for Now!

John.

"The Natural Philosopher" wrote in message
...
On 27/07/14 23:49, Tim Watts wrote:
I thought it was more the issue that you cannot phase lock France and
the UK? So DC is a natural choice if you have to re-invert it. And if
you have to do that, might as well transmit in DC too.

Not saying your reason is not a good reason - I just thought it was a
secondary reason to a fairly immutable primary problem.

No. I visited the first link to france at the UK end and the issue is
primarily one of losses

'we can draw an arc for 30 minutes off the capacitance in that cable'

To drive that capacitance takes a LOT of out of phase current and that
suffers resistive losses.

Big ones


Drivel
Capacitance does not cause any losses.
It does cause phase shift and instability.
When the cable is under load it will actually help with phase shift.
Only resistance causes losses.

Where exactly is this arc drawn for 30 minutes and for what purpose.?

On 28/07/2014 08:23, harryagain wrote:
"The Natural Philosopher" wrote in message
...
On 27/07/14 23:49, Tim Watts wrote:
I thought it was more the issue that you cannot phase lock France and
the UK? So DC is a natural choice if you have to re-invert it. And if
you have to do that, might as well transmit in DC too.

Not saying your reason is not a good reason - I just thought it was a
secondary reason to a fairly immutable primary problem.

No. I visited the first link to france at the UK end and the issue is
primarily one of losses

'we can draw an arc for 30 minutes off the capacitance in that cable'

To drive that capacitance takes a LOT of out of phase current and that
suffers resistive losses.

Big ones


Drivel
Capacitance does not cause any losses.
It does cause phase shift and instability.
When the cable is under load it will actually help with phase shift.
Only resistance causes losses.

Whenever current is passing through any imperfect conductor.

On an AC transmission line, current is constantly being drawn to alter
the voltage across the line capacitance to earth. So the capacitance is
the cause of the resistive losses.

Loading a cable has no effect on the phase shift you mention, unless
that load is reactive, and depending on whether it is inductive or
capacitive, it can then either worsen or improve the situation.

Where exactly is this arc drawn for 30 minutes and for what purpose.?


The arc *can* be drawn by using the DC charge stored in the cable. No
claim was made that it ever had been drawn either deliberately or
otherwise, though it's the kind of trick that installation engineers
have been known to pull as a joke, or that happens when things go wrong
when commissioning plant of this sort.

The greens are planning to use this effect to store energy in their
proposed long distance links for their beloved European renewables
Supergrid, all of which *you* have mentioned here in the past.

Should we add memory loss to your minimal comprehension skills?

--
Tciao for Now!

John.

On 28/07/14 13:29, John Williamson wrote:
On 28/07/2014 08:23, harryagain wrote:
"The Natural Philosopher" wrote in message
...
On 27/07/14 23:49, Tim Watts wrote:
I thought it was more the issue that you cannot phase lock France and
the UK? So DC is a natural choice if you have to re-invert it. And if
you have to do that, might as well transmit in DC too.

Not saying your reason is not a good reason - I just thought it was a
secondary reason to a fairly immutable primary problem.

No. I visited the first link to france at the UK end and the issue is
primarily one of losses

'we can draw an arc for 30 minutes off the capacitance in that cable'

To drive that capacitance takes a LOT of out of phase current and that
suffers resistive losses.

Big ones


Drivel
Capacitance does not cause any losses.
It does cause phase shift and instability.
When the cable is under load it will actually help with phase shift.
Only resistance causes losses.

Whenever current is passing through any imperfect conductor.

On an AC transmission line, current is constantly being drawn to alter
the voltage across the line capacitance to earth. So the capacitance is
the cause of the resistive losses.

Loading a cable has no effect on the phase shift you mention, unless
that load is reactive, and depending on whether it is inductive or
capacitive, it can then either worsen or improve the situation.

Where exactly is this arc drawn for 30 minutes and for what purpose.?


The arc *can* be drawn by using the DC charge stored in the cable. No
claim was made that it ever had been drawn either deliberately or
otherwise, though it's the kind of trick that installation engineers
have been known to pull as a joke, or that happens when things go wrong
when commissioning plant of this sort.

No, They said that once they had switched off each end, that was what
they did to discharge the cable


The greens are planning to use this effect to store energy in their
proposed long distance links for their beloved European renewables
Supergrid, all of which *you* have mentioned here in the past.

Should we add memory loss to your minimal comprehension skills?



--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On 28/07/2014 13:29, John Williamson wrote:
On 28/07/2014 08:23, harryagain wrote:


Where exactly is this arc drawn for 30 minutes and for what purpose.?


The arc *can* be drawn by using the DC charge stored in the cable. No
claim was made that it ever had been drawn either deliberately or
otherwise, though it's the kind of trick that installation engineers
have been known to pull as a joke, or that happens when things go wrong
when commissioning plant of this sort.


If you watch footage of repairs to 400 kV overhead lines (typically shot
from helicopters) you will see long and impressive arcs being drawn from
these for several seconds while they are brought down to earth potential.

On Sun, 27 Jul 2014 23:49:49 +0100, Tim Watts wrote:

I thought it was more the issue that you cannot phase lock France and
the UK?

No real reason why you couldn't make the channel link work using AC, there are a
number of transmission lines, some with part undergrounded sections in the UK
that are significantly longer overall. No true underwater AC connections
though, the ones that do run underwater (on the grid system at least) all run in
tunnels.

However, the impact of a single point connection and the loss of that connection
was more of a concern, hence the UK - France link is comprised of 2 x 1000MW
links and the AC/DC conversion process permits presettable defined levels of
power transfer regardless of most external conditions so the interconnector is
essentially despatchable 'generation' at the entry point to the respective
countries.

There is also a degree of isolation from system disturbances when
interconnecting at DC which can be very useful from a system stability point of
view.

--

On 28/07/14 10:40, The Other Mike wrote:
On Sun, 27 Jul 2014 23:49:49 +0100, Tim Watts wrote:

I thought it was more the issue that you cannot phase lock France and
the UK?

No real reason why you couldn't make the channel link work using AC, there are a
number of transmission lines, some with part undergrounded sections in the UK
that are significantly longer overall. No true underwater AC connections
though, the ones that do run underwater (on the grid system at least) all run in
tunnels.

However, the impact of a single point connection and the loss of that connection
was more of a concern, hence the UK - France link is comprised of 2 x 1000MW
links and the AC/DC conversion process permits presettable defined levels of
power transfer regardless of most external conditions so the interconnector is
essentially despatchable 'generation' at the entry point to the respective
countries.

There is also a degree of isolation from system disturbances when
interconnecting at DC which can be very useful from a system stability point of
view.


How are you going to phase lock the French Grid to the UK Grid?

The "length" is a red herring...

On Mon, 28 Jul 2014 10:44:43 +0100, Tim Watts wrote:

On 28/07/14 10:40, The Other Mike wrote:
On Sun, 27 Jul 2014 23:49:49 +0100, Tim Watts wrote:

I thought it was more the issue that you cannot phase lock France and
the UK?

No real reason why you couldn't make the channel link work using AC, there are a
number of transmission lines, some with part undergrounded sections in the UK
that are significantly longer overall. No true underwater AC connections
though, the ones that do run underwater (on the grid system at least) all run in
tunnels.

However, the impact of a single point connection and the loss of that connection
was more of a concern, hence the UK - France link is comprised of 2 x 1000MW
links and the AC/DC conversion process permits presettable defined levels of
power transfer regardless of most external conditions so the interconnector is
essentially despatchable 'generation' at the entry point to the respective
countries.

There is also a degree of isolation from system disturbances when
interconnecting at DC which can be very useful from a system stability point of
view.


How are you going to phase lock the French Grid to the UK Grid?

The "length" is a red herring...

How do you phase lock the French grid to the Belgian Grid to the Dutch Grid to
the German one to the Swiss one etc etc. It doesn't in the main involve DC and
at AC it is no real problem.

There is potentially 'as big a problem' with the UK grid system if it splits
during a major disturbance into a two or more islands of load and generation. In
this case (since the late 80's or so) the synchronising systems have an
additional mode where they just sit there primed until the phase angle and
voltage discrepancy falls with certain limits (which are deliberately set wider
than for normal operation) and the breaker then closes, a successful closure
could potentially take a few hours.

As the grid system operator is separate from the generation operator in the UK
there is no generation intervention required as such, just switchgear that can
take occasionally take a bit of abuse and generator governors that can hold a
set point frequency.

P.S. There is a long established 400kV AC link between Spain and Morocco with a
similar distance underwater to the UK - French link.

--

On 28/07/14 12:30, The Other Mike wrote:

How do you phase lock the French grid to the Belgian Grid to the Dutch Grid to
the German one to the Swiss one etc etc. It doesn't in the main involve DC and
at AC it is no real problem.

Are they phased locked? I don't know.

If they are, then they have relatively long borders which means lots of
interconnections.

You aren't going to hold 2 disparate grids in sync with one or two cross
channel links - National Grid has enough trouble holding the UK grid in
phase between Scotland and the South (they have (or had at Bankside at
least) a phase indicator colloquially known as the "scottish wobblemeter".

If it started oscillating, a certain amount of panic ensued...

There is potentially 'as big a problem' with the UK grid system if it splits
during a major disturbance into a two or more islands of load and generation. In
this case (since the late 80's or so) the synchronising systems have an
additional mode where they just sit there primed until the phase angle and
voltage discrepancy falls with certain limits (which are deliberately set wider
than for normal operation) and the breaker then closes, a successful closure
could potentially take a few hours.

Yes. There's an interesting document that you can google for which
outlines the Nat Grid Blackstart procedure. Rather involved...

As the grid system operator is separate from the generation operator in the UK
there is no generation intervention required as such, just switchgear that can
take occasionally take a bit of abuse and generator governors that can hold a
set point frequency.

P.S. There is a long established 400kV AC link between Spain and Morocco with a
similar distance underwater to the UK - French link.

"Nightjar "cpb"@" "insert my surname here wrote in message
...
On 27/07/2014 22:24, newshound wrote:
On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, which is
why our links to the continent are HVDC. Even then, the cables are very
specialised and there are IIRC only three manufacturers of HVDC underwater
cables in the world. That is a limiting factor on how many offshore wind
farms can be built, as they need HVDC underwater cable, the manufacturers
have a limited capacity and well filled order books, while increasing
capacity is not something that can be done overnight.


Our main link to Europe is not underwater, it is via the channel tunnel.
http://www.independent.co.uk/news/bu...l-2289665.html

On 28/07/2014 08:16, harryagain wrote:
"Nightjar "cpb"@" "insert my surname here wrote in message
...
On 27/07/2014 22:24, newshound wrote:
On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, which is
why our links to the continent are HVDC. Even then, the cables are very
specialised and there are IIRC only three manufacturers of HVDC underwater
cables in the world. That is a limiting factor on how many offshore wind
farms can be built, as they need HVDC underwater cable, the manufacturers
have a limited capacity and well filled order books, while increasing
capacity is not something that can be done overnight.


Our main link to Europe is not underwater, it is via the channel tunnel.
http://www.independent.co.uk/news/bu...l-2289665.html


That is a quarter of the capacity of the Sellindge to Les Mandarins
underwater link and they were still only talking about it in April this
year.

--
Colin Bignell

harryagain wrote:

Our main link to Europe is not underwater, it is via the channel tunnel.

"IS" are you sure?

http://www.independent.co.uk/news/bu...l-2289665.html

I read lots of "PLANS", "PROPOSED", "WILL" and "COULD". Facts harry, you
should try some ...

ElecLink doesn't even seem to have started yet, and may never do

http://www.ft.com/cms/s/0/2f563bce-9e1a-11e3-b429-00144feab7de.html#axzz38kOXxdkI

Proposed capacity is 1000MW (doubled from the initial 500MW capacity you
linked to), isn't the existing French connector 2000MW, and the existing
Dutch connector 1000MW, how would that make it the main link?

Seems they have greased enough palms as rules have now been bent for them

https://www.ofgem.gov.uk/ofgem-publications/87163/eleclinkdecisioncoverletter.pdf

In article , Andy
Burns scribeth thus
harryagain wrote:

Our main link to Europe is not underwater, it is via the channel tunnel.

"IS" are you sure?

http://www.independent.co.uk/news/bu...r-link-to-run-
through-channel-tunnel-2289665.html

I read lots of "PLANS", "PROPOSED", "WILL" and "COULD". Facts harry, you
should try some ...

ElecLink doesn't even seem to have started yet, and may never do

http://www.ft.com/cms/s/0/2f563bce-9...ml#axzz38kOXxd
kI

Proposed capacity is 1000MW (doubled from the initial 500MW capacity you
linked to), isn't the existing French connector 2000MW, and the existing
Dutch connector 1000MW, how would that make it the main link?

Seems they have greased enough palms as rules have now been bent for them

https://www.ofgem.gov.uk/ofgem-publi...ncoverletter.p
df


This is how we're going to get around power shortages in the UK look too
that forward thinking country France and pipe their nuclear power in by
that tunnel!..

Now we know the real reason why it was built, perhaps theres a 4th bore
there somewhere;?....
--
Tony Sayer

On Mon, 28 Jul 2014 08:16:50 +0100, "harryagain"
wrote:

Our main link to Europe is not underwater, it is via the channel tunnel.
http://www.independent.co.uk/news/bu...l-2289665.html

The two HVDC links to Europe, the 1000MW UK-NED and te 200MW UK-FRA are both
partly underwater, the latter in operation for the last 28 years. Neither use
any part of the channel tunnel.


--

On 28/07/2014 10:42, The Other Mike wrote:
On Mon, 28 Jul 2014 08:16:50 +0100, "harryagain"
wrote:

Our main link to Europe is not underwater, it is via the channel tunnel.
http://www.independent.co.uk/news/bu...l-2289665.html

The two HVDC links to Europe, the 1000MW UK-NED and te 200MW UK-FRA are both
partly underwater, the latter in operation for the last 28 years. Neither use
any part of the channel tunnel.


Don't confuse him with facts, or it'll all end in tears.

--
Tciao for Now!

John.

harryagain wrote:

Our main link to Europe is not underwater, it is via the channel tunnel.

The channel tunnel, that well known airborn structure, eh?

--
Scott

Where are we going and why am I in this handbasket?

On Sun, 27 Jul 2014 22:33:37 +0100, "Nightjar \"cpb\"@" "insert my surname
here wrote:


Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, which is
why our links to the continent are HVDC. Even then, the cables are very
specialised and there are IIRC only three manufacturers of HVDC
underwater cables in the world. That is a limiting factor on how many
offshore wind farms can be built, as they need HVDC underwater cable,
the manufacturers have a limited capacity and well filled order books,
while increasing capacity is not something that can be done overnight.

Some, but I'm not sure exactly what proportion of UK offshore wind farms have AC
substations located offshore and an AC connection to the existing grid /
distribution network. I can see DC being of use in the truly offshore arrays
that Germany operates that are out of sight and over the horizon but for the UK
wind farms that are located much closer to shore there seems little or no point
in converting to DC.

There is mention in this article clearly implying an AC connection and
interconnection regime for the London Array the biggest of the white elephant
monstrosities to pollute our country.

http://www.theengineer.co.uk/sectors...012971.article

There is a 600kV 2.2GW mainly underwater DC connection currently under
construction between Scotland and the Wirral to carry the output from wind
generation but this is a point to point grid connection with no directly
connected generation.




--

Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?

jgh

On 28/07/14 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?

It doesn't. BUT it has a higher out of phase current flowing through it
due to greater capacitance to earth.



jgh



--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On 28/07/14 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?


It doesn't. It has a lower Line-Earth reactance (capacitative) - at
least that is what's being claimed.

However, the cables are all armoured so it does not really matter if
they are in a dry tunnel or under the sea.

The big difference is a cable vs overhead lines.

On 28/07/14 16:06, Tim Watts wrote:
On 28/07/14 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not
capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?


It doesn't. It has a lower Line-Earth reactance (capacitative) - at
least that is what's being claimed.

However, the cables are all armoured so it does not really matter if
they are in a dry tunnel or under the sea.

Mmm. Not sure that is so, but cant put a finger on why I feel its
different..


The big difference is a cable vs overhead lines.


Yes.


--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

In article , The Natural Philosopher
scribeth thus
On 28/07/14 16:06, Tim Watts wrote:
On 28/07/14 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not
capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?


It doesn't. It has a lower Line-Earth reactance (capacitative) - at
least that is what's being claimed.

However, the cables are all armoured so it does not really matter if
they are in a dry tunnel or under the sea.

Mmm. Not sure that is so, but cant put a finger on why I feel its
different..


The big difference is a cable vs overhead lines.


Yes.



Yep water -v- air cooled...

--
Tony Sayer

On 28/07/14 19:16, The Natural Philosopher wrote:
On 28/07/14 16:06, Tim Watts wrote:
On 28/07/14 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables
too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not
capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?


It doesn't. It has a lower Line-Earth reactance (capacitative) - at
least that is what's being claimed.

However, the cables are all armoured so it does not really matter if
they are in a dry tunnel or under the sea.

Mmm. Not sure that is so, but cant put a finger on why I feel its
different..

A normal armoured cable would have a pretty effective faraday screen
around the live cores.

Not sure about mega HV cables but I'd be surprised if they were not
fully screened too.


The big difference is a cable vs overhead lines.


Yes.

On 28/07/2014 16:06, Tim Watts wrote:
On 28/07/14 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not
capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?


It doesn't. It has a lower Line-Earth reactance (capacitative) - at
least that is what's being claimed.

However, the cables are all armoured so it does not really matter if
they are in a dry tunnel or under the sea.

I would have thought that enclosing a cable in water would change the
relative permeability of the arrangement, and hence the inductance per
metre of the cable?

(too long since I played with transmission line theory!)

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 04/08/14 02:26, John Rumm wrote:
On 28/07/2014 16:06, Tim Watts wrote:
On 28/07/14 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables
too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not
capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?


It doesn't. It has a lower Line-Earth reactance (capacitative) - at
least that is what's being claimed.

However, the cables are all armoured so it does not really matter if
they are in a dry tunnel or under the sea.

I would have thought that enclosing a cable in water would change the
relative permeability of the arrangement, and hence the inductance per
metre of the cable?

(too long since I played with transmission line theory!)


I guess it might - I was considering the capacitative effects only.

But assuming the armour is steel wire - how much I wonder?

On 04/08/2014 07:47, Tim Watts wrote:
On 04/08/14 02:26, John Rumm wrote:
On 28/07/2014 16:06, Tim Watts wrote:
On 28/07/14 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables
too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not
capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?


It doesn't. It has a lower Line-Earth reactance (capacitative) - at
least that is what's being claimed.

However, the cables are all armoured so it does not really matter if
they are in a dry tunnel or under the sea.

I would have thought that enclosing a cable in water would change the
relative permeability of the arrangement, and hence the inductance per
metre of the cable?

(too long since I played with transmission line theory!)


I guess it might - I was considering the capacitative effects only.

But assuming the armour is steel wire - how much I wonder?

Where is Mr Wade when you need him? - sounds like his kind of sum ;-)

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 28/07/2014 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?

The *resistance* doesn't change. When transmitting AC, however, the
*impedance* does change, both due to the capacitance to earth, and, if
the frequency is high enough, "skin effect", which is where the current
flow is restricted to the areas near the surface of the strands also
starts to come into play.


--
Tciao for Now!

John.

On 28/07/14 17:11, John Williamson wrote:
On 28/07/2014 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not
capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?

The *resistance* doesn't change. When transmitting AC, however, the
*impedance* does change, both due to the capacitance to earth,


so far so good


and, if
the frequency is high enough, "skin effect", which is where the current
flow is restricted to the areas near the surface of the strands also
starts to come into play.


Yep a couple of hundred megahertz is what we transmit power at these days!



--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On 28/07/2014 19:18, The Natural Philosopher wrote:
On 28/07/14 17:11, John Williamson wrote:
On 28/07/2014 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables
too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not
capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?

The *resistance* doesn't change. When transmitting AC, however, the
*impedance* does change, both due to the capacitance to earth,


so far so good


and, if
the frequency is high enough, "skin effect", which is where the current
flow is restricted to the areas near the surface of the strands also
starts to come into play.


Yep a couple of hundred megahertz is what we transmit power at these days!

It can seriously affect stuff as low as 455kHz, if you remember the Litz
wire we had to use for IF transformers in AM radios back in the day.

The thicker the conductor, the stronger the effect, and it is even
noticeable at 20kHZ, according to this site:-

http://www.belden.com/blog/broadcast...-Frequency.cfm

Not noticeable in the special case we're discussing here, but I did say
"if the frequency is high enough" in the general case.
--
Tciao for Now!

John.

wrote in message
...
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?

jgh

All AC circuits have resistance, inductance and capacitance.
With long cables, the resisatance and capacitance effects become more
important.

Capacitance depends on the nature of the insulation, the surface area of the
conductors and the distance apart they are.
With overhead cable, the distance apart is much greater than with
underground cables.
So capacitance is much more with the latter.
Also the permitivity of air is much less than plastic insulation.

On 27/07/14 22:24, newshound wrote:
On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

No it really isn't dielectric losses.


--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On 28/07/2014 01:56, The Natural Philosopher wrote:
On 27/07/14 22:24, newshound wrote:
On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

No it really isn't dielectric losses.


It's capacitive loading. The capacitance per kilometre of overhead lines
between cable and ground is low, while the capacitance per kilometre of
underground cables is at least two orders of magnitude higher.

At the lengths of cable used for power distribution, some of the effects
that normally only affect HF signals come into play at 50Hz.

--
Tciao for Now!

John.

On 28/07/14 08:19, John Williamson wrote:
On 28/07/2014 01:56, The Natural Philosopher wrote:
On 27/07/14 22:24, newshound wrote:
On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

No it really isn't dielectric losses.


It's capacitive loading.

Exactly.

But the power isn't lost *in* the dielectric, its lost in driving the
current down the wires *to* the dielectric.

The capacitance per kilometre of overhead lines
between cable and ground is low, while the capacitance per kilometre of
underground cables is at least two orders of magnitude higher.

At the lengths of cable used for power distribution, some of the effects
that normally only affect HF signals come into play at 50Hz.



--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On 28/07/2014 11:52, The Natural Philosopher wrote:
On 28/07/14 08:19, John Williamson wrote:
On 28/07/2014 01:56, The Natural Philosopher wrote:
On 27/07/14 22:24, newshound wrote:
On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

No it really isn't dielectric losses.


It's capacitive loading.

Exactly.

But the power isn't lost *in* the dielectric, its lost in driving the
current down the wires *to* the dielectric.

You are quite right, of course. But the term "dielectric loss" is the
one which CEGB trainers used to use when lecturing mixed classes of
mathematicians, physicists, and all types of engineer to explain why
pylons were the prefered method of transmission, with underground cables
the least desirable from the viewpoint of efficiency.

It's a convenient if not very precise shorthand.