Bass Link HV DC?

[Bazil]

Hi,

I just found out that the Bass Link cable between Vic and Tas uses HV DC.

I was always under the impression that AC had less transmission loss.

Is that only under certain circumstances? Is there some other reason for
using DC?

Cheers,

Bryan

 

[Nicholas Sherlock]

Bazil wrote:

I just found out that the Bass Link cable between Vic and Tas uses HV DC.

I was always under the impression that AC had less transmission loss.

Is that only under certain circumstances? Is there some other reason for
using DC?

DC has less transmission losses than AC since the capacitive and
inductive losses for AC are quite significant at these voltages.

Cheers,
Nicholas Sherlock

 

[Bazil]

Nicholas Sherlock wrote:

Bazil wrote:

I just found out that the Bass Link cable between Vic and Tas uses HV DC.

I was always under the impression that AC had less transmission loss.

Is that only under certain circumstances? Is there some other reason
for using DC?


DC has less transmission losses than AC since the capacitive and
inductive losses for AC are quite significant at these voltages.

My understanding was that AC was used in transmission because there were
lower % line losses at HV, and that it was of course easier to step up /
down.

What sort of rectifiers / inverters are they going to be using at each
end??? What sort of efficiency can they expect? What sort of losses will
the cable have?

I thought I heard that overall only 25% efficiency. Might be wrong on
that one. But even at 50% efficiency one would have to question the
viability...

Intrigued.

 

[Jasen Betts]

On 2005-12-11, j.l@octa4.net.au <j.l@octa4.net.au> wrote:

I thought I heard that overall only 25% efficiency. Might be wrong on
that one. But even at 50% efficiency one would have to question the
viability...

Intrigued.

A 300km long submarine cable would not work at all with 50Hz AC, the
charging current would actually exceed the capacity of the cable. So
therefore DC is the only way it can work.

so it's a characteristic impedance issue?

DC is mainly used for
underground lines longer than 50km and AC lines longer than say 500km
with no t-offs.

AC? do you mean Aerial ?

Bye.
Jasen

 

I thought I heard that overall only 25% efficiency. Might be wrong on
that one. But even at 50% efficiency one would have to question the
viability...

Intrigued.

A 300km long submarine cable would not work at all with 50Hz AC, the
charging current would actually exceed the capacity of the cable. So
therefore DC is the only way it can work. DC is mainly used for
underground lines longer than 50km and AC lines longer than say 500km
with no t-offs.

DC is much more efficient in transmission lines, its just more
difficult to change voltage levels. HVDC transmission lines themselves
are also cheaper to build per km than an equivalent HV AC transmission
line. The catch is at the converter stations - these are very
expensive.

At the end of the day the determining factor for how and why things are
built the way they are is overall cost.

Other advantaes of DC links are:
The systems at each end dont need to be synchronised
The amount of power transferred through the line is easily controlled
A DC link does not increase the fault levels of the system

The converter stations are made of massive series-parralell banks of
thyristors, theres a fair bit of information on the net about these.

DC transmission links are in use in Australia connecting SA and VIC -
Murry Link and think there is one between QLD and NSW, that may only an
assynchronous tie in.

Cheers

 

[Phil Allison]

<j.l@octa4.net.au>

A 300km long submarine cable would not work at all with 50Hz AC, the
charging current would actually exceed the capacity of the cable.

** This would be a very puzzling fact to many folk.

An under sea HV, 50 Hz cable has capacitance to the a metal sheath over the
conductor and insulation.

Just as a guess, imagine that capacitance were kept to a MERE * 33.3 pF *
per meter.

Then, for 300km, the total C = 10 uF.

Now, Xc = 1 / ( 2.pi.50.10 exp-6 ) = 318 ohms.

Say the cable was designed to operate at 300 KVAC and 1000 amps = 300 MW.

The current flow due to C alone would be 300 exp3 / 318 = 943 amps.

Big problem.




............ Phil

 

[Jasen Betts]

On 2005-12-11, Bazil <nospam@spamzfree.net> wrote:

Hi,

I just found out that the Bass Link cable between Vic and Tas uses HV DC.

I was always under the impression that AC had less transmission loss.

AC has higher transmission loss, skin effect, capacitative and inductive
losses, it has lower voltage-conversion loss.

Is that only under certain circumstances?

yeah for short runs the energy lost converting to HV DC and back to AC isn't
compensated for by transmission efficiency gains.

Is there some other reason for
using DC?

it gives frequency isolation between the power grids which may be an
advantage in some circumstances

Bye.
Jasen

 

[Poxy]

j.l@octa4.net.au wrote:

I thought I heard that overall only 25% efficiency. Might be wrong on
that one. But even at 50% efficiency one would have to question the
viability...

Intrigued.

A 300km long submarine cable would not work at all with 50Hz AC, the
charging current would actually exceed the capacity of the cable. So
therefore DC is the only way it can work. DC is mainly used for
underground lines longer than 50km and AC lines longer than say 500km
with no t-offs.

DC is much more efficient in transmission lines, its just more
difficult to change voltage levels. HVDC transmission lines themselves
are also cheaper to build per km than an equivalent HV AC transmission
line. The catch is at the converter stations - these are very
expensive.

At the end of the day the determining factor for how and why things
are built the way they are is overall cost.

Other advantaes of DC links are:
The systems at each end dont need to be synchronised
The amount of power transferred through the line is easily controlled
A DC link does not increase the fault levels of the system

The converter stations are made of massive series-parralell banks of
thyristors, theres a fair bit of information on the net about these.

DC transmission links are in use in Australia connecting SA and VIC -
Murry Link and think there is one between QLD and NSW, that may only
an assynchronous tie in.

Interesting stuff - thanks for the info.

 

[Poxy]

Phil Allison wrote:

j.l@octa4.net.au



A 300km long submarine cable would not work at all with 50Hz AC, the
charging current would actually exceed the capacity of the cable.


** This would be a very puzzling fact to many folk.

An under sea HV, 50 Hz cable has capacitance to the a metal sheath
over the conductor and insulation.

Just as a guess, imagine that capacitance were kept to a MERE *
33.3 pF * per meter.

Then, for 300km, the total C = 10 uF.

Now, Xc = 1 / ( 2.pi.50.10 exp-6 ) = 318 ohms.

Say the cable was designed to operate at 300 KVAC and 1000 amps =
300 MW.

The current flow due to C alone would be 300 exp3 / 318 = 943 amps.

Big problem.

Cheers for the detail Phil.

 

[Bazil]

Phil Allison wrote:

j.l@octa4.net.au


A 300km long submarine cable would not work at all with 50Hz AC, the
charging current would actually exceed the capacity of the cable.



** This would be a very puzzling fact to many folk.

An under sea HV, 50 Hz cable has capacitance to the a metal sheath over the
conductor and insulation.

Just as a guess, imagine that capacitance were kept to a MERE * 33.3 pF *
per meter.

Then, for 300km, the total C = 10 uF.

Now, Xc = 1 / ( 2.pi.50.10 exp-6 ) = 318 ohms.

Say the cable was designed to operate at 300 KVAC and 1000 amps = 300 MW.

The current flow due to C alone would be 300 exp3 / 318 = 943 amps.

Big problem.

OK,

just for the exercise, what would be the typical capacitance / meter for
shielded HVDC cable?

And thx to everyone for your answers. Learn a little every day...

Bryan

 

[Phil Allison]

"Bazil"

just for the exercise, what would be the typical capacitance / meter for
shielded HVDC cable?

** Irrelevant.




.......... Phil