Transmission Line power Ratings.

Anyone know why or what the mechanism is that causes large power
transmission lines to differant power carrying capacities depending on
which way the power is flowing.
Was reading some Nemmco reports regarding the power outages of the
last few days and discovered that the main power interconnector
between Victoria and SA is rated at 500 MW in the direction of SA >
VIC but only 160 MW Vic > SA.
Others are similar, VIc > NSW is more than NSW to VIC over the same
power line.
I would have thought that wires dont care about direction.

 

Is the report referring to the capacity of the networks to supply one
another rather than the capacity of the transmission lines themselves
perhaps?

 

Obviously you don't know any audiophiles then!

 

On an AC power system, the power transmitted down a long (several
100kms) transmission line (or series of lines) is limited by system
stability, not the thermal rating of the line. The real power
transmitted down an AC transmission line depends on the phase difference
between each end of the transmission line. It is important to keep the
phase angle across the whole network less than 90 degrees, or
instability can occur.

The phase angle can be reduced by capacitors / static compensators etc.
These might only be present at one end of the line. This is why the line
has an unsymmetrical power transfer. Also different generators can cope
with higher reactive power.

High voltage DC power transmission is now often used, as this does not
have the stability issues AC transmission has (an also allows systems
with different frequencies to be connected).

David

 

The Basslink cable between Vic and Tas runs at 400kV DC. More
figures are at http://www.basslink.com.au/home/index.php?id=6

 

The cable itself can physically carry the same in either direction,
its whats at the end that counts, or different state laws/standards on
what can be sent / received ?


Generator might not be sufficient at one end to supply the local and
remote loads, but in the reverse situation the generator is adequate.
same with the switchgear at one end to handle the function, compared
to the other.


I doubt its likely - but its maybe possible that it depends where it
enters the system, it might run directly from a power station at end
A and straight TO a substantial LOAD at end B, (meaning full power can
be sent straight to the load at B from station A if available) before
it can get to the power station at end B The power station at end B.
(on the other side of the load - and the load can't be switched out,
and the wire through it isnt capable of carrying power to both the
load it passes through and the load at end A

IE:

LOAD A <-------- GEN A <----------------> LOAD B <--------- GENB


Therefore all that can be sent back to end A - is generating capacity
of B less the load at end B in the middle of the line. if there is no
need for a higher current than specified to ever flow from B to A then
no need to upgrade the cable through load B ?


Might be a shitload of greenies at one end running endless CFL's and
TOTALLY stuffing the power factor (derating the line carrying capacity
in one direction) too

 

wires don't but if it's a DC line the inverters at each end may differ
in capacity (or even presence).

 

Thanks for the responses.
In this case its AC , and its the actual power line and the sub
station equipment at each end of the line thats the problem, not the
generating capacity.
When the line was first built it was 500 MW in both directions , but
has been slowly derated over the years but in only the one direction.
Thats what seemed bizzarre.

 

It's probably politics and big-business at play then.
Just like the California power crisis
http://en.wikipedia.org/wiki/California_electricity_crisis

Dave.

 

They used Monster cable ?

Graham

 

Another advantage of HV DC transmission is that there's no peak voltage to
worry about. I can't find it now (it was on Wikipedia) but for part of the
Itaipu dam output, something like 700kV is used. It also generates 50 and 60
Hz via multiple separate turbines.

Interesting economics here....
"The final cost of ITAIPU amounts to US$ 20 billion, 50% of this value are
direct investments and balance financial charges.

If whole area of the lake - at nominal level - would be covered by solar
modules the power of the would be 135 000 MWp, which would produce 230 TWh a
year. For the same yearly output as ITAIPU a solar PV-plant would cost US$
132 billion."

http://www.solar.coppe.ufrj.br/itaipu.html

Also
http://en.wikipedia.org/wiki/HVDC

Graham

 

It might not be a limit so much as a constraint. One issue is what
happens to the system if the line trips out. If 500MW were being supply
to SA over the line and it were to trip, that would mean SA suddenly
lost 17% of of its power (at peak time today). This may be more than the
system could handle. Since Victoria uses much more power, a loss of
500MW would not be so significant.

Sylvia.

 

**One huge advantage of HVDC transmission is that there is no loss due to
capacitance, inductance nor skin effect. It is brilliant for very long
distance transmission, or, in the case of Basslink, undersea use. The usual
losses in DC transmission are approximately 3% per 1,000km.