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Transmission Line power Ratings.

Discussion in 'Hobby Electronics' started by Mauried, Feb 1, 2009.

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  1. Mauried

    Mauried Guest

    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.
     
  2. K Ludger

    K Ludger Guest

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

    Bob Parker Guest


    Obviously you don't know any audiophiles then! :)
     
  4. David

    David Guest

    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
     
  5. Bob Parker

    Bob Parker Guest

    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
     
  6. kreed

    kreed Guest


    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 ;)
     
  7. Jasen Betts

    Jasen Betts Guest

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

    Mauried Guest

    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.
     
  9. 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.
     
  10. Eeyore

    Eeyore Guest

    They used Monster cable ?

    Graham
     
  11. Eeyore

    Eeyore Guest

    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
     
  12. Sylvia Else

    Sylvia Else Guest

    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.
     
  13. **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.
     
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