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RE: Why no corona? AC / DC transmission lines
Original poster: "boris petkovic by way of Terry Fritz <twftesla-at-qwest-dot-net>" <petkovic7-at-yahoo-dot-com>
--- Tesla list <tesla-at-pupman-dot-com> wrote:
> Original poster: "Malcolm Watts by way of Terry
> Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
> Hi Gary,
> On 11 Sep 2002, at 10:30, Tesla list wrote:
> > Original poster: "Lau, Gary by way of Terry Fritz
> > While these HV lines clearly exhibit corona, what
> I can't figure out is why
> > the ground under them isn't littered with bird
> carcasses. Corona coming
> > off their little beaks has GOT to hurt should they
> try to perch on the
> > lines! Is it just a case of being too large to
> perch on?
> I've never seen birds alight (NPI) on EHV power
> lines. I imagine
> that, just like the cat getting too close to the Van
> de Graaf
> machine, the discomfort level they experience as
> they approach sends
> them scurrying.
(btw,I have seen them on 420 kV lines.And not a small
Gary ,I assume their beaks are not the problem.I would
think beaks in birds are,in some way,equivalent of
nails in humans-hard insensitive parts of a body and
if any corona there that shouldn't hurt them.
Birds' bodies are covered with feathers and that
should suppress formation & prickling action of corona
at other parts of birds body.
What might bother you is something else:amount of
capacitive current a bird draws from HV line.
Consider ,for instance,a big crow or seagull sitting
on 500 kV power line (290 kV with respect to ground).
A capacity of average human body at 30' hight above
ground is taken to be about 75 pF.
For big bird you may take 15 pF.
If you calculate a capacitive current at 60 Hz tru
bird you'll get 1.64 mA.
Not having mentioned treshold of perception at 60 Hz
(for humans about 0.5 mA),what birds fell is
questionable.For humans 1.5 mA tru two finger causes
unpleasant sensation of fingers trembling.
After careful consideration I must abandon the idea
that predominantely corona charge obtains the function
of charging bird at 60 Hz if legs are connected to
line.The legs must carry significant part of that
Another ,more curious thing to me,is that some arc
discharge should arise when bird decide to take off or
approach HV line to sit on it.
For example :consider a power line worker in basket
wearing protective suit constituted of metal fibres in
it.When he approaches HV power line he has to hook
himself and basket to line with metal cable in order
to be at same potential.
Short moment before connecting ,a visible and audible
arc jumps between HV line and cable.
A nature of the arc has much in common with that of
disruptively discharged actions of condenser
(familiar terminology to coilers).
Meanwhile,it seems birds doesn't care,they keep coming
to HV lines and taking off apparently without problems