On 2/1/07, Mark Peterson <mpeterson@cannontech.com> wrote:
> I'm working on a 480V distribution relay protection project that
> requires the application of a shunt trip device at the time of an
> overload condition.  I need to detect when the load exceeds 500A, and at
> that time apply a short across the 480V source in order to force the
> circuit's 2000A main breaker to trip.  This is similar to using a
> grounding switch as the initiator for a transfer trip scheme in the
> electric utility world.  Crude, but simple and effective.  I have argued
> for other methods but this method has been dictated by the client, the
> government client.

You have argued for a good reason. I was working on 6/30KV plant for
almost 10 years but I never seen this solution in my country (and it
was a big plant with an equivalent electrical distribution equipment
of a city with 200.000 people). All current breakers have a DC
connectig-disconnecting coil driven by an automation which is
measuring the current using a current transformer and disconect the
breaker exactly at the programmed current.


>
> Detecting the current is no problem.  My effort now is to determine the
> best method to apply the short.  The available fault current is in the
> neighborhood of 50kA.  A near-zero ohm short would be ridiculous.  A
> fault current of approximately 3000A is sufficient and it can be limited
> to that magnitude if I use a 425uH (160mohm) inductor for the shorting
> conductor.  Such an inductor would be roughly 15cm in diameter and 30cm
> long, with 90 turns of wire that is capable of withstanding the 3000A
> surge current for a maximum time of about 10 cycles, or 160 ms.
>
> The question now is:  What is the best way to switch the inductor across
> the 480V supply?  Traditional clapper or mercury wetted relay?  Triac?
> SCRs?  Whadayathink?

Forget about any triac or SCR. I have doubts there is any mercury
relay rated at 3KA. Such switchers are usualy mechanical, having
different switching and conducting plates (contacts), just because the
peak to peak current is impossible to be predicted (even using a
calibrated inductor in series) and the switching contacts are
producing huge flames, so it require a special chamber to purge the
flame.


best wishes and take care,
Vasile
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist