Vaccum c/bs and transformers

Status
Not open for further replies.
Restrike and Ratings

Restrike and Ratings

Maybe I missed it, but is the vacuum breaker on the source side of the transformer or on the load side? If there is significant capacitive load (harmonic filter, power factor correction, etc.) are the vacuum breakers rated for the current? It is common for a vacuum breaker to be rated ~10A for capacitive switching current but 1200A load current. This means that the vacuum bottle can restrike and voltage will skyrocket. That will rapidly destroy a transformer. Are the transformer failures due to over-voltage or overload? If oil-filled, when wes the oil last tested and dried?

Is on the transformer primary, is it rated for interrupting the peak assymetrical current? With high X/R ratios for source current, during a fault, the assymetrical current may exceed the vacuum bottle rating. Is there a chance the vacuum bottle is unable to successfully interupt the magnetizing current of the transformer?
 
mxslick said:
John:

Yeah, kind of. The "chopping" is like the effect of rapidly turning a switch on and off, causing repeated high inrush/current spikes but at very high rate. (At least that's how I got it based on the reference book I quoted in my second post. #12)

Perhaps Laszlo can explain it better than I can..
Click to expand...

I don't know it is going to be better, but basically the right question was asked.

The conventional contacts open in free air. Components of the air ionized by the arc, thus become somewhat conductive, sustaining the arc. The objective is to lengthen this arc until the available voltage can not sustain it and eventually opening the circuit. There are various tricks employed both using the arc's magnetic energy and other mechanical means to lengthen the arc. The arcing, which takes place over a period of time - relatively speking - of 3-5 cycles, 'eats' up the inertia/magnetic energy that is built up in rotating machinery. In the case of transformers the motors connected to the secondary side count. Basically think of this that a certain amount of energy needs to be dissipated with two components, V and I. Force the I down and the V will rise.


Enter vacuum breakers. Virtually no arcing since no ionizable air present. The voltage therefore will rise very rapidly - subcycle - compared to the above and could be damaging to the equipment.
 
weressl said:
Enter vacuum breakers. Virtually no arcing since no ionizable air present. The voltage therefore will rise very rapidly - subcycle - compared to the above and could be damaging to the equipment.
Click to expand...

Also the way I understood it, the dielectric between contacts was propotional to the pressure in the air surrounding the contacts. Therfore the vaccum contacts create a low pressure surrounding the contacts and thus drive down the dielectric between the contacts thus increaseing the voltage recovery level.
 
mull982 said:
Also the way I understood it, the dielectric between contacts was propotional to the pressure in the air surrounding the contacts. Therfore the vaccum contacts create a low pressure surrounding the contacts and thus drive down the dielectric between the contacts thus increaseing the voltage recovery level.
Click to expand...

Vacuum means the nearly total absence of any dielectric substance.
 
Status
Not open for further replies.
Top