I am aware of several switchgear manufacturers that have developed, an

[inventorman]

I am aware of several switchgear manufacturers that have developed, and are marketing, mechanical &quot;zero-volt closing&quot; capacitor switches. they claim that this eliminates transients when energizing capacitors. These switches energize capacitors on the zero-volt crossing of the line voltage.<br>
<br>
My question is:<br>
<br>
How will these switches react to energizing capacitors that have residual voltage on them from &quot;zero-current&quot; switch openings, which is a natural occurence?<br>
<br>
It is my understanding that zero-volt closing is only practical when energizing completely de-energized capacitors.

 

[bfuchs]

I may be wrong, but I believe the discharge resistor does the trick. It is not uncommon for modern capacitors to have these resistors in parallel to the cap. When the cap is disconnected, the residual energy on the capacitor can discharge progressiveley through the impedance of the resistor. Most power factor bank controllers manage this by preventing a disconnected cap from being reconnected within a certain time limit (a couple of minutes say), to enable the capacitor to discharge itself completely (or sufficiently). These controllers memorise the switching history of the capacitor, and even measure how efficient it is, and if it doesn't compensate as it should, it can actually warn you that the bank is faulty ...<br>
<br>
Otherwise you are right, the residual energy could create some voltage transients. That is why it is so important to verify that those resistors are connected, or even better, built into the capacitor.<br>
<br>
Hope this was helpful !<br>

 

[inventorman]

Thanks bfuchs. This has been very valuable information to me. I should probably get hold of someone at a local electrical utility and observe capacitor switching first hand. thanks again.