Cap Bank contactors melted 4

[capuchi]

Hi to all!

We have a 500 KVAR Auto Correcting Capacitor consisting of 2 x 50 KVAR and 10 x 40 KVAR, 460 V, 3 phase, 60HZ. Five (5) of 12 switching contactors had contacts melted (some 2 poles melted, others 1 pole). Continuous current rating of the contactors is 80 amps at 460 V. The Cap Bank is still functioning since the demand is only 240 KW out of 2 MVA Main Transformer. What seems to be the problem? Is the 80 amps rating insufficient to withstand the inrush current? I believe it is sufficient? I stand to be corrected. Also, how would you calculate the capacitor inrush current (in the IEEE world coz the IEC has its own calculation) in order to determine the correct size of contactor for the application. Would appreciate your comments/reply

capuchi

 

[pwrtran]

May consider to change to solid state switching device c/w zero voltage switching...

 

[LionelHutz]

There should be data for the contactors which will tell you teh capacitor current. Also, there is often a special pre-charge block that is added to the contactor for capacitor switching.

 

[dpc]

Insertion (damping) resistors can be used in this application. Switching caps is tough duty for contactors. If you can copy and paste this long URL into your browse, there is a good technical publication from ABB on the subject.

http://library.abb.com/global/scot/...7dec12570850041a49a/$File/1SBC101140C0202.pdf

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[capuchi]

Thanks for your advise guys. I really appreciate it. Will consider your recommendations.

capuchi

 

[KJLRSA]

We have often seen burnt contactors on capacitor banks due to a few reasons.

1. Discharge resistors not present
2. Lose connection - most likely
3. Manufacturer uses kW rating of contactors to size. This is incorrect, as normal contactors not rated for capacitive switching. You need to get specific capacitor switching contactors with IEC 60831-1 compliance such as the TeSys LC1D contactors from Schneider.