Fault on a Star/Delta Starter

In my experience, this kind of flashover happens when the arc chute of an opening contactor has some ionised air still when the other contactor closes, causing a flashover. i have seen it quite often in open transition ΥΔ starters without the 50ms transition delay between star contactor open and delta contactor close situation. Can't say whether a similar situation can happen in closed transition ΥΔ....

The O/L's (P&B Goulds) trip the OCB rather than the main contactor. In normal operation the contactors stop the drive. We found the fault to be the star contactor being slow to open, it was only partially open as the delta closed. Hence the almighty flash and bang. I'd got the panel door open when it failed, I nearly had to go and change my underpants!

The fault was a worn pivot bearing, and a bad connection on one of the opening coils capacitor.

The contactors are old BHI things left over from the building of the ark. The contactors have 3 coils, 1 per phase to pull them in and opening coils fed by capacitors. It's DC control just to add to the fun.

Thanks for the site, it was already in my favourites list but I didn't think to look at it before this little adventure.

PS the OCB's are well maintained. Not that they're anything to do with me, I just got involved in this as a favour.

It probably was ionised air in the star contactor that caused the flashover. The air gap wasn't wide enough due to the slow operation of the contactor at the moment full voltage was applied by the delta contactor.

If you've worked on BHI gear you'd know it will stand up to anything you throw at it, totally over engineered. Considering this gear is near 50 years old it just took things in it's stride.

As for breakers not withstanding repeated operation, how can we use whatever type of breaker for repeated starting of large drives?

For sure it was ionizing in the contactor. If there were vents in the doors you can see the blue flash when it happens. Not that uncommon with air contactors. I once replaced one with vacuum contactors, the user had a hard time believing it was working because he didn't see a blue arc flash.

And you sir have major kahunas for being willing to operate a MV star-delta starter built with air contactors when the door is open! I've done it on a 5kV solid state soft starter, there was an eerie creeping blue plasma phenomenon crawling all over the conductor insulation like a living spider web. Freaked me out...

It was a bit daunting to operate the gear with the doors open, especially as we were expecting a bang. I will admit I didn't stand to close. And as I said I was very close to having to change my underpants when it did go bang! But we soon found the problem area!

I've a lot of faith in BHI gear, I once came across a crane long travel control panel that someone had maintained (supposedly), they had left every arc chute on the floor! The LT drive motors were 2 X 60HP wound rotor both wired to one set of contactors, normal braking operation would be to reverse the motors while still travelling. The panel was working quite happily, although it did look like a firework party.

You would have loved these crane panels. Each of the crane main booms were switch rooms each had open contactor panels over 60' long. The only thing to stop you falling on to the panels was a single handrail.