Autotransformer retrofit - fuse or Circuit breaker

[jimmy2times]

I’d like to gauge opinion on following, we have a MV motor (bow thruster on ship), 2MW 6.6kV. Due to poor original system design we have issue with voltage drop on starting across ship when on min generators. Temporary work around being bring more capacity online to start motor. Anyway we are looking to install AT starter as a fix.

Currently the DOL starting uses VCB at switchboard that is approx 200m from motor. Now, due to lack of space in switchroom, looking to install starter more locally to motor. What are views on incorporating fuse in the local starter or just omitting it and keeping the VCB for short circuit protection.

I quite like the idea of remaining fuseless (time to replace and safety) but also thought the fuse would be of some benefit in event of fault (less stress, arc damage in situ). Possibly an unnecessary cost though. There is no existing diff protection.

Control cables need to be run between the two locations regardless of which way we go.

Would it be simplist solution for VCB to clear the overload faults as well? Or leave that locally to contactor?

(Still wondering if it is feasible to replace the VCB with fused softstarter but waiting for switchboard OEM to advise on that), that could be another topic in itself though. Think it is a non-starter (excuse the pun!) due to space though.

Thanks as always.

 

[jraef]

How are you accomplishing the motor protection, and how are you planning on accomplishing the contactor / changeover required for an AT Korndorfer starter scheme? VCBs would not be capable of having mechanical interlocking (I don't believe) and even so, would be physically huge when trying to use them for an RVAT scheme.

The reason I ask is because if you plan on doing this with vacuum contactos and an overload relay, you will find that those MUST be coordinated with fuses, not VCBs (at least in any that I have seen). When you were going ATL (DOL) with the VCB as the controller, that was not a concern. Now that you are introducing new hardware, you are introducing new issues.

BTW, any RVSS option will not likely be any larger than the RVAT option in my experience.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[jimmy2times]

It would employ two MV contactors down at the starter regardless as per normal (just in one option it would have fuses and the other option no fuses).

Protection, in cases of not employing fuses locally at the starter. Then the existing vacuum circuit breaker at main switchboard would be arranged so that it would clear all faults at fault currents above locked rotor limit (contactors remaining closed). For other faults lower than locked rotor, I still wasn't sure on this, whether to still have VCB open as well(i.e. trip signals to the existing switchboard VCB from the starter) or otherwise command the contactor to open locally. either way ensure contactor does not attempt to interrupt beyond its capability.

I need to check the making duty though of contactors as this could be an issue though if ship engage more generation than required when starting motor and there is a fault on initial closing