Replacing A High speed Dc Air circuit breaker with VCB

[mash98]

Dear guys we have a dc motor with a capacity of 1000 kw with following specs.
- Rated Voltage : 700 vdc
- Rated current : 1700 Amperes
- Type : Seperately excited
-Drive System : Motor generated set( sure its an old system but nevertheless useful) with 4th quadrant control, the motor is being used for reverseable plate rolling mill.
The motor is fitted with a single pole High speed circuit breaker of below specs.
- rated voltage : 1000 vdc
- Current range 3000-4000 amperes presettable and our setting is 3800 amperes.
So far everything is fine but we want to have an spare breaker set for the above rating but the cost of the same seems reasonably high.Now my question is if we can use a VCB of 2000 amperes,12000 ac volts/50 hz (As I have one spare in my store) by connecting in parallel arrangements to match the current requirement while using the same existing sensing and tripping devices.I am aware of the fact about the natural tendency of alternating current's arc quenching capabilty but as the dc voltage is only the 1/17th of VCB,s rated voltage so curious about its possibility.Kindly let me know the possibilty of this option.

regards

 

[ScottyUK]

I would be really really careful with this. Big DC contactors almost always use a magnetic blowout to drive the arc up the arc chute where it weakens and extinguishes. Without the mag blowout the breaking capacity is much reduced; an AC breaker won't have a mag blowout.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

VCBs absolutely require the presence of current zeros an only interrupt on/near a current zero. With dc you don't have any current zeros. Don't try it. There is a very real physical reason that dc breakers are very more expensive than ac breakers.

 

[Zogzog]

I agree with david, not a good idea. Just get a spare of the type you have, should be easy.

 

[mash98]

Can somebody tell me what exactly are the difference between ac and dc vaccum breakers construction.

Thanks

 

[dpc]

I'd check manufacturer's data for more details. Interrupting dc is MUCH more difficult than ac because the dc current never goes through zero, like ac current. You should never apply any device in dc service unless it has a published dc service rating. It could fail in a really bad way.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[mash98]

Dear guys I am still waiting. can somebody let me know the source from where I can get knowledge about ac and dc vcbs .

thanks

 

[Zogzog]

AC VCB's exist, DC VCB's do not exist.

 

[mash98]

you are right after posting I went on to the net to find any dc vcb but could not find one actually i been told by some supplier about a dc vcb( not vcb supplier).But I went through some japanese site who are working on it anyway thanks.
regards

 

[oldfieldguy]

Somebody already caught the answer for you, but that's never stopped us before.

The arc quenching on a vacuum circuit breaker in AC service depends on the current dropping to zero during each cycle. At zero volts, the arc dies.

In DC service, this isn't going to happen. No arc quench, the heat rises, resulting in interesting phenomena involving high temperatures, molten metals etc.

old field guy