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 Post subject: Calculations with downstream motor contribution
PostPosted: Wed Jan 09, 2013 1:28 pm 
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Joined: Fri Jun 08, 2012 10:53 am
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I have a calculation challenge involving a main switchboard fed from two parallel connected generators. (Ship system). Two large thruster motors are connected to the main switchboard and will contribute to the short circuit fault current.

Analysing the main switchboard then becomes difficult. I have calculated the branch currents (15 kA from each generator and 5 kA from each of the thrusters). The generator breakers are time delayed 0.5 seconds and the thruster breakers at 0.3 seconds. Available short circuit on the main switchboard is thus 40 kA.

How do we calculate this? Using a power system calulation tool for arc flash gives out the result for 40 kA disconnected after 0.5 seconds, but is that really correct? The thruster breakers would disconnect after 0.3 seconds thus leaving only 30 kA for the remaining 0.2 second until the generator breakers react.


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PostPosted: Wed Jan 09, 2013 1:39 pm 
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Beo wrote:
How do we calculate this? Using a power system calulation tool for arc flash gives out the result for 40 kA disconnected after 0.5 seconds, but is that really correct? The thruster breakers would disconnect after 0.3 seconds thus leaving only 30 kA for the remaining 0.2 second until the generator breakers react.

The current flowing through the generator breakers is always 15 kA for each generator (don't know if there's only one breaker or one per generator), regardless of the state of the thruster breakers. They will (should, based on testing and maintenance) react only to that current (unless you have a relay scheme).


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PostPosted: Wed Jan 09, 2013 1:53 pm 
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There is one breaker for each generator.

I calculated the branch currents and the duration is for the branch currents.
The arc flash incident and boundary for the main bus bars is what I'm looking for.


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PostPosted: Fri Jan 11, 2013 10:47 am 
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It sounds like you have to do this in steps. Step on for the first 0.3 seconds is all short circuit current on line. Then step 2 would be from 0.3 to 0.5 second with only generators on line. Would the thruster motors contribute short circuit current of 0.3 seconds? Doesn't it decay? I would think the generator current will also decay but not as much.


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PostPosted: Fri Jan 11, 2013 12:06 pm 
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Yes, the calculations are based on IEC 363. The currents are decaying fast and are in steady-state after about 0.2 second. The IEC 363 only accounts for the first 0.1 seconds so doing the calculations in steps is not really possible.

I think this is the drawback of using power calculation tools, they do not really analyse situations like these, and a "one-click-solutions" to arc flash calculations (and all other calculations as well) can do more damage than good if used the wrong way.

IEEE 1584 does not say anytning about decaying currents and if to use the 1/2 period sub-transient values, transient values or steady state currents. My generators deliver 15 kA sub-transient, 5 kA steady state by the way.


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PostPosted: Mon Jan 14, 2013 9:14 am 

Joined: Tue Mar 02, 2010 1:50 pm
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It appears to me that your original .5 seconds at 40 kA is conservative, so it'd be safe to use that value for an arc flash calculation.
I think the software I use would assume the same. As I understand it, for a bus to "clear" the fault current has to be reduced below a certain level, say 80%. So 80% of the fault current has to be tripped to clear the arc.


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PostPosted: Mon Jan 14, 2013 9:57 am 

Joined: Wed Aug 25, 2010 7:15 am
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Location: St. Paul, MN
In SKM, if you have the thruster overcurrent modeled, and it is set to trip at 0.3 secconds based on the 5kA that the thrusters are contributing back into the upstream fault, then they should open when you run the arc flash study. You can also specify a reduced generator contribution after a specific number of cycles. If I understand your situation correctly, the study would do a calculation for the first 0.2 seconds based on 40ka of bolted fault, then reduce the generators and do another calculation for 0.1 seconds based on the reduced generator contribution plus the full motor contribution, and finally do another calculation for 0.2 seconds with just the reduced generator contribution.

Isn't that what you want it to do?


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PostPosted: Mon Jan 14, 2013 12:00 pm 

Joined: Sat Aug 06, 2011 2:08 pm
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I would like to ask for more information on the thruster motors - which might impact what assumptions need to be made -
Are the thruster motors directly powered 3 phase AC power - or are they powered from variable speed drives (VFDs - or inverters)? (If powered from variable speed drives - then there is probably no provisions for the VFD to have 'regen' power put power back into the AC bus, in which case, there would be no power contribution from the motors to be considered.)
Are the thruster motors DC motors powered from an AC to DC (SCR) drive? (And again - an SCR drive might not have regen provisions - negating the need to consider any power contribution from the motor to the arc flash computations.)


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PostPosted: Mon Jan 21, 2013 9:38 am 
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The motors are 3 phase AC.

I didn't know that SKM has such a feature. I use EDSA's Paladin DesignBase, and it seems to not be a very good tool for analysing arc flash. It does nothing with my motors, and leave the fault for the entire duration of the generator breakers.I also encounter various bugs, giving out arc durations that are impossible with my settings.

bvadams: how does the reduced generator contribution work? I always wondered wether to use the 1/2 period bolted short circuit currents or to use the stationary steady-state currents, or maybe the current at the time of tripping? Still confuses me alot.

What I did find in my calculations, was that the minimum current (stationary steady state for ship systems) is the most important of all. Lock the switchboard and make it impossible to open if the system is energized seems to be the best personell protection. But the minimum currents at steady state need to be in the tripping range of the breakers. It seems most folks are interested in worst case ppe levels, boundary zones and so on, but forgets the minimum currents.


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PostPosted: Tue Feb 05, 2013 8:12 am 
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I tried doing this with PTW from SKM as well now.
When doing multiple branches the software leaves the entire fault until cleared by the latest breaker.

This seems to me to be a very inaccurate solution.


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PostPosted: Wed Feb 06, 2013 3:51 pm 
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Beo wrote:
There is one breaker for each generator.

I calculated the branch currents and the duration is for the branch currents.
The arc flash incident and boundary for the main bus bars is what I'm looking for.


Id you are using Easy Power, there is a check box to use the "integrated method" ( I always use it). If your motor parameters are entered correctly I believe this will take care of your problem?


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