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 Post subject: AC Generator to SCR Drive to DC Motor
PostPosted: Tue Aug 13, 2013 8:44 am 

Joined: Tue Mar 12, 2013 11:01 am
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I am attempting to perform arc flash studies for stand alone cranes using SKM PTW. The crane is powered by a 480V generator, goes to a main distribution panel where it is seperataed into the 5 direction functions, each having an isolation transformer, then each function has an SCR drive with regeneration braking and DC motors. Is there any way to model this correctly? I have tried searching the forum but was unable to find anything.
Thank You for your help.

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PostPosted: Tue Aug 13, 2013 1:12 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
SKM can easily model everything on the AC side. I believe SKM has added a DC arc flash model recently but I don't have it so I can't tell you what it does. After that point, look at your maximum current through the SCR and/or the SCR fusing because above that point, the SCR turns into a fuse (explodes). This gives you the amount of DC current available. Use this and the equation in Annex D.8 of 70E to calculate arc flash.

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PostPosted: Mon Aug 19, 2013 8:52 am 
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Joined: Mon Sep 19, 2011 10:28 am
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Location: LLynchburg, VA
Jeff, the SKM dc and ac tools are only "loosely linked" and the other tools (ETAP, EasyPower, etc) are similar in that the admittance matrix used to solve the ac network solution for fault currents isn't interactive with the equations on the dc side, so you may need to iterate when anything changes in either system and manually supply data into the dialog box for the rectifiers and inverters. There are some dc fault phenomenon you should become familiar with also like the initial rise time constant, peak, and steady state values for dc fault current due to the inductances the distribution cables. The values for these cables are not in the SKM library so you need to get it from the manufacturer if you need a validated solution.

BE AWARE that SKM shows a graphical connection between the ac and dc systems at the drive (rectifier) but that if you re-arrange the ac side to change its impedance or available ac fault current, the dc side of the rectifier model doesn't get modified until you actually re-open the dialogue box and do that manually. BTW, SCRs fail in a shorted mode prior to erupting open so worst case you may see some ac on the dc side of the rectifier.

Note that in the SKM dialoge box for a rectifier that you need to define the input system impedances for the ac supply, the isolation transformer, and information about the rectifier to determine its equivalent impedance. In one of the systems I modeled the only protective devices were on the ac side and SKM did not clear these due to a dc fault prior to the "2 second" limit, so I had to take the dc fault current translated back into AC equivalent fault current then look at the TCC to find the clearing time. This was compounded for that project by potentially cascading arcing faults on the dc side fed by multiple rectifiers...lots of work!
I'm not sure that you'd get much contribution to an ac arc flash from the regenerative side of your model. While the rotational inertia of the rotor and load can regenerate ac to brake the motor, during a fault on the ac side of the drive the controller would normally see a low voltage on one or all phases and shut down on undervoltage.

I did an analysis for almost an identical project as yours for a nuclear lab that had eight 1MW dc drives with a spreadsheet while the SKM DC Arc Flash beta version first came out. Eventually the released version of SKM dc arcflash values came within 10-20% of the IEC and ANSI calculations I did by spreadsheet, so I ended up using my own IEC calculations for SCR rectifiers as they values were more conservative than the ANSI method.

I assumed the drive controller would initially try to respond to a dc fault by minimizing the firing angle of the SCRs before the a current limit would be reached, possibly prior to a protective device clearing. Out of my conservative respect for OSHA interpretations, I chose not to count on the controller's current limiting and relied only on validated protective device models.

I'm working on a draft for couple of papers on this subject for the 2014 IEEE ESW Workshop and I&CPS Technical Conference, but hope this portion of experience is helpful to you and others.

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