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Lighting Inverter and Short Circuit / Incident Energy
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Author:  engineerintraining12 [ Tue Jan 16, 2018 11:32 am ]
Post subject:  Lighting Inverter and Short Circuit / Incident Energy
Hey All,

Hope everyone is doing well. I recently came across a project where we are dealing with a 15 KW 3 phase lighting inverter and I wanted to know how this might effect short circuit current and incident energies for the lighting panels downstream and whether or not my assumptions are correct. I don't have the capability to model direct current on my power systems modeling program and my assumptions have only accounted for 3 phase alternating current. Is it safe to assume that I would see a worse short circuit current at my panel downstream when the inverter is not in emergency mode using 3 phase alternating current as opposed to when it is in emergency mode using the DC battery pack? Also would it be safe to assume that I would see a worse incident energy at my panel downstream when the inverter is not in emergency mode using 3 phase alternating current as opposed to when it is using the DC battery pack? Again, I don't have the ability to model direct current and maybe I should leave this part out of my study because of that, but I just wanted to know your thoughts. A picture of the inverter and panels are attached.

Best Regards,
Engineer in Training

Attachments:
inverter.PNG
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Author:  PaulEngr [ Tue Jan 16, 2018 7:33 pm ]
Post subject:  Re: Lighting Inverter and Short Circuit / Incident Energy
I'm not following. An inverter is a DC to AC converter whereas a converter is an AC to DC conversion aka a rectifier.

One way to model things with VFD's is recognizing that the VFD has a finite maximum amount of current that it can draw on the AC side usually expressed like 250% of rated current for 3 seconds or some similar value, and that this gives you an upper limit on the current that it can actually supply/draw on the AC side of things since we're dealing with semiconductors and not busbars. Then you don't need to model DC at all.

Aside from that, modelling DC is almost trivial. You can use the DC version of Lee which relies on the concept of maximum power transfer. In ANY system the maximum power occurs when the load resistance equals the system resistance. At that point the maximum power draw is 0.5*V*I. So if we know the maximum current output of the DC converter ("inverter") or say resistance of a battery or any similar information and we know the system voltage, we can calculate the maximum arc power in kilowatts. Then just multiple by time to get to energy and divide by the surface area of a sphere where the radius is equal to the working distance and convert units and ou get the incident energy. This sounds complicated but it's really not. Just look at Annex D.8 in NFPA 70E. I have yet to run into a system so complicated that I need more than this. Ammerman has an improved model that relies on the actual power arc curves as determined from a number of sources but it's a lot more complicated to use and although it gives a lower value, it's not that much better. Both results are off by roughly a factor of 2 to 4 compared to measured incident energies according to a study commissioned by Duke Power (looking at batteries). There's not enough data to do anything like an empirical formula so right now the best we can do is theoretical models.
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