DC Bus modeling in SKM 1

[JohnLewis123]

Hello
Could somebody tell me how to model a DC bus in SKM.
The system i have right now is AC rectified to DC and inverted to AC back again running 10-12 motors.
The inverters are fed from the DC bus, I am trying to find out the short circuit current available at the DC bus also taking the capacitors in the inverters into effect.
I am aware of the industry standard which says that capacitors can be neglected, but in my case i have 42 drives each with 6 caps of 5600 micro farad. I believe this would make difference to the short circuit at the DC bus. I cannot find a way to model this stuff. I have also attached a picture of the setup
Any help would be appreciated.
Thanks!!

 

[JohnLewis123]

Any Ideas??

 

[iop995]

I don't know SKM but I think it have all needed components / blocks. Your question is related to SKM tips or generaly speaking, about "translation" of real devices in a simulating environments? Also, "DC bus" - you refer to DC bus strictly (cooper busbars, joint points / cables, etc) or to entire DC system (Xfers, rectifiers and so on)?

 

[JohnLewis123]

I was referring strictly about the DC bus, In SKM as a matter of fact i think in any software we cannot model AC and DC together, so i am trying to see how to model the system only using DC components and also considering capacitance into effect from the inverters.

 

[Skogsgurra]

I have tried the PSIM simulator in mixed power, control systems for a couple of weeks now and I have built a lot of confidence in the system. Of course, it can simulate AC and DC in the same run - doesn't your SKM allow that?
Good value for the money, around USD 1k for the entry level system, which is what I use.

http://www.gke.org/pub/files/AC sixpulse rectifier DC bus start and transient 1.jpg

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LiteYear]

Bugger. My replies from Opera are disappearing this morning. Seem to work from Firefox. Let's see if I can remember what I said...

I'm not familiar with SKM, but FWIW I agree that the capacitors will have an effect.

If you're modelling your short circuit as a fixed (low) resistance, then I think you'll get fairly accurate results using superposition. Use SKM to model the fault current due to the AC sources, then add in the discharge current of the capacitors: I = (Vbus / R) * e ^ (-t/RC)

For R, add the short circuit resistance, the average series resistance of the DC bus between the caps and the fault location, and the series/parallel combination of the ESR of the bus caps.

 

[JohnLewis123]

Thanks LiteYear
I will try to calculate it by hand and see how the results are going to look.

@Skogsgurra-Looked at PSIM simulator online, looks interesting. but i don't think this has the ability to run different kinds of studies like the short ckt, harmonic, load flow, etc etc..
but i think this is a good software for simualtions

 

[iop995]

In this large DC bus is very important to know distances and busbar dimensions, material type (resistivity) and maybe operating temperature, to compute correct segment resistance between rectifier ouptputs and invertes inputs. Also, very important is short-circuit point. Regarding inverter capcitors current effect, maybe input fuse of invertes trip at a busbar short-circuit and reduce effect (regarding time; this current capacitor effect may be eliminate by a "isolating" diode connected to input of each inverter. Sure, this diode, increase cost and loses.

 

[mikekilroy]

Sounds like what you are asking is how much energy do your capacitors hold?

IF so, simple: E=.5*c*v^2

so why not just add up all ur c (in farads), stick in formula with v (volts) of your dc bus, and calculate the energy stored there?

 

[mikekilroy]

sorry, I have no idea what SKM is (software?) but maybe this helps you make a block or something to handle it?

 

[dpc]

I would contact SKM tech support and ask them if the software treats capacitors as a fault source on the dc side. If they say no, (as I anticipate), you're out of luck. If they say yes, then ask them how. You might as well take advantage of the annual maintenance fee your company is paying.

SKM Power Tools is not an electrical transient analysis modeling program. It basically models steady-state conditions for short-circuits and power flows. It has a transient motor starting analysis, but that is not the same thing at all.

Determine capacitor contributions for faults is problematic due to the high-frequency ringing of the capacitor current that typically occurs.

 

[waross]

How about a drawing of the VFD circuits. The placement of the capacitors in the drive circuit is important.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JohnLewis123]

@dpc---you are right. That is what i did and i am waiting on their output
@Waross--attached find the rough sketch