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 Post subject: Fault contribution from VFD or soft starter
PostPosted: Tue Apr 15, 2008 10:59 am 
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I was told that there will be contribution from VFD or soft starter when I was doing the arc flash study for an industrial plant. However, I don't have VFD or soft starter module in PTW32. How to simulate those IGBT devices?

Thank in advance!


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PostPosted: Tue Apr 15, 2008 5:49 pm 
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Not sure how to simulate it, but can tell you that VFD or any gate device is going to block fault current. Most VFD's will only pass 2 x FLA into a bolted fault.


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PostPosted: Tue Apr 15, 2008 7:14 pm 
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So if VFD will block the fault current, how come it still has 2xFLA to the fault point?


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PostPosted: Wed Apr 16, 2008 7:21 am 
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Noah wrote:
So if VFD will block the fault current, how come it still has 2xFLA to the fault point?

They are designed to limit the current into the motor up to 2xFLA. This is something that can be programmed if you wish, however it is not recommended as it may lead to nuicance tripping.

As for Soft-Starters it is not as clear cut. You can buy soft-starters with by-pass contactors, which do as the name says, when the motor is finished starting it closes the by-pass and now the motor is being fed directly from the source.

If there is a fault upstream of the soft-start and the motor is running in by-pass then it WILL contribute to the fault until the fault is cleared.

Hope that helps....


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PostPosted: Wed Apr 16, 2008 7:52 am 
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Thanks for all the inputs! So the VFD will limit up to 2xFLA fault current to the motor as it is designed. What if there is a fault at load side of VFD, does VFD limit 2xFLA to the line side or upstream as well?


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PostPosted: Wed Apr 16, 2008 4:20 pm 
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The VFD is a conversion device. AC input is full wave recified to supply the DC bus, which also has a decent quantity of capacitors. The DC is then inverter to AC through a pulse width modulation process. You don't get an sinewave output, you get pulsed AC square waves, whose summation approximates an AC sinewave of the appropriate frequency. The thyristors that invert the DC do not have as low resistance as metal (that's why there is always a .7V forward bias across a diode. If you short circuit a thyristor with the gate open, you get zero current flow. The control processor will limit the gate on duration to limit current flow. I would guess that under some very abnormal fault conditions you may be able to get more than 2 x current. But you would be overloading the input bridge rectifier and the output thyristors. Solid state devices will not respond well to overload and will blow like a fuse. If you wanted to solve for max let through you could add the resistance of both the rectifier SCR and the thyristor to get max current flow, and then look at each component device to see its damage point. I would have to guess that value may be higher than 2, but still really low, maybe 6 x.


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PostPosted: Mon Apr 21, 2008 12:29 pm 
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Drives and softstarts without a bypass

VFD's and softstarts without a bypass contactor are not shown to add motor contribution back to their feed circuits. With a bypass contactor, the motor is connected "across the line" as old timer electricians like to say. At the time of the fault, as voltage collapses and the motor begins to re-generate back to the feeder circuit, the bypass contactor will drop out, but you'll still see voltage from the motor for several cycles. This is also the case for motors connected without a starting device, or "across the line".

Without a bypass contactor, the motor is connected to the circuit through the power electronic device, be it an IGBT or SCR. Going back to basic electronics, these devices need to be gated on, as mentioned previously in this thread, but also must be forward biased to conduct. Theoretically, they should block any voltage coming from the motor, as that voltage would be applied to the anode of the device.

There is mention of this in the 1584 if I recall correctly, but I can't cite where just this minute, and my book is elsewhere. I'll follow-up when I fish it out.


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PostPosted: Mon Apr 21, 2008 1:10 pm 
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Thankyou Dave!


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PostPosted: Wed Apr 23, 2008 6:54 pm 
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According to SKM which does not model VFDs, simply add a motor instead of VFD with a very high Xd'' (say 999). Hope that will help.


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PostPosted: Thu Apr 24, 2008 5:56 am 
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Thank you Calabala, I also have checked it with SKM, they gave me the same instruction. I was told that in next version they will add VFD and soft-starter in their program, and a lot of other features.


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