VFD fault contribution

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mull982

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When considering VFD's for contribution to a fault in a short circuit analysis what magnitude of fault current is usually considered. For example lets say I have an MCC which feeds a 200HP VFD which in turn feeds a disconnect and 200HP motor.

I know if the VFD has a bypass switch then the fault current contributions from the line and load side (motor contribution) is considered to simply bypass the VFD and is unaffected and usually represents the worst case for fault analysis.

What about when a VFD bypass does not exist? Will the VFD pass all or a portion of the line side contribution through the VFD? Is there only a certain percentage of the fault current that is let through from the line side since the line side voltage is converted to a DC voltage first before is it converted back to an AC voltage?

Typically I have heard that most VFD's will not pass fault contribution from the load side (motor contribution) unless the VFD is a regenerative type VFD. Does this mean that if a VFD is not of the re-generative type that it will not pass any current from the load side to line side? Is this due to the DC conversion in the VFD?
 
Besoeker

Besoeker

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mull982 said:
Typically I have heard that most VFD's will not pass fault contribution from the load side (motor contribution) unless the VFD is a regenerative type VFD. Does this mean that if a VFD is not of the re-generative type that it will not pass any current from the load side to line side? Is this due to the DC conversion in the VFD?
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Pretty much on the money. A plain rectifier can't reverse power flow.
 
Jraef

Jraef

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I don't agree with the "pass all or a portion of the line side contribution through the VFD" either, unless maybe the entire VFD melts in such a way as to conduct from line to load but not line to line...
 
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mull982

Senior Member
Besoeker said:
Pretty much on the money. A plain rectifier can't reverse power flow.
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When you order a drive that can reverse power flow I'm assuming you have to order a specific Regenerative type drive? I'm assuming that there is a special type rectifier in these drives to allow the reverse power flow?

Are there particular applications where this type of drive is beneficial?

Jraef said:
I don't agree with the "pass all or a portion of the line side contribution through the VFD" either, unless maybe the entire VFD melts in such a way as to conduct from line to load but not line to line...
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For typical drives what value of fault current can be passed through from line side to load side? Most commercial analysis software I have seen gives you an option for the VFD to either consider 100% Let-through on the load side or manually enter a 3-Phase and L-G contribution to the load side as a percentage of rating (assuming drive rating) as well as an X/R ratio.
 
Jraef

Jraef

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You will know when you BUY a regen capable drive, it will cost you twice as much, because you are essentially buying two drives for one motor. Most of the time this is only done for the purpose of being able to brake the load to a stop repeatedly because you don't need braking resistors, the motor energy is pumped back into the line rather than burned off as heat. There is a new trend of using AFE drives to mitigate harmonics now too, so that may add to the mix to increase the incidence, but those are typically going to be larger HPs. If you come across one already installed, it will be very difficult to discern if they are regen capable by looks alone, you will have to check catalog data on the model numbers.

When I said I DISagreed with the statement about a non-regen drive passing through line faults, is because I cannot see how it can do that at all. A fault on the line side may damage the front end of a drive, but everything going OUT of the drive has to be COMMANDED by the drives CPU. So even if the front end is damaged, that would not result in the drive sending anything to the motor, it just dies. My comment about "melting" was meant sarcastically. If anything melted as a result of a line fault, everything in the front end becomes conductive and makes it another bolted fault on the LINE side.
 
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mull982

Senior Member
Jraef said:
When I said I DISagreed with the statement about a non-regen drive passing through line faults, is because I cannot see how it can do that at all. A fault on the line side may damage the front end of a drive, but everything going OUT of the drive has to be COMMANDED by the drives CPU. So even if the front end is damaged, that would not result in the drive sending anything to the motor, it just dies. My comment about "melting" was meant sarcastically. If anything melted as a result of a line fault, everything in the front end becomes conductive and makes it another bolted fault on the LINE side.
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Perhaps my question above regarding a drive passing through faults was confusing, I apologize.

I was referring to a case where a fault occurs on the load side of the drive (motor side) and wondering how much of the available fault current is passed through the drive. For instance lets say the fault occur at the motor terminals and without a drive in place the fault contribution from the system is 20kA. Now with a drive in place how does this affect the fault contribution from the system for a fault occurring at the motor? Do most drives limit the fault contribution through the drive? I am guessing because most analysis software allows you to input a contribution percentage that most drives somehow limit the fault contribution for faults on the load side of the drive?
 
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jim dungar

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mull982 said:
Do most drives limit the fault contribution through the drive?
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There is no guaranteed answer.
In most cases the power electronics will sense the output fault and shut down the inverter.
In some cases the drive will act as a 'fuse' when the current flow through it causes drive components to fail in an 'open' mode.
Then there where be the rare occasions when the drive components fail in a shorted mode and the fault contribution can be substantial.
 
Besoeker

Besoeker

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jim dungar said:
In some cases the drive will act as a 'fuse' when the current flow through it causes drive components to fail in an 'open' mode.
Then there where be the rare occasions when the drive components fail in a shorted mode and the fault contribution can be substantial.
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In my experience, power semiconductor components usually fail in shorted mode. I could probably count the number of OC failures I've come across on the fingers of one hand.
But what do I know.....
 
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Ingenieur

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Besoeker said:
The AC supply absorbs it.
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DC bus caps
then inverted to the AC grid

technically the grid load absorbs it, not the supply per se. but it does convey on the supply conductors, but if it is transmitting power is it still a supply?
similar to a braking resistor on the DC bus with the converter, just waste heat at that point though
 
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jim dungar

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Besoeker said:
In my experience, power semiconductor components usually fail in shorted mode. I could probably count the number of OC failures I've come across on the fingers of one hand.
But what do I know.....
Click to expand...

I think you may be forgetting about when they fail violently (e.g. blow to pieces).
 
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