Medium Voltage VFD Short Circuit Rating

[philly]

I was looking at a datasheet for a 4.16kV Siemens VFD and I noticed that the VFD listed a Short Circuit Current Rating of 50kA for "100mSec maximum time release".

To me this looks like the VFD has a short circuit withstand rating of 50kA for approximately (6) cycles or 100mSec. Is this correct? I haven't seen many of these drives in application so I'm not sure if this is a somewhat typical value.

Does a MV VFD not have a "Momentary" rating similar to other MV devices which have both a withstand (or Interrupting) and a momentary or Close & Latch rating?

 

[LMAO]

I was looking at a datasheet for a 4.16kV Siemens VFD and I noticed that the VFD listed a Short Circuit Current Rating of 50kA for "100mSec maximum time release".

To me this looks like the VFD has a short circuit withstand rating of 50kA for approximately (6) cycles or 100mSec. Is this correct? I haven't seen many of these drives in application so I'm not sure if this is a somewhat typical value.

Does a MV VFD not have a "Momentary" rating similar to other MV devices which have both a withstand (or Interrupting) and a momentary or Close & Latch rating?

Never heard of a "short circuit rating" for a VFD. Maybe they are referring to short circuit rating of internal fuses protecting the diode bridge?

 

[Jraef]

That's the withstand rating, not an interrupt rating. It just means that whatever you use to feed this drive must limit the current to 50kA and clear within 100ms. After that, parts within the drive can become shrapnel. 40kA and 50kA are fairly common 100ms specs for MV drives.

In a nutshell, it's telling you that you will likely have to use fuses to protect it, even if you feed it with a power breaker.

 

[philly]

That's the withstand rating, not an interrupt rating. It just means that whatever you use to feed this drive must limit the current to 50kA and clear within 100ms. After that, parts within the drive can become shrapnel. 40kA and 50kA are fairly common 100ms specs for MV drives.

In a nutshell, it's telling you that you will likely have to use fuses to protect it, even if you feed it with a power breaker.

Thanks, this is pretty much what I thought but wanted to make sure I understood correctly.

What about for fault currents less then 50kA (Say 10kA for example), is there are longer time duration that the equipment can withstand these lower faults. I'm used to seeing a "short time withstand" rating for most MV equipment which typically has a duration of 1 or 3 seconds which tells me that that any fault current under the rated amount can persist for the rated 1 or 3 second duration without any issues. Not sure if the MV drives have a similar rating?

 

[Jraef]

Thanks, this is pretty much what I thought but wanted to make sure I understood correctly.

What about for fault currents less then 50kA (Say 10kA for example), is there are longer time duration that the equipment can withstand these lower faults. I'm used to seeing a "short time withstand" rating for most MV equipment which typically has a duration of 1 or 3 seconds which tells me that that any fault current under the rated amount can persist for the rated 1 or 3 second duration without any issues. Not sure if the MV drives have a similar rating?

The 1-3 second withstand rating is still something that applies to equipment involved in distribution, because you want the gear to hold up long enough for the OCPD to clear, IF down stream OCPDs don't do it first. A VFD is different. As far as the fault current is concerned, it IS the "load", because it s a converter. Then it is a new power source for another load down stream of it and it has it's own protection scheme for the downstream load. So the only real possible place a fault could take place as far as the OCPD feeding the VFD is concerned, would be the VFD itself. If the fault inside of the VFD takes place at a level lower than 50kA, and it takes longer than 100ms for the OCPD to clear it, so be it. Nothing turned to shrapnel and damaged people or things around it. But your VFD was ALREADY toast by definition, so you and your boss are sad, but safe.

 

[victor.cherkashi]

Every equipment has damage curve (time and current). I used current limiting fuse/c.b. Put protection curve of the fuse on top of damage curve of equipment to see if there is problem

Sent from my Nexus 5 using Tapatalk

 

[philly]

In a nutshell, it's telling you that you will likely have to use fuses to protect it, even if you feed it with a power breaker.

Was thinking about this comment again when I realized that most MV breakers have a 5 cycle max clearing time which is 83ms. If you were protecting the drive with a MV breaker and set the Instantaneous to pickup below 50kA wouldn't it adequately clear the fault and protect within the required 100mS window?

Or maybe once you consider relay time to send signal to breaker and any other time error margins this may be too close to 100mS for comfort?