Variable Frequency drive and Short circuit current rating 1

[power2engineer]

Available fault current at input terminals of VFD is 40KA where as VFD is rated 10KA. What is the solution to limit the fault current to VFD so it does not get destroyed in case of internal fault?

 

[waross]

A line reactor will give you some filtering as well as dropping the available fault current.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Mr. power2engineer (Electrical)(OP)17 Jun 23 02:13
"....Available fault current at input terminals of VFD is 40KA where as VFD is rated 10KA. What is the solution to limit the fault current to VFD so it does not get destroyed in case of internal fault?"
Use only the Semiconductor fuse recommended by the VFD manufacturer. It should be fine. Attention: Semiconductor fuses (NOT normal motor fuses) are specially designed for VFD and electronic devices. All major fuse manufacturer have them in their catalogue.
Che Kuan Yau (Singapore)

 

[dpc]

If you are in the US and under the National Electrical Code, there are not many good options. The simplest solution will be to replace the VFD with one that has an adequate rating. Second option is to contact the VFD manufacturer to see if they have tested their unit with upstream current-limited fuses or will certify that an upstream fuse will protect the VFD. Or perhaps they have a version with a higher-rated circuit breaker on the input. The available SC current should be specified when the drive is purchased.

But installing the VFD such that its SC rating (on the nameplate) is exceeded is an NEC violation. If you don't need to meet the NEC, I'd install a fused disconnect ahead of the VFD with the smallest sized fuse you can get away with - and take your chances.

 

[waross]

Hi Jeff. Would an isolating transformer ahead of the VFD do the trick and meet NEC?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

Not Jeff, but yes that is another option I should have mentioned, although these days, a new drive might be easier to obtain. It would depend on the size of the drive, but if the resulting fault current on the load side of the xfmr was less than 10 kA, that would certainly take care of the short circuit rating issue.

Cheers,

Dave

 

[power2engineer]

Dave thanks for the suggestions. I saw Schneider Electric specifies Current limiting fuses or line reactors specific to each of their drives - I am not sure Rockwell international or other US drive manufacturers provide such recommendations.

https://download.schneider-electric...1-PMUY8AYQgtHdymfdqEs5o8QfDCILLBoCjE4QAvD_BwE

 

[che12345]

Mr. power2engineer (Electrical)(OP)17 Jun 23 20:56
".... I saw Schneider Electric specifies Current limiting fuses or line reactors specific to each of their drives - I am not sure Rockwell international or other US drive manufacturers provide such recommendations."
1. Most VFD manufacturers recommend semi-conductor fuse to protect their VFD product. Attention: The listed Class J fuse is NOT a semi-conductor fuse.
2. Any fuse faces the possibility of single-phasing.
3. There are current limiting circuit breakers in the market which have the following properties:
a) eliminates single-phasing,
b) current limiting e.g. a normal breaker would have a I[sub]p[/sub]84kA, instead of 16.2kA with a short-circuit current of 40kA [sub]rms[/sub],
c)low let-through energy (A[sup]2[/sup] s )
4. Any form of line reactor would be costly and bulky.
Che Kuan Yau (Singapore)

 

[power2engineer]

Che - so are you saying Schneider Elec solution to mitigate VFD SCCR rating of adding line reactors or current limiting fuses is not correct?

 

[FacEngrPE]

VFD semiconductor fuses are usually an internal component of a drive. They do a reasonably good job of protecting the solid state components. If the OEM allows a current limiting fuse to allow a drive with a SCCR of 10kA to connect to a source with available 40kA, and the drive is ITL certified (UL/CSA/FM etc) the case has has been tested per the ITL rules.
Or you can get a copy of UL 508A Supplement SB
EATON On UL 508A, Supplement SB (determining SCCR based on the components in the power circuit)

We all need to learn to specify required SCCR on ordering, as the default (5kA) is way too low.

 

[che12345]

@ Mr. power2engineer (Electrical)(OP)18 Jun 23 18:33
"...Che - so are you saying Schneider Elec solution to mitigate VFD SCCR rating of adding line reactors or current limiting fuses is not correct?"
1. No, you had mistaken what I mean. The VFD manufacturer knows better what is good for protecting their product. The listed line reactor and the current limiting fuses ARE CORRECT for his product. In addition, the basic principle is also correct.
2. What I intended to raise the attention are:
a) strongly support the use of semi-conductor fuse recommended by the VFD manufacturer,
b) the listed Class J fuse is NOT a semi-conductor fuse,
c) line reactors are costly and bulky,
d) as an alternative, there are current limiting circuit breakers in the market that eliminate single-phasing and with other useful characteristics.
Che Kuan Yau (Singapore)

 

[che12345]

@ Mr. FacEngrPE (Mechanical)18 Jun 23 21:45
".... #1. VFD semiconductor fuses are usually an internal component of a drive. .. #2. They do a reasonably good job of protecting the solid state components. If the OEM allows a current limiting fuse to allow a drive with a SCCR of 10kA to connect to a source with available 40kA, and the drive is ....."
1. No. VFD semiconductor fuses are usually NOT an internal component. Most VFD does NOT allow the user to add any thing into their housing/casing. Any alteration/addition to their design involves heat flow calculation and fire safety etc....
2. By installing line reactor or semi-conductor fuse recommended by the VFD manufacturer would be fine. BTW a semi-conductor fuse would be better than a current limiting e.g. Class J fuse.
3. Install line reactor or semi-conductor fuse or current limiting circuit breaker ahead/input of the VFD. NOT inside or after/output of the VFD.
Che Kuan Yau (Singapore)

 

[LionelHutz]

I assume you have done some digging? Lots of drives have a higher rating but it's not on the rating plate of the drive and often hidden in a separate document or just a small obscure blurb in the manual.

I have seen quite a few drives that do not require semiconductor fuses for a higher short circuit rating.

Adding semiconductor fuses without the manufacturer publishing the combination as having a higher fault rating does nothing to help meet UL or NEC requirements.

The drive will probably be destroyed anyways if something bad happens. Typically, the protection clears when the drive fails and by that time the drive is usually scrap unless it's around 150-200hp or bigger since the smaller drives are all single boards for all the power components while the larger ones become more modular inside.

 

[LionelHutz]

1. No. VFD semiconductor fuses are usually NOT an internal component.

Many bigger sizes of VFDs have internal fuses.

 

[che12345]

@ Mr. LionelHutz (Electrical)19 Jun 23 12:54
"....Many bigger sizes of VFDs have internal fuses".
In general:
1. most LV VFD are in a compact plastic housing without any contactor or breaker in it. There is NO room/not intended for any additional devices to be installed by the user in it. To protect the switching device, semiconductor fuses are installed externally out side , ahead at the input of the VFD.
2. most MV VFD are in multiple cubical panels. It usually comprising of a fused vacuum contactor and transformer etc. The current limiting Class E fuse is good for the protection of the vacuum contactor and transformer but NOT the electronic device, which requires a much lower let-through energy (I[sup]2[/sup] t) fuse e.g. a semiconductor fuse.
3. Take note of the characteristic differences between a current limiting fuse and a semiconductor fuse. They serve different purposes.
Che Kuan Yau (Singapore)

 

[Colin Flatters]

Having adequately rated VFDs and sutable input protection fuses to limit the I2t is perhaps the best approach rather than limiting the upstream fault level (unless it is excessivly high). Looking at the IEEE1584 arcflash you may well get a faster disconnection and a lower IE as well as a lower available I2t if he fuses operate more quickly and lestt I2t energy to cause damage asa resut).

As a separate issue he drive harmonics will also be higer if the supply impedance increases as the higher Theveninin impedance )if you follow that principle) will produce more harmonic voltage distortion.

 

[LionelHutz]

che12345 - Why do you insist on arguing about things you know nothing about? MANY larger LOW VOLTAGE VFDs 100% have MANUFACTURER INSTALLED internal fuses.

You post about cute little plastic housing as if that's the only way you can get a low voltage VFD, which is laughable to me, but does explains your wrong position if that's all you understand about VFDs.

 

[jraef]

The ONLY way a current limiting fuse can be used in this situation is if the VFD MANUFACTURER has tested and LISTED the VFD at a higher SCCR in a series listing. You cannot do this on your own in the field. This has been unsuccessfully argued may times in Code groups and forums, the bottom line is that the NEC does not allow for it. The closest is that IF a registered PE were to sign off on it, that might be acceptable. But in the 3 or 4 cases where this was tried near me, I have never come across a PE willing to risk his license for a little thing like this.

That said, most of the RESPONSIBLE VFD manufacturers have done this and will provide you with the necessary documentation showing you the EXACT fuse that the VFD has been listed with. By the way, there are often "Semiconductor" fuses, so be aware that if not provided as part of the VFD package, it can be challenging to find fuse holders for them that work in fused disconnect switches.

Under the old UL-508A rules for VFDs, this was more of a "dealer's choice" as to whether a VFD mfr wanted to provide a higher SCCR than the "courtesy" 10kA listing they can get without having to test. But in 2019, UL changed to "harmonize" with the IEC standards and came out with UL 61800-5-1 (the number matches the relevant IEC spec BTW) which REQUIRES that the VFD be tested and listed at a higher SCCR (usually 65kA or higher). So any drive still using the original UL-508A listing and ignoring this changed requirement is selling old product. If so, you might want to consider updating to something that will not be obsolete quite as soon. Even if it is older, it's still worth checking with the VFD mfr to see if they do have a series listing with specific fuses, many do.

I'll also throw in here that most of the time, under the new UL 61800-5-1 rules that are now more stringent, it has to be fuses, not circuit breakers. So you will now see a lot of VFD mfrs installing these fuses downstream of circuit breakers, regardless of the apparent redundancy. The fuses are generally over sized so that they don't clear unless it is a dead short circuit, leaving the day-to-day thermal over current protection to the breaker.

And YES, adding a Drive Isolation Transformer ahead of the VFD is a valid alternative solution, albeit expensive. I have done it several times. A Line Reactor however does not cut it.


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