VFD fault contribution

[Ingenieur]

You can't miss something that wasn't there in the first place.

Apparently you can...twice
because it was in a prior post
0 for 2

 

[mull982]

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.

So I guess for fault analysis purposes if there is no bypass switch you just assume that there is 100% let-through on load side of drive? I cant really think of a reason why you would only want to use a smaller % of the let-through expect perhaps if you were evaluating Arc Flash at a downstream disconnect and wanted to look at a minimum case? Like you mentioned though the drive will likely shut itself down on a fault before an external protective device so I don't know what you would use for clearing time to calculate Incident Energy on the load side of VFD.

It sounds like for analysis purposes if there is not a bypass you don't consider regenerative contribution on the line side from the motors unless you know specifically that the drive is a regenerative drive.

 

[mull982]

After posting I played around with the VFD component in an SKM model and I noticed that for a VFD without a bypass switch if you uncheck the 100% let-through option and manually pick a % of VFD rating for let through that the fault current on the load side of the drive is extremely small compared to 100% let through.

For example with a 300HP VFD the 100% let through value is 45kA on the load side of drive but if you choose 100% of VFD rating (instead of 100% let through) the value on the load side if reduced to 0.4kA.

Does anyone know how this load side value is calculated as a percentage of the drive value? I'm assuming that the drive Hp rating is somehow converted to a full load rating which is used as the base for the percentage rating?

 

[Besoeker]

:thumbsup:
.................
But when you are away, does he miss you?

He does. I just need teach him to aim better.

 

[jim dungar]

So I guess for fault analysis purposes if there is no bypass switch you just assume that there is 100% let-through on load side of drive? I cant really think of a reason why you would only want to use a smaller % of the let-through expect perhaps if you were evaluating Arc Flash at a downstream disconnect and wanted to look at a minimum case? Like you mentioned though the drive will likely shut itself down on a fault before an external protective device so I don't know what you would use for clearing time to calculate Incident Energy on the load side of VFD.

It sounds like for analysis purposes if there is not a bypass you don't consider regenerative contribution on the line side from the motors unless you know specifically that the drive is a regenerative drive.

Wasn't the OP question about fault current from the motor?

As far as let through to a fault, I would treat it as if nothing flows through the VFD, unless there is a bypass contactor.
If the VFD components are functional then its electronics will prevent fault current from flowing towards the motor, in a manner similar to how they prevent startup current from flowing. There fore the incident energy would usually be based on 1.1 times the drive rating (maybe as high as 1.5x). BTW, I use the same philosophy with a UPS, although bypass is pretty common on medium and large units.

You are correct I don't normally worry about the motor contribution. Even if the drive is a regenerative type, is would have to be in a decelerating mode and the current would pretty much be limited to 1.5x.

 

[Jraef]

Wasn't the OP question about fault current from the motor?

... Therefore the incident energy would usually be based on 1.1 times the drive rating (maybe as high as 1.5x). ...

I'm with you on this, but just want to add that most drives will allow at least 200% current for about 3 seconds, long enough to be noticeable in an incident energy situation. The 150% rating is for 1 minute (on constant torque rated drives).

 

[mull982]

Wasn't the OP question about fault current from the motor?

My OP was asking about fault contribution through the VFD from the line side (system) and load side (motor). I may have posed the question in a confusing manner so I apologize.

As far as let through to a fault, I would treat it as if nothing flows through the VFD, unless there is a bypass contactor.

I think this is a good assumption. As mentioned below the let through would be limited by the drive to such a small amount that it would be negligible for any fault calculations/results

If the VFD components are functional then its electronics will prevent fault current from flowing towards the motor, in a manner similar to how they prevent startup current from flowing. There fore the incident energy would usually be based on 1.1 times the drive rating (maybe as high as 1.5x). BTW, I use the same philosophy with a UPS, although bypass is pretty common on medium and large units..

Were does the 1.1 or 1.5 times the drive rating come from? Is this the typical maximum current that a drive will output even during a fault?

You are correct I don't normally worry about the motor contribution. Even if the drive is a regenerative type, is would have to be in a decelerating mode and the current would pretty much be limited to 1.5x.

With a regenerative drive, during a fault on the line side of the drive the motor would pass fault contribution through the drive regardless if it was decelerating or not?

I'm with you on this, but just want to add that most drives will allow at least 200% current for about 3 seconds, long enough to be noticeable in an incident energy situation. The 150% rating is for 1 minute (on constant torque rated drives).

 

[jim dungar]

Were does the 1.1 or 1.5 times the drive rating come from? Is this the typical maximum current that a drive will output even during a fault?

With a regenerative drive, during a fault on the line side of the drive the motor would pass fault contribution through the drive regardless if it was decelerating or not?

The 1.1 or 1.5, or the 2X (from jraef) are based on the amount of current the electronics can supply to the load. Check with the manufacturer, but the value is small.
If the fault is on the line side of, or internal to, the VFD then no current will flow towards the motor.
If the fault is on the load side of the VFD, its electronics will usually shut down the drive resulting in no current flow. If the drive does not shut down, its electronics will limit the current to its design parameters, in a manner similar to how it limits motor starting current.

A regenerative drive primarily only feeds back into the grid when the DC bus voltage is too high, which typically only occurs during deceleration (although an overhauling load could also be a factor).
A line side fault would be seen as a loss of incoming power so most drives will turn off and 'coast to a stop'.

 

[mull982]

The 1.1 or 1.5, or the 2X (from jraef) are based on the amount of current the electronics can supply to the load. Check with the manufacturer, but the value is small.
If the fault is on the line side of, or internal to, the VFD then no current will flow towards the motor.
If the fault is on the load side of the VFD, its electronics will usually shut down the drive resulting in no current flow. If the drive does not shut down, its electronics will limit the current to its design parameters, in a manner similar to how it limits motor starting current.

A regenerative drive primarily only feeds back into the grid when the DC bus voltage is too high, which typically only occurs during deceleration (although an overhauling load could also be a factor).
A line side fault would be seen as a loss of incoming power so most drives will turn off and 'coast to a stop'.

Very informative! Thank You!

Does a similar concept to what you mention above apply to inverters as well? If there is a fault on the DC side of a PV inverter will any contribution from the AC side pass through inverter to DC side? I know inverters similar to VFD will limit AC output to 2-3x inverter continuous rating so for short circuits on AC side, the inverter will contribute very little (almost negligible)