MV VFD Feeder Protection

[Pitt03]

I am reviewing an application where a 4.16kV MV VFD is being fed from an upstream MV MCC through a fused contactor. The contactor has a multilin 735 relay on the feeder to the VFD. It also appears that a multilin 469 relay will be located at the VFD enclosure for providing motor protection in addition to the protection that the drive provides. The VFD can be bypassed when needed to operate the motor (300hp) across the line thus a need for a separate motor protection relay. The drawings currently show the 469 CT's located on the drive primary however I believe they should be located on the load side of the VFD in order to "see" the motors current thermal state when operating on the VFD (based on when I read in another post on the forum)

My question relates to the protection settings on the 735 relay in this application. In terms of setting the pickup level is it similar to primary protection on a LV drive where the pickup level should be set at 125% of drive input rating. Drive input rating in this case is 70A so I'm thinking a pickup of around 90A or so should be adequate.

-Would an Instantaneous pickup be recommended for this application? I saw in another thread that it was recommended to set the Instantaneous setting above the
secondary fault current from the drive? Is there a reason for this or good way of estimating this fault current if it is not stated in the drive information?

-In terms of setting the feeder relay time delay are there any special considerations other than coordination with upstream protection and the downstream 469
relay? The VFD appears to have an internal 600kVA transformer so I don't know if any consideration needs to be given for protection of it related to through-
faults, etc..

-I'm assuming that 469 relay is set as you typically would for an across the line motor application. Are there any special considerations related to the pickup,
OL curve, and INST settings that need to be considered?

- Both the 735 and 469 relays both trip the upstream MV fused contactor. Are there any special considerations in terms of coordination since they all trip same
device? Any special considerations with the fuse?

 

[ZeroSeq]

See comments:

I am reviewing an application where a 4.16kV MV VFD is being fed from an upstream MV MCC through a fused contactor. The contactor has a multilin 735 relay on the feeder to the VFD. It also appears that a multilin 469 relay will be located at the VFD enclosure for providing motor protection in addition to the protection that the drive provides. The VFD can be bypassed when needed to operate the motor (300hp) across the line thus a need for a separate motor protection relay. The drawings currently show the 469 CT's located on the drive primary however I believe they should be located on the load side of the VFD in order to "see" the motors current <correct, CTs should be on load side of VFD> thermal state when operating on the VFD (based on when I read in another post on the forum).

My question relates to the protection settings on the 735 relay in this application. In terms of setting the pickup level is it similar to primary protection on a LV drive where the pickup level should be set at 125% of drive input rating. Drive input rating in this case is 70A so I'm thinking a pickup of around 90A or so should be adequate. <check with the VFD manufacturer to see what current levels can be run at and for what amount of time>

-Would an Instantaneous pickup be recommended for this application? I saw in another thread that it was recommended to set the Instantaneous setting above the
secondary fault current from the drive? Is there a reason for this or good way of estimating this fault current if it is not stated in the drive information? <Use an instantaneous pickup not far above the VFD maximum operating current (~125%) and provide an instantaneous delay that will coordinate with the drive transformer inrush. Check if the drive has a primary switched reactor that will reduce inrush>

-In terms of setting the feeder relay time delay are there any special considerations other than coordination with upstream protection and the downstream 469
relay? The VFD appears to have an internal 600kVA transformer so I don't know if any consideration needs to be given for protection of it related to through-
faults, etc.. <coordinate any inverse time curve with the fuses, or coordinate the instantaneous delay with the fuse at the contactor interrupting capacity>

-I'm assuming that 469 relay is set as you typically would for an across the line motor application. Are there any special considerations related to the pickup,
OL curve, and INST settings that need to be considered? <yes, you need to de-rate the motor thermal curve to account for reduced operating speeds from the VFD. You also need to ensure the 469 is using rms current and not fundamental to account for harmonic heating content.>

- Both the 735 and 469 relays both trip the upstream MV fused contactor. Are there any special considerations in terms of coordination since they all trip same
device? Any special considerations with the fuse? <See point three re. fuse coordination>

 

[LionelHutz]

Typically, there a transformer on the input of the VFD and you can't compare protecting a transformer to protecting a low voltage VFD.

If by VFD secondary fault current you mean it's output current capability then forget about that approach. Assuming a transformer input on the VFD, just set it like you would for any other transformer.

I'd expect the 469 should just stop the VFD and not trip the main power.

 

[Pitt03]

Thanks for the responses.

Zeroseq - Is there rule of thumb for de-rating the motor thermal curve to account for reduced operating speeds? That is a very good point. When you mention coordinating the relays inverse time curve with the fuses do you mean having the relay time curve below the fuse curve and then have the relay INST delay cut the fuse curve so that the fuse protects/coordinates the contactor interrupting rating?

LionelHutz - I was able to get inrush information from manufacturer. They were also able to give me size of transformer but were only able to give me approximate impedance values of 6-8%. They were not able to provide any damage curve. I guess for the purposes of what I need to do however I can just use damage curve from software for the given rating (doesn't depend on impedance) and set the relay to protect it like you would any other transformer.

Good point about having the 469 relay trip the drive. The drive also has a bypass contactor so it would need to trip the main disconnecting means in the drive and not just provide a shutdown signal the drive itself.