Protection for VFD controlled motor
Protection for VFD controlled motor
(OP)
Hi,
I have a SEL 710 protection relay that protects a VFD (with an isolation transformer) and 700HP motor. There are fuses on the line side of the VFD for short circuit protection and contactors( controlled by SEL 710)for opening/closing the circuit. I am not an expert, so please correct me if I am wrong
1) SEL 710 will provide thermal overload protection to the motor and the cable. Fuses protect the VFD and motor against short circuits. The VFD has built in protection for motor overloads, so it can protect itself against motor overloads. So when designing protection logic for the SEL 710, I should focus on providing thermal overload protection for the motor and the cable.
2) Does anyone know if I need to take into account the inrush of the isolation transformer?
Any help would be appreciated
Thanks
I have a SEL 710 protection relay that protects a VFD (with an isolation transformer) and 700HP motor. There are fuses on the line side of the VFD for short circuit protection and contactors( controlled by SEL 710)for opening/closing the circuit. I am not an expert, so please correct me if I am wrong
1) SEL 710 will provide thermal overload protection to the motor and the cable. Fuses protect the VFD and motor against short circuits. The VFD has built in protection for motor overloads, so it can protect itself against motor overloads. So when designing protection logic for the SEL 710, I should focus on providing thermal overload protection for the motor and the cable.
2) Does anyone know if I need to take into account the inrush of the isolation transformer?
Any help would be appreciated
Thanks






RE: Protection for VFD controlled motor
Without knowing where and how you are connecting the CTs for the SEL-710, its impossible to know if you need to factor in transformer inrush. But if it is an isolation transformer AHEAD of the VFD, and the CTs are connected BEHIND the VFD, then the SEL-710 will see zero effects of ANYTHING happening on the line side of the VFD, save a complete power failure. It will see, and monitor, the exact same thing the VFD is seeing and monitoring going to the motor. If however you have the CTs AHEAD of the isolation transformer, then transformer inrush will play a part.
"Will work for salami"
RE: Protection for VFD controlled motor
I believe the SEL 710 might be providing a back up protection for the motor incase the VFD has to be taken out of service. Also, may be they dont want to rely on the VFD to provide overload protection. The CTs for the relay are located ahead of the VFD and isolation transformer. If I am using only the thermal model, then I dont think I might have to worry about inrush since it last only a few cycles. Any thoughts?
Thanks
RE: Protection for VFD controlled motor
A standard overcurrent relay would be more appropriate, IMO.
RE: Protection for VFD controlled motor
It provides overcurrent protection, thermal overload, undercurrent, current unbalance, current differential, phase loss, ground fault, and over or under frequency and much more.
RE: Protection for VFD controlled motor
RE: Protection for VFD controlled motor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Protection for VFD controlled motor
So given the stretch sceario, the CTs for the MPR need only be ahead of a point of common coupling for the two controllers. I have done this several times with 469s on the load side of the VFD and bypass isolation contactors, but Multilin specifically says they are capable. Not having used a 710 yet (although I have a project in house with 4 of them right now), I can't comment on their viability downstream of the VFD, so that only leaves upstream. Then if the isolation transformer is not sized to be able to start a 700HP motor across the line, (because it was sized for the VFD which eliminates inrush), then that only leaves the line side of the transformer as the PCC for the CT connection. But then in that case, your thermal data will be skewed by the transformer losses. You can compensate for that in the settings, but when you bypass the VFD, that compensation will now render your protection curves too high for the motor directly! Does the 710 allow for multiple curves that can be selected by a digital input? If so, that would be a good solution.
But see how complicated the scenarios get when you only provide partial information?
"Will work for salami"
RE: Protection for VFD controlled motor
RE: Protection for VFD controlled motor
I have confirmed that the SEL710 would provide back up 51 protection only, although the VFD would also be able to perform these protection functions as well. There are zero sequence CTs available as well that could be used for detecting current unbalances( excluding during transformer inrush).
The VFD design drawings show capacitor banks being used after the isolation transformer. Do you guys think it would be a good idea to limit the number of VFD power ups in a minute to give enough time for the capacitors to charge/discharge
I have also attached a snapshot of the 4160V single line with the SEL 710. The isolation transformers are not shown in the single line.
RE: Protection for VFD controlled motor
I wouldn't bother personally, but it's not my money...
"Will work for salami"