MV VFD protection. 4

[slavag]

Dear All.
We need emergency provide protection for MV VFD feeder
( 11kV, 2MW). On this moment, in the stock, I have only OC/EF (51,51N) relay for emergency cases. For cable protection is O.K. for MV VFD, I don't know.
What do you recommend: use this relay as emergency protection and order ASAP standard motor protection relay or use this relay for ever?
Cost is not issue.
Thanks for your help.
Best Regards.
slava

 

[davidbeach]

If the VFD is doing the motor protection functions, it would seem that overcurrent protection of the VFD should be fine. But then I've not dealt with that particular protection issue before.

 

[dpc]

You should not really need a motor protection relay on the input to the VFD. Any standard overcurrent relay should be OK. The VFD should provide motor overload protection.

You can set the feeder instantaneous overcurrent above the maximum short circuit output of the drive.

The feeder relay will see a fair level of current harmonics, depending on the type of drive. So digital relay with some type of harmonic filtering or selectable fundamental vs rms might work out well.

The Multilin 469 can supposedly be used on the OUTPUT of a VFD for motor protection.

 

[slavag]

Hello David and DPC.
Thanks a lot for your help.

DPC, used 469 on the OUTPUT of a VFD, it's new for me.
I always thinked that unpossible used standard motor protection ( 50, 60 Hz) on the OUTPUT of a VFD.
Best Regards.
Slava

 

[dpc]

Slavag,

It surprised me too until I ran into one on a project. Then I looked in the 469 manual, FAQ, and posted a couple of questions here, I think. The 469 has a special filtering mode for non-sinusoidal waveform and GE says it will work.

I asked SEL about their motor relays on a drive output and they said "don't even think about it".

 

[jraef]

I have been involved in many projects with Robicon MV VFDs and Multilin 469 relays, they are a proven combination. While the VFD has some motor protection features, the MPRs are far far better. The 469 is the only one I am aware of that is proven downstream of VFDs, although to be precise, I have never tried any others.

 

[CoryA]

There are some other options out there too.

The Startco MPU-32 is a current-sensing motor-protection relay that can be set to monitor 50 Hz, 60 Hz, or varying frequencies. It samples at 32 samples per cycle, and can adjust its center frequency based on the detected zero crossings to maintain this rate. See section 6.1 in the MPU-32 manual for more information.

The Startco MPS is a current- and voltage-sensing motor-protection relay that can utilize its 4-20-mA analog input to synchronize its sampling rate with an ASD. See section 5.24.2 and 7.1 in the MPS manual for more information.

---
Cory Anderson

http://startco.ca/

 

[slavag]

Hi.
CoryA: it's intresting. Thank you.
Jraef: Sorry for misunderstanding. You recommend use MPR also for VFD INPUT or only on OUTPUT?
Regards.
Slava

 

[jraef]

slavaq,
We used the 469s on the output side of the VFDs. One of the reasons is that on critical path equipment where they need a DOL (or Soft Starter) bypass operation capability in case the VFD goes down, the 469 is always in the circuit so that it knows the motor status at all times. That means that it must be on the load side of the VFD, below the point of common coupling with the bypass circuit. Besides, motor protection monitoring from the line side of a VFD is hardly accurate enough to bother doing.

CoryA, the Startco MPS relay can be used on Medium Voltage.