Smart questions
Smart answers
Smart people
Join Eng-Tips Forums

Member Login

Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

gdeep (Electrical) (OP)
9 Jan 13 16:46

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
jraef (Electrical)
9 Jan 13 17:51
Honestly, I'm not sure what the SEL-710 is doing for you here, most modern VFDs provide equal or near equal protection for their motors AND are inherently copmpatible with the complex output waveforms of VFDs.

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"

gdeep (Electrical) (OP)
9 Jan 13 19:32
Thanks for the reply.
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?

dpc (Electrical)
9 Jan 13 19:37
Last time I checked, SEL specifically stated that this relay is not intended for use with VFD-driven motors. The GE Multilin 469 supposedly can be applied with a VFD, but as Jeff said, I don't think it really does much for you. I don't think the SEL 710 will provide much overload protection in this situation. But you should check with SEL.

A standard overcurrent relay would be more appropriate, IMO.
sunnysky (Electrical)
10 Jan 13 0:13
SEL-710 may be redundant, but it calculates thermal energy in the motor to determine maximum safe start times, with full motor protection. Accurate temperature tracking also minimizes the time needed between starts.

It provides overcurrent protection, thermal overload, undercurrent, current unbalance, current differential, phase loss, ground fault, and over or under frequency and much more.
davidbeach (Electrical)
10 Jan 13 0:22
The drive provides all that. If there were no drive between the motor and the CTs connected to the relay then you would have a point. The 710 is not appropriate on a VFD circuit. For better or worse, you are dependent on the VFD to protect the motor; if you want to protect the circuit to the VFD then the SEL-751 or SEL-751A would be a much better choice than the SEL-710.
waross (Electrical)
10 Jan 13 6:32


SEL-710 may be redundant, but it calculates thermal energy in the motor to determine maximum safe start times, with full motor protection. Accurate temperature tracking also minimizes the time needed between starts.

It provides overcurrent protection, thermal overload, undercurrent, current unbalance, current differential, phase loss, ground fault, and over or under frequency and much more.
Yes but to do most of that, it must be monitor the motor, not the transformer feeding the drive feeding the motor.

"Why not the best?"
Jimmy Carter

jraef (Electrical)
10 Jan 13 10:52
Do you have a bypass starter for the drive then? If so, is your power system even capable of starting that 700HP motor without the VFD? If there is no existing bypass starter, by the time you unwire the VFD and connect something in it's place, the thermal condition of the motor is going to be irrelevant. Seems a waste of money unless you have a parallel VFD or maybe a soft starter as a bypass for this VFD, plus the absolute need to restart this motor prior to the cool down time. If that is the case, then it does make sense. But if so, then you asked for an engineering opinion on something where you only gave us 30% of the relevant data. For shame...

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"

dpc (Electrical)
10 Jan 13 11:58
It is questionable the the SEL-710 will really provide any real thermal protection in this instance. The thermal model used in this relay relies heavily on the motor slip in its calculations and assumes it is looking at a sine wave.
gdeep (Electrical) (OP)
10 Jan 13 15:59
Thank you all for your inputs. This design has been passed down to me and I am trying to figure out the design details to program the relays. So sorry for any incomplete data.

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.
jraef (Electrical)
10 Jan 13 17:15
Looking at that diagram, those relays are all but useless. They are not going to see ANYTHING going on inside of those motors, they will ONLY see the current going into the VFD bridge rectifier. You have to remember, the line is powering the VFD, the VFD is powering the motor. What happens on the line side is only indirectly related to what is happening in the motor, because the VFD is like a new power source for that motor, the line is just supplying the raw material for it.

I wouldn't bother personally, but it's not my money...

"Will work for salami"

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close