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 Post subject: Recommended overcurrent settings for transformers and motors
PostPosted: Wed Dec 27, 2017 11:35 am 
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Joined: Thu Jan 29, 2015 12:40 pm
Posts: 16
Good day! I am doing a coordination study for a power plant (auxiliary/station service loads). I am noticing that many of the transformer time overcurrent settings are over 2X FLA, and many of the instantaneous overcurrent settings are over 2X Inrush. For motors, I am also seeing 51's and LTPU's above 200%. To me, these are set too high. I can see that we are well into the damage curves. But I know what reaction I will receive once I suggest to lower settings on a bunch of relays. Because as you know, the generation mentality will often sacrifice equipment for the sake of power generation. And I can understand that because the equipment sacrificed might not be as valuable as keeping the process going. So I guess what I'm looking for here is some support in my decision to lower the settings. Do you agree or disagree?

I've copied two links to SKM's suggestions because I think they've done a good job at wrapping up guidelines from IEEE 242, C37.91, C37.96 and some other standards. My recommendations to the plant would fall in line with these guidelines. Thanks in advance.

http://www.skm.com/applicationguides22.html
http://www.skm.com/applicationguides21.html


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 Post subject: Re: Recommended overcurrent settings for transformers and mo
PostPosted: Thu Dec 28, 2017 1:36 pm 

Joined: Mon Dec 07, 2015 9:45 am
Posts: 33
Location: Massachusetts
Josiah Halverson wrote:
Good day! I am doing a coordination study for a power plant (auxiliary/station service loads). I am noticing that many of the transformer time overcurrent settings are over 2X FLA, and many of the instantaneous overcurrent settings are over 2X Inrush. For motors, I am also seeing 51's and LTPU's above 200%. To me, these are set too high. I can see that we are well into the damage curves. But I know what reaction I will receive once I suggest to lower settings on a bunch of relays. Because as you know, the generation mentality will often sacrifice equipment for the sake of power generation. And I can understand that because the equipment sacrificed might not be as valuable as keeping the process going. So I guess what I'm looking for here is some support in my decision to lower the settings. Do you agree or disagree?


Primary or Secondary for transformer OCPD?
For Primary, if it's within NEC 450.3 table requirements then I would not be too concerned, especially if it means replacing fuses. If it's just a trip unit or relay setting it can be changed and as you note as long as you avoid the in rush point you should not have any issues.
For Secondary I would definitely recommend lowering the settings to prevent transformer damage in the event of a fault down stream from the OCPD.

We have a facility we've been working at for years, where they had not, until recently, performed any coordination study, and had several transformers explode in the 5 yrs prior due to poor maintenance and age (1940's and earlier vintage). This prompted their safety department to budget roughly $2million over the course of the following few years to perform coordination and arc flash studies and pay for initial mitigation suggestions.
We found that almost every transformer effectively had no protection from a secondary protective device or even OCPD below that OCPD, from downstream faults. This immediately identified why the transformers that did explode did so because of downstream faults.

If this company truly does not care about the loss of equipment and the lost profit from the resulting damage, there is always the human safety aspect that they may be willing to listen to. If this too is not something that concerns them, then there may not be much you can do.

The reality is if their systems cannot run without the OCPD for the secondary on those transformers being 2x FLA then their system needs to be upgraded to safely provide the necessary load distribution.


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 Post subject: Re: Recommended overcurrent settings for transformers and mo
PostPosted: Thu Dec 28, 2017 4:42 pm 
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Joined: Thu Jan 29, 2015 12:40 pm
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Thanks, SteveA! I'm still getting used to Table 450.3(A). What I don't get is why there is no specification for instantaneous or time-overcurrent. Let's take an example.

Transformer rated at:
10MVA
13200/4160V
Pri FLA 546.7 A
Sec FLA 1734.8 A
Inrush Factor 12.0
Z = 6.94%
SEL-487 relay on the HV side
SEL-387 relay in series with an SEL-751A on the LV side
"Any location" assumed

Table 450.3(A) says 400% on primary with 250% on secondary (% of FLA). Is this a 50 or 51? If it's a 50, then the relay will trip on inrush. If it's a 51, then it should work. But they should say something about adjusting the time dial to move the trip curve out of the damage area. Is my understanding correct? At least for Table 430.52, for motors they specify instantaneous and inverse time. I wonder why not for transformers.


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 Post subject: Re: Recommended overcurrent settings for transformers and mo
PostPosted: Thu Dec 28, 2017 4:50 pm 

Joined: Mon Dec 07, 2015 9:45 am
Posts: 33
Location: Massachusetts
the requirement is for Long Time Pickup, as it is all that the NFPA cares about for their purposes. all other settings are up to the engineer in order to provide coordination and asset protection. The code book is not a design guide or course book, if you need to learn how to coordinate and protect your system look into the IEEE books and papers that explain this.


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 Post subject: Re: Recommended overcurrent settings for transformers and mo
PostPosted: Thu Dec 28, 2017 5:25 pm 
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Joined: Thu Jan 29, 2015 12:40 pm
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Thank you. Actually, I have quite a bit of experience with coordination and protection which is why NEC threw me a little because it leaves out all the details I am used to seeing (time delays, inrush, curve types, CT ratios, 50 vs 51 etc.) You're point about NFPA only caring about LTPU made it clear. Thanks again. :)


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 Post subject: Re: Recommended overcurrent settings for transformers and mo
PostPosted: Fri Dec 29, 2017 7:24 am 

Joined: Mon Dec 07, 2015 9:45 am
Posts: 33
Location: Massachusetts
Ah, yea, gotcha.

I always tell our young engineers to remember, the National Fire Protection Agency basically only cares about safety (ie, preventing fires). The essential purpose of the National Electrical Code is as such, a minimum set of standards to ensure the safety of the installation. View what is in the code from this perspective and things generally make sense as to why it is written the way it is.


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