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 Post subject: Infinite Secondary or Modeled Primay
PostPosted: Fri Feb 12, 2021 6:25 am 

Joined: Mon Mar 09, 2020 9:27 am
Posts: 6
I am building a model in SKM. Our utility gives us Max Primary Three Phase Fault, Max Primary Single Phase Fault, and Infinite Three Phase Fault Current at Secondary Terminals. they also give me the transformer size and impedance. I know from experience they use oil filled naturally ventilated transformers at the POC.

A few questions to help me understand this stuff:

If I model the transformer and use their primary currents, would this not be more accurate than using the secondary infinite bus?

Is the infinite bus not the worst type of currents to use, i.e. it would produce the lowest IE?

Is there a reason they would give me the calculated primary currents but infinite secondary? Would this not just be the primary currents increased due to the voltage decrease(ohms law)?

I am no engineer so forgive me if these are basic questions.


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 Post subject: Re: Infinite Secondary or Modeled Primay
PostPosted: Fri Feb 12, 2021 8:20 am 

Joined: Thu Apr 18, 2019 11:42 am
Posts: 41
Recommendations:

Use infinite bus methods to size the equipment from a short circuit current withstand perspective. Don't forget X/R ratios...

Use actual (not max) utility short circuit current availability for arc flash calculations. Infinite bus may or may not be worse case. In many cases that I've seen (from some consultants AF studies), they will use infinite bus and the AFIEs are lower than if they had used actual SCCA from the utility. Big problem...

If the utility won't give you actuals, then you're going to have to do max/min calculations to see where the worse case is and use that.


Mike


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 Post subject: Re: Infinite Secondary or Modeled Primay
PostPosted: Mon Feb 22, 2021 6:58 am 

Joined: Wed Jul 18, 2018 12:35 am
Posts: 4
Location: UK
If using SKM I would suggest that using multiple scenarios is your easy solution, so set BASE SCENARIO as the model with the populated network fault level data / impedances and create another scenario for the infinite bus option etc

SKM will then be able to evaluate all the scenarios concurrently and advise which gives the worst case

I agree totally with the previous response, the infinite model frequently doesnt give the worst case IE but SKM will tell you which scenario does give worst case and what the values are. Putting realistic impedance and X/R values is crucial to getting meaningful results.


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 Post subject: Re: Infinite Secondary or Modeled Primay
PostPosted: Mon Feb 22, 2021 8:03 am 

Joined: Tue Jul 26, 2016 8:01 am
Posts: 2
As already mentioned, the infinite bus will aid in equipment evaluations.
Use the primary fault current values with their respective X/R values for IE/Arc Flash calculations. I always request primary fault current contributions regardless if primary or secondary metered. I'll even request their upstream protection, many times I'll model the upstream pole fuses or internal bay-o-net fusing to identify any coordination issues.
There are some utility companies that still won't provide contribution for various reasons. In those cases I'll perform a sensitivity analysis (max/min), choosing values based on voltage levels from experience and taking into considering the distance from the source. Always note in the report or within the study one-line that the utility contribution is assumed or estimated in those cases.


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 Post subject: Re: Infinite Secondary or Modeled Primay
PostPosted: Mon Feb 22, 2021 1:10 pm 

Joined: Tue Mar 02, 2010 1:50 pm
Posts: 26
I have performed a number of arc flash studies for utility owned powerplants and powerplant auxiliary equipment, and when I got started I got seriously burned by using the wrong fault currents from our system protection department. Since then, I typically model the transmission or distribution systems myself for maximum and minimum fault levels. If you are working distribution transformers those seem to have really low impedances and could overstate the arc flash hazard depending on where you are in the system if you only consider the infinite bus on the utility side, or could severely under estimate the incident energy due to the breaker setting characteristics. You also will need the utility's overcurrent protection (usually a fuse) if you ever hope to be able to operate your main service disconnect without calling the utility to de-energize the system from the primary.


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