System study information from utility

[JBD]

What is the sense in purchasing and using power system analysis software, like SKM, if utilities will not provide accurate "as currently installed" information?

Yes I understand that at some future date they may upgrade the transformer and other equipment feeding a facility, but how about today's data. Yes their grids are very dynamic with some clearing times, Z, and X changing constantly, but they must use some "fixed" values to set their own relays and fusing. If they feed a location with a dedicated transformer, can't they be more definitive with the values at the secondary of the transformer?

Maximum design (future) fault currents are absolutely correct for choosing equipment with the proper fault ratings. But maximum ratings may not provide the worst case incident energy for arc-flash. Even device coordination may be easier to accomplish if some fault levels are unattainable. Fault clearing times are not as important but if I have to tell the utility the TCC of my equipment why can't they tell me any thing about theirs?

In a recent situation a facility asked for an 800A 208Y/120V 3phase 4wire service. The utility said it had to be designed for a potential fault current of 57kA based on the maximum potential transformer that might ever be installed. The actual utility installation is a dedicted 75kVA padmount based on the estimated load profile of the service. The utility refuses to provide %Z, X/R or fault data for the installed transformer.

 

[stevenal]

mc5w,

That 75kVA and it's nameplate may be changed out with whatever 75kVA unit happens to be in stock when it fails. If an overload is suspected, a 100kVA might even be installed. IEEE and NEMA have no impedance standards for most distribution transformers, so calcs based on today's data are valid for today only.

 

[cflatters]

Having recently completed a whole set of loadflow and fault level studies in the UK, I have not had any problem getting sequence impedance & fault level information from the DNO's concerned.

They have provided output data from their DINIS models without a problem.

 

[ScottyUK]

cflatters,

I wonder if that is because the UK is, to my mind, a long way behind the US in assessing arc flash hazard in detail. The DNO's might be more reticent if their data was likely to be used in life-safety calculations which could leave them open to prosecution if the data was later found to be inadequate. Anyone from a UK DNO care to comment?



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