Utility Contribution calculation

[calabala]

Hi,
How does utility companies calculate the 3-phase, line to ground fault utility contribution to a facility, as well as X/R ratio.
Suppose that this facility is powered from a 44 KV line and is 2000 KVA, 44kV/600V, delta-wye ground connected with a % impedance of 6%. Say the distance between the utility feed and the facility is 2.3 Km

 

[jghrist]

Usually, the utility will have their system modelled on a computer program that calculates fault currents.

Who owns the transformer and the line from the transformer to the facility?

 

[richanton]

Usually you have a contact from your utility company who can give you that info.

 

[calabala]

Thanks guys for your replies. I usually do that whenever needed, but I just wanted to do that myself. What type of equations and info needed to get those calculations?

 

[jghrist]

What type of equations and info needed to get those calculations?

Only all of the line impedances, transformer impedances, and source impedances on the utility network and a fault program that can handle however many buses the utility has. Basically, there is no way that you can calculate the available utility faults yourself.

 

[davidbeach]

What you need for this calculation is the phone number or email address of the utility service planner who you will be working with. As jghrist said, you, as a customer will never have enough information to make the calculation.

 

[cuky2000]

If you are trying to calculate the SC at the secondary side of the transformer, the effect of SC available may not have significant impact.
Usually an infinity source it is assume in many practical applications. If not data is available rom the utility X/R=20 and the rating of the circuit breaker of the nerarest substation (40 kA) may be assumed.

See the enclosed link that show the graphically the effect of the utility SC available with the distance on the secondary feeder.

http://cuky2000.250free.com/SC_Calc_Dist_Tranf_1.jpg

 

[davidbeach]

It depends a lot on what you need the numbers for. If all you are doing is selecting interrupting ratings, infinite source on the primary of the worst possible transformer plus secondary conductors is what you will get from most utilities for secondary service. If you are trying to do protection coordination or, even worse arc flash, you may find it difficult to get real numbers but as said there is no way you will ever be able to calculate the true utility contribution for a utility connected to the regional transmission grid.