Short Circuits Calcs w/ Transformer Taps 1

[AFerguson]

Given:

10 MVA, 46 kV - 12.47 kV Transformer
Z = 10.4%
Primary Tap -2.5% (45 kV)
Secondary Tap +5% (13.09 kV)
3P Secondary Short Circuit Current = 4290 A
1P Secondary Short Circuit Current = 4620 A

I am trying to determine the utility (primary) short circuit contributions but seem to be missing something in my equations. Most appreciated if someone could outline the method to solve the above problem (either using SCC or MVA method).

 

[davrom]

musicguy1800, no offense was intended; I appologize if it sounded like that. It is only that you had a correct intuition (from my point of view).

To Aferguson: I think your actual question is: "What is wrong with my equations when I consider the voltage taps?".
Indeed, performing the calculation considering the voltage taps, the utility impedance gets negative in value (for my equations at least - I haven't checked the reactive's equations). I believe (I am about 90% sure about that) the Utility calculated the shc currents for rated voltages (46/12.47 kV) disregarding the voltage taps; the equations are OK. The question is how the TX is used in normal operation: at rated voltages or at adjusted voltages?

 

[davrom]

Aferguson,

It is actualy the "MVA method" you referenced the "proportional units (pu) method" or the "common base power (MVA) method"?

 

[jbartos]

Comment on reactive (Electrical) Jan 27, 2004 marked ///\\All AF asked for was the fault level on the HV. I have no idea what he wants it for.
///HV parameters are needed as a starting point for the downstream short circuit and voltage drop analyses. Normally, systems impedance Zsys, short circuit MVA, Utility voltage, etc. can be obtained from the Utility to proceed with the power distribution modeling by a suitable software.\\\

 

[CJFlatters]

What you do need to take account of in addition to the utility's declared s/c value of secondary s/c fault level is any fault contribution from rotating plant, motors & embedded generation etc.



_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager

 

[DanDel]

AFerguson, the 3-phase value you have been given is only a little less than the calculation based on an infinite bus on the primary.
If = (VA/1.73V)(1.0/0.104) = (10,000,000/1.73*12470)(9.6154) = 4457A
This doesn't take into account the impedance changes that will occur with the 10.4%Z when you are using tap changes as you mentioned.
I don't believe that you can accurately calculate the primary fault current from the values you have been given.