When doing power systems study, it always the predicament of Electrical Engineers where to get the values of the utility short circuit MVA. I was just talking to a senior engineer a few weeks back and he told me that he always consider an infinite bus supply when doing his studies. Despite the fact that it is partially correct, if the fault current values are used in the selection of equipment, then it will be very expensive.
The available short circuit varies significantly from location to location even for a particular utility, thus the utility provides the more accurate values for these values. This is not always an easy tasks for the electrical engineer though. Talking to the utility can sometime be very difficult. If you ask me, I would rather not unless I know anybody with in their organization.
If you faced a blank wall, what will you do? Use infinite bus as an assumption? No way, especially if you are trying to reduce the cost of your project.
It is here where IEC 60076-5 Power transformers – Part 5: Ability to withstand short circuit comes into rescue. Table 5 of this standard provides the short-circuit apparent power of the system for any particular voltage. As a bonus, it provides for both current European and North American practice.
|Highest voltage for equipment, Um
|Short-circuit apparent power, MVA|
|Current European practice||Current North American practice|
|7.2, 12, 17.5 and 24||500||500|
|36||1 000||1 500|
|52 and 72.5||3 000||5 000|
|100 and 123||6 000||15 000|
|145 and 170||10 000||15 000|
|245||20 000||25 000|
|300||30 000||30 000|
|362||35 000||35 000|
|420||40 000||40 000|
|525||60 000||60 000|
|765||83 500||83 500|
Source: IEC 60076-5
For Australia, use AS 2374.5-1982 Power transformers - Ability to withstand short-circuit which is a modified version of the IEC standard to suit Australian practice.
In above table, it is very noticeable that North American practice have higher fault MVA ratings for their equipment thus significantly larger in size.