iwire said:
It seems to me if it was reduced it could not be infinite.
I always assumed 'infinite bus at the transformer' meant that for the sake of calculations the supply assumed to have no limit.
This is a very common assumption. However, in power system analysis it actually means that it's an ideal voltage source that maintains constant voltage magnitude, constant phase, and constant frequency. It is modeled as an infinite source behind the system equivalent impedance, which is predominantly reactance. An infinite bus can be placed/considered anywhere on a system.
For example, say I am installing new 100MVA power generation unit which is connected through a generator step-up transformer to a 230kV substation. The transformer is located 0.25mi from the substation, and we are responsible for the distribution line from the HV side of the GSU out to the substation. I contact the utility, and they say that for long-term planning purposes assume an infinite bus at the substation. What does that really mean? Well, I find out that the equipment in the substation has a short circuit rating of 21kAIC. Since the utility cannot exceed the fault level of the equipment I can safely calculate my maximum (worst case) short circuit MVA. Therefore at 230kV that equates to a maximum of 8366MVA. That MVA number is a combination of the combined system impedance at the point of interconnection and the ideal voltage source. I can then calculate the system impedance "z".
However, the line between the HV terminals of the generator step-up transformer and substation is now added to the substation impedance as series impedance. This, will end up reducing the amount of available fault current at the HV terminals of the transformer. If, the reduction is minimal, then the amount of short circuit MVA could still be high enough that it would appear to be an infinite source. Let's say we have a 100MVA transformer with z = 10%, then the short circuit MVA capability of the transformer is 1000MVA, e.g. acts as a choke.
So, if the impedance of the line doesn't drop the short circuit MVA down to 1000MVA or so, then the contribution from the substation would still appear to be an infinite source at the HV terminals of the transformer, even though it has been reduced by the transmission line. This being the case, then to determine equipment ratings on the LV side if the transformer, you can use the MVA short circuit capability of the transformer as the maximum worst case available fault current.
You could just as easily develop a scenario where the line is much longer, or theres an air core reactor used to limit fault current, and it does reduce the fault current to a level below the 1000MVA, and then it would not be an infinite source at the transformer, even though you started at the substation with an infinite source.
In regards to the bussman link, thanks anyway, but I've been doing power system studies for 16 years, and my understanding of them is far beyond the basics.
