Why autotransformer can handle more power than conventional transformer?

Perhaps you're referring to the size of the core? Strictly speaking, a 30MVA autotransformer can handle the same amount of power as a conventional transformer.

But an autotransformer will be smaller than an equivalent conventional transformer. The reason is an autotransformer essentially has one winding on a core, with one tap including the whole winding, and another tap including only part of the winding. In a conventional transformer, power is transferred from one winding to the other by induction. In an auto, some is by conduction, and some is by induction.

Here's an example comparison:

Say we have a 10MVA 100kV/50kV 3 winding auto transformer. There are two parts of the winding - the series winding (the section of winding between the 100kV connection and 50kV connection), and the common winding (the section of winding between the 50kV connection to the neutral). If powered at 10MVA (full load), there will be:

10M / (100k * sqrt(3)) = 57.7 Amps flowing into the primary, and

10M / (50k * sqrt(3)) = 115.5 amps flowing out of the secondary

So, 57.7A flow into the primary, down through the series winding and out the secondary. At the same time 57.7A flow up from the neutral through the common winding and out the secondary, resulting in a total secondary current of (57.7 + 57.7) = 115.5 (sorry, rounding is making my math look wrong!).

Thus, the series winding is handling a total of:

(100k - 50k)* 57.7A*sqrt(3) = 5MVA, and the common winding is handling a total of:

(50k - 0) *57.7A*sqrt(3) = 5MVA

So the series and common windings only have to be made to handle 1/2 the total load(for this example - it is 1/2 because the primary / secondary voltage is 2/1). In a conventional transformer, each winding carries the full load (10MVA), because it operates on induction (no conduction).

If the votlage ratio between primary and secondary is too high, there is no real net savings in size (and thus cost), so a conventional transformer is often used. The cutoff point is around 4.