If you go and put a cobble stone block at Y=15 then use following command(works for all power)(/summon Fireball ~ ~2 ~ {ExplosionPower:50,direction:[0.0,0.0,0.0]}) Then hit it on to that block, it destroys blocks in a pattern. Should be more random to look like actual explosion.
The explosion algorithm is only made to work with vanilla size explosions. Larger ones may work, but are not guaranteed to.