I have a electric dryer that is 220 volt. When I turn it on, after about 15 minutes it doesn't blow it's own 30 amp breaker, but blows the 100 amp main breaker. I can shut off the 30 amp breaker specifically for the dryer and it shuts it off. Any ideas??

I think, based on experience, is that there is a loose connection at the Main breaker. This can be either the incoming Line wires feeding the Main, or can even be the bolted busbar connection. Many Main breakers are the plug-in style, and this plug-in connection can also be the cause.

Any loose or corroded connection will create heat, and this heat can migrate into the circuit breaker, and cause the breaker to trip, for no apparent reason, since breakers have a thermal component inside.

The Main breaker may need to be replaced, since the heat can also damage the breaker or connections. The Main breaker cannot be replaced by a DIY'er, since the meter needs to be pulled to de-energize the feed to the Main breaker, so to have this repaired an electrician, and possibly the utility company will need to come out and effect repairs.