THE BALLOON ENVELOPE COLLIDED WITH POWER LINES AFTER DRAGGING THE BASKET & OCCUPANTS APRX 200 YARDS ALONG AN OPEN FLD AND THROUGH A FENCE WHICH DAMAGED THE BASKET CONTAINING THE OCCUPANTS. THE BALLOON PLT SAID THAT THE WIND WAS ONLY 5 KTS AT TAKEOFF BUT AFTER 30 MINUTES OF FLT A WIND DESCRIBED BY THE PLT AS 'SHEER (SIC) FROM APRX 150 FT TO SURFACE, 30-35 MPH-FLAPGS ON FLAG POLES STANDING STRAIGHT OUT-LOOKED FOR LNDG AREA-VENTED THE BALLOON & PREPARED FOR LNDG.' FURTHER ATATEMENT BY THE PLT CONTAINED INFO THAT THE VENT OPENING CREATED A SAIL EFFECT TO INCREASE THE RESULTANT DRAG. PLT ALSO SAID, 'TRIED PULLING TOP OUT OF BALLOON BUT ONLY 1/3 OF TOP RELEASED CAUSING A DRAGGING EFFECT.' THE BALLOON ENVELOPE FINALLY CONTACTED & DRAPED OVER THE POWER LINES & WAS DAMAGED DURING THE CONTACT & LATER DURING REMOVAL. INTERVIEWS WITH SEVERAL COMMERCIAL BALLOON PLTS & A REPRESENTATIVE OF RAVEN INDUSTRIES, SIOUX FALLS, SD, REVEALED THAT MOST BALLOON PLTS CAN EXPECT A SAFE LNDG IN WINDS OF 10 MPH OR LESS & A RISK IN HIGHER WINDS.