DC loss smaller than AC transmission the transmission failed. Two AC systems with AC lines interconnection system when a short circuit occurs when one side, on the other side to deliver short-circuit current to one side of the fault. On both sides of the system the ability to switch off short-circuit currents are threatened and need to replace the switch. In DC, because SCR device, power can be quickly and easily adjusted, DC power lines communication system essentially to short circuit short circuit current, fault short-circuit current of the AC system and when there is no interconnection. So you don't have to replace the existing switches and current-carrying devices on both sides.
DC transmission lines, is adjusted independently and work at various levels, did not affect each other. So, when a failure occurs, simply failure of stopping, much can still be less than half the power of electricity. AC transmission lines, failure of any permanent, must be full blackout.