AST-411 digital transformer differential protection device is suitable for Volumes I and II of 35kV and below voltage substation main transformer, as the transformer primary protection. Main functions: a) differential quick-break protection function: split-phase differential protection, the operating time is not greater than 25ms; b) differential protection function: split-phase differential protection (with harmonic restraint, the ratio of system dynamic characteristics and the low-pressure side of the CT transformation ratio adjustment function); c) CT disconnection detection and differential current is too large alarm function: real-time detection of CT disconnection, locking differential protection, run-time when the differential current limit alarm; d ) body protection function: the action of the heavy gas, the heavy gas regulator action, light gas alarm, the regulator light gas alarm; e) to start the ventilation function: When the high-pressure side of the B-phase current is greater than the the start ventilation current setting start the ventilation outlets; e ) event logging, fault recording; g) of CAN data communication functions.