Features of flowgates in real-life power systems are analyzed， and definitions of initial flowgates， transmission flowgates and critical flowgates are proposed as well. A method to discover flowgates automatically based on electrical zone division is proposed. Firstly， electrical zone division of power system is obtained from real-time power flow solutions， and the initial flowgates which are the minimal cut sets of power grid are obtained based on graph theory. Then， transmission flowgates can be calculated from the initial flowgates based on the constraints of power flow direction and line outage distribution factor（LODF）. Finally， the critical flowgates can be discovered based on the security margin of transmission flowgates. Tests on two real-life provincial power systems validate that the method can effectively locate not only the flowgates given by experts but also new flowgates. The test results are reasonable and comprehensive， which show the method has strong adaptability to changes in power system operations. At the same time， the tests show that flowgates of a provincial power system can be discovered in less than one minute. This effectively validate that the proposed method can be applied in real time.