In order to quickly eliminate overload caused by power flow transfer

an emergency control strategy based on virtual control units and heuristic search is proposed. Firstly

the concept of virtual control unit is introduced

i. e. the control unit that can completely eliminate the overload

whose bus power adjustment equal to maximum overload of lines. Then

the two-level search strategy is presented to find the optimal control unit. At the first level

by setting a rational threshold

the bus pairs with stronger load shedding capacity is selected to create the priority control unit set. At the second level

the heuristic searching procedure based on virtual control units is given to obtain the optimal controllable unit within the priority control unit set

where the load shedding effectiveness and load shedding capacity are integrated. If the control unit can not be found in the above threshold range

the first level search scope should be appropriately enlarged until satisfactory control units are found. Verification of power flow is avoided by taking into account the normal line redundancy constraining the adjustment calculated. The power flow of lines and bus power are amended after a tune-up according to sensitivity relations

while the control speed is improved. The simulation on an IEEE 30-bus system has verified the effectiveness of the proposed algorithm.