New expression of operation margin for mho relays is established by decomposing the line flow to the line impedance vector. Preventive control model based on the sensitivities of operation margin to power injection is proposed with the help of load flow Jacobian matrix. Based on the simulation results on IEEE 14-bus test system

it is found that: (1) the operation margin can be effectively controlled via sensitivity method; (2) the active and reactive power control at the same bus will yield different or even opposite control effects; (3) the proposed method is especially effective for stressed power system; (4) the relative control error compared with that of load flow method is very small; (5) the calculation efficiency is much higher than that of load flow method.