Abstract: It is difficult to meet both the voltage and reactive control demands between provincial and prefecture power networks in extreme heavy load flow mode.The voltage mismatch of middle voltage side of the transformer is proposed as a form of the mismatching of voltage and reactive control between provincial and prefectural power networks.A new model to evaluate the mismatch risk is proposed and the voltage mismatch criterion is considered in this model.An anti-mismatching strategy is proposed to avoid the mismatch of control measures in the prefectural dispatch center.A real-time slack control optimal method is proposed to meet both the voltage and reactive control demands between provincial and prefectural power networks.With this method,the getaway reactive power,getaway voltage and voltage of the middle voltage side of the transformer are taken as control parameters to realize real-time slack optimization of the running state of power system without the reactive power optimization of the whole network.Simulation results of applying the proposed method to a certain regional power grid show that the proposed method not only has the advantage of real-time and high reliability,but can ensure security and economics of the power system.