Network parameter errors of transmission line grounding capacitance may influence the performance of the optimal reactive power flow and voltage control system in an energy management system（EMS）. An approximate grounding capacitance estimation method is briefly addressed and the applicable conditions of the method are discussed in detail. Then， a new idea of grounding capacitance estimation based on the reactive power flow measurement transposition method is presented. The new method has taken into account both the reactive power exchange with system and the reactive power loss. The performance of the estimation method is not affected by the grounding capacitance error in the transmission line while tolerating definite errors in the transmission line impedance parameters. Test results of the novel method show that the method proposed has good prospects for application.