It becomes an important issue to maintain voltage profile within the allowable range in distribution systems with a large number of distributed generators (DGs), such as Photovoltaic (PV). Although reactive power control by power conditioning system (PCS) is effective to mitigate the voltage increase, there is a possibility that active power curtailment is needed to provide enough amount of reactive power due to current limitation. Here, it is expected that the active power curtailment can be avoided if a cooperative voltage control between load ratio tap controller (LRT) and PCS is realized. To this end, system operators have to know the amount of reactive power controlled by PCS. Hence, in this paper, we developed a new state estimation method of the reactive power using the measurement data obtained by the switchgears with sensors. In addition, a probabilistic approach was applied because an accurate estimation is theoretically impossible only by using the limited number of measurement data. The proposed method was tested in a hierarchic distribution system model and its effectiveness was shown through simulations.