Since an output of Photovoltaic generation (PV) depends on weather conditions, it is concerned that the introduction of a large amount of PVs causes voltage fluctuation in power system. For this reason, in recent years, many investigations on voltage issues in distribution systems have been reported. Most of them assume that primary voltage of a distribution substation is constant under the condition that the primary voltage is properly managed using several facilities in transmission system. However, when a large amount of PVs is introduced in the distribution system, it is expected that the primary voltage of the distribution substation largely fluctuates and hence voltage fluctuation significantly occurs in the upper transmission system. Therefore, it is necessary to analyze in detail how much the introduction of PVs in distribution systems influences the voltage fluctuation in the transmission system. In this paper, a power system model in which both transmission and distribution systems are included is proposed, and the voltage fluctuation in the transmission system is analyzed in case that a large amount of PVs is connected into the distribution systems. Moreover, a reactive power control method is proposed in order to reduce the voltage fluctuation in the transmission system using Static Var Compensator (SVC) installed in the primary side of the transformer at the distribution substation. The effectiveness of proposed method is verified using some numerical simulations.