This paper presents the impact of photovoltaic power plants on distribution grid for voltage stability and economic values by using the central region of Thailand as the simulation system. This region has maximum load of demand with high potential of solar energy development, through varying load characteristic and solar characteristic which used actual existing data parameters from the current photovoltaic power plant in Thailand. Besides, the selected central region system without photovoltaic power plant is simulated as a base case then the contained photovoltaic power plants are also installed to investigate the impact of each case on distribution grid in term of the voltage stability and the total system energy losses by varying six connected buses and three installed distance locations. The simulation results show that the optimal location of photovoltaic power plant can enhance the economic operation of distribution utility over 12.3 million units a year at the same time the voltage stability does not has any effect to distribution grid. Finally, the impact of photovoltaic power plant should be studied before approve the installed location to optimize the social benefit.