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Abstract--This paper presents the analysis and the control of shunt-compensator for improving unbalanced voltages of power distribution systems in Thailand. The shunt-compensator is connected to the low voltage level in power distribution system. The analysis detail and the design of shunt-compensator obtain by using the voltage vector control (mathematical model with the stationary reference frame and synchronous reference frames). The unbalanced voltages are identified by using positive and negative sequence detection that can be shown in the synchronous reference frame. This method also presents the decoupling control including feed-forward control to compensate both positive and negative sequence component of grid voltage. The power circuit of the shunt-compensator and the distribution network are modeled by Provincial Electricity Authority (PEA) of Thailand that is the same as worldwide standard of power distribution system. By using computer program, the simulation results showed the benefit and the performance of proposed methodology which obviously ensure the performance of shunt-compensator for mitigating the unbalanced voltages. Index Terms-- Shunt-compensator, synchronous reference frames, faults, unbalance and decoupled control