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Along With the increased installation of the distributed energy resources (DER) in distribution system, long-term voltage stability at the transmission level can be improved if proper coordination is developed between DERs and the existing grid controllers, e.g., load tap changer (LTC). This paper analyzes the problem of long-term voltage instability and presenting possible countermeasures. The study summarizes essential aspects for modelling, including LTC, DERs, and especially voltage dependent load. A coordination between DERs and LTC is proposed to allocate better power supply, especially at the coupling points between transmission and distribution networks, i.e. primary substation. A case study is performed using a modified CIGRE medium-voltage benchmark network. The simulation results have shown that the coordination has substantially improved the long-term voltage stability in the way of using …