This study presents a non-linear integral higher-order sliding mode controller (I-HOSMC) design approach to ensure islanded operations of T-type three-phase inverters which are interfaced with distributed energy resources (DERs). The main control objective is to ensure the desired voltage level during islanded operations of DERs. The proposed controller is designed to provide robustness against external disturbances and reduce chattering while guaranteeing the overall stability of the system. The performance of the proposed controller is analysed under different balanced and unbalanced loading conditions by considering both linear and non-linear loads. Simulation results demonstrate the effectiveness of the proposed I-HOSMC as compared to different existing standard sliding mode controllers and other control strategies.