In this manuscript, an off-grid or isolated microgrid (IMG) model has been considered, which consists of renewable power generating sources (RPGS) like solar photovoltaic (PV), solar thermal, wind and fuel cell (FC) along with diesel engine generator (DEG) and battery energy storage system (BESS). The paper emphasizes the analytical study of the impact of RPGS in the frequency control of IMG system. In an IMG integrating distributed energy sources (DES), proper frequency regulation is important because some RPGS do not take part in the frequency control technique and depend on weather conditions like solar insolation and wind speed. For frequency control of the IMG system, a integral controller (LQR-I) has been used Linear quadratic regulator with an for its robust characteristics. For designing the LQR-I controller, the IMG system has been represented as a state-space model. The effectiveness of the proposed LQR-I controller has been validated through the dynamic response of the IMG system for a small load perturbation. Robustness of the controller has been verified by sensitivity analysis of the system for varying IMG system parameters such as K-WTG, K-pv, K-s, K-T, K-AE, K-FC, K-DEG, K-BESS, T-FC, K-p, T-p as well as randomly varying load, varying wind speed and seasonal variation of solar insolation level. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.