The main goal of the Load Frequency Control (LFC) system is to reduce the deviation of frequency and stabilize the power system against disturbances. In this paper, a Variable-Order interval type-II Fuzzy Fractional PID (VOFFPID) controller with hybrid optimization is proposed for frequency control of multi-source two-area interconnected power system. Interval type-II Fuzzy Inference System (FIS) determines the coefficients of the Fractional Order PID (FOPID) controller and the derivative and integral order online, to give a higher degree of freedom to the controller. This model includes hydro, thermal and gas generation in each area. To improve the performance of the system, the initial control parameters of the FOPID controller have been obtained using Whale Optimization Algorithm (WOA). Moreover, Stochastic Gradient Descent (SGD) is used to optimize the consequent part of the fuzzy controller during control procedure. To verify the robustness of the proposed control system, the designed controller is applied in the presence of a wide range of disturbances, uncertainties and nonlinearity. The controller is capable of guaranteeing the integrity since when the controller of one area is dropped, the other controller can control the whole system.