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Remarkable advancement in wave energy conversion technology has taken place in recent years. Due to its simplicity, the Wells turbine has been one of the most widely used power take-off mechanisms in an oscillating water column type wave-energy conversion device. However, the turbine suffers from several challenges due to its narrow operating range, which hinders the commercial feasibility of the system. Several aerodynamic applications have successfully used passive control methods to modify the flow conditions. This work applied a combination of stall fences and casing grooves for passive flow control of a Wells turbine. The computational fluid dynamics (CFD) technique is used to analyze the modified turbine numerically. The casing groove modified the tip-leakage vortices, interacted with local vortices created by the stall fences, and helped reattach the flow at higher flow coefficients. As a result, the modified turbine increases the operating range up to 33.3%. In addition, the peak-to-average (PTA) power ratio decreased by up to 27.7%.