This paper aims at specifying the optimal allocation of a hybrid supercapacitor-vanadium redox flow battery (VRB) energy storage system (ESS) for maintaining power balance of active distribution networks (ADNs) for wind power applications. Correspondingly, an optimal allocation approach for the hybrid ESS considering the application scenarios of supercapacitor ESS and VRB ESS was proposed. The supercapacitor ESS was used for restraining the high-frequency components of net load power and a net load power spectrum analysis method based on dividing the variations of net load power was used to obtain the optimal capacity of supercapacitor ESS. For the allocation of hybrid ESS, a mathematical framework was proposed considering both the economy and reliability of ADNs associated with VRB dynamic operation characteristics. Finally, the proposed optimal allocation approach was validated and tested by a modified IEEE 33-bus system. Test results have demonstrated the effectiveness of the proposed optimal allocation approach associated with the power flow operation characteristics of ADNs.