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Electrical energy storage (EES) can improve the flexibility and reliability of electric power systems. At the same time, they can supply different ancillary services. The profit of the energy storage operation can be maximized by deciding the best level of each service. Merchant-owned facilities require a profit-maximizing formulation for grid-connected energy storage systems with multiple ancillary services. This paper proposes a new linear profit-maximizing formulation for grid-connected merchant-owned energy storage systems operating with multiple ancillary services. All technical characteristics of EES have been modelled, including cycle life loss due to fast charge/discharge and low depth of discharge operations. A piece-wise linear model of an EES converter's capability curve has also been included for reactive power modeling. The model was assessed considering a battery EES, a flywheel EES, and a compressed air EES, with the results demonstrating the benefits of the formulation. From case studies, it is clear that the merchantowned battery EES and the flywheel EES can generate profits, especially from voltage regulation and frequency regulation services. The case studies prove that the proposed model can be used as an optimal planning and operation tool for any type or size of EES. INDEX TERMS Energy storage, ancillary markets, energy storage scheduling. NOMENCLATURE A. ACRONYMS DOD Maximum depth of discharge of the EES operation EES Electrical energy storage SOC State of charge of the EES B. INDICES i Index for denoting sections of the cycle life vs DOD curve q Index for denoting quarters t Index for denoting hourly time periods of the year, t = 1 : NT C. PARAMETERS A q Factor for equally distributing investment cost for all quarters of all years (unitless factor) c 1 , c 2 , c 3 Constants for representing operation and maintenance cost equation

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