The ancillary services markets for the electric power system have been increasing relevance the last decade. Inside, the secondary regulation market is increasing because the electric generation sources based on power electronics converters produces a decrease in the inertia of the electric power system, instability of the frequency and changes in the automatic generation control systems. This research evaluates the market power for secondary regulation reserve and quantify it through the Herfindhal-Hirschman index. The methodology of grey clustering and entropy weight allows distinguishing which generation companies gather the market power and quantify it according to five criterions: total duration accumulated during the annual regulation, average energy of the maximum registered reserve during the annual regulation, maximum reserve capacity registered during the annual regulation, minimum reserve capacity registered during the annual regulation and average reserve capacity during the annual regulation. Moreover, the result shows a multicriteria Herfindhal-Hirschman index calculation.