In Step 2, a frequency doubling operation is performed on the phase-locked results sin θ cos θ of the power conversion cell to generate a phase of the Mth-order AC harmonic of the power conversion cell, as shown in step 2 of FIG. 5. Here, the frequency doubling operation may be performed according to a formula, such as cos 2θ=2 cos 2θ-1, sin 2θ=2 sin θ cos θ, etc.
In step 3, the first information di of the power conversion cell for which the mean value is to be obtained (or the first information di multiplied by an agreed coefficient K, K is a positive number, for example, any positive number such as 1, 1.5, 3) is set as the amplitude of the Mth-order AC harmonic of the power conversion cell, as shown in step 3 of FIG. 5. The first information di represents a variable value of the first information of the i-th (n=1, 2, 3 . . . ) power conversion cell. The first information may be, for example, a value of an actual physical quantity such as the voltage, the current, the temperature or the power of the corresponding power conversion cell, or an integral value of any physical quantity obtained by the controller of the corresponding power conversion cell (for example, the integral value Intvdci of the voltage at the first terminal of the corresponding power conversion cell in FIG. 8), or a value of a virtual physical quantity such as a state of a battery, but the present disclosure is not limited thereto.
