In this paper, a new online capacitance estimation
method for dc-link capacitors in a three-phase ac/dc/ac
pulsewidth-modulation converter is proposed. A controlled ac
voltage with a lower frequency than the line frequency is injected
into the dc-link voltage, which then causes ac power ripples at
the dc output side. By extracting the ac power component on the
dc output side using digital filters, the capacitance can then be
calculated using a support vector regression method. A function
that defines the relation between a given capacitor power and its
corresponding capacitance is determined using a set of training
data. This function is then used to predict the output for the given
input which is not included in the training set. The proposed
method can be simply implemented with only software and no
additional hardware. Experimental results confirm that the estimation
error is less than 0.15%.