Power capacitors are widely used for reactive power compensation, harmonic filtering, carrier transmission, and high-frequency protection in power systems. In practical operating conditions, the voltage applied to power capacitors contains significant harmonic components, resulting in complex loss characteristics. To accurately characterize the active power losses of capacitors under harmonic voltages, a wideband distributed equivalent model based on the extended Debye model is proposed, taking into account the actual physical structure of the capacitor. Analytical expressions for the voltage and current distributions on the capacitor plates are derived, and their spatial characteristics are systematically analyzed. By considering both electrode (plate) losses and dielectric losses, the distribution of active power loss within the capacitor is investigated, and the corresponding rules are summarized. The results indicate that harmonic voltages increase the amplitude of the current flowing on the capacitor plates, leading to a nonlinear current distribution. Moreover, as the voltage frequency increases, the active power loss of the capacitor rises significantly, and in the high-frequency range, the plate losses become the dominant contributors to the total active power loss. Based on these findings, a loss reduction strategy is proposed, in which the number of voltage input points is increased to promote a more uniform electrode current distribution, thereby effectively reducing the active power loss of the capacitor.