Three-phase Buck-type dynamic capacitor（D-CAP）

in case of normal reactive power compen-sation will cause resonance under influence of background harmonics of grid. In this paper

the basic struc-ture and principle of Buck type D-CAP is analyzed

the reason for the resonance generation is analyzedthrough the modeling of self-impedance of D-CAP and impedance of grid and the corresponding resonancepoint is calculated. At the same time

as for the harmonics near its resonance frequency

it is proposed tobuild the feedback loop by way of detecting capacitance and voltage of D-CAP so to achieve the controlmethod of active resonance damping. Finally

the achievement of composite control strategy for harmonicsuppression and resonance damping by the D-CAP is proposed at the same time of performing reactive pow-er compensation. The current harmonic distortion of grid side due to resonance is remarkably suppressedthroughout the strategy

and the current harmonic contents at the grid side reaches power quality standard.The correctness of theoretical analysis is verified by the experimental result.