In wind turbine generators system, lacking of inertia is a common problem. In AC collecting system, some researches have been done to solve this problem. However, in DC collecting system, few researches were done. This paper analyzes the analogies between AC and DC collecting system and the virtual capacitor control corresponding to the virtual synchronous generator control. Small signal models of different precision are proposed considering the LLC resonant converter used for stepping-up. The virtual capacitance provided by the control is calculated using the small signal model and this verifies the effectiveness of the control quantitatively and demonstrates that the small signal models are useful.