In a DC microgrid (DCMG), the DC bus voltage is vulnerable to the power fluctuations caused by the variations of intermittent renewable energy sources and local loads. A concept is developed to operate a bidirectional DC-DC converter as a DC machine to mimic the inertia characteristic of a DC machine and regulate the DC bus voltage fluctuations during power fluctuations of the DCMG. The concept of operating a bidirectional DC-DC converter as a DC machine can be termed as virtual DC machine (VDCM). The proposed concept of a VDCM is basically a control technique for a bidirectional DC-DC converter, which is derived from the theory of the DC machine operation and its speed control technique. The implementation, operation, controller design and dynamics of VDCM are also discussed. A VDCM can be easily operated in both generating and motoring mode also. Being free from comparatively slower mechanical devices, a VDCM can switch over from one mode to another very quickly. Hence, it provides a useful way to control the storage devices in microgrid applications.

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In this study, energy management and damping improvement of a DC microgrid are proposed by using an interconnection and damping assignment-passivity-based control (IDA-PBC) technique. The control technique is applied for (i) the grid connected inverter and (ii) the DC-DC converters connected with fuel cell and battery of the DC microgrid to control the DC bus voltage. The IDA-PBC generates the control laws with the desired energy function for the current controller of the converters based on the generation from the renewable power generators and load demand. In addition, an integral action is added with the IDA-PBC control laws to reduce the steady-state error in the DC bus voltage. The parameters of the IDA-PBC technique are tuned based on the state of charge of the battery and grid availability for a smooth transition between the operating modes and better energy management. It is proved that the derived energy function satisfies the Lyapunov stability criterion. Simulations are carried out in MATLAB/Simulink to test the performance of the proposed control technique. The results show that the proposed technique provides the desired damping and energy management.