In this paper, a coordinated droop control method through virtual voltage axis is proposed for voltage restoration and energy management of DC microgrids. Droop control is a control method that operates a DC microgrid using only local information. However, there are several drawbacks in that output voltage deviations occur, and the utilization priorities of various sources are not considered. To solve these problems, the voltage compensation term is defined as the virtual axis voltage value. It can minimize the voltage deviation of the DC microgrid and enable energy management by determining the operation band. However, the value of the virtual voltage axis is affected by communication delays. Therefore, its influence on the values of the virtual voltage axis was analyzed. The reference voltage and the droop gain of the droop control were designed, and the validity of the proposed method was verified using a lab-scale DC microgrid.

A distributed control method is proposed to improve the power sharing performance of bidirectional distributed generators and the voltage regulation performance of a dc bus in a dc microgrid. To analyze the structural characteristics of a dc microgrid, voltage sensitivity analysis based on power flow analysis is presented. Based on this analysis, a distributed control method using a voltage sensitivity matrix is proposed. The proposed method uses information received through the communication line and performs the droop gain variation method and voltage shifting method without additional controllers. This approach achieves improved power sharing and voltage regulation performance without output transient states, even at slow communication speed. The proposed method is implemented through an experimental environment consisted of two bidirectional distributed generators, a load, and a non-dispatchable distributed generator in a four-bus ring type model. The experimental results demonstrate improved power sharing accuracy and voltage regulation performance.

In this paper, impedance modeling of a DC microgrid system consisting of a source and load converter, including an input filter, is performed. Impedance-based modeling has been used to derive mathematical models of the output impedance of the source converter and the input impedance of the load converter. The correlation between the converter interaction and system stability is analyzed based on the mathematical model. An impedance-based stability analysis is used to determine the system stability by analyzing the interactions among the converters in the DC microgrid system. Middlebrook’s stability criterion, which uses the impedance transfer function, is applied to determine system stability. Moreover, in this paper, a stability enhancement control algorithm is proposed to resolve the system instabilities resulting from interaction among the converters and the distortion caused by the harmonics emanating from the AC input. The proposed stability enhancement control algorithm consists of a feed-forward type virtual impedance (VI) and a proportional-resonant (PR) controller. The validity of the proposed method is demonstrated by the results of the response characteristics in the frequency domain, and the effectiveness of the proposed control algorithm is verified via simulations and prototype experimental models.

Compared to the conventional boost power factor correction (PFC) converter, a totem-pole bridgeless PFC has high efficiency because it does not have an input diode rectifier stage, but a current spike may occur when the polarity of the grid voltage changes. This paper proposes a digital control algorithm for bridgeless totem-pole PFC with a simple control structure based on the phase angle of grid voltage. The proposed algorithm has a PI-based double-loop control structure and performs DC-link voltage and input inductor current control. Rectifying switches operate based on the proposed rectification algorithm using phase angle information calculated through a single-phase phase-locked loop (PLL) to prevent current spikes. The feed-forward duty ratio value is calculated according to the polarity of the grid voltage and added to the double-loop controller to perform appropriate power factor control. The performance and feasibility of the proposed control algorithm are verified through a 3 kW hardware prototype.

In this article, a new control structure is proposed to improve the scalability of dc microgrids. Because conventional control structures require implementation of higher control level, subsystems that do not include them cannot be added in a microgrid. However, there are many subsystems in the field that do not support higher control level, which hinders microgrid scalability. For such subsystems to perform these functions, their local controller should be modified, which requires a lot of time and economic effort. In this article, by adding a new controller that performs higher control level without changing the local controller of subsystems, a structure to expand the scalability of a dc microgrid is proposed. The proposed flexible control structure consists of a local controller of subsystems, a flexible controller that performs higher control level, and microgrid central controller. A new communication network is required between local controller and flexible controller. However, due to a communication delay, the overall system stability is reduced. Therefore, problems related to the additional communication network were analyzed and a compensator was designed to overcome these issues. Through the flexible control structure, a subsystem that does not support higher control level can be added to dc microgrid.