Microgrids' performance and stability mostly depend on power-flow control strategy. In order to allow for coordinated control while maintaining reliable operation, decentralized control methods based on P and Q droop characteristics have been utilized. Inherently, the power droop control methods have slow dynamics. In this paper, a novel control method based on V-I characteristics is introduced to exploit the flexibility and fast dynamics of the inverter-based distributed energy resources. In the proposed method, the direct and quadrature axis voltage components are drooped with the corresponding currents according to a piecewise linear droop function. Eigenvalue analysis of a sample microgrid shows that the proposed method features faster dynamics and improved damping compared to the conventional droop scheme. Simulation results are presented to verify the efficacy of the proposed method.

Unbalanced load currents not only give rise to unbalanced voltages but also adversely affect the performance of the conventional current-limiting mechanisms. The latter might result in overcurrent stress on the distributed energy resources (DERs) or current harmonics. In this paper, a novel decentralized control method is proposed to improve the power quality and protect DERs from overload. The proposed controller makes use of the model predictive control (MPC) technique to minimize the voltage unbalance, improve current limiting, and prevent active power overload. The MPC is combined with the V- I droop method to realize coordinated operation with fast dynamic response. The proposed method is tested on the CIGRE benchmark microgrid. Simulation results demonstrate that the proposed method improves power quality but also allows for operation close to the maximum load capacity without imposing DERs to overload.

This paper proposes a new cooperative control framework for coordination of energy storage units (ESUs), photovoltaic (PV) panels, and controllable load units in single-phase low voltage microgrids (MGs). The control objectives are defined and acted upon using a two level structure; primary and secondary control. Unlike conventional methods, a V-I droop mechanism is utilized in the primary control level. A distributed strategy is introduced for the secondary control level to regulate the MG voltage and manage state of charge (SoC) and power among the ESUs. The distributed secondary controllers are coordinated based on a leader-follower framework, where the leader restores the MG voltage to the rated value and the followers manage the sharing of power between the ESUs so as to balance the SoCs. Once the ESUs reach the minimum charge level, the information state increases above a positive critical value, at which point load control units perform load shedding. Similarly, fair PV curtailment is conducted in case the ESUs reach the maximum charge level. Experimental results are presented to demonstrate the efficacy of the proposed method.

This paper proposes a distributed control strategy for coordination of distributed energy resources (DERs) in low-voltage resistive microgrids. The proposed framework consists of two level structures; primary and secondary control. Unlike the existing distributed control solutions, the proposed method is based upon the practical assumption of network impedance being resistive. The primary control level consists of a V-I droop mechanism, where GPS timing is used to synchronize the control agents. A consensus-based distributed secondary control method is introduced to improve the voltage regulation and load sharing accuracy of the V-I droop method. In the proposed approach, the d-axis component of the voltage is altered so as to regulate the average microgrid voltage to the rated value while guarantying proper sharing of active power among the DERs. Additionally, the q-axis component of voltage is adjusted to perform proper current and, accordingly reactive power sharing. The proposed control methodology accounts for the distribution line impedances. It features a plug-and-play environment; prior system knowledge is not required, and an arbitrary DER can enter the microgrid without any need for additional synchronization. An ac microgrid is prototyped to experimentally demonstrate the efficacy of the proposed method.

In this paper, a distributed control scheme is proposed for voltage control and power management of coupled microgrids (MGs). The control scheme is implemented in a two-level structure. In the primary level, a voltage-current droop controller is adopted in the dq reference frame to facilitate proportional load sharing among the distributed energy resources (DERs) with fast dynamic response. A novel distributed secondary control scheme is introduced to regulate the power exchange among MGs and restore the average voltage to the nominal value. The secondary controller is comprised of a network of control agents interconnected through low bandwidth communication links. Each control agent alters the offset of d and q axis droop characteristics to change the current output of the DERs within each cluster until the current exchanges among the clusters are regulated at the set points and the average voltage is restored to the nominal value. If the load of an MG cluster is increased above its total generation capacity, the secondary controller alters the current exchange among the clusters to prevent overload. Simulation and experimental results are presented to demonstrate the efficacy of the proposed method.

Coordinated control of distributed energy resources (DER) is essential for the operation of islanded microgrids (MGs). Conventionally, such coordination is achieved by drooping the frequency of the reference voltage versus active (or reactive) power. The conventional droop method ensures synchronized operation and even power sharing without any communication link. However, that method produces unwanted frequency fluctuations, which degrade the power quality. In order to improve the power quality of islanded MGs, a novel decentralized control method is proposed in this paper. In this method, the GPS timing technology is utilized to synchronize the DERs to a common reference frame, rotating at a nominal frequency. In addition, an adaptive Q-f droop controller is introduced as a backup to ensure stable operation during GPS signal interruptions. In the context of the common reference frame, even sharing of active (i_d) and reactive (i_q) components of the current are achieved based on v_d - i_d and v_q - i_q droop characteristics. The method has been tested on a laboratory-scale MG. Experimental results demonstrate the efficacy of the proposed method in terms of dynamics, power quality, and robustness with respect to GPS interruptions.