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In this paper, the implementation of inverter-dominated dynamic microgrids (MGs) in hybrid AC/DC distribution systems is discussed. Compared to conventional networked MGs structure, dynamic MGs operate with varying electric boundaries, which enables more flexible system operating topologies and power flow options. A control framework is developed for hybrid AC/DC distribution systems. The entire feeder is sectionalized into multiple dynamic MGs that interact at varying points of interconnection (POIs). A set of distributed controllers is proposed to coordinate multiple inverter-interfaced distributed generators (DGs) and interlinking converters (ICs) across the hybrid AC/DC feeders under autonomous operation. Various control modes are developed for the system to operate under either static topology or reconfigure as requested. Secondary regulations over the system operating frequency and voltage are enabled in both AC and DC subgrids, along with proportional power sharing among DGs. Dedicated control efforts are developed to achieve seamless system topology transitions, and the operational stability of the proposed controllers is analyzed. The performance of the proposed work in augmenting the operation resiliency of a hybrid AC/DC test feeder is validated using comprehensive case studies. [ABSTRACT FROM AUTHOR]

The state-of-the-art dynamic consensus-based microgrid (MG) secondary controls suffer from the communication delay effect. Specifically, the system could not converge to the desired operating points with time-delayed communications. Such deviations are hard to detect in a decentralized manner and could destabilize the system. This paper proposes an accurate distributed secondary controller for DC MGs based on the surplus consensus algorithm. The proposed controller achieves accurate proportional power sharing and average voltage regulation among distributed generators (DGs) with the presence of variable and bounded communication delays. A surplus consensus-based observer is developed. The developed observer is proved robust against variable and bounded communication delays; it tracks the average of a group of dynamic states with zero steady-state deviations, which cannot be done using the conventional dynamic consensus-based observer. The convergence speed of the developed observer is analyzed and a parameter design procedure is presented. Moreover, the delay-dependent stability analysis of DC MG operation with the proposed secondary controller is derived. The marginal delay that leads the system to instability is calculated. At last, the performance of the proposed secondary controller and the developed stability analysis are validated under various scenarios using MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

Network reconfiguration (NR) has received attention due to its potential to improve grid resilience by realizing self-healing microgrids (MGs). This paper proposes a new strategy for the real-time frequency regulation of a reconfigurable MG, wherein the feedforward control of synchronous and inverter-interfaced distributed generators (DGs) is achieved in coordination with the operations of sectionalizing and tie switches (SWs). This enables DGs to compensate more quickly, and preemptively, for a forthcoming variation in load demand due to NR-aided restoration. An analytical dynamic model of a reconfigurable MG is developed to analyze the MG frequency response to NR and determine the desired dynamics of the feedforward controllers when integrated with feedback loops for inertial response emulation and primary and secondary frequency control. A small-signal analysis is conducted to analyze the contribution of the feedforward control to the MG frequency regulation. Simulation case studies of NR-aided load restoration are also performed. The results of the small-signal analysis and case studies confirm that the proposed strategy is effective for the improvement of the MG frequency regulation under various conditions of load demand, model parameter errors, and communication time delays. [ABSTRACT FROM AUTHOR]

Microgrids (MGs) with dynamic boundaries provide an alternative solution to serve distribution systems without energization from the utility. Adopting the concept of dynamic MGs enables a flexible operation of service restoration toward resilient distribution networks by changing system topology through smart switches and conventional circuit breakers. This work proposes a novel restoration framework including three control phases including initial, restorative and corrective actions for the distribution network (DN) with full utilization of the flexibility of distributed generators in various operational modes. Comparing with existing works that only consider MGs as a constant negative load and ignore the dynamics of different components within MGs, each MG is equipped with a model predictive control (MPC)-based controller that is responsible for optimally scheduling available power to the DN based on its local generation and load demand. Aided by the MPC framework, in this article, the conventional restoration strategy is extended that further includes the operational dynamic behavior of MGs during restoration process guaranteeing a more flexible and reliable operation. Several numerical cases are studied using the IEEE 8500-node test system to verify the effectiveness and scalability of the proposed framework. [ABSTRACT FROM AUTHOR]

The extensive usage of open communication mechanisms in distributed secondary control of islanded microgrids (MGs) suggests that the effects of the communication network and time delays on system performance cannot be negligible. Based on the relationship between convergence and algebraic connectivity, this paper proposes an optimal design for a directed communication topology and pinning location through the mirror operation and global propagation rates, respectively. Alternatively, a small-signal dynamic model of an MG equipped with time-delayed distributed secondary voltage control is derived, to investigate the delay-dependent stability of MG system in terms of the controller, network and pinning conditions. An analytical formula is then developed to determine the individual delay margins with respect to different sets of controller gains and pinning. Through a series of trials, the qualitative effects of controller parameters and pinning conditions on delay margins can be used to guide the design of both controllers and pinning for the improved system performance. The effectiveness of proposed methodology is verified via the real-time hardware-in-the-loop (HIL) simulations. [ABSTRACT FROM AUTHOR]

To enhance the redundancy and reliability for a distributed generation system, a grid-tied photovoltaic (PV) generation system based on series-connected module-integrated inverters (SC-MIIs) is presented in this paper. In the grid-tied SC-MII system, each PV panel is interfaced with an MII with independent maximum power point tracking to harvest maximum solar energy. The outputs of MIIs are at ac line frequency and are connected in series to fulfill the voltage requirement in the utility grid. Since the high step-up power conversion stage in the conventional microinverter is avoided, the grid-tied SC-MII system is easier to implement and features high efficiency. Meanwhile, a distributed control strategy for SC-MIIs is proposed, in which the active power sharing among the MIIs depends on the individual maximum power available from PV panels, while the reactive power of the system can be regulated by any of the MIIs according to the utility grid command. Simulation and experimental results verify the feasibility and effectiveness of the proposed system and its corresponding control strategy. [ABSTRACT FROM AUTHOR]

In this paper, a new strategy for service restoration of de-energized loads is proposed using advanced technologies of microgrids (MGs) that accommodate distributed energy resources (DERs). In particular, backup distributed generators (DGs), initially used for critical loads within a building, have recently been operated under normal grid conditions while exporting excess power to other loads on the same or different feeders. In addition, smart switches (SSWs) have been installed in a distribution grid to reduce the frequency and duration of power outages. Using flexible communication links, the sensors of the SSWs exchange control and measurement signals with grid management systems, namely, an advanced distribution management system, an MG energy management system, and a DER management system. This paper focuses on developing a methodological framework to determine the operating modes of synchronous-machine DGs and the on–off operation of SSWs, so as to change dynamically the boundaries of MGs that are formed in a distribution grid for fault isolation and load restoration. Simulation case studies demonstrate that the proposed strategy is effective in mitigating the influence of a network fault and restoring de-energized loads while successfully maintaining frequency and voltage levels in MGs. [ABSTRACT FROM AUTHOR]

Today, interests on hybrid ac/dc microgrids, which contain the advantages of both ac and dc microgrids, are growing rapidly. In the hybrid ac/dc microgrid, the parallel-operated ac/dc bidirectional interfacing converters (IFCs) are increasingly used for large capacity renewable energy sources or as the interlinking converters between the ac and dc subsystems. When unbalanced grid faults occur, the active power transferred by the parallel-operated IFCs must be kept constant and oscillation-free to stabilize the dc bus voltage. However, under conventional control strategies in unbalanced grid conditions, the active power transfer capability of IFCs is affected due to the converters’ current rating limitations. Moreover, unbalanced voltage adverse effects on IFCs (such as output power oscillations, dc-link ripples, and output current enhancement) could be amplified by the number of parallel converters. Therefore, this paper investigates parallel operation of IFCs in hybrid ac/dc microgrids under unbalanced ac grid conditions and proposes a novel control strategy to enhance the active power transfer capability with zero active power oscillation. The proposed control strategy employs a new current sharing method which introduces adjustable current reference coefficients for parallel IFCs. In the proposed control strategy, only one IFC, named as redundant IFC, needs to be designed and installed with higher current rating to ensure the constant and oscillation-free output active power of parallel IFCs. Simulation and experimental results verify the feasibility and effectiveness of the proposed control strategy. [ABSTRACT FROM PUBLISHER]

This paper considers an optimal control problem to improve dc microgrid stability while minimizing its operation cost. A dc microgrid consists of various components, such as renewable energy sources, loads, and power lines. Every component may change its role during operation by switching to a different mode in real time. A switched system approach is employed to ensure the stability of a dc microgrid with a rich array of operation modes. Meanwhile, an optimal control algorithm is designed to improve the system performance by appropriately selecting the operation modes. A typical dc microgrid with three source buses and one load bus is implemented. The effectiveness of the algorithms is verified by MATLAB/Simulink time-domain tests and numerical studies. [ABSTRACT FROM PUBLISHER]

This paper proposes an improved distributed secondary control scheme for dc microgrids (MGs), aiming at overcoming the drawbacks of conventional droop control method. The proposed secondary control scheme can remove the dc voltage deviation and improve the current sharing accuracy by using voltage-shifting and slope-adjusting approaches simultaneously. Meanwhile, the average value of droop coefficients is calculated, and then it is controlled by an additional controller included in the distributed secondary control layer to ensure that each droop coefficient converges at a reasonable value. Hence, by adjusting the droop coefficient, each participating converter has equal output impedance, and the accurate proportional load current sharing can be achieved with different line resistances. Furthermore, the current sharing performance in steady and transient states can be enhanced by using the proposed method. The effectiveness of the proposed method is verified by detailed experimental tests based on a 3 × 1 kW prototype with three interface converters. [ABSTRACT FROM AUTHOR]

This paper presents a review of control strategies, stability analysis, and stabilization techniques for dc microgrids (MGs). Overall control is systematically classified into local and coordinated control levels according to respective functionalities in each level. As opposed to local control, which relies only on local measurements, some line of communication between units needs to be made available in order to achieve the coordinated control. Depending on the communication method, three basic coordinated control strategies can be distinguished, i.e., decentralized, centralized, and distributed control. Decentralized control can be regarded as an extension of the local control since it is also based exclusively on local measurements. In contrast, centralized and distributed control strategies rely on digital communication technologies. A number of approaches using these three coordinated control strategies to achieve various control objectives are reviewed in this paper. Moreover, properties of dc MG dynamics and stability are discussed. This paper illustrates that tightly regulated point-of-load converters tend to reduce the stability margins of the system since they introduce negative impedances, which can potentially oscillate with lightly damped power supply input filters. It is also demonstrated that how the stability of the whole system is defined by the relationship of the source and load impedances, referred to as the minor loop gain. Several prominent specifications for the minor loop gain are reviewed. Finally, a number of active stabilization techniques are presented. [ABSTRACT FROM AUTHOR]

DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of dc distribution when compared to its ac counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources, and electronic loads and energy storage systems. With rapid emergence of these components in modern power systems, the importance of dc in today's society is gradually being brought to a whole new level. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular, and distributed power systems can be classified under dc MG framework and ongoing development, and expansion of the field is largely influenced by concepts used over there. This paper aims first to shed light on the practical design aspects of dc MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in dc MG protection and grounding is provided. Owing to the fact that there is no zero-current crossing, an arc that appears upon breaking dc current cannot be extinguished naturally, making the protection of dc MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes, which discusses both design of practical protective devices and their integration into overall protection systems, is provided. Closely coupled with protection, conflicting grounding objectives, e.g., minimization of stray current and common-mode voltage, are explained and several practical solutions are presented. Also, standardization efforts for dc systems are addressed. Finally, concluding remarks and important future research directions are pointed out. [ABSTRACT FROM AUTHOR]

In this paper, a converter-based dc microgrid is studied. By considering the impact of each component in dc microgrids on system stability, a multistage configuration is employed, which includes the source stage, interface converter stage between buses, and common load stage. In order to study the overall stability of the above dc microgrid with constant power loads (CPLs), a comprehensive small-signal model is derived by analyzing the interface converters in each stage. The instability issue induced by the CPLs is revealed by using the criteria of impedance matching. Meanwhile, virtual-impedance-based stabilizers are proposed in order to enhance the damping of dc microgrids with CPLs and guarantee the stable operation. Since droop control is commonly used to reach proper load power sharing in dc microgrids, its impact is taken into account when testing the proposed stabilizers. By using the proposed stabilizers, virtual impedances are employed in the output filters of the interface converters in the second stage of the multistage configuration. In particular, one of the virtual impedances is connected in series with the filter capacitor, and the other one is connected at the output path of the converter. It can be seen that by using the proposed stabilizers, the unstable poles induced by the CPLs are forced to move into the stable region. The proposed method is verified by the MATLAB/Simulink model of multistage dc microgrids with three distributed power generation units. [ABSTRACT FROM PUBLISHER]

In this paper, a double-quadrant state-of-charge (SoC)-based droop control method for distributed energy storage system is proposed to reach the proper power distribution in autonomous dc microgrids. In order to prolong the lifetime of the energy storage units (ESUs) and avoid the overuse of a certain unit, the SoC of each unit should be balanced and the injected/output power should be gradually equalized. Droop control as a decentralized approach is used as the basis of the power sharing method for distributed energy storage units. In the charging process, the droop coefficient is set to be proportional to the n th order of SoC, while in the discharging process, the droop coefficient is set to be inversely proportional to the n th order of SoC. Since the injected/output power is inversely proportional to the droop coefficient, it is obtained that in the charging process the ESU with higher SoC absorbs less power, while the one with lower SoC absorbs more power. Meanwhile, in the discharging process, the ESU with higher SoC delivers more power and the one with lower SoC delivers less power. Hence, SoC balancing and injected/output power equalization can be gradually realized. The exponent n of SoC is employed in the control diagram to regulate the speed of SoC balancing. It is found that with larger exponent n , the balancing speed is higher. MATLAB/simulink model comprised of three ESUs is implemented and the simulation results are shown to verify the proposed approach. [ABSTRACT FROM PUBLISHER]

This paper presents the coordinated control of distributed energy storage systems in dc microgrids. In order to balance the state-of-charge (SoC) of each energy storage unit (ESU), an SoC-based adaptive droop control method is proposed. In this decentralized control method, the droop coefficient is inversely proportional to the nth order of SoC. By using a SoC-based droop method, the ESUs with higher SoC deliver more power, whereas the ones with lower SoC deliver less power. Therefore, the energy stored in the ESU with higher SoC decreases faster than that with lower SoC. The SoC difference between each ESU gradually becomes smaller, and finally, the load power is equally shared between the distributed ESUs. Meanwhile, the load sharing speed can be adjusted by changing the exponent of SoC in the adaptive droop control. The model of the SoC-based adaptive droop control system is established, and the system stability is thereby analyzed by using this model. Simulation and experimental results from a 2 \times 2.2 kW parallel converter system are presented in order to validate the proposed approach. [ABSTRACT FROM AUTHOR]

In this paper, a hierarchical control system for parallel power electronics interfaces between ac bus and dc bus in a hybrid microgrid is presented. Both standalone and grid-connected operation modes in the dc side of the microgrid are analyzed. Concretely, a three-level hierarchical control system is implemented. In the primary control level, the decentralized control is realized by using the droop method. Local ac current proportional-resonant controller and dc voltage proportional-integral controller are employed. When the local load is connected to the dc bus, dc droop control is applied to obtain equal or proportional dc load current sharing. The common secondary control level is designed to eliminate the dc bus voltage deviation produced by the droop control, with dc bus voltage in the hybrid microgrid boosted to an acceptable range. After guaranteeing the performance of the dc side standalone operation by means of the primary and secondary control levels, the tertiary control level is thereafter employed to perform the connection to an external dc system. Meanwhile, the impact of the bandwidth of the secondary and tertiary control levels is discussed. The closed-loop model including all the three control levels is developed in order to adjust the main control parameters and study the system stability. Experimental results of a 2 \times 2.2~\ kW parallel ac-dc converter system have shown satisfactory realization of the designed system. [ABSTRACT FROM PUBLISHER]