Tomislav Dragicevic

Supervisory Control of an Adaptive-Droop Regulated DC Microgrid With Battery Management Capability

DC power systems are gaining an increasing interest in renewable energy applications because of the good matching with dc output type sources such as photovoltaic (PV) systems and secondary batteries. In this paper, several distributed generators (DGs) have been merged together with a pair of batteries and loads to form an autonomous dc microgrid (MG). To overcome the control challenge associated with coordination of multiple batteries within one stand-alone MG, a double-layer hierarchical control strategy was proposed. 1) The unit-level primary control layer was established by an adaptive voltage-droop method aimed to regulate the common bus voltage and to sustain the states of charge (SOCs) of batteries close to each other during moderate replenishment. The control of every unit was expanded with unit-specific algorithm, i.e., finish-of-charging for batteries and maximum power-point tracking (MPPT) for renewable energy sources, with which a smooth online overlap was designed and 2) the supervisory control layer was designed to use the low-bandwidth communication interface between the central controller and sources in order to collect data needed for adaptive calculation of virtual resistances (VRs) as well as transit criteria for changing unit-level operating modes. A small-signal stability for the whole range of VRs. The performance of developed control was assessed through experimental results.

DC Microgrids—Part I: A Review of Control Strategies and Stabilization Techniques

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.

DC Microgrids—Part II: A Review of Power Architectures, Applications, and Standardization Issues

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 todays 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.

Robust Networked Control Scheme for Distributed Secondary Control of Islanded Microgrids

Distributed secondary control (DSC) is a new approach for microgrids (MGs) by which frequency, voltage, and power can be regulated by using only local unit controllers. Such a solution is necessary for anticipated scenarios that have an increased number of distributed generators (DGs) within the MG. Due to the constrained traffic pattern required by the secondary control, it is viable to implement a dedicated local area communication functionality among the local controllers. This paper presents a new wireless-based robust communication algorithm for the DSC of MGs. The algorithm tightly couples the communication and the control functionality, such that the transmission errors are absorbed through an averaging operation performed in each local controller, resulting in a very high reliability. Furthermore, transmissions from each DG are periodic and prescheduled broadcasts, and in this way, contention over the shared wireless medium is avoided. Real-time simulation and experimental results are presented in order to evaluate the feasibility and robustness endowed by the proposed algorithm. The results indicate that the proposed algorithm is very robust with respect to communication impairments, such as packet delays and random packet losses.

A Distributed Control Strategy for Coordination of an Autonomous LVDC Microgrid Based on Power-Line Signaling

In a microgrid (MG), an energy management control is essential in order to handle the variety of prime movers, which may include different types of renewable energy sources (RESs) and energy storage systems (ESSs). Specifically, the recharging process of the secondary battery, i.e., the most prominent ESS, should be done in a specific manner to preserve its lifetime, the common MG bus voltage must be kept within the bounds, and the energy offered by RES should be utilized as efficiently as possible. This paper proposes a method for coordination of an autonomous low-voltage direct-current (LVDC) MG that consists of a number of sources using power-line signaling (PLS), which is a distributed control strategy in which the units inject sinusoidal signals of specific frequency into the common bus in order to communicate with each other. The control structure that allows the application of this method is revealed, and the optimal range of operating PLS frequencies is specified. In order to achieve a zero steady-state error of injected signals in the common bus, primary control of batteries has been extended with dedicated proportional-resonant controllers that are switched on only during injection period. Finally, a method for coordination among the units using the PLS concept was developed and experimentally tested, confirming its applicability for autonomous LVDC MGs.

Hierarchical Control for Multiple DC-Microgrids Clusters

This paper presents a distributed hierarchical control framework to ensure reliable operation of dc microgrid (MG) clusters. In this hierarchy, primary control is used to regulate the common bus voltage inside each MG locally. An adaptive droop method is proposed for this level, which determines droop coefficients according to the state-of-charge (SOC) of batteries automatically. A small-signal model is developed to investigate effects of the system parameters, constant power loads, as well as line impedance between the MGs on stability of these systems. In the secondary level, a distributed consensus-based voltage regulator is introduced to eliminate the average voltage deviation over the MGs. This distributed averaging method allows the power flow control between the MGs to be achieved at the same time, as it can be accomplished only at the cost of having voltage deviation inside the system. Another distributed policy is employed then to regulate the power flow among the MGs according to their local SOCs. The proposed distributed controllers on each MG communicate with only the neighbor MGs through a communication infrastructure. Finally, the developed small-signal model is expanded for MG clusters with all the proposed control loops. The effectiveness of the proposed hierarchical scheme is verified through detailed hardware-in-the-loop simulations.

Autonomous Active Power Control for Islanded AC Microgrids With Photovoltaic Generation and Energy Storage System

In an islanded ac microgrid with distributed energy storage system (ESS), photovoltaic (PV) generation, and loads, a coordinated active power regulation is required to ensure efficient utilization of renewable energy, while keeping the ESS from overcharge and overdischarge conditions. In this study, an autonomous active power control strategy is proposed for ac-islanded microgrids in order to achieve power management in a decentralized manner. The proposed control algorithm is based on frequency bus-signaling of ESS and uses only local measurements for power distribution among microgrid elements. Moreover, this study also presents a hierarchical control structure for ac microgrids that is able to integrate the ESS, PV systems, and loads. Hereby, basic power management function is realized locally in primary level, while strict frequency regulation can be achieved by using additional secondary controller. Finally, real-time simulation results under various state of charge (SoC) and irradiance conditions are presented in order to prove the validity of the proposed approach.

Intelligent Distributed Generation and Storage Units for DC Microgrids—A New Concept on Cooperative Control Without Communications Beyond Droop Control

Low voltage dc microgrids have been widely used for supplying critical loads, such as data centers and remote communication stations. Consequently, it is important to ensure redundancy and enough energy capacity in order to support possible increments in load consumption. This is achieved by means of expansion of the energy storage system by adding extra distributed energy storage units. However, using distributed energy storage units adds more challenges in microgrids control, since stored energy should be balanced in order to avoid deep discharge or over-charge in one of the energy storage units. Typically, voltage droop loops are used for interconnecting several different units in parallel to a microgrid. This paper proposes a new decentralized strategy based on fuzzy logic that ensures stored energy balance for a low voltage dc microgrid with distributed battery energy storage systems by modifying the virtual resistances of the droop controllers in accordance with the state of charge of each energy storage unit. Additionally, the virtual resistance is adjusted in order to reduce the voltage deviation at the common dc bus. The units are self-controlled by using local variables only, hence, the microgrid can operate without relying on communication systems. Hardware in the loop results show the feasibility of the proposed method.

Advanced LVDC Electrical Power Architectures and Microgrids: A step toward a new generation of power distribution networks.

Current trends indicate that worldwide electricity distribution networks are experiencing a transformation toward direct current (dc) at both the generation and consumption level. This tendency is powered by the outburst of various electronic loads and, at the same time, the struggle to meet the lofty goals for the sharing of renewable energy sources (RESs) in satisfying total demand. RESs operate either natively at dc or have a dc link in the heart of their power electronic interface, whereas the end-point connection of electronic loads, batteries, and fuel cells is exclusively dc. Therefore, merging these devices into dedicated dc distribution architectures through corresponding dc?dc converters is an attractive option not only in terms of enhancing efficiency because of reduction of conversion steps but also for realizing power quality independence from the utility mains. These kinds of systems generally provide improved reliability in comparison to their alternating current (ac) counterparts since the number of active elements in dc?dc power electronic devices is smaller than in dc-ac converters. Control design in dc systems is also significantly simpler since there are no reactive and harmonic power flows or problems with synchronization.

Modeling and Sensitivity Study of Consensus Algorithm-Based Distributed Hierarchical Control for DC Microgrids

Distributed control methods based on consensus algorithms have become popular in recent years for microgrid (MG) systems. These kind of algorithms can be applied to share information in order to coordinate multiple distributed generators within a MG. However, stability analysis becomes a challenging issue when these kinds of algorithms are used, since the dynamics of the electrical and the communication systems interact with each other. Moreover, the transmission rate and topology of the communication network also affect the system dynamics. Due to discrete nature of the information exchange in the communication network, continuous-time methods can be inaccurate for this kind of dynamic study. Therefore, this paper aims at modeling a complete dc MG using a discrete-time approach in order to perform a sensitivity analysis taking into account the effects of the consensus algorithm. To this end, a generalized modeling method is proposed and the influence of key control parameters, the communication topology, and the communication speed are studied in detail. The theoretical results obtained with the proposed model are verified by comparing them with the results obtained with a detailed switching simulator developed in Simulink/Plecs.