DC Microgrids

This paper presents a battery integrated Power Flow Controller (PFC) which is found effective for the interconnection of several dc microgrids. The configuration offers delicate control over load-flow and also provides a way for the integration of Common Energy Storage (CES) to the adjacent grids. The CES is more effective when both the grids have surplus or deficit of power compared to their individual storage capacity (if any). In this paper, a Universal Active Power Control Converter (UAPCC) is proposed (which is basically a three-port converter), where port-1 is connected in parallel with the line, port-2 is connected in series with the line, and port-3 is connected to the CES through a bidirectional dc-dc converter. Relevant control algorithms have been developed for the operation of such system satisfying various system requirements that are inevitable for the interconnection of dc microgrids. The proposed control methods allow decoupled operation of three ports to control power flow between dc grids and CES independently. The complete system along with control methods are initially verified through computer simulation using MATLAB/SIMULINK. Thereafter, a prototype is developed in the laboratory at 380 V level to experimentally validate the concept. The results show effectiveness of the UAPCC for interconnection of dc microgrids with CES. INDEX TERMS Common Energy Storage (CES), DC microgrids, DC power flow controller, DC-DC converters , renewable energy sources (RES).

A single-bus DC microgrid can represent a wide range of applications. Control objectives of such systems include high-performance bus voltage regulation and proper load sharing among multiple distributed generators (DGs) under various operating conditions. This paper presents a novel decentralized control algorithm that can guarantee both the transient voltage control performance and realize the predefined load sharing percentages. First, the output-constrained control problem is transformed into an equivalent unconstrained one. Second, a two-step backstepping control algorithm is designed based on the transformed model for bus-voltage regulation. Since the overall control effort can be split proportionally and calculated with locally-measurable signals, decentralized load sharing can be realized. The control design requires neither accurate parameters of the output filters nor load measurement. The stability of the transformed systems under the proposed control algorithm can indirectly guarantee the transient bus voltage performance of the original system. Additionally, the high-performance control design is robust, flexible, and reliable. Switch-level simulations under both normal and fault operating conditions demonstrate the effectiveness of the proposed algorithm.

DC power distribution has become a topic of interest in the recent past, despite being given up a long time. A number of research efforts have been carried out in this field; especially the idea of DC distribution for microgrids has witnessed a significant amount of time and attention. System efficiency is one of the under-research areas of this field that has witnessed a lot of research efforts which were generally simulation based studies. Detailed mathematical efficiency analysis is missing in the present body of knowledge. In this regard, the current research effort aims to present a foundation level mathematical efficiency analysis set-up that may be used as an analytical testbed for different efficiency studies especially those related to the comparative evaluation of DC and AC systems. The current effort highlights the strong dependence of AC and DC distribution system efficiency on the factor of time. The efficiency advantage of DC over AC or vice versa within a specified duration depends on the ratio of DC and AC grid powers summed over the duration.

In the last years, renewable energy (RE) is increasing widely in the energy sector, and microgrid technology is overgrowing. In this paper, stand-alone microgrid using solar photovoltaic (PV) energy as a source of renewable energy is simulated to provide power for direct current (DC) loads with hybrid energy storage system (HESS) which consists of battery and supercapacitor bank. The proposed microgrid system is tested under various cases of load and variable irradiance to confirm and validate the proposed management strategy to remain the DC bus voltage within a stable limit. The performance of DC microgrid is comparing with and without supercapacitor (SC) bank and notes a desirable decrease in the magnitude of transient voltage when using HESS. The sun power SPR-E19-320 standard was simulated to analyze system performance taking into account the constant load demand. Note that HESS helps reduce transient of DC voltage very effectively in all situations. Very large transients arise...

This paper proposes a novel simultaneous control interface and protection scheme for DC microgrids (DCµG) interconnected systems, although the interconnections could be possible via two power electronic systems such as DC/DC converter. Each DCµG presented in this paper is supplied by AC grid as well as battery bank unit, distributed generation (DG) unit and AC load unit. The proposed control interface is based on a developed suitable small signal model for each DCµG that provides proper power flow control and cancellation of interactions between DCµG. Also, the presented protection scheme for fast fault detection in DC link is based on fuzzy inference system (FIS), so that faults could be detected as quickly as possible in few milliseconds. In fact, the aim of this paper is simultaneous protection and control interface for DC multi microgrids, so that the performance of the controller and protection system is separate, yet integrated together. As such, DC multi microgrid performance under different events, such as change in active power of DGs and short circuit is not in trouble and disorder. Simulation results indicate the remarkable effectiveness of the proposed control interface and protection scheme for DCµG interconnected systems. For validation of the capability and feasibility of FIS fault detection scheme, simulated network and protection algorithm are implemented in laboratory-scale. The implementation results demonstrate that FIS protection scheme can swiftly detect faults within a few milliseconds.

This paper presents a new protection method for LVDC ring-bus microgrid systems based on Multi-Criterion System (MCS) and Neural Network (NN). The proposed method aimed at high-speed detecting line-to-ground (LG) and line-to-line (LL) low impedance faults without using a definite threshold of differential current by using specific rules and multi-criterion system. MCS protection showed speed and accuracy compared to differential protection. Also, NN estimated fault location in percent of line length acceptably as a secondary controller. In order to evaluate the reliability and the enforceability of fault detection and location schemes, simulated network and protection algorithms are implemented and tested in laboratory-scale. The implementation results indicate that the MCS and NN protection scheme can consistently detect and estimate fault locations in the order of a few milliseconds. To reach this goal, a loop type LVDC microgrid with proper power electronic equipment like solid-state bidirectional breakers and the multi-level inverter is fulfilled.

A distributed control paradigm based on reverse droop primary controller is established to formulate a control framework for the problem of achieving voltage regulation and proportionate load current sharing in a DC microgrid scenario. It utilizes the hierarchical control approach to form a cyber-physical interaction layer of the DC distribution system. The adjustment/correction factors for the DC voltage is generated by the secondary control layer wherein the voltage and current observers process the neighboring agents' dataset to effectively regulate the DC voltage. The proposed control achieves faster dynamics as compared to a conventional V-I droop controller thereby enabling improved transient and steady state dynamics. The controller is subjected to various test cases like plug-n-play capability, switched networks, remote and local load changes, resiliency to communication delays, etc. in order to investigate its robustness to disturbances and uncertainties. A low voltage DC microgrid test-bench with underlying buck converters with dispatchable energy source inputs is employed to validate the aforementioned features. The paper considers the functioning of the microgrid test-bench under various load dynamics such as constant power loads (CPLs) and resistive loads.

This article presents a distributed periodic eventtriggered (PET) optimal control scheme to achieve generation cost minimization and average bus voltage regulation in DC microgrids. In order to accommodate the generation constraints of the distributed generators (DGs), a virtual incremental cost is firstly designed, based on which an optimality condition is derived to facilitate the control design. To meet the discrete-time (DT) nature of modern control systems, the optimal controller is directly developed in the DT domain. Afterward, to reduce the communication requirement among the controllers, a distributed event-triggered mechanism is introduced for the DT optimal controller. The event-triggered condition is detected periodically and therefore naturally avoids the Zeno phenomenon. The closedloop system stability is proved by the Lyapunov synthesis for switched systems. The generation cost minimization and average bus voltage regulation are obtained at the equilibrium point. Finally, switch-level microgrid simulations validate the performance of the proposed optimal controller.