There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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.

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

Droop control is the basic control method for load current sharing in dc microgrid applications. The conventional dc droop control method is realized by linearly reducing the dc output voltage as the output current increases. This method has two limitations. First, with the consideration of line resistance in a droop-controlled dc microgrid, since the output voltage of each converter cannot be exactly the same, the output current sharing accuracy is degraded. Second, the dc-bus voltage deviation increases with the load due to the droop action. In this paper, in order to improve the performance of the dc microgrid operation, a low-bandwidth communication (LBC)-based improved droop control method is proposed. In contrast with the conventional approach, the control system does not require a centralized secondary controller. Instead, it uses local controllers and the LBC network to exchange information between converter units. The droop controller is employed to achieve independent operation, and the average voltage and current controllers are used in each converter to simultaneously enhance the current sharing accuracy and restore the dc bus voltage. All of the controllers are realized locally, and the LBC system is only used for changing the values of the dc voltage and current. Hence, a decentralized control scheme is accomplished. The simulation test based on MATLAB/Simulink and the experimental validation based on a 2 × 2.2 kW prototype were implemented to demonstrate the proposed approach.

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An Improved Droop Control Method for DC Microgrids Based on Low Bandwidth Communication With DC Bus Voltage Restoration and Enhanced Current Sharing Accuracy

Gallium nitride (GaN) power devices are an emerging technology that have only recently become available commercially. This new technology enables the design of converters at higher frequencies and efficiencies than those achievable with conventional Si devices. This paper reviews the characteristics and commercial status of both vertical and lateral GaN power devices, providing the background necessary to understand the significance of these recent developments. In addition, the challenges encountered in GaN-based converter design are considered, such as the consequences of faster switching on gate driver design and board layout. Other issues include the unique reverse conduction behavior, dynamic $R_{\mathrm {{ds}},\mathrm {{on}}}$ , breakdown mechanisms, thermal design, device availability, and reliability qualification. This review will help prepare the reader to effectively design GaN-based converters, as these devices become increasingly available on a commercial scale.

Review of Commercial GaN Power Devices and GaN-Based Converter Design Challenges

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 × 2.2 kW parallel converter system are presented in order to validate the proposed approach.

/pdf/state-of-charge-balance-using-adaptive-droop-control-for-1br9lt0y7v.pdf

State-of-Charge Balance Using Adaptive Droop Control for Distributed Energy Storage Systems in DC Microgrid Applications

With the emerging installations of multitype renewable energy sources and energy storage elements, the dc electronic distribution systems in residential buildings (dc nanogrid) are becoming an alternative future system solution, achieving a zero net-energy consumption and optimized power management. The concept of the energy control center (ECC), which interconnects the dc system to the traditional ac utility grid, is introduced, and the operation function of ECC converter suitable for dc nanogrid application is defined. This paper investigates a two-stage topology using a full bridge in series with a bidirectional synchronous rectifier dc–dc converter as a single-phase ECC for dc nanogrid, with a significant reduction of the dc-link capacitor value. The operation analysis and the design of passive components are provided. A bidirectional control system and the design process are also presented in terms of the system requirement and the small dc-link capacitor.

Grid-Interface Bidirectional Converter for Residential DC Distribution Systems—Part One: High-Density Two-Stage Topology

This paper investigates gate drive design for high voltage gallium nitride (GaN) high electron-mobility transistors (HEMT) in a cascade structure. High dv/dt and di/dt switching characteristics of GaN device and its influences on high-side gate drive are analyzed on an 8.4kW bidirectional multi-channel buck/boost battery charger operating in critical conduction mode (CRM). Driving candidates for high-side gate drive are reviewed, and digital isolator based driving architecture is proposed with discussion of PCB layout and package parasitics. Experimental results are conducted in each step for concepts validation.

Gate drive design considerations for high voltage cascode GaN HEMT

It is well-known that load sharing among paralleled sources in dc grid can be achieved by droop control. However, practical factors that influence the load sharing and voltage regulation are seldom discussed, like the sensor discrepancy and cable resistance. In this paper, the impact from cable resistance is analyzed and verified by experiment. To suppress the impact from practical factors, instead of using constant droop resistance, a nonlinear droop method is proposed. In the proposed method, the value of droop resistance is a function of output current. Its value increases when the converter's output current increases. The proposed method needs only the local output current information so the advantage of traditional droop control is preserved. Different second-order droop functions are evaluated and compared. The effectiveness of proposed droop method is verified by simulation and experiment. The experimental result proves the proposed nonlinear droop method has better load sharing at heavy load and tighter bus voltage regulation at light load. The necessary droop voltage range to fully utilize the capacity of paralleled sources is also reduced.

A nonlinear droop method to improve voltage regulation and load sharing in DC systems

The architecture of an electric power system is a major determinant for the system structural practicabil-ity, safety, size, weight, energy efficiency, reliability, and ultimately the cost of energy utilization. Although most of these are actually quantifiable physical variables, the final choice of the electrical architecture when building or design-ing new electric power systems is, quite logically, very often made as a result of either different engineering traditions and experiences, or higher confidence in using proven and established practices. This paper examines a dc nanogrid - hypothetical future sustainable home electronic power distribution system with dc-based distribution and the presence of renewable energy sources. Description of the nanogrid and the testbed for its system-level operation analysis is presented in this paper together with initial experimental results and conclusions.

A testbed for experimental validation of a low-voltage DC nanogrid for buildings

Making electricity grids “smarter” and facilitating them with integration of renewable energy sources (RES) are fairly accepted as the necessary steps to achieve a sustainable power industry. However, serious concerns over reliability and performance require integration of energy storage as a critical part to buffer energy or provide arbitrage. Lithium-based battery offers high specific power/energy density, and gains popularities in many applications, such as (Plug-in) hybrid electric vehicle ((P)HEV) and renewable energy system. This paper discusses two issues: (a) integrating lithium-based battery into a multi-renewable-energy-source-feeding DC-distributed renewable energy system (DRES); (b) floating charge safety for a high voltage series-connected Li-ion battery pack. Based on the analysis of this two issues, this paper then proposes an integration strategy for lithium-based energy storage management, and the maximum cell voltage V Cell_max controlled floating charge profile for long string series connected lithium-ion cells pack. An 8.4kW bi-directional multi-phase dc-dc converter is designed to validate above mentioned concepts

Wei Zhang

Papers

Grid-Interface Bidirectional Converter for Residential DC Distribution Systems—Part One: High-Density Two-Stage Topology

Gate drive design considerations for high voltage cascode GaN HEMT

A nonlinear droop method to improve voltage regulation and load sharing in DC systems

A testbed for experimental validation of a low-voltage DC nanogrid for buildings

Lithium-based energy storage management for DC distributed renewable energy system