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 and AC Microgrids are key elements to integrate renewable and distributed energy resources as well as distributed energy storage systems. In the last years, efforts toward the standardization of these Microgrids have been made. In this sense, this paper present the hierarchical control derived from ISA-95 and electrical dispatching standards to endow smartness and flexibility to microgrids. The hierarchical control proposed consist of three levels: i) the primary control is based on the droop method, including an output impedance virtual loop; ii) the secondary control allows restoring the deviations produced by the primary control; and iii) the tertiary control manage the power flow between the microgrid and the external electrical distribution system. Results from a hierarchical-controlled microgrid are provided to show the feasibility of the proposed approach.

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Hierarchical control of droop-controlled DC and AC microgrids — a general approach towards standardization

Energy management means to optimize one of the most complex and important technical creations that we know: the energy system. While there is plenty of experience in optimizing energy generation and distribution, it is the demand side that receives increasing attention by research and industry. Demand Side Management (DSM) is a portfolio of measures to improve the energy system at the side of consumption. It ranges from improving energy efficiency by using better materials, over smart energy tariffs with incentives for certain consumption patterns, up to sophisticated real-time control of distributed energy resources. This paper gives an overview and a taxonomy for DSM, analyzes the various types of DSM, and gives an outlook on the latest demonstration projects in this domain.

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Demand Side Management: Demand Response, Intelligent Energy Systems, and Smart Loads

The enabling of ac microgrids in distribution networks allows delivering distributed power and providing grid support services during regular operation of the grid, as well as powering isolated islands in case of faults and contingencies, thus increasing the performance and reliability of the electrical system. The high penetration of distributed generators, linked to the grid through highly controllable power processors based on power electronics, together with the incorporation of electrical energy storage systems, communication technologies, and controllable loads, opens new horizons to the effective expansion of microgrid applications integrated into electrical power systems. This paper carries out an overview about microgrid structures and control techniques at different hierarchical levels. At the power converter level, a detailed analysis of the main operation modes and control structures for power converters belonging to microgrids is carried out, focusing mainly on grid-forming, grid-feeding, and grid-supporting configurations. This analysis is extended as well toward the hierarchical control scheme of microgrids, which, based on the primary, secondary, and tertiary control layer division, is devoted to minimize the operation cost, coordinating support services, meanwhile maximizing the reliability and the controllability of microgrids. Finally, the main grid services that microgrids can offer to the main network, as well as the future trends in the development of their operation and control for the next future, are presented and discussed.

Control of Power Converters in AC Microgrids

The increasing interest in integrating intermittent renewable energy sources into microgrids presents major challenges from the viewpoints of reliable operation and control. In this paper, the major issues and challenges in microgrid control are discussed, and a review of state-of-the-art control strategies and trends is presented; a general overview of the main control principles (e.g., droop control, model predictive control, multi-agent systems) is also included. The paper classifies microgrid control strategies into three levels: primary, secondary, and tertiary, where primary and secondary levels are associated with the operation of the microgrid itself, and tertiary level pertains to the coordinated operation of the microgrid and the host grid. Each control level is discussed in detail in view of the relevant existing technical literature.

Trends in Microgrid Control

Distributed control methods for microgrid systems become a popular topic in recent years. Distributed algorithms, such as consensus algorithms, can be applied for distributed information sharing. However, by using this kind of algorithms the stability analysis becomes a critical issue since the dynamics of electrical and communication systems interact with each other. Apart from that, the communication characteristics also affect the dynamics of the system. Due to discrete nature of information exchange in communication network, Laplace domain analysis is not accurate enough for this kind of dynamic study. The aim of this paper is to model the complete DC microgrid system in z- domain and perform sensitivity analysis for the complete system. A generalized modeling method is proposed and the system dynamics under different control parameters, communication topologies and communication speed are studied. The responses derived from the analytical model are verified by comparing them with Simulink/PLECS based simulation results.

Modeling and Sensitivity Analysis of Consensus Algorithm Based Distributed Hierarchical Control for

The virtual synchronous generator (VSG) has been extensively applied for the integration of distributed generators. For the benefit of analysis, both the reduced-order small-signal model (RSM) and full-order small-signal model (FSM) have been proposed; however, because of the different modeling approaches, the modeling accuracy may differ a lot. To provide a guideline for selecting a suitable model, an accuracy assessment has been conducted in this paper, where different grid strength cases and equilibrium points are considered. More specific, the eigenvalue analysis is adopted for comparing the dominant dynamics of the RSM and FSM. Afterwards, the responses of the RSM and FSM are compared with the nonlinear model (NM) in the Digsilent/Powerfactory to show the modeling accuracy. It has been found that the RSM is mainly suitable for the weak and relatively strong grids, while the FSM shows better accuracy in the case of an extremely strong grid and around the equilibrium point that is close to VSG’s rated output.

Accuracy Assessment of Reduced- and Full-Order Virtual Synchronous Generator Models Under Different Grid Strength Cases

This paper offers a comparative analysis of Maximum Power Point Tracking (MPPTs) techniques under the spinning conditions for small satellite applications. The Nano Satellite (NanoSat) is constrained on volume and mass with body-mounted solar panels and where it is dedicated to some special orientation scenarios, depending on specific mission requirements. This results in irregular solar irradiance on the PV system; thus, it needs an efficient and effective strategy to extract the maximum power transferred to the loads in the NanoSat Electrical Power System (EPS). While the EPS can be regarded a space microgrid due to control and coordination in a small electric network of distributed generations (DGs), storage, and loads. Therefore, some well-known MPPT techniques are investigated where different scenarios are carried out among conventional Perturb and Observe (P&O), Incremental Conductance (IC), and Ripple Correlation Control (RCC) to highlight the optimal MPPT extractions for NanoSat applications. The system was developed and modeled in MATLAB/SIMULINK and the final findings demonstrate that the RCC provides smooth power compared to other ones, while the other ones offer an oscillated power under the effect of radiations change and spinning conditions for the system under test.

A Comparative Study of MPPTs for Nano-Satellite Microgrid Applications under Spinning Flight Scenarios

Power quality issues are challenging to deal with by conventional harmonic suppression methods in a virtual synchronous generator-based system connected to the distorted grid. To address this issue, a phase-compensated harmonic suppression method is proposed to effectively suppress the grid harmonic current for the virtual synchronous generator in the distorted grid. The phase-compensated harmonic suppression method is developed according to the Nyquist diagram to improve the harmonic control bandwidth and the system stability. With this method, the grid harmonic component is effectively suppressed, the Nyquist sensitivity distance of the system is maximized, the total harmonic distortion (THD) of the grid current is reduced from 8.53% to 2.97%, and the harmonic suppression response time is reduced from 10 cycles to 2 cycles, showing a better harmonic suppression performance.

A phase-compensated harmonic suppression method for virtual synchronous generator system in distorted grid

The external droop control loop of I-V droop control is designed as a voltage loop with embedded virtual impedance, so the internal current loop plays a major role in the system bandwidth. Thus, in this paper, the influence of internal current loop on transient response performance of I-V droop controlled paralleled dc-dc converters is analyzed, which is guided and significant for its industry application. The model which is used for dynamic analysis is built, and the root locus method is used based on the model to analyze the dynamic response of the system by shifting different control parameters. Analysis results show that any converter's inner current loop can influence all converters' dynamic responses in the system and the one with larger Virtual Resistance (VR) has greater effect. Increasing the proportional terms can decrease the fluctuations of current and voltage, and key parts of errors can be reduced more rapidly during dynamical process, but adverse effect on completely eliminating errors can be caused; increasing integral terms can enlarge current fluctuations during dynamical process, but the errors can be eliminated more rapidly. The experiments are performed to verify the analysis.

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Juan C. Vasquez

Papers

Modeling and Sensitivity Analysis of Consensus Algorithm Based Distributed Hierarchical Control for

Accuracy Assessment of Reduced- and Full-Order Virtual Synchronous Generator Models Under Different Grid Strength Cases

A Comparative Study of MPPTs for Nano-Satellite Microgrid Applications under Spinning Flight Scenarios

A phase-compensated harmonic suppression method for virtual synchronous generator system in distorted grid

The influence of internal current loop on transient response performance of I-V droop controlled paralleled DC-DC converters