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

This paper performs an extensive review on control schemes and architectures applied to dc microgrids (MGs). It covers multilayer hierarchical control schemes, coordinated control strategies, plug-and-play operations, stability and active damping aspects, as well as nonlinear control algorithms. Islanding detection, protection, and MG clusters control are also briefly summarized. All the mentioned issues are discussed with the goal of providing control design guidelines for dc MGs. The future research challenges, from the authors’ point of view, are also provided in the final concluding part.

/pdf/review-on-control-of-dc-microgrids-and-multiple-microgrid-2wzqd7rmgl.pdf

Review on Control of DC Microgrids and Multiple Microgrid Clusters

Microgrids are a suitable, reliable and clean solution to integrate distributed generation into the mains grid. Microgrids can present both AC and DC distribution lines. The type of distribution conditions the performance of distribution line and implies different features, advantages and disadvantages in each case. This paper analyses, in detail, all this parameters for AC and DC microgrids in order to identify and describe the available alternatives for building and configuring a microgrid. Elements and issues involved in the implementation and development, such as protections, power converters, economic analysis, and availability are discussed and described. This analysis constitutes a tool for selecting a suitable configuration of a microgrid adapted to the needs in each situation. In addition, the paper provides a picture of the current situation of microgrids, and identifies and proposes future research lines.

AC and DC technology in microgrids: A review

The modern electric power systems are going through a revolutionary change because of increasing demand of electric power worldwide, developing political pressure and public awareness of reducing carbon emission, incorporating large scale renewable power penetration, and blending information and communication technologies with power system operation. These issues initiated in establishing microgrid concept which has gone through major development and changes in last decade, and recently got a boost in its growth after being blessed by smart grid technologies. The objective of this paper is to presents a detailed technical overview of microgrid and smart grid in light of present development and future trend. First, it discusses microgrid architecture and functions. Then, smart features are added to the microgrid to demonstrate the recent architecture of smart grid. Finally, existing technical challenges, communication features, policies and regulation, etc. are discussed from where the future smart grid architecture can be visualized.

Enhancing smart grid with microgrids: Challenges and opportunities

Since the inception of competitive power markets two decades ago, electricity price forecasting (EPF) has gradually become a fundamental process for energy companies’ decision making mechanisms. Over the years, the bulk of research has concerned point predictions. However, the recent introduction of smart grids and renewable integration requirements has had the effect of increasing the uncertainty of future supply, demand and prices. Academics and practitioners alike have come to understand that probabilistic electricity price (and load) forecasting is now more important for energy systems planning and operations than ever before. With this paper we offer a tutorial review of probabilistic EPF and present much needed guidelines for the rigorous use of methods, measures and tests, in line with the paradigm of ‘maximizing sharpness subject to reliability’. The paper can be treated as an update and a further extension of the otherwise comprehensive EPF review of Weron [1] or as a standalone treatment of a fascinating and underdeveloped topic, that has a much broader reach than EPF itself.

/pdf/recent-advances-in-electricity-price-forecasting-a-review-of-4h3q1ho3b5.pdf

Recent advances in electricity price forecasting: A review of probabilistic forecasting

Distributed generation is emerging as a new technology for supplying the increasing demand for electricity. Microgrids are attracting a great deal of attention since they integrate distributed generation in the main grid reliably and cleanly. When designing the control system of a microgrid, several functions must be considered, such as the management of electrical and thermal energy, load management, synchronisation with the main grid, etc. Both companies and institutions have carried out research into the control of microgrids over recent years and many proposals can be found in the literature. Thus, the design of the control system of a microgrid is a complex task due to its multiple functions and the large number of proposed solutions. This paper presents a complete description of the main features of a microgrid and describes the characteristics of the control systems used. Details are provided of the control tasks involved and of the main types of controls proposed in the literature. In addition, this paper describes the controls used in existing microgrids all over the world and proposes future areas for research.

General aspects, hierarchical controls and droop methods in microgrids: A review

The droop control method is usually selected when several distributed generators (DGs) are connected in parallel forming an islanded microgrid. This is because of the advantages it offers such as flexibility, absence of critical communications etc. Besides, several studies add a fictitious impedance to improve the performance of the original droop method. However, only a few studies deal with the design of this fictitious impedance, which is necessary to ensure an improvement in the dynamics and stability of the microgrid. In addition, these studies do not consider load variations for the design of the fictitious impedance, which is a habitual event in these systems. On the other hand, some studies propose a restoration control to bring the frequency and voltage amplitude of the microgrid to their nominal values. However, these do not deal with the design of the dynamics of this control to maintain a good transient and to ensure the stable performance of the microgrid. This study proposes the design of a fictitious impedance that ensures the stable operation of an experimental microgrid without power oscillations during load jumps and throughout its entire load range. This study also proposes a new restoration control that allows to take into account the possible inertias, delays etc. of the DGs and reduces the bandwidth of the required communications. Moreover, the proposed restoration control is properly designed to guarantee a good transient and the satisfactory performance of the microgrid. Experimental results confirm the validity of the proposed controls.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-rpg.2012.0319

Design and implementation of a droop control in d–q frame for islanded microgrids

Merit order of renewable generation has produced an increasing, relevant number of zero values in Spanish electricity spot prices since 2010, in contrast with the rest of markets which rarely present zero values. This letter shows that existing methodologies for normalizing the price distribution fail to yield a proper normalization. This letter proposes a new algorithm based on Nataf transformation which addresses this issue and ensures a correct normalization. Numerical results validate the proposed algorithm.

A Note on the Normalization of Spanish Electricity Spot Prices

DC transmission and distribution systems have several advantages compared to classical AC system. This paper presents a review of DC technology, doing a special mention in HVDC. It addresses some issues like HVDC converter types, DC- DC converter topologies, DC transmission and distribution topologies, transmission cables and DC circuit breakers with the main manufacturers and commercial devices.

/pdf/overview-of-dc-technology-energy-conversion-240y8bgm9x.pdf

Estefanía Planas

Papers

AC and DC technology in microgrids: A review

General aspects, hierarchical controls and droop methods in microgrids: A review

Design and implementation of a droop control in d–q frame for islanded microgrids

A Note on the Normalization of Spanish Electricity Spot Prices

Overview of DC technology - Energy conversion