• Energy Research

Microgrids are envisioned as one of the most suitable alternatives for the integration of distributed generation units in the utility grid, as they efficiently combine generation, energy storage and loads in the same distribution network. In this context, hybrid ac/dc microgrids are arising as an interesting approach as they combine the advantages of ac and dc networks and do not require excessive modifications in the distribution network. However, they require more complex control strategies as they need to control the ac and dc networks and the interface power converter simultaneously. This paper identifies and analyses the control strategies that can be implemented in hybrid microgrids for an adequate power management in grid-tied and islanded modes of operation. The review is focused on hierarchical controls as they are the most extended approach in the literature. A classification has been elaborated, which covers the three main levels of hierarchical control strategies (primary, secondary and tertiary). Each of the levels has been independently studied in order to provide a comprehensive analysis of the alternatives found in the literature. The future trends related to this topic show that a higher research effort is required regarding the control of the interface device and the ancillary services that the management strategy must provide—e.g. blackstart, transition between islanded and grid-connected modes of operation, interconnection of microgrids, etc.

In this paper, a smart machine-learning-based energy management system (MLBEMS) is developed for a hybrid energy storage system (HESS). This HBESS consists of batteries with high-energy (HE) and high-power (HP) characteristics, to provide grid-supporting services. The aim of the MLBEMS is to improve the overall battery lifetime and achieve state-of-charge (SoC) balancing for two different use cases (UC). UC1 involves enhanced frequency regulation for the Pan-European grid, while UC2 pertains to an electric vehicle (EV) charging station with photovoltaic (PV) generation. The designed MLBEMS is compared with a rule-based energy management system (RBEMS) from the literature with similar use cases. To ensure optimal power sharing between the battery modules, an optimization model is created using real battery aging data. Using a genetic algorithm, optimal power sharing is achieved for various initial SoC conditions. The generated dataset is subsequently utilized to train a machine-learning regression model, and the resulting prediction function is imported into MATLAB/Simulink. In UC1, MLBEMS achieved a 39.3% better SoC balancing compared to RBEMS, along with 36.5% and 22.6% higher battery lifetimes for HE and HP batteries, respectively. Similarly, for UC2, MLBEMS achieved a 68.5% improvement in SoC balancing, along with 53.6% and 45.8% higher battery lifetimes for HE and HP batteries, respectively.

Electric grids are undergoing several changes, mostly driven by the replacement of classical highly-inertial generators by converter-interfaced generation and storage systems. This entails the reduction of inherent inertia levels and might lead to instability issues. In a future scenario formed by grids of different natures and characteristics, power electronic converters will play a key role on grid tying applications. These converters are known as interlinking converters (ICs), and they enable total control over the power flow between interconnected grids. Therefore, they are envisioned to take part not only tying hybrid ac/dc systems but also in ac/ac connections. This paper presents a novel control strategy for ICs named dual inertiaemulation (DIE), that improves the dynamic response of tied grids by emulating inertia at both sides of the converter, and which can be employed at any IC regardless of the interconnected grid type (ac or dc). The proposed control is tested by means of time-domain simulations of WSCC 9-bus and IEEE 14-bus benchmark systems. The obtained results demonstrate that the proposed technique increases the equivalent inertial response of the interconnected grids, hence reducing frequency oscillations and the rate of change of frequency (RoCoF), and improving the frequency nadir.

Microgrids have been widely studied in the literature as a possible approach for the integration of distributed energy sources with energy storage systems in the electric network. Until now the most used configuration has been the ac microgrid, but dc-based microgrids are gaining interest due to the advantages they provide over their counterpart (no reactive power, no synchronization, increasing number of dc devices, etc.). Therefore, hybrid ac/dc microgrids are raising as an optimal approach as they combine the main advantages of ac and dc microgrids. This paper reviews the most interesting topologies of hybrid ac/dc microgrids based on the interconnection of the ac and dc networks and the conventional power network. After performing a description and analysis of each configuration, a comparative evaluation has been performed to highlight the most important features of each one. The future trends identified during the study also show that several features such as the scalability, modeling or design require further research towards the integration of hybrid microgrids in the power network.

Abstract Classical power systems, which are typically structured in top-down topologies, are gradually evolving towards more decentralized systems comprised by clusters of smart subgrids to cope with the increasing penetration of distributed generation, energy storage systems and controllable loads. These clusters will increase the overall reliability, optimize resource usage and reduce investments in back-up systems. However, tying subgrids via passive devices (tie lines or power transformers) poses certain problems from the point of view of modularity and controllability. They also limit the connection capability of subgrids, as it is expected that systems with different voltage natures (ac and dc) will coexist in the future network. In this context, interlinking converters (IC) have emerged as a universal approach for the interconnection of such subgrids regardless of their characteristics. These power converters not only provide power flow control, but they also improve the power quality of networks through different ancillary services. Therefore, ICs are expected to be the energy routers of the future, smartly connecting and managing the interaction among grids. In the literature several topologies and control techniques have been proposed for this type of converters to transfer power between grids and provide support under contingencies. However, there are no classifications from the point of view of the participation of ICs in the primary regulation of the power system. The aim of this paper is to 1) identify the main characteristics of ICs compared to conventional interconnections based on passive devices, 2) review the most usual IC topologies depending on the nature of the grids they are interconnecting (ac-ac, ac-dc or dc-dc) and 3) analyse and compare the different control approaches for the primary regulation via ICs and propose a general classification based on this analysis regardless of the number of conversion power stages of the IC and the nature and characteristics of tied grids.

During the last decades the number of microgrids and their research have increased notably, as they offer a high versatility for the integration of distributed generation (DG) units and renewable energies. Among the different types of microgrids, dc systems are becoming more popular due to their advantages over conventional ac systems. However, their control techniques differ from the ones employed at ac networks, and depending on the device type-either generation, storage systems or loads-alternative control techniques need to be developed. Therefore, in this paper an experimental dc platform is developed with the aim of testing and evaluating these kind of control strategies. This test bench consists of a bidirectional four switch buck-boost converter together with a TMS320F28335 DSP for the implementation of the control strategy on each power converter. The proposed platform is suitable for integrating these systems at a 48 V dc bus and facilitates the evaluation of a wide range of control strategies as they are implemented in the Matlab/Simulink ® environment. In this case, the validation of the experimental platform has been carried out by implementing a virtual-impedance-based control technique. The experimental results included in the paper corroborate the suitability of the platform for the evaluation of control techniques for dc microgrids.