Review of a disruptive vision of future power grids: a new path based on hybrid AC/DC grids and solid-state transformers

Power grids are evolving with the aim to guarantee sustainability and higher levels of power quality for universal access to electricity. More specifically, over the last two decades, power grids have been targeted for significant changes, including migration from centralized to decentralized paradigms as a corollary of intensive integration of novel electrical technologies and the availability of derived equipment. This paper addresses a review of a disruptive vision of future power grids, mainly focusing on the use of hybrid AC/DC grids and solid-state transformers technologies. Regarding hybrid AC/DC grids in particular, they are analyzed in detail in the context of unipolar and bipolar DC grids (i.e., two-wire or three-wire DC grids), as well as the different structures concerning coupled and decoupled AC configurations with low-frequency or high-frequency isolation. The contextualization of the possible configurations of solid-state transformers and the different configurations of hybrid transformers (in the perspective of offering benefits for increasing power quality in terms of currents or voltages) is also analyzed within the perspective of the smart transformers. Additionally, the paper also presents unified multi-port systems used to interface various technologies with hybrid AC/DC grids, which are also foreseen to play an important role in future power grids (e.g., the unified interface of renewable energy sources and energy storage systems), including an analysis concerning unified multi-port systems for AC or DC grids. Throughout the paper, these topics are presented and discussed in the context of future power grids. An exhaustive description of these technologies is made, covering the most relevant and recent structures and features that can be developed, as well as the challenges for the future power grids. Several scenarios are presented, encompassing the mentioned technologies, and unveiling a progressive evolution that culminates in the cooperative scope of such technologies for a disruptive vision of future power grids.This work has been supported by FCT—Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020. This work has been supported by the FCT Project newERA4GRIDs PTDC/EEI-EEE/30283/2017, and by the FCT Project DAIPESEV PTDC/EEIEEE/30382/2017

Design of a Modular Multilevel Converter with 400 kWh of Integrated Batteries

In this paper the electromechanical design and the cascaded signal processing platform of a modular multilevel converter with integrated, second-life battery modules are presented. With its 120 full-bridge submodules, the prototype is designed for a maximum in- and output power of 100 kW and to store up to 400 kWh

A review of power electronics equipment for all-electric ship MVDC power systems

Medium Voltage DC (MVDC) distribution Power Systems for all-electric ships (AES) can be regarded as functionally composed of three subsystems, namely the power sources, the load centers and the distribution network. Extensive use of power electronics is required for connecting power sources and load centers to the MVDC bus and for protecting the MVDC power system through properly placed DC circuit breakers. In this paper, an overview is given of the power electronics equipment found in the literature and on the market that could be suitable for use in future AES MVDC power systems. Some industrial experiences regarding DC generator systems, energy storage apparatus and solid-state DC circuit breaker prototypes are reported in the paper as examples of state-of-the-art realizations. Different DC/DC converters, which can be employed as solid-state transformers, are also discussed and a structure obtained by combining them is proposed

Modular multilevel converter with partially rated integrated energy storage suitable for frequency support and ancillary service provision

Grid scale Energy Storage Systems (ESSs) have received significant interest in recent years due to their ability to reduce/defer investment in transmission/distribution networks, as well their ability to act as primary reserve sources and provide emergency support to the transmission system. This paper investigates the dual-purposing of a HVDC-Scale Modular Multilevel Converter (MMC), allowing it to also act as an ESS. This has potential application in primary frequency response provision and other services such as de-coupled power oscillation damping. In the proposed topology a certain percentage of submodules (SM) within the MMC have their capacitor interfaced through a DC-DC converter to an Energy Storage Element (ESE), formed of a battery or ultracapacitor. By applying appropriate control, energy can be exchanged from the ESE to the main SM capacitor of each ESE-SM, and from there to either the AC or DC bus. It was found that for some operating points, an injection of circulating current was required to facilitate exchanging energy with the ESE-SMs. Analysis shows that, for instance, an additional power injection to AC or DC terminal of 0.1 p.u. (10%) is possible with only 4% of the SMs replaced by Full-Bridge ESE-SMs, and no additional SMs added

The future of electrical power grids: a direction rooted in power electronics

Electrical power grids are changing with a focus on ensuring energy sustainability and enhanced power quality for all sectors. Over the last few decades, there has been a change from a centralized to a decentralized paradigm, which is the consequence of a large-scale incorporation of new electrical technologies and resultant equipment. Considering the foreseeable continuation of changes in electrical power grids, a direction rooted in power electronics with a focus on hybrid AC/DC grids, including the support of solid-state transformers and unified systems, is presented in this paper. Converging on hybrid AC/DC grids, DC grids (structured as unipolar and bipolar) and coupled and decoupled AC configurations are analyzed. On the other hand, in the context of solid-state transformers, feasible structures are analyzed, including the establishment of hybrid AC/DC grids, and the assessment of gains for boosting power quality is presented. Unified power electronics systems are also of fundamental importance when contextualized within the framework of future power grids, presenting higher efficiency, lower power stages, and the possibility of multiple operations to support the main AC grid. In this paper, such subjects are discussed and contextualized within the framework of future power grids, encompassing highly important and modern structures and their associated challenges. Various situations are characterized, revealing a gradual integration of the cited technologies for future power grids, which are also known as smart grids.This work was supported by FCT—Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020

Energy Pulsation Reduction in Modular Multilevel Converters Using Optimized Current Trajectories

In power electronics, the modular multilevel converter (MMC) is an easily scalable topology with an high output voltage quality. It is suitable for the transmission of large amounts of electrical power over long distances, which supports the realization of the ongoing energy transition. State-of-the-art methods require a comparatively large total cell capacitance in the system for energy pulsations during operation. In the present paper, in order to minimize this total capacitance, first a new method is developed to model the system, and second, by help of this model, optimal current trajectories are calculated. These currents are used for control to reduce the energy pulsation over the complete operating range, and thus, to better utilize the hardware. The new method independent on the Clarke transformations is implemented on a laboratory scale setup, and measurement results are presented which validate the new method. Furthermore, the new method is compared to the state-of-the-art method of the compensation of the 2nd harmonic and outperforms the latter significantly. This applies to the entire operating range for different power factors. A total reduction of up to 44% of the energy pulsations is achieved

Hourly Dispatching Wind-Solar Hybrid Power System with Battery-Supercapacitor Hybrid Energy Storage

This dissertation demonstrates a dispatching scheme of wind-solar hybrid power system (WSHPS) for a specific dispatching horizon for an entire day utilizing a hybrid energy storage system (HESS) configured by batteries and supercapacitors. Here, wind speed and solar irradiance are predicted one hour ahead of time using a multilayer perceptron Artificial Neural Network (ANN), which exhibits satisfactory performance with good convergence mapping between input and target output data. Furthermore, multiple state of charge (SOC) controllers as a function of energy storage system (ESS) SOC are developed to accurately estimate the grid reference power (PGrid,ref) for each dispatching period. A low pass filter (LPF) is employed to decouple the power between a battery and a supercapacitor (SC), and the cost optimization of the HESS is computed based on the time constant of the LPF through extensive simulations. Besides, the optimum value of depth of discharge for ESS considering both cycling and calendar expenses has been investigated to optimize the life cycle cost of the ESS, which is vital for minimizing the cost of a dispatchable wind-solar power scheme. Finally, the proposed ESS control algorithm is verified by conducting control hardware-in-the loop (CHIL) experiments in a real-time digital simulator (RTDS) platform