System configuration, fault detection, location, isolation and restoration: a review on LVDC Microgrid protections

Low voltage direct current (LVDC) distribution has gained the significant interest of research due to the advancements in power conversion technologies. However, the use of converters has given rise to several technical issues regarding their protections and controls of such devices under faulty conditions. Post-fault behaviour of converter-fed LVDC system involves both active converter control and passive circuit transient of similar time scale, which makes the protection for LVDC distribution significantly different and more challenging than low voltage AC. These protection and operational issues have handicapped the practical applications of DC distribution. This paper presents state-of-the-art protection schemes developed for DC Microgrids. With a close look at practical limitations such as the dependency on modelling accuracy, requirement on communications and so forth, a comprehensive evaluation is carried out on those system approaches in terms of system configurations， fault detection, location, isolation and restoration

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

Integrated charging of EVs using existing LVDC light rail infrastructure : a case study

This paper outlines an approach to integrating electric vehicle (EV) charging systems to existing low voltage direct current (LVDC) public electrical transport infrastructure. Existing utility networks face challenges of accommodating a multitude of new connections associated with the adoption of EV charging infrastructure but when present, electrical light rail or tram networks represent a good opportunity to provide fast construction and less disruptive city centre charging implementation. Light rail network operation requires immediate power capacity to be available from any point on the network but if this margin were to be relaxed it opens up opportunities for sharing the available capacity with EV charging systems. This paper presents an electrical capacity assessment based on four separate charging control strategies applied to the public tram system in the City of Edinburgh, Scotland. The results of these studies, earthing and wider system protection requirements are considered and preliminary findings made

Smart electric vehicle charging strategy in direct current microgrid

This thesis proposes novel electric vehicle (EV) charging strategies in DC microgrid (DCMG) for integrating network loads, EV charging/discharging and dispatchable generators (DGs) using droop control within DCMG. A novel two-stage optimization framework is deployed, which optimizes power flow in the network using droop control within DCMG and solves charging tasks with a modified Djistra algorithm. Charging tasks here are modeled as the shortest path problem considering system losses and battery degradation from the distribution system operator (DSO) and electric vehicles aggregator (EVA) respectively. Furthermore, a probabilistic distribution model is proposed to investigate the EV stochastic behaviours for a charging station including time-of-arrival (TOA), time-of-departure(TOD) and energy-to-be-charged (ETC) as well as the coupling characteristic between these parameters. Markov Chain Monte Carlo (MCMC) method is employed to establish a multi-dimension probability distribution for those load profiles and further tests show the scheme is suitable for decentralized computing of its low burn-in request, fast convergent and good parallel acceleration performance. Following this, a three-stage stochastic EV charging strategy is designed to plug the probabilistic distribution model into the optimization framework, which becomes the first stage of the framework. Subsequently, an optimal power flow (OPF) model in the DCMG is deployed where the previous deterministic model is deployed in the second stage which stage one and stage two are combined as a chance-constrained problem in stage three and solved as a random walk problem. Finally, this thesis investigates the value of EV integration in the DCMG. The results obtained show that with smart control of EV charging/discharging, not only EV charging requests can be satisfied, but also network performance like peak valley difference can be improved by ancillary services. Meanwhile, both system loss and battery degradation from DSO and EVA can be minimized.Open Acces

Improved voltage-based protection scheme for an LVDC distribution network interfaced by a solid state smart transformer

The increasing electrification of transport and heat will place increasing demand on low voltage (LV) networks with the potential to overload medium voltage (MV)/LV transformers and LV cables. Deployment of a solid-state transformer (SST) at MV/LV substations and using LV direct current (LVDC) distribution systems offer great potential to address such challenges. However, the SST deployment in addition to the introduction of LVDC will fundamentally change LV fault behaviour and protection requirements due to the limited short-circuit capabilities of such technologies. The SST will deliver limited fault currents, making current-based protection (widely used in LV networks) less reliable. Therefore, this study presents an advanced communication-less protection scheme which can effectively detect and locate DC faults even with reduced fault levels. The developed protection scheme overcomes the selectivity limitations in LVDC voltage-based protection solutions by using a combination of DC voltage magnitude, voltage concavity (sign of d2v/dt2) and the sign of the rate of change of current (di/dt) regardless of the current magnitudes. The credibility of the developed protection algorithm is tested against different fault scenarios applied on an active LVDC network model built in PSCAD/EMTDC. Noise signals have been included in the simulation to appraise the resilience of the developed scheme

Protection strategy in active DC power distribution networks

Environmental incentives to combat climate change are providing the motivation to improve the energy efficiency of power distribution systems and integrate state-of-the-art renewable technologies. DC distribution networks are receiving considerable attention in the literature because they offer a simple and flexible interface between these modern resources and consumers. However, many technical challenges relating to the design and standardisation of DC protection devices still exist that must be overcome prior to widespread adoption. Since DC fault current develops rapidly, many high-speed protection schemes tailored for DC networks have been proposed. However, few of them have considered the difficulties in practical implementation. This thesis will present the implementation challenges and propose corresponding protection schemes to address the issues. In seeking to achieve this aim, the work presented within this thesis makes three main contributions. This thesis has fi�rstly improved the reliability of the high-speed DC differential protection scheme. The main implementation challenge of this scheme is that a short time synchronisation error may cause a signi�ficant current difference error, resulting in a false-trip problem when a fault occurs outside the protected zone. This thesis has proposed a "multi-sample differential (MSD) protection scheme" to ensure the protection stability for external zone faults (i.e., the relays must not operate) whilst maintaining sensitivity for internal zone faults (i.e., the relays must operate) by examining multiples measurement samples. Secondly, the difficulty in realising high-speed DC distance protection is that measurement of rate-of-change of current can be severely affected by even low-level noise, resulting in a failure in fault detection. This thesis has presented the methodology for selecting the appropriate sampling time of the numerical derivative as well as the cut-off frequency of low-pass current measurement �lfiters. Although high-speed protection schemes can effectively isolate faults quickly, their implementation requires many advanced devices, which may not be economical for lowpower and low-cost DC networks. Finally, this thesis has proposed a "modulated low fault-energy (MLE) protection scheme" that employs fault current limiters (FCL) at the grid energy sources and mechanical circuit breakers (MCB) elsewhere throughout the distributed network. This deployment can constrain the fault current to a lowenergy level that enables a longer time window for the downstream MCBs to realise protection with a lower total implementation cost. Drawing conclusions from this PhD research, the author advocates that more consideration should be given to implementation challenges when designing protection schemes in DC distribution networks. Excessive pursuit of ultrafast fault isolation speeds can lead to over-cost and protection instability issues in practice. A prospective protection scheme must compromise between the high-speed protection requirements in theory and the reliable but economical requirements in practice, to accelerate the realisation of large-scale DC grids in future.Environmental incentives to combat climate change are providing the motivation to improve the energy efficiency of power distribution systems and integrate state-of-the-art renewable technologies. DC distribution networks are receiving considerable attention in the literature because they offer a simple and flexible interface between these modern resources and consumers. However, many technical challenges relating to the design and standardisation of DC protection devices still exist that must be overcome prior to widespread adoption. Since DC fault current develops rapidly, many high-speed protection schemes tailored for DC networks have been proposed. However, few of them have considered the difficulties in practical implementation. This thesis will present the implementation challenges and propose corresponding protection schemes to address the issues. In seeking to achieve this aim, the work presented within this thesis makes three main contributions. This thesis has fi�rstly improved the reliability of the high-speed DC differential protection scheme. The main implementation challenge of this scheme is that a short time synchronisation error may cause a signi�ficant current difference error, resulting in a false-trip problem when a fault occurs outside the protected zone. This thesis has proposed a "multi-sample differential (MSD) protection scheme" to ensure the protection stability for external zone faults (i.e., the relays must not operate) whilst maintaining sensitivity for internal zone faults (i.e., the relays must operate) by examining multiples measurement samples. Secondly, the difficulty in realising high-speed DC distance protection is that measurement of rate-of-change of current can be severely affected by even low-level noise, resulting in a failure in fault detection. This thesis has presented the methodology for selecting the appropriate sampling time of the numerical derivative as well as the cut-off frequency of low-pass current measurement �lfiters. Although high-speed protection schemes can effectively isolate faults quickly, their implementation requires many advanced devices, which may not be economical for lowpower and low-cost DC networks. Finally, this thesis has proposed a "modulated low fault-energy (MLE) protection scheme" that employs fault current limiters (FCL) at the grid energy sources and mechanical circuit breakers (MCB) elsewhere throughout the distributed network. This deployment can constrain the fault current to a lowenergy level that enables a longer time window for the downstream MCBs to realise protection with a lower total implementation cost. Drawing conclusions from this PhD research, the author advocates that more consideration should be given to implementation challenges when designing protection schemes in DC distribution networks. Excessive pursuit of ultrafast fault isolation speeds can lead to over-cost and protection instability issues in practice. A prospective protection scheme must compromise between the high-speed protection requirements in theory and the reliable but economical requirements in practice, to accelerate the realisation of large-scale DC grids in future

Review on Control of DC Microgrids and Multiple Microgrid Clusters

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

Power Electronics Converters for the Internet of Energy: A Review

This paper presents a comprehensive review of multi-port power electronics converters used for application in AC, DC, or hybrid distribution systems in an Internet of Energy scenario. In particular, multi-port solid-state transformer (SST) topologies have been addressed and classified according to their isolation capabilities and their conversion stages configurations. Non-conventional configurations have been considered. A comparison of the most relevant features and design specifications between popular topologies has been provided through a comprehensive and effective table. Potential benefits of SSTs in distribution applications have been highlighted even with reference to a network active nodes usage. This review also highlights standards and technical regulations in force for connecting SSTs to the electrical distribution system. Finally, two case studies of multi-port topologies have been presented and discussed. The first one is an isolated multi-port bidirectional dual active bridge DC-DC converter useful in fast-charging applications. The second case of study deals with a three-port AC-AC multi-level power converter in H-Bridge configuration able to replicate a network active node and capable of routing and controlling energy under different operating conditions