Low Voltage DC Grids Research Articles

An Improved Three-Level Neutral Point Clamped Converter System With Full-Voltage Balancing Capability for Bipolar Low-Voltage DC Grid

An Improved Three-Level Neutral Point Clamped Converter System With Full-Voltage Balancing Capability for Bipolar Low-Voltage DC Grid

Cyber–physical system for fast prototyping of power electronic converters in EMI shaping context

Dynamic development of power systems, which utilize power electronic converters as interfaces between the renewable energy sources, as well as complex control and measuring arrangements, forces the cooperation of specialists representing various fields to assure electromagnetic compatibility (EMC) of the so-called smart industrial systems. Low voltage DC grid applications involving multiple power electronics devices may turn the grid susceptible to noise injection and impair the signal integrity and power quality due to the electromagnetic interference (EMI) emission and due to devices and grid interactions. The EMI propagation leads to misreading measurements and faulty conditions of equipments and in ICT (Information and Communications Technology) systems within the conducted emission frequency range, which impacts the hosting capacity of the grid. This paper proposes a cyber–physical system based on the NI-PXI platform for fast-prototyping and EMC assessment as pre-compliance tool of low-voltage DC grid in smart industrial systems. Thus, the LabVIEW-based design as well as the EMC-testbench system validation were performed with a DC/DC converter through the pulse-width modulation control algorithms. In addition, a case study has been investigated to evaluate the interferences between the converter and Power Line Communication. Hence, the theoretical control algorithms, hardware and software details, and experimental results are discussed, highlighting the paradigms of one flexible and collaborative cyber–physical system from the area of smart industrial systems.

A Flexible Rooftop Photovoltaic-Inductive Wireless Power Transfer System for Low-Voltage DC Grid

A rooftop photovoltaic (PV) system is a significant solution of building-integrated centralized generation in the low-voltage (LV) DC grid. The drilling-free rooftop PV-inductive wireless power transfer (PV-IWPT) system for the LVDC grid can reduce the installation and post-maintenance costs, with the elimination of physical cable connections and preventing breaking of thermal bridge and rain penetration between the rooftop and the inside of the building. Moreover, it has inherent isolation properties between the input and the output terminals. However, it is challenging for the PV-IWPT system to simultaneously achieve the maximum power point tracking (MPPT) for the PV panels and the maximum efficiency point tracking (MEPT) for the system even under diverse irradiance conditions. In this paper, we propose an efficient PV-IWPT system, which consists of a series-series IWPT converter, driven by the PV panels and loaded by the LVDC grid directly. Moreover, we apply in the proposed system a particle swarm optimization with an auxiliary Perturb and Observe (P&O) algorithm and a load matching method without wireless feedback communication to realize MPPT and MEPT in the overall PV-IWPT system simultaneously. Finally, we analyze theoretically and verify experimentally the proposed PV-IWPT system in a 500-W experimental platform under different irradiance conditions.

Design Space Analysis of the Dual-Active-Bridge Converter for More Electric Aircraft

In the literature, different DC/DC power electronic converters (PECs) have been found to interconnect high-voltage DC and low-voltage DC grids in the electric power distribution networks of aircraft. In this scenario, the dual-active-bridge (DAB) converter has been shown to be one of the most promising topologies. The main disadvantages of this PEC are the large output capacitance required to satisfy more electric aircraft (MEA) requirements and the high conduction losses produced in low-voltage power devices of (LV). Therefore, this paper proposes analytical models to determine the voltage ripple and root-mean-square (RMS) current in DC bus capacitors of DABs considering different modulation strategies. Moreover, an analysis of the design space in an MEA case study is performed to evaluate the influence of the design variables in power losses of power devices and peak-to-peak voltage ripple in DC bus capacitors. These models are useful for the design stage of this PEC, as well as to enable multi-objective optimization procedures by reducing the computational cost of these methodologies. Furthermore, the exploration of the switching frequency and limit of the modulation angle aid in reducing the resulting volume of the low-voltage DC capacitor.

Robust nonlinear control of a power flow controller for meshed DC grids

The studied power flow controller is an electronic device capable of controlling the power flows at a node in a meshed low voltage DC grid. It is made of m buck-boost converters connected in parallel on their high voltage-side to a reservoir capacitor. This paper proposes a nonlinear control method achieving robust regulation of the power in each line and of the reservoir voltage inside the converter. This control law only requires measurements of local currents and voltages, and the knowledge of the maximum possible resistance of the grid, whereas prior control laws required the full knowledge of some nominal parameters. Proofs of stability are given for every possible uncertain parameters of the studied grid model whereas prior control laws were only valid at nominal parameters and their neighbourhood. The performance of this control method is asserted through a tenth-scale experimental set-up. Stochastic simulations are conducted to estimate the basin of attraction.

1,500Vdc 급 DC 아크 발생 시험장비를 이용한 Smart Grid 선로의 안전성 분석

A lot of research on DC arc accidents is being conducted because the current during DC arc occurrence is within normal operation, and since it can pose a serious threat to human safety as the overcurrent circuit breaker and earth leakage breaker cannot block the arc accident in advance. especially the safety of many accidents, failures, and fires occurring in LVDC (Low Voltage DC) and Smart Grid lines is currently important in the field of new and renewable energy. In addition, the performance of new and renewable energy facilities applied to LVDC and Smart Grid lines, as well as accidents and fires caused by arcing, are emphasized and the safety of facilities is emphasized. Therefore, in this paper, a DC arc simulator that satisfies the UL1699B international standard was manufactured while simulating the Smart Grid. A circuit that can accurately detect information such as arc voltage and arc current according to arc generation separation distance and separation acceleration change in simulated Smart Grid line with 1~1,500Vdc and 1~10A power section using the manufactured device. And, through the configured circuit, when an arc occurs, the status diagnosis and safety of the 1,500Vdc class Smart Grid line are analyzed and reliability is sought.

Flooded Lead Acid Battery SOC Estimation for Energy Conscious LVDC Building: Warm and Humid Climate

Microgrids make it easier to integrate Renewable Energy Sources (RESs) and Energy-Storage Systems (ESSs) at the consumer level, with the intent of enhancing power quality, reliability, and efficiency. This microgrid concept at the nano grid level is championed by a low voltage DC (LVDC) grid, facilitating the direct integration of several distributed generators, storage and loads that are almost DC source/load. Since the State of Charge (SOC) of the battery is an essential parameter for building energy management systems, careful monitoring of SOC is essential. The SOC of a battery directly reflects its performance; thus, straightforward, reasonably accurate, and timely estimation of SOC protects against overcharging or discharging issues, with fewer processor requirements and computations. A novel SOC estimation method is proposed based on the literature assessment of different existing SOC estimation methods. In regions with a tropical climate, the temperature range over the year is small, and the sunshine is relatively intense and fairly even throughout the year. Also, for all practical purposes, it is a fact that the model that takes inputs at the point of use gives accurate results. Considering the 2 facts mentioned above, the current model uses a less complicated algorithm, within acceptable accuracy for the purpose and relatively user-friendly.
 HIGHLIGHTS
 
 State Of Charge (SOC) estimation of 48V Lead Acid battery for Low Voltage DC (LVDC) building energy management systems
 Straightforward, reasonably accurate, and timely estimation of SOC which protects against overcharging or discharging issues, with fewer processor requirements and computations
 For the SOC estimation, the open-circuit voltage is obtained by measuring the terminal voltage, current and first-order model of the battery developed under similar climatic condition
 Development and testing of an algorithm incorporating temperature-independent first-order battery model for the tropical climate with lesser temperature variation
 
 GRAPHICAL ABSTRACT

Planning and design of urban low-voltage DC grids

Accompanied by further cost degression and technological development in the field of power electronic devices and DC technology, low-voltage DC (LVDC) grids for public power supply gain practical relevance for the future grid design, promising a solution to the technical challenges in the course of energy transition and digitization. In this paper, a comprehensive planning framework for urban public power supply LVDC grids is presented. Based on an analysis of degrees of freedom in such systems including potential for power flow control, the structural design of LVDC grids is investigated by explicitly planning radial, ring and meshed structures applying heuristic and metaheuristic algorithms on georeferenced supply data. Key to accuracy of the developed line courses is the orientation to existing traffic routes, establishing additional practical significance of the proposed approach. Three optimized LVDC grids as result of the case study are compared in terms of technical and economic aspects.

Fault control and line protection strategy for LVDC microgrids based on modified high-frequency-link DC solid state transformer

Fault current with high amplitude and rate of change, together with the vulnerability of electronic devices, poses a great challenge to the line protection in low voltage DC (LVDC) microgrids. A fast fault control and protection method are proposed for LVDC microgrids, based on the modified topology of the high-frequency-link DC solid state transformer (DCSST). By installing two bidirectional antiparallel thyristors in the DCSST and isolation devices at the interface converter side, the freewheeling diodes are protected and current zero-crossings occur due to the submodule capacitor discharge during the fault. Then the line protection could be realized by improved current differential protection, and the mechanical circuit breaker can be tripped around current zero-crossings. It alleviates the high requirement for fast tripping devices (such as solid-state circuit breakers) and fault identification within several milliseconds. The scheme combines the converter fault control with line protection to ensure slow but more economic and reliable fault identification for LVDC grid protection. The proposed strategy is verified by PSCAD/EMTDC simulations and the results show that the fault-tripping-recovery process only takes few tens of milliseconds.

A stepwise methodology for the design and evaluation of protection strategies in LVDC microgrids

Over the past years, there has been an increasing scientific interest in Low Voltage DC grids as an alternative to traditional Low Voltage AC grids, driven by the energy transition. However, fault detection and protection of LVDC microgrids still poses an important challenge for their breakthrough on a large scale. Due to the required speed and reliability of LVDC microgrid protection, an increasing amount of research is focussing on local, measurement-based protection algorithms. While several solutions for such a local, measurement-based fault protection algorithm have been proposed in literature, the design process of these algorithms is often unclear. As a result, it is often not straightforward to an LVDC grid designer how to tackle the design of an LVDC microgrid protection strategy. Furthermore, the framework within which the proposed algorithms are designed is often not discussed, as well as how well the algorithm will perform when the fault conditions deviate from this reference framework. Therefore, this paper presents a stepwise methodology for the design of fault protection strategies in LVDC microgrids with the aim of making this design process more straightforward and transparent. Furthermore, the proposed methodology allows to evaluate the robustness and boundaries of the designed protection strategy easily. The different steps of the methodology are discussed in detail and applied to a case study, followed by a sensitivity analysis of the developed protection algorithm to investigate its boundaries and improve its robustness.

DAB Based DC-DC High Frequency Link PET for Interconnecting MVDC-LVDC Grids

This paper proposes dual active bridge (DAB) based high frequency power electronic transformer (PET) for interconnecting medium voltage dc (MVDC) and low voltage dc (LVDC) grids for dc power distribution. The above proposed concept works on dual active phase shift principle and square wave HF modulation technique for bidirectional power transfer. Compared to the traditional dc transformer scheme, The proposed power electronic transformer (PET) can disconnect from LVDC distribution grid effectively as a dc breaker when a short circuit fault occurs in the distribution grid. The isolated DC-DC PET topology with a wide range of voltage conversion ratio is useful for High Voltage DC tapping. The DAB based on switched capacitor is connected to the medium voltage DC side and acts as an inverter. The proposed topology has the ability to transfer higher power, and lower circulating power, lower high frequency link voltage, and RMS current and peak values with the same transmission power in the MVDC side. This paper analyzes the topology, voltage and power characterization, control strategy in detail. Increase in the intermediate AC frequency will reduce the size of the transformer and other passive elements significantly in the circuit. The theoretical analysis is supported by MATLAB simulation.

Interleaved, Switched Inductor and High-Gain Wide Bandgap Based Boost Converter Proposal

Many applications (electric vehicles, renewable energies, low-voltage DC grids) require simple, high-power density and low-current ripple-boost converters. Traditional step-up converters are limited when large transformation ratios are involved. In this work is proposed a step-up converter that brings together the characteristics of high gain, low ripple, and high-power density. From the converter proposal, a mathematical analysis of its operation is first performed, including its static transfer function, stress of components, and voltage and current ripples. Furthermore, it provides a design example for an application of Vin = 48 V to Vo = 270 V and 500 W. For its implementation, two different wide bandgap (WBG) semiconductor models have been used, hybrid GaN cascodes and SiC MOSFETs. Finally, the experimental results of the produced prototypes are shown, and the results are discussed.

A Modified Hybrid DC Circuit Breaker With Reduced Arc for Low Voltage DC Grids

The increasing demand for the development and utilization of renewable energy resources has lead to a growing interest in the development of LVDC systems. The dc grid provides more flexibility in integrating different forms of renewable energy sources effectively. However, the lack of a reliable protection mechanism remains the main drawback in the growth of dc distribution systems. Unlike ac systems, the absence of a natural zero crossing in dc systems for arc extinction makes the use of a conventional mechanical circuit breaker (MCB) a less reliable solution. The use of a solid-state circuit breaker (SSCB) results in fast fault interruption but reduces the overall efficiency of the system due to the on-state voltage drop of the semiconductor devices. A hybrid circuit breaker (HCB) combining MCB and SSCB yields better static and dynamic performances but the main challenges remains in the demagnetization of the transmission line inductance after a fault interruption and an arc formation between the MCB contacts. This paper proposes a hybrid circuit breaker (HCB) which is suitable for fast fault interruption in low voltage dc (LVDC) systems while alleviating the above issues. Proposed topology employs a semiconductor switch as well as an actively switched capacitor branch in parallel with the main mechanical breaker to facilitate fast current commutation during a fault. The mechanical breaker forming the main branch is turned off at zero voltage. This eliminates the arc formation across the moving contacts of the breaker. Moreover, the fault interruption process does not require a varistor for network demagnetization following the fault current commutation. This paper also presents a discharging mechanism for the capacitor in a practical implementation. Operation of the proposed dc circuit breaker is evaluated through a prototype tested in the laboratory.

Three-Port Bidirectional Operation Scheme of Modular-Multilevel DC–DC Converters Interconnecting MVDC and LVDC Grids

The modular-multilevel-based isolated dc–dc converter (MMDC) is an attractive solution for the interconnection of medium-voltage (MV) and low-voltage (LV) dc grids. With a modular-multilevel converter (MMC) on the MV side and full bridges (FBs) on the LV side, the MMDC can be flexibly scaled to different voltages and power ratings. However, the existing MMDC is limited to the two-port operation where the dc-link voltages and powers of FBs must be always kept identical. This letter proposes an operation scheme that enables fully independent three-port bidirectional operation of MMDC capable for the interconnection of MVDC and two isolated LVDC grids with different nominal voltages. A comprehensive operation strategy is proposed, which contains two operation modes with different modulation schemes for the MMC to optimize the transformer current in the full load range. Compared with the most popular three-port isolated dc–dc converter triple active bridge, the power flow among three ports follows a simple mathematical rule and can be regulated bidirectionally with small computational efforts as well as intrinsically eliminated circulating power. The experimental results validate the feasibility and effectiveness of the proposed operation scheme.

A Selective Power Droop Control for Hybrid Interlinking Converter in AC/LVDC Microgrid

This article proposed a selective power droop control for a hybrid interlinking converter (HIC) in connecting ac grid with low-voltage DC (LVDC) grid. The direct connection to the LVDC means relatively lower converter capacity, lower switching loss, higher efficiency, and lower cost in HIC. However, the small capacity converter and large capacitive coupling impedance of HIC can lead to a relatively lower reactive power compensation range. Therefore, the practical overcapacity situation may be more likely to occur in HIC, which deteriorates the system performance. To address this issue, the unique P-δ and Q-P-V droops are proposed for HIC with automatic selective factors adjustment control. With the proposed control method, the HIC can effectively regulate active/reactive powers and improve power quality. To demonstrate the validity of the proposed structure and its control method, a 5-kW HIC is built, and results are presented for poor power quality situations. The results convincingly indicate that the proposed HIC is efficient and capable of low capacity advantage, multifunction control strategy, and overcapacity operation.

Decentralized Plug-and-Play Protection Scheme for Low Voltage DC Grids

Since the voltages and currents in dc grids do not have a natural zero-crossing, the protection of these grids is more challenging than the protection of conventional ac grids. Literature presents several unit and non-unit protection schemes that rely on communication, or knowledge about the system’s topology and parameters in order to achieve selective protection in these grids. However, communication complicates fast fault detection and interruption, and a system’s parameters are subject to uncertainty and change. This paper demonstrates that, in low voltage dc grids, faults propagate fast through the grid and interrupted inductive currents commutate to non-faulted sections of the grid, which both can cause circuit breakers in non-faulted sections to trip. A decentralized plug-and-play protection scheme is proposed that ensures selectivity via an augmented solid-state circuit breaker topology and by utilizing the proposed time-current characteristic. It is experimentally shown that the proposed scheme provides secure and selective fault interruption for radial and meshed low voltage dc grids under various conditions.

Switched Reluctance Generator for Low Voltage DC Microgrid Operation: Experimental Validation

This paper presents the control of a Switched Reluctance Generator (SRG) for low voltage DC grid with the objective of efficiency maximizing. Analysis of the energy conversion, including electrical machine losses (Joule, magnetic, mechanical) and power converter losses (switching and conduction), has shown that there is an optimal combination of control variables (turn-on and conduction angles, phase current reference), which maximizes the drive efficiency. The control variables are derived from a Finite Element Analysis and parametric optimization algorithm for all of the operating points in the torque-speed plane and stored in lookup tables. The performances are evaluated with intensive numerical simulations and experimental tests with a 8/6 SRG feeding a DC resistive load for different rotational speeds. The results show good performances of the output DC voltage control with low ripples, even in the presence of speed and load variations. Thanks to the optimization, simulation results show that beyond 1500 rpm, drive efficiency is higher than 60 % and almost reaches 70 % at nominal speed. The experimental results show that, for light loads and beyond rated speed, the drive efficiency lies in the range between 60 % and 80 % .

RETRACTED ARTICLE: Power sharing scheme in interconnected DC microgrids – A new approach

While the techno-economic feasibility of Solid State Transformer (SST) is constantly under question, some of the leaders in the power sector have recognised the potential of the technology and have started efforts of commercialisation. One of the important advantages of SST is its ability to integrate the low voltage DC grids technology. The key areas of research in this field is to use the accurate model of SST between two dc grids. In this article, an accurate composite model of SST that incorporates high switching frequency is developed. The model is controlled using a conventional voltage control technique and with a proposed power control technique. Thus developed SST is employed in transferring the power between two ring main dc microgrids (composed with distributed energy resources) using dual active bridge configuration. This article provides detailed insight of power-sharing scheme between two interconnected ring dc microgrids while maintaining its individual bus voltages within prescribed limits with supervisory communication control approach. Experiments have been conducted on the proposed method by using proposed control approach, and its results are explored.

Fuse-Based Short-Circuit Protection of Converter Controlled Low-Voltage DC Grids

Low-voltage dc (LVdc) microgrids emerge as a viable alternative to ac microgrids. A large research interest is noted toward fast and selective protection of dc grids, typically focusing on hybrid or full solid-state solutions. In this article, the use of fuses as short-circuit protection in low-voltage dc microgrids is evaluated. The main advantage of fuses is that they are simple, cheap, standardized, and have low steady-state losses. A theoretical basis is formed to model dc short-circuit currents in grids with a limited short-circuit availability. The outcomes are applied to evaluate the possibilities of fuse protection in LVdc grids. It was found that fuses are an effective means of protection, although the required amount of capacitance at the output of the voltage balancing converter can be high, which impacts the total system cost. A fuse-based protection strategy is presented that highlights the need for additional capacitance to clear faults compared to the necessary capacitance for system stability. An experimental setup was built to validate the claims.

Control and Protection in Low Voltage DC Grids

Many office applications are low power such as computers, monitors, laptops, phones and even LED lighting. Powering them from a 48 volt DC grid is sufficient and has one great advantage. Namely, the 48volt is safe regarding touching. However, protection to prevent excessive short circuit currents has to be provided. In this paper we discuss a 48 volt DC grid that is implemented as a living lab. In the living lab, droop control and short circuit protection are implemented. The 48 volt DC grid is connected to a 350-400 volt DC grid via a bidirectional DCDC converter. The 350-400 volt DC grid is used for powering higher power levels such as air conditioning, battery storage, photovoltaic or an AC grid-tied inverter for power exchange. The theory behind and implementation of droop control, via a hybrid analog/digital controller is discussed along with measurements. Predictive short-circuit protection is discussed to prevent excessive short circuit currents. Earth leakage detection via monitoring is discussed as well as its advantage for DC grids. Droop control and protection are combined in a Grid Manager that regulates each load individually.