Small-scale DC Research Articles

A Centralized Shifted Voltage Control Method for Accurate Power Sharing in DC Islanded Microgrids

Due to line impedance mismatch among renewable energy sources (RESs), it is hard to realize accurate power sharing in the DC microgrid system. To solve this issue, a distributed power sharing strategy for adjusting the RES output voltage is developed by adding shifted output voltage into each local controller. Thanks to the shifted voltage, the influence of voltage drop caused by the droop controller is effectively mitigated, so that the DC bus voltage is constantly balanced regardless of the load changes. The proposed method is realized with a centralized approach, and all the required control variable to determine the reference voltage is transmitted through low-bandwidth communication. The controller design and system stability are analyzed in detail with a simplified microgrid model. Small-scale DC microgrid is simulated to verify the effectiveness of the centralized shifted voltage control method.

Composite Switched Lyapunov Function-Based Control of DC–DC Converters for Renewable Energy Applications

Renewable energy sources play a pivotal role in the pursuit of sustainable and eco-friendly power solutions. While offering environmental benefits, they present inherent challenges. Photovoltaic systems rely on surrounding conditions, wind systems contend with variable wind speeds, and fuel cells are both costly and inefficient. Furthermore, the energy injected by renewable energy sources (RES) exhibits unpredictable behavior. To tackle these problems, researchers employ diverse power electronic devices and converters like inverters, power quality filters, and DC–DC choppers. Among these, DC–DC converters stand out for effectively regulating DC voltage and enhancing the efficiency of RESs. The meticulous selection of a suitable DC–DC converter, coupled with the integration of an efficient control technique, significantly influences overall power system performance. This paper introduces a novel approach to the design of switching controllers for DC–DC converters, specifically tailored for application in renewable energy systems. The proposed controller leverages the power of composite switched Lyapunov functions (CSLF) to enhance the efficiency and performance of DC–DC converters, addressing the unique challenges posed by renewable energy sources. Through comprehensive analysis and simulation, this study demonstrates the efficacy of the controller in optimizing power transfer, improving stability, and ensuring reliable operation in diverse renewable energy environments. Moreover, the small-scale DC–DC converter experiment’s findings are presented to confirm and validate the proposed scheme’s practical applicability.

An Advanced Power Flow Control in Small Scale DC Power Structure by Using Multilevel Converter

Because they combine outstanding harmonic performance with low switching frequencies, multilevel transforms are attractive options in Small-Scale DC Power Ne2rks. High dependability can also be obtained by including redundant submodules into the cascaded transform chain. DC microgrids are developing as the next generation of smallscale electric power structures, with very low line impedance. This phenomena creates high currents in microgrids even with little voltage changes; hence, a power flow controller must have quick transient reaction and accurate power flow management. Multi-level transforms are used as power flow controllers in this work to provide high speed and high accuracy power flow management in a dc microgrid. Because a multi-level transform is employed, the output filter can be tiny. The linear controller, such as PI or PID, is established and widely used in the power electronics sector, but its performance degrades as system parameters change. In this paper, a neural structure (NN) based voltage management technique for a DC-DC transform is developed. This project also shows how to construct the output LC filter of a multi-level transform to meet a current ripple requirement. In comparison to typical 2-level transforms, we can demonstrate that a multilevel transform with a smaller filter may provide high-speed and high-precision power flow management for low line impedance situations. MATLAB/Simulink Simulation results are used to evaluate the control performance of each output current in the step response while accounting for transient variations in the power flow.

Design and Comparison of Different Types of Synergetic Controllers for Islanded DC Microgrids

In this paper, different types of synergetic controllers (SCs), such as simple SC, piecewise linear function form of SC, linear synergetic controller, improved synergetic controller with terminal SC, and fast terminal SC have been designed and applied to an islanded DC microgrid. To be able to make the design of the controllers independent from system component values, a generalized mathematical method has been derived for all power electronics-based subsystems of the microgrid system. A small-scale DC microgrid has been designed and implemented to verify experimentally the robustness of the control law from the different synergetic controllers. Moreover, a generalized synergetic controller design approach has been derived in this paper for the islanded DC microgrids. The obtained results from both simulation and experiment tests have been compared and included in this paper. It is concluded that definition of macro variables is affecting robustness and tracking performance of the designed controllers.

An Advanced Power Flow Control in Small Scale DC Power Structure by Using Multilevel Converter

Because they combine outstanding harmonic performance with low switching frequencies, multilevel transforms are attractive options in Small-Scale DC Power Ne2rks. High dependability can also be obtained by including redundant submodules into the cascaded transform chain. DC microgrids are developing as the next generation of smallscale electric power structures, with very low line impedance. This phenomena creates high currents in microgrids even with little voltage changes; hence, a power flow controller must have quick transient reaction and accurate power flow management. Multi-level transforms are used as power flow controllers in this work to provide high speed and high accuracy power flow management in a dc microgrid. Because a multi-level transform is employed, the output filter can be tiny. The linear controller, such as PI or PID, is established and widely used in the power electronics sector, but its performance degrades as system parameters change. In this paper, a neural structure (NN) based voltage management technique for a DC-DC transform is developed. This project also shows how to construct the output LC filter of a multi-level transform to meet a current ripple requirement. In comparison to typical 2-level transforms, we can demonstrate that a multilevel transform with a smaller filter may provide high-speed and high-precision power flow management for low line impedance situations. MATLAB/Simulink Simulation results are used to evaluate the control performance of each output current in the step response while accounting for transient variations in the power flow.

Utilising SMES-FCL to improve the transient behaviour of a doubly fed induction generator DC wind system

Wind energy is seen as one of the main pillars of renewable energy. However, the intermittent nature of these sources still poses as a major challenge. Moreover, sensitivity to grid faults and response to load changes are also main concerns.Superconducting devices have been introduced to solve grid faults and energy storage problems associated with renewable energy sources. Nevertheless, the cost of superconducting materials was still a major drawback for their application in power grids.In this paper, a novel power electronics circuit is used to connect the superconducting magnetic energy storage (SMES) to a DC system based on a doubly fed induction generator wind turbine. The proposed system merges energy storage function and the fault current limiting function into one device which is referred to as SMES-FCL in this paper.The role played by the SMES-FCL is studied under various scenarios that may affect the whole system. The study of the system is carried in MATLAB/SIMULINK where the system is simulated in standalone and grid-connected modes. In the end, the proposed SMES-FCL control circuit is tested in a small-scale DC system experimentally.

Intelligent Operation of Small-Scale Interconnected DC Grids via Measurement Redundancy

Interconnected dc grids are studied in this paper, which comprise resistive and constant-power loads (CPLs) fed by photovoltaic (PV) units. All the sources and CPLs are connected to the grid via dc–dc converters. Nonlinear behavior of PV units in addition to the effect of negative-resistance CPLs can destabilize the dc grid. Thus, the decentralized nonlinear model and intelligent control are proposed using adaptive output-feedback controller to stabilize the grid. The use of the output-feedback control makes possible the utilization of other available signals, in case of loss of main signal, at the converter location and creates measurement redundancy that improves reliability of the dc network. The switching between measured signals of different types are performed through using the neural network (NN) controllers without the need to further tuning. The stability of the entire network is assured through Lyapunov stability method while each converter employs only local measurements. The adaptive NNs are utilized to overcome the unknown dynamics of the dc–dc converters at distributed energy resources and CPLs and those of the interconnected network imposed on the converters. Simulation and experimental results are provided on a small-scale dc grid to show the effectiveness of the developed model and the proposed controller.

Application of Multi-Level Converter for Fast Current Control in Small-Scale DC Power Network

DC microgrids have been emerging as next-generation small-scale electric power networks, where the line impedance is very low. This phenomenon causes large currents in the microgrids, even for a slight change in voltage; therefore, it is critical for a power flow controller to have faster transient response and precise power flow control. In this study, multi-level converters are applied as the power flow controllers to realize high-speed and high-precision power flow control in a dc microgrid. The output filter can be small, as a multi-level converter is used. This paper also presents the design of the output LC filter of a multi-level converter to satisfy a requirement of current ripple. We experimentally verified that a multi-level converter with a smaller filter can realize high-speed and high-precision power flow control for low line impedance conditions compared with the conventional two-level converters.

Mathematical representation, stability analysis and performance improvement of DC microgrid system comprising hybrid wind/battery sources and CPLs

A microgrid is a well known solution to increase the technical and economical capability of distributed generations. Recently, DC microgrids, as small-scale DC networks, have attracted more attention due to lower losses and simple control structure. Stability of DC microgrids can be an important issue under high penetration of constant power loads (CPLs). In this study, stability analysis of the DC microgrid system including hybrid wind/battery and CPLs is studied, and then three different types of stabilising compensators are presented in two groups to increase the stability margin and to stabilise the system under high penetration of CPLs. The proposed stabilising compensators modify the voltage reference, current reference and duty cycle by adding active damping signals in the external, middle and internal parts of the interfaced converter control loops. These active damping signals are obtained by the feedback of the output voltage and current and modify the DC microgrid control structure. To evaluate the system stability under different stabilising compensators, small signal and frequency response analyses are done with and without active damping signals. Simulation results based on a detailed DC microgrid model are given to verify the effectiveness of the proposed compensators.

A Dual-Window DC Bus Interacting Method for DC Microgrids Hierarchical Control Scheme

Hierarchical control schemes have been commonly employed in dc microgrid controls, which combine local control, dc bus voltage coordination, and communication links to guarantee smart operation of dc microgrids. Conventional dc bus voltage regulation cannot conduct signal exchange. Therefore, external communication links are usually needed for hierarchical control schemes. However, once the communication link fails, the hierarchical control system will lose its ability to coordinate the distributed power smartly. This paper proposed a dual-window dc bus interacting (DBI) method to exchange information between the distributed energy sources, for the situations where communication link fails or is not available. A small-scale dc microgrid experimental system was set up, and a simple master-slave control scheme was implemented without communication link to demonstrate the feasibility of the proposed DBI method for dc microgrid controls. The expandability and immunity of the proposed DBI method were also evaluated.

Stability of the Small-Scale Interconnected DC Grids via Output-Feedback Control

A decentralized nonlinear model and controller is proposed to stabilize the interconnected small-scale islanded dc grids in the presence of renewable energy sources with proven stability in this paper. The dc interconnected network comprises dc sources along with resistive and constant-power loads (CPLs). Though the dc sources are photovoltaic (PV) in this paper, the proposed controller can be applied to other types of low-inertia intermittent sources as well. All sources and/or CPL loads are connected to the grid through simple dc–dc converters (DDCs) to avoid power electronic complexities. The negative-resistance CPLs can destabilize the grid in the presence of the DDC dynamical components. The decentralized nonlinear output-feedback controller mitigates rapid voltage and power oscillations caused by the disturbances and measurement noises, and stabilizes the grid. Since the proposed output-feedback controller needs only partial knowledge of the local converter states, the number of measure points reduces leading to a simple implementation. Simulation results on a small-scale dc grid are provided to show the performance of the proposed controller.

A Control Method of a Small-Scale DC Power System Including Distributed Generators

This paper describes a dc micro-grid system interconnecting distributed power generators. The system consists of five generation and control units; a solar-cell generation unit, a wind-turbine generation unit, a battery energy-storage unit, a flywheel power-leveling unit, and an ac grid-interconnecting power control unit. The control method is proposed for suppressing the circulating current by detecting only the dc grid voltage. This method brings high reliability, high-flexibility and maintenance-free operation to the system. The method pays attention to dc output voltage performance of each unit. Each of the power control unit and the energy-storage unit is controlled to act as a voltage source with imaginery impedance. On the other hand, each of the two generation units is controlled to act as a current source. The power-leveling unit is controlled to act as a current source having the function of frequency selectivity like a high-pass filter. A 10-kW prototype system verifies experimentally the validity and effectiveness of the proposed control method for the dc-grid system.

Arc characteristics in small-scale DC plasma arc furnaces using graphite cathodes

The characteristics of electric arcs using graphite cathodes operating in small-scale DC transferred-arc furnaces and an in-flight plasma reactor have heen investigated. The variation oh arc voltage with arc length at currents up to 1.5 kA has been studied. These characteristics are relevant to the engineering design and evaluation of DC plasma furnaces and reactors for the development of chemical and metallurgical processes.

Magnetron sputtering of Cu 55Ni 45

We present some preliminary results on the variation of the properties of sputtered Cu 55Ni 45 alloy films as a function of deposition conditions. The films were prepared on Pyrex substrates in a uhv getter sputter deposition system using a small-scale dc magnetron source. Deposition rates were of order of 1 nms −1. The sputtering gas pressure, target-substrate distance and film thickness have all be systematically varied. It has been found that the thermo-electric powers of films are a strong function of the film structure, so a simple comparative measure of this property has been made. X-ray diffraction studies have been used to measure the average grain sizes of the films. The objective of the study is to relate the film properties to their growth conditions and to make a comparison with the zone model proposed by Movchan and Demshishin ( Fiz Met Metaloved, 28, 653 (1996)) and extended by Thornton ( J Vac Sci Technol, 11, 666 (1974)). In particular an attempt has been made to quantify the effect of the arrival of energetic atoms at the surface of the growing film.