Search Results (135)

Enhancing the resilience of renewable energy systems in ultra-weak grids is crucial for promoting sustainable energy adoption and ensuring a reliable power supply during disturbances. Ultra-weak grids characterized by a very low Short-Circuit Ratio, less than 2, and high grid impedance significantly impair voltage and frequency stability, imposing challenging conditions for Inverter-Based Resources. To address these challenges, this paper considers a 110 KVA, three-phase, two-level Voltage Source Converter, interfacing a 700 V DC link to a 415 V AC ultra-weak grid. X/R = 1 is controlled using Sinusoidal Pulse Width Modulation, where the Grid-Connected Converter operates in Grid-Forming Mode to maintain voltage and frequency stability under a steady state. During symmetrical and asymmetrical faults, the converter transitions to Grid-Following mode with current control to safely limit fault currents and protect the system integrity. After fault clearance, the system seamlessly reverts to Grid-Forming Mode to resume voltage regulation. This paper proposes an improved control strategy that integrates voltage feedforward reactive power support and virtual capacitor-based virtual inertia using Active Disturbance Rejection Control, a robust, model-independent controller, which rapidly rejects disturbances by regulating d and q-axes currents. To test the practicality of the proposed system, real-time implementation is carried out using the OPAL-RT OP4610 platform, and the results are experimentally validated. The results demonstrate improved fault current limitation and enhanced DC link voltage stability compared to a conventional PI controller, validating the system’s robust Fault Ride-Through performance under ultra-weak grid conditions. Full article

Microgrids (MGs) technologies, with their advanced control techniques and real-time monitoring systems, provide users with attractive benefits including enhanced power quality, stability, sustainability, and environmentally friendly energy. As a result of continuous technological development, Internet of Things (IoT) architectures and technologies are becoming more and more important to the future smart grid’s creation, control, monitoring, and protection of microgrids. Since microgrids are made up of several components that can function in network distribution mode using AC, DC, and hybrid systems, an appropriate control strategy and monitoring system is necessary to ensure that the power from microgrids is delivered to sensitive loads and the main grid effectively. As a result, this article thoroughly assesses MGs’ control systems and groups them based on their degree of protection, energy conversion, integration, advantages, and disadvantages. The functions of IoT and monitoring systems for MGs’ data analytics, energy transactions, and security threats are also demonstrated in this article. This study also identifies several factors, challenges, and concerns about the long-term advancement of MGs’ control technology. This work can serve as a guide for all upcoming energy management and microgrid monitoring systems. Full article

Recurrent catastrophic inverter failures significantly undermine the reliability and economic viability of utility-scale photovoltaic (PV) power plants. This paper presents a comprehensive investigation of severe inverter destruction incidents at the Kopli Solar Power Plant, Estonia, by integrating controlled laboratory simulations with extensive field monitoring. Initially, detailed laboratory experiments were conducted to replicate critical DC-side short-circuit scenarios, particularly focusing on negative DC input terminal faults. The results consistently showed these faults rapidly escalating into multi-phase short-circuits and sustained ground-fault arcs due to inadequate internal protection mechanisms, semiconductor breakdown, and delayed relay response. Subsequently, extensive field-based waveform analyses of multiple inverter failure events captured identical fault signatures, thereby conclusively validating laboratory-identified failure mechanisms. Critical vulnerabilities were explicitly identified, including insufficient isolation relay responsiveness, inadequate semiconductor transient ratings, and ineffective internal insulation leading to prolonged arc conditions. Based on the validated findings, the paper proposes targeted inverter design enhancements—particularly advanced DC-side protective schemes, rapid fault-isolation mechanisms, and improved internal insulation practices. Additionally, robust operational and monitoring guidelines are recommended for industry-wide adoption to proactively mitigate future inverter failures. The presented integrated methodological framework and actionable recommendations significantly contribute toward enhancing inverter reliability standards and operational stability within grid-connected photovoltaic installations. Full article

The rapid development of direct current (DC) grids poses significant challenges to the speed, reliability, and selectivity of fault protection systems. These systems are required to identify and distinguish between internal and external faults despite the constraints of limited information and time. This study introduces a non-unit protection scheme based on the classification of two-dimensional feature parameters utilizing the Hilbert–Huang transform (HHT) and a support vector machine (SVM). Through time–frequency analysis of the voltage waveform following DC faults, critical information within the high-frequency component of the fault voltage, specifically, the instantaneous frequency and amplitude of the wavefront, is extracted to distinguish internal from external faults. Two-dimensional feature parameters are associated with signal attenuation and distortion during fault propagation via the transmission path, thereby providing a foundation for precise fault identification. The employment of an SVM ensures the selectivity of this scheme without relying on protection settings. The efficacy of the scheme is validated through simulations conducted using PSCAD/EMTDC across various fault scenarios. Full article

This paper proposes a dynamic current limit for modular multilevel converters (MMCs) that maximizes the injection of current during grid faults in order to mitigate the voltage dip, reduce voltage imbalances in case of an asymmetrical fault, and ensure the proper operation of protective relays. The reduced short-circuit capacity of MMCs, and power converters in general, is one of their main limitations. In the event of a fault, the converter’s current is significantly lower than that of the synchronous generators, which may impact both the performance of power system protective relays and the mitigation of voltage drops during faults. Usually, to protect the MMCs themselves, their output current is limited by their control. However, the current flowing through the power semiconductors is the arm current, not the output current, and this consists of an AC and a DC component. A new current saturation strategy aiming at maximizing fault current injection, in compliance with the most recent grid codes, is proposed. This strategy limits the arm currents by dynamically adjusting the output current limit while injecting reactive currents (both positive- and negative-sequence) and active current according to the grid codes, the fault type, and voltage sag level. A theoretical analysis is carried out to determine the maximum current injection that will not exceed the arm limits, and this is then validated through detailed PSCAD simulations. With the proposed strategy, the supplied current can be increased by approximately 40%. Full article

The insufficient durability of solar energy systems is an important problem in low-voltage situations in the electrical grid. This problem can cause PV systems to become difficult to operate during periods of low voltage and may disconnect PV systems from electrical grids. In this study, a hybrid protection system combining a DC chopper and a capacitive bridge fault current limiter (CBFCL) and based on a machine learning (ML) approach is proposed as a protection strategy to improve the low voltage ride-through (LVRT) capability of a grid-connected PV power plant (PVPP) system. To forecast the best control parameters using real time, including both the fault and normal operation conditions of the grid-connected PVPP system, the ML approach is trained on historical data. Among 20 classifier algorithms, the Coarse Tree classifier and Medium Gaussian SVM classifier have the best accuracy and F1-score for the DC chopper and DC chopper + CBFCL protection systems. The Medium Gaussian SVM classifier has the highest accuracy (98.37%) and F1-score (99.17%) for the DC chopper and CBFCL protection method among the 20 classifier methods. In comparison to another protection system, the simulation results show that a proposed hybrid protection system using SVM offers optimum protection for the grid-connected PVPP system. Full article

This paper presents a controller for a direct current (DC) grid-connected single-stage solar photovoltaic (PV) converter. The proposed controller provides both static and dynamic voltage support to the grid voltage. Unlike the common practice, it allows a small and controlled offset in the PV voltage in proportion to the power flowing through the converter, which enhances the system’s stability margins. A novel feedback branch from the grid voltage is introduced to enable grid voltage support. Additionally, the controller includes a current-limiting feature to protect the converter switches from overcurrent transients. The proposed approach combines and designs the voltage and current controllers using an optimal full-state feedback approach. This results in a systematic design with optimal and robust properties. Detailed simulations, comparisons, and experimental results are presented in this paper to verify the effectiveness of the proposed approach. Particularly, the experimental findings demonstrate improved stability during local load disturbances and grid fluctuations, with lower voltage drops, reduced grid current variations, lower stress on the grid, and reduced losses in the grid network compared to conventional controllers. Full article

With the widespread integration of distributed energy resources and novel loads, the DC attributes of distribution networks are becoming increasingly pronounced. Multi-terminal flexible DC distribution networks have emerged as a trend for future distribution grids due to lower line losses, better power quality, etc. However, owing to their low damping and inertia, the multi-terminal flexible DC distribution network is vulnerable to DC faults. Analyzing the fault characteristics and calculating the fault current level is of great significance for the design of relay protection systems and the optimization of associated parameters. Throughout the fault process, the discharge paths of multiple converters are mutually coupled, and the fault characteristics are complex, which poses a great challenge to short-circuit calculations. This paper proposes a method for calculating the characteristic quantities of the whole network throughout the asymmetric short-circuit fault in a multi-terminal flexible DC distribution network. During the capacitor discharge stage, an equivalent model of the fault port is established before the control response. During the fault ride-through stage, a transfer matrix that takes into account the electrical constraints on both the AC and DC sides of the converters is proposed by combining the equivalent circuit of fully controlled converters. Finally, a simulation model of a six-terminal flexible DC distribution network is developed in PSCAD/EMTDC, and the simulation results demonstrate that the proposed method expands the calculation range from faulty branch to network-wide characteristic quantities throughout the process of asymmetric short-circuit faults, with the maximum relative error remaining below 5%. Full article

As renewable energy sources (RES) continue to expand and the use of power inverters has surged, inverters have become crucial for converting direct current (DC) from RES into alternating current (AC) for the grid, and their security is vital for maintaining stable grid operations. This paper investigates the security vulnerabilities of photovoltaic (PV) inverters, specifically focusing on their internal sensors, which are critical for reliable power conversion. It is found that both current and voltage sensors are susceptible to intentional electromagnetic interference (IEMI) at frequencies of 1 GHz or higher, even with electromagnetic compatibility (EMC) protections in place. These vulnerabilities can lead to incorrect sensor readings, disrupting control algorithms. We propose an IEMI attack that results in three potential outcomes: Denial of Service (DoS), physical damage to the inverter, and power output reduction. These effects were demonstrated on six commercial single-phase and three-phase PV inverters, as well as in a real-world microgrid, by emitting IEMI signals from 100 to 150 cm away with up to 20 W of power. This study highlights the growing security risks of power electronics in RES, which represent an emerging target for cyber-physical attacks in future RES-dominated grids. Finally, to cope with such threats, three detection methods that are adaptable to diverse threat scenarios are proposed and their advantages and disadvantages are discussed. Full article

Modular multilevel converter–high-voltage direct current (MMC-HVDC) systems are a key technology for integrating large-scale offshore wind farms due to their flexibility, controllability, and decoupled active and reactive power characteristics. However, offshore wind farms rely on power electronic converters, resulting in low inertia, which can worsen frequency fluctuations and affect system stability during major disturbances. Additionally, the decoupled power control of MMC-HVDC systems limits wind farms’ inertia contribution to the AC grid, exacerbating inertia deficiency. To address this, a coordinated inertia support strategy is proposed, utilizing a DC voltage–frequency mapping method that enables wind farms to perceive frequency variations without communication and rapidly provide inertia response. This strategy coordinates wind farms and MMC-HVDC systems to enhance frequency support. Simulations demonstrate that the proposed strategy overcomes MMC-HVDC’s decoupling effect, accelerates frequency recovery, and improves the inertia response speed, achieving faster power support and higher peak power output, thereby enhancing frequency stability. Furthermore, PSCAD/EMTDC simulations were conducted to analyze the transient characteristics of MMC-HVDC under AC-side faults, verifying that braking resistors (BRs) effectively suppress DC overvoltage, reducing wind farm power curtailment and improving system security. This study provides a new approach for frequency stability control in MMC-HVDC-based offshore wind integration and serves as a reference for further optimization of inertia support and fault protection strategies. Full article

The growing demand for electricity, integration of renewable energy sources, and recent advances in power electronics have driven the development of HVDC systems. Multi-terminal HVDC (MTDC) grids, enabled by Voltage Source Converters (VSCs), provide increased operational flexibility, including the ability to reverse power flow and independently control both active and reactive power. However, fault propagation in DC grids occurs more rapidly, potentially leading to significant damage within milliseconds. Unlike AC systems, HVDC systems lack natural zero-crossing points, making fault isolation more complex. This paper presents the implementation of a wavelet-based protection algorithm to detect faults in a four-terminal VSC-HVDC grid, modelled in MATLAB and SIMULINK. The study considers several fault scenarios, including two internal DC pole-to-ground faults, an external DC fault in the load branch, and an external AC fault outside the protected area. The discrete wavelet transform, using Symlet decomposition, is applied to classify faults based on the wavelet entropy and sharp voltage and current signal variations. The algorithm processes the decomposition coefficients to differentiate between internal and external faults, triggering appropriate relay actions. Key factors influencing the algorithm’s performance include system complexity, fault location, and threshold settings. The suggested algorithm’s reliability and suitability are demonstrated by the real-time implementation. The results confirmed the precise fault detection, with fault currents aligning with the values in offline models. The internal faults exhibit more entropy than external faults. Results demonstrate the algorithm’s effectiveness in detecting faults rapidly and accurately. These outcomes confirm the algorithm’s suitability for a real-time environment. Full article

DC microgrids are revolutionizing energy systems by offering efficient, reliable, and sustainable solutions to modern power grid challenges. By directly integrating renewable energy sources and eliminating the inefficiencies of AC-DC conversion, these systems simplify energy distribution and enhance performance in critical applications such as data centers, electric vehicle charging, and telecommunications. This review paper comprehensively examines the design, implementation, and performance of DC microgrids in real-world settings. Key components, including distributed energy resources (DERs), energy storage systems (ESSs), and control strategies, are analyzed to highlight their roles in ensuring reliability and operational efficiency. This review also explores the challenges facing DC microgrids, such as stability issues, protection mechanisms, and high initial costs, while offering insights into advanced control strategies and modular designs to overcome these obstacles. Through an evaluation of global case studies, this article bridges the gap between theoretical research and practical deployment and also demonstrates how DC microgrids can enhance energy efficiency, support sustainable power generation, and provide resilience in various applications. The findings highlight the potential of DC microgrids as a cornerstone of future energy systems, enabling clean, reliable, and decentralized energy solutions. Full article

More recently, researchers and the industrial community have started researching DC appliances and DC microgrids as a means of increasing the end-to-end efficiency of systems. Given the fluctuating nature of renewable resources, energy storage becomes mandatory in powering households with minimal AC grid supply, and rechargeable battery packs with maximum power point tracking controllers with inverters are used. However, this approach is not the most efficient due to losses in the power converters used in the energy supply path, while short life and environmental concerns of battery storage also come into play. With the rapid development of commercial super-capacitors, with longer life, higher power density and wider operational temperature range, this device family can be at the center of a new development era, for power converters for DC homes and DC appliances. The new family of converters and protection systems known as supercapacitor-assisted techniques is a unique new approach to minimize or eliminate batteries while improving the ETEE. These new SCA techniques are based on a new theoretical concept now published as supercapacitor-assisted loss management theory. In this paper, we will demonstrate how we extend SCALoM theory to develop SCA converters for whiteware, with the example of a DC-converted commercial double-door refrigerator with implementation details. Full article

As renewable energy resources are increasingly deployed on a large scale in remote areas, their share within the power grid continues to expand, rendering direct current (DC) transmission essential to the stability and efficiency of power systems. However, existing transmission line protection principles are constrained by limited fault feature quantities and insufficient correlation exploration among features, leading to operational refusals under remote and high-resistance fault conditions. To address these limitations in traditional protection methods, this study proposes an innovative single-ended protection principle based on Principal Component Analysis (PCA) and Kernel Density Estimation (KDE). Initially, PCA is employed for multidimensional feature extraction from fault data, followed by KDE to construct a joint probability density function of the multidimensional fault features, allowing for fault type identification based on the joint probability density values of new samples. In comparison to conventional methods, the proposed approach effectively uncovers intrinsic correlations among multidimensional features, integrating them into a comprehensive feature set for fault diagnosis. Simulation results indicate that the method exhibits robustness across various transition resistances and fault distances, demonstrates insensitivity to sampling frequency, and achieves 100% accuracy in fault identification across sampling time windows of 0.5 ms, 1 ms, and 2 ms. Full article

Due to the development of power semiconductors and the increase in digital loads, DC microgrids are receiving attention, and their application scope is rapidly expanding. As the technological stability of high-voltage direct current (HVDC) continues to rise, the potential of low-voltage direct current (LVDC) distribution systems is becoming increasingly intriguing. Many researchers are actively conducting safety and efficiency research on DC distribution systems and power grids. In LVDC distribution systems, small-scale DC microgrids are formed by renewable energy sources supplying DC power. This paper analyzes the efficiency improvement that can be achieved by integrating a fire protection system into a DC microgrid. This research analyzed the changes when fire protection systems such as receivers, transmitters, fire alarms, emergency lighting, and evacuation guidance, which have traditionally used AC power, were converted to DC circuits. As a result, the power supply infrastructure within the DC microgrid can be simplified, energy loss can be reduced, and the stability of the power system can be improved. The research results of this paper suggest that DC circuit-based fire protection facilities can positively impact future smart grid and renewable energy goals. Full article