DC System Research Articles

Analysis of the Monitoring and Identification Effect of Cognitive Service Technology on Dc System in Power Grid

This study investigates the impact of cognitive service technology on the monitoring and identification capabilities within a direct current (DC) system in a power grid. This paper introduces a novel Pattern Recognition Neural Network (PRNN) by integrating a Pattern Recognition Algorithm (PRA) and Convolutional Neural Networks (CNN) to analyse patterns in the DC system's data. With the increasing complexity and interconnectivity of modern power systems, the need for advanced monitoring and identification solutions becomes crucial. Cognitive service technology, known for its adaptability and learning capabilities, offers a promising avenue for enhancing the performance and reliability of DC systems. The research employs a comprehensive approach, incorporating data analysis, modelling, and simulation to assess the effectiveness of cognitive service technology in monitoring and identifying critical parameters within the DC system. The study aims to contribute valuable insights into the application of cognitive services for improving the overall efficiency and resilience of power grids operating on direct current, thereby fostering advancements in smart grid technologies

The Study of Multi-Terminal DC Systems in an Offshore Wind Environment: A Focus on Cable Ripple Analysis

This paper studies the THD and AC losses on the DC cables of offshore wind farm-based multi-terminal HVDC systems when they extract and deliver power from and to more than one connection point. In the paper, the study of a full system PLECS + Simulink model with two branches, including a wind resource, a wind turbine, a Permanent Magnet Synchronous Generator (PMSG), a Pulse Width Modulation (PWM) rectifier, a Single Active Bridge (SAB) DC–DC converter, an Input Parallel Output Series (IPOS) DC–DC converter, HVDC cables, and a simplified onshore system, is presented. It focuses on the investigation of the output ripple content of multiple DC–DC converters on DC cables under different wind conditions with different voltage and power ratings. The importance of the study is providing a general understanding of the operation of the innovative offshore wind farm-based DC system, as well as the interaction between different DC–DC converters and their influence on cable ripple content under different situations.

Transient characteristics and protection coordination strategy of hybrid AC / DC system based on modular multilevel converters under AC/DC transmission lines touching fault

Rapid development of hybrid AC/DC transmission systems has increased the probability of the touching faults between AC and DC overhead lines. The mechanism of coupling interaction is so complex that the adaptability of the existing hybrid system protection is affected. Previous studies mainly focus on the line-commutated converter based high-voltage direct current (LCC-HVDC) system and AC system, this paper concentrates on the fault coupling characteristics analysis and coordination protection of modular multilevel converter based on high voltage direct current(MMC-HVDC) and AC transmission systems. The analytical expressions of fault current are deduced under the different operating states of the converters and the valve-side AC circuit breaker. Subsequently, the fault current coupling characteristics and generation mechanism are revealed. On this basis, the adaptability of existing hybrid system protection is assessed. Further, by comparing and analyzing the influence and risk of mal-operation of AC transmission line protection, the coordination reclosing protection strategies for hybrid system is proposed. Finally, the simulation tests were carried out based on PSCAD/EMTDC. The simulation results show that the proposed coordination of AC and MMC-HVDC system reclosing strategy is effective for improving the reliability of hybrid system.

Bipolar Current-Fed DC–DC Converter With Automatic Voltage Balance and Full Range ZVS for Bipolar DC System

Bipolar Current-Fed DC–DC Converter With Automatic Voltage Balance and Full Range ZVS for Bipolar DC System

Three-Level DC-DC Boost Converter for Wind Energy Conversion

In recent years, advancements in power converter technology for wind energy conversion systems have aimed to enhance power output, increase reliability, and improve overall efficiency. However, achieving a fixed and high-power DC output remains a challenge. This study explores three modes of operation for a DC-DC boost converter to enhance fixed DC output, essential for various applications like battery charging and electronics. The research categorizes DC system topologies based on factors such as setup complexity, cost-effectiveness, and performance metrics. It proposes a DC/DC conversion approach that offers cost-effectiveness and efficiency, aiming to set a new standard in topology for future converters. The study addresses key challenges in the efficiency of the step-up process of DC/DC converters, focusing on parameter design, operational principles, and control strategies. Simulation results confirm the effectiveness of the suggested approach, emphasizing its future potential for non-isolated high step-up DC/DC converters. Reduced device voltage stress and increased efficiency represent the two advantages of the suggested three-level DC-DC boost converter, which combines a standard boost converter with a switched capacitor function to supply three capacitors in series. Simulation results demonstrate the converter’s effectiveness in a wind energy conversion system, showcasing its potential for high-power applications.

An optimized Closed-loop Precharging Strategy for Modular Multilevel Converters Based on Power Balancing Control under start or fault restart conditions

Modular multilevel converter (MMC)-based high-voltage DC (HVDC) systems have found rapid growth in power grids. To reduce the inrush current, it is necessary to precharge the submodule (SM) capacitors to the rated voltage before normal operation. In this paper, an optimized pre-charging strategy for MMC is proposed based on power balancing . First, the charging power is obtained by a designed closed-loop control of the SM capacitors. Then the total power released by the AC or DC side could be calculated by power conservation and feedback by controlling the charging current. At the same time, the power difference between the upper and lower bridge arm of each phase and the internal voltage balancing of bridge arms are also considered by the proposed bridge arm and SM voltage algorithms. By comparing with conventional closed-loop pre-charging strategies, the pre-charging speed is greatly enhanced as the achievement of synchronous charging of upper and lower bridge arm SMs. Moreover, as the same voltage and current double closed-loop control applied in both MMC pre-charging and normal state, the state transition time could be reduced. Finally, a series of simulations and prototype experiments are conducted to verify the feasibility of the proposed pre-charging strategy.

Overvoltage Suppression Strategy of LCC-HVDC Delivery System Based on Hydropower Phase Control Participation

In a high-voltage direct current (HVDC) transmission system, commutation failure at the receiving end may lead to transient overvoltage at the sending end converter bus of the weak alterative current (AC) system. Firstly, the principle calculation method of overvoltage generation at the sending end after commutation failure is analyzed. Combined with the output characteristics of the hydroelectric excitation system, a coordinated control strategy for hydroelectric and DC systems is proposed. Since the voltage and current values at the DC outlet of the rectifier side change first after a fault occurs at the receiving end, the relationship equation between DC voltage and AC bus voltage is derived and it is used as an input signal to construct additional excitation control for hydropower stations. The proposed strategy is verified by establishing a simulation hydrogen–wind–solar model bundled via a DC sending system in PSCAD/EMTDC. The simulation results illustrate that the transient overvoltage suppression rates are all more than 35%, and the maximum is 38.53%. The proposed strategy can reduce the overvoltage by 0.126 p.u. compared with the International Council on Large Electric Systems (CIGRE) standard control strategy.

Enhanced energy balancing and optimal load curtailment strategy for DC microgrid integration in hybrid AC/DC distribution networks

AbstractUnleashing the potential of distributed renewable energy sources (RESs), intelligent and autonomous microgrids are becoming pivotal in attaining net‐zero carbon emission goals. Hybrid AC/DC microgrids rise as cutting‐edge microgrid topologies, capitalising on the best of both AC and DC systems. However, the integration of intermittent renewables and uncertainties in loading poses stability challenges. Advanced bidirectional converter controls provide efficient power exchange, but in extreme contingencies, a resilient supervisory control framework (load management/load curtailment approach) is inevitable to withstand/avoid unplanned renewable disruptions/blackouts. Moreover, the operational paradigm shift towards achieving net‐zero emissions, isolated operation of RESs, and conventional load shedding methods are anticipated to encounter substantial challenges, necessitating the development of alternative strategies. In order to improve the stability of hybrid microgrid systems in islanding scenarios, this research presents an energy balancing and load curtailment strategy. The proposed method aims at optimising resource utilisation, prioritising essential loads, and executing an optimal load curtailment strategy (if required), thereby augmenting the stability of systems. Unlike a meta‐heuristic or exhaustive search, which depends on 2n − 1 possible combinations and become unworkable as load numbers increase, the suggested methodology is based on a mathematically modelled load restriction method. By including load criticality, this strategy effectively prevents blackouts even with an increasing number of loads, providing a significantly more useful and practical solution. Additionally, the proposed charging algorithm ensures that the energy storage system imports energy from the grid during off‐peak hours and maximises power generation from the DC subgrid. The efficacy of the proposed strategy is validated using a modified IEEE‐33 bus system as a test case for a hybrid AC/DC microgrid. Simulation results demonstrate the effectiveness of the MILP‐based load curtailment approach in maintaining system stability and preventing blackouts during unforeseen events.

Fault Detection and VSC-HVDC Network Dynamics Analysis for the Faults in Its Host AC Networks

High-voltage direct current (HVDC) transmission is preferred over high-voltage alternating current (HVAC) for long power lines for asynchronous power grid interconnection and high-level renewable energy integration. The control and protection functions associated with HVDC systems help with fast and secure clearance of faults. The control and protection challenges in the embedded HVDC network are of great concern for the stable and secure operation of an HVDC network. The DC fault current may reach an extremely high level in a rather short period because of the low impedance in a DC system, which is dangerous for converters, and disturbances in the AC network directly influence the performance of the HVDC system. Sometimes, faults on the AC side may lead to disconnection or failure of the DC link, causing reliability problems as well as huge economic losses. AC and DC protection solutions are being developed for HVDC systems to enhance their sustainability and reliability. As such, AC and DC faults should be detected and cleared at a faster rate. Therefore, in this article, the feasibility of the synchro-squeezed transform (SST) is analyzed for detection purposes. For more accurate and faster detection, the signal is first decomposed using the empirical mode decomposition (EMD) technique, and then the SST is applied. A discrete Teager energy (DTE) spectrum is obtained with the processed signal, which works as the detection index. The algorithm shows low sampling frequency requirements, with higher efficiency and reliability for the purpose. PSCAD/EMTDC version 4.6 software and MATLAB 2022a software is used for the modeling and simulation.

Frequency Stability Control Strategy for Voltage Source Converter-Based Multi-Terminal DC Transmission System

The voltage source converter-based multi-terminal DC transmission (VSC-MTDC) system can use additional frequency control to respond to the frequency change of faulty AC system. However, the control coefficient of traditional additional frequency control is mostly fixed, and the control flexibility is insufficient, so it cannot be adjusted adaptively according to the frequency change of the system. Therefore, a frequency control strategy of the VSC-MTDC system based on fuzzy logic control is proposed. Based on the DC voltage slope controller, this strategy introduces an additional frequency controller based on fuzzy logic control, takes the frequency deviation and frequency change rate as the additional controller input, and dynamically adjusts the control quantity through the fuzzy logic control link to realize the adaptive adjustment of the VSC-MTDC system to the AC system’s frequency. Finally, a three-terminal flexible HVDC system is built on the PSCAD/EMTDC simulation platform for simulation verification. The results show that the proposed control strategy can effectively use the flexible DC system to support the frequency of the AC system and significantly improve the frequency stability of the faulty AC system.

Ground fault detection in substation DC system based on improved differential current method

DC system is the main electrical equipment and control signal system in the substation, and its stability and reliability are directly related to whether the power system can operate safely. This paper firstly introduces the common methods of insulation detection of DC power system in substation and its advantages and disadvantages, and proposes the differential current method based on unbalanced bridge, i.e., using the combination of unbalanced bridge and leakage current sensor to detect the insulation condition of DC system, which makes up for the defects of the traditional differential current method that can only detect the faults of simple branch circuits, and realizes the detection of multi-branch grounding under complex situations. Finally, the feasibility of this method is verified by building a simulation model through simulink.

A Comprehensive Stability Assessment System for EV DC Charging Station Based on Multitimescale Impedance Model

The existing all-in-one impedance-based stability analysis only qualitatively reveal the instability factors by the graphic Nyquist criterion while the stability margin is not considered. In this article, a design-oriented multiattribute and multidimensional based comprehensive assessment system (MAMD-CAS) is proposed to evaluate the stability quantitatively and guide the parameter design as well as ensure the desired dynamic response and stability margin. This MAMD-CAS mainly includes the loop-gain-based multi-timescale impedance estimation to analyze the low-frequency oscillation (LFO), phase margin (PM) calculation of different timescales and dynamic performance indicators as the time-domain quantitative expression of impedance estimation. In detail, the multi-timescale impedance model is established and its loop-gain representation is deduced to implement the impedance estimation. Combined with the known PM, the influence of the parameters on dynamic performance and stability margin can be comprehensively evaluated, then the conflict between dynamic performance and stability margin is explored. It is found that the voltage-loop parameters do not have the coupling problem between dynamic performance and stability margin while <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">vir</sub> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">svr</sub> present a conflicting effect between the inertia-loop LFO dynamic performance and stability margin and their interaction affects the severity of inertia-loop LFO. The appropriate control parameters can be thus selected for a good stability margin while suppressing the impedance peak. This MAMD-CAS analysis result is verified by simulation and experiment and this assessment system provides theoretical fundamental for the online stability monitor of multi-timescale power electronics system.

Automatic transfer switch for DC system application

Automatic transfer switches (ATS) have the potential to be important in ensuring uninterruptible power supply across a wide range of sectors, enabling smooth transitions between utility and emergency RES power sources, generators, and more. Utilizing DC electricity provides a cost advantage and is suitable for short-distance transmission. The author proposes a 12-volt DC backup system that corresponds to a minimum input voltage of more than 10 Vdc and can be applied in various DC load applications. The ATS design incorporates multiple inputs and uses algorithms to prevent simultaneous operation of all sources and reduce feedback. These developments in backup system technology, coupled with the shift to renewable energy, are contributing to the creation of a sustainable and resilient electricity grid. In experiments, the ATS system demonstrated its ability to provide 24 hours of backup power with a specified load (lighting bulbs, light-emitting diodes (LEDs), motors, and fans) discharging 0.27–2.25 Amperes. Using the millis programming language, it supports the performance of a constant switch transition speed of around 0.6453 seconds for a switch from buck to SCC. 0.658 seconds for solar charge controller (SCC) to power supply (PSU), 0.656 seconds for PSU to CC PSU, and 0.61 seconds for PSU to OFF.

Coordinated DC voltage control strategy for the receiving end hybrid LCC-VSC system

The receiving end hybrid line commutated converter (LCC) — voltage source converter (VSC) system has the obstacle that when an AC fault occurs at the sending LCC side, the DC voltage of the rectifier side will fall, resulting in the power transmission reduction of the whole system. Particularly, when the rectifier side DC voltage falls below the inverter side DC voltage, the power transmission is interrupted, posing a huge threat to the stability of the power grid at both the sending and receiving ends. To tackle these problems, a coordinated DC voltage control strategy for the receiving end is proposed. By controlling the DC voltage of the receiving VSC, the proposed method can improve the DC system power transmission capability when voltage drops at the sending side. The proposed control strategy is simple but practical, which is good for the stable and secure operation of both the sending and the receiving power system.

Study on the topology of a system for delivery of electricity from deep sea wind through voltage source converter

It is a difficult problem to maintain high reliability while designing the topology of the voltage source converter (VSC) for deep-sea wind power in a compact manner. In this paper, the topology of the converter, the main wiring type and grounding method, and the DC system wiring scheme are studied. The advantages and disadvantages of each scheme are compared in offshore wind power application scenarios. Finally, the half-bridge converter topology and the true bipolar with metal return line topology are recommended for the topology design of the VSC system for deep and remote offshore wind power. The power operating range is also presented. The scheme proposed in this paper paves the way for promoting the lightweight and highly reliable system for the delivery of electricity from deep sea wind through VSC.

Advanced Learning Technique Based on Feature Differences of Moving Intervals for Detecting DC Series Arc Failures

DC microgrids are vital for integrating renewable energy sources into the grid, but they face the threat of DC arc faults, which can lead to malfunctions and fire hazards. Therefore, ensuring the secure and efficient operation of DC systems necessitates a comprehensive understanding of the characteristics of DC arc faults and the implementation of a reliable arc fault detection technique. Existing arc-fault detection methods often rely on time–frequency domain features and machine learning algorithms. In this study, we propose an advanced detection technique that utilizes a novel approach based on feature differences between moving intervals and advanced learning techniques (ALTs). The proposed method employs a unique approach by utilizing a time signal derived from power supply-side signals as a reference input. To operationalize the proposed method, a meticulous feature extraction process is employed on each dataset. Notably, the difference between features within distinct moving intervals is calculated, forming a set of differentials that encapsulate critical information about the evolving arc-fault conditions. These differentials are then channeled as inputs for advanced learning techniques, enhancing the model’s ability to discern intricate patterns indicative of DC arc faults. The results demonstrate the effectiveness and consistency of our approach across various scenarios, validating its potential to improve fault detection in DC systems.

Investigation on different materials after pulsed high field conditioning and low-energy H- irradiation

During operation, the radio-frequency quadrupole (RFQ) of the LINAC4 at CERN is exposed to high electric fields, which can lead to vacuum breakdown. It is also subject to beam loss, which can cause surface modification, including blistering, which can result in reduced electric field holding and an increased breakdown rate. First, experiments to study the high-voltage conditioning process and electrical breakdown statistics have been conducted using pulsed high-voltage DC systems in order to identify materials with high electric field handling capability and robustness to low-energy irradiation. In this paper, we discuss the results obtained for the different materials tested. To complement these, an investigation of their metallurgical properties using advanced microscopic techniques was done to observe and characterize the different materials and to compare results before and after irradiation and breakdown testing.

Multifaceted Functionalities of Bridge-Type DC Reactor Fault Current Limiters: An Experimentally Validated Investigation

With the ongoing expansion and interconnection of electrical power systems, alongside the rapid proliferation of renewable distributed generations (DGs), the short-circuit extent in the power grid is experiencing a significant rise. Fault current limiters (FCLs) have been introduced in an effort to address this issue, ensuring the robustness and sustainability of expensive power system components when confronted with short-circuit faults. Among the various types of FCLs, bridge-type DC reactor fault current limiters (BDCR-FCLs) have emerged as one of the most promising options. While BDCR-FCLs have shown excellent properties in limiting harmful short-circuit currents, they are also advantageous in other respects. This paper investigates the supplementary functionalities of BDCR-FCLs as a multifaceted device towards the enhancement of the quality of supplied energy in terms of total harmonic distortion (THD) reduction, power factor (PF) correction, peak current reduction for nonlinear loads, and soft load variation effects, as well as their capability to limit fault current. To this aim, the capabilities of BDCR-FCLs have been studied through various simulated case studies in PSCAD/EMTDC software V5.0.1, in addition to experimental tests considering an AC microgrid connected to a DC system. The experimental and simulation investigations verify the superior multifaceted functionalities BDCR-FCLs introduce in addition to their excellent fault current-limiting capabilities. The results show that PF improved by 6.7% and 7%, respectively, in simulation and experimental tests. Furthermore, the current THD decreased by 20% and 18% in the simulation and experiment, respectively.

DC Series Arc Fault Diagnosis Scheme Based on Hybrid Time and Frequency Features Using Artificial Learning Models

DC series arc faults pose a significant threat to the reliability of DC systems, particularly in DC generation units where aging components and high voltage levels contribute to their occurrence. Recognizing the severity of this issue, this study aimed to enhance DC arc fault detection by proposing an advanced recognition procedure. The methodology involves a sophisticated combination of current filtering using the Three-Sigma Rule in the time domain and the removal of switching noise in the frequency domain. To further enhance the diagnostic capabilities, the proposed method utilizes time and frequency signals generated from power supply-side signals as a reference input. The time–frequency features extracted from the filtered signals are then combined with artificial learning models. This fusion of advanced signal processing and machine learning techniques aims to capitalize on the strengths of both domains, providing a more comprehensive and effective means of detecting arc faults. The results of this detection process validate the effectiveness and consistency of the proposed DC arc failure identification schematic. This research contributes to the advancement of fault detection methodologies in DC systems, particularly by addressing the challenges associated with distinguishing arc-related distortions, ultimately enhancing the safety and dependability of DC electrical systems.

A ZVS Branch-Sharing Partial Power Converter With Bipolar Voltage Regulation Capability

The isolated input-parallel-output-series (IPOS) partial power converter (PPC), which is composed of a dc transformer (DCX) cell and a voltage regulation (VR) cell, is regarded as a promising solution for dc distribution power systems for only a partial power is delivered by the VR cell under different voltage conditions. In this paper, a branch-sharing IPOS-PPC with bipolar voltage regulation capability is proposed, which is composed of two cells: 1) A half-bridge LLC resonant cell works at fixed switching frequency as a DCX with desired performance to realize isolation and main power conversion. 2) An auxiliary full-bridge/push-pull (FB/PP) cell is applied to realize the bipolar voltage regulation and the rest partial power conversion. Two cells share the lagging-leg of FB/PP cell to guarantee zero voltage switching (ZVS) in the whole load range from zero to full load. Compared with the traditional unipolar voltage regulation VR cell, the FB/PP cell can not only reduce the power processed by itself within the same voltage regulation range, but also reduce the component count and achieve a compact structure. Finally, a 180-300 V/ 48 V/ 1 kW experimental prototype is built to verify the feasibility and advantages of the proposed converter.