DC Microgrids E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
DC MICROGRIDS Written and edited by a team of well-known and respected experts in the field, this new volume on DC microgrids presents the state-of-the-art developments and challenges in the field of microgrids for sustainability and scalability for engineers, researchers, academicians, industry professionals, consultants, and designers. The electric grid is on the threshold of a paradigm shift. In the past few years, the picture of the grid has changed dramatically due to the introduction of renewable energy sources, advancements in power electronics, digitalization, and other factors. All these megatrends are pointing toward a new electrical system based on Direct Current (DC). DC power systems have inherent advantages of no harmonics, no reactive power, high efficiency, over the conventional AC power systems. Hence, DC power systems have become an emerging and promising alternative in various emerging applications, which include distributed energy sources like wind, solar and Energy Storage System (ESS), distribution networks, smart buildings, remote telecom systems, and transport electrification like electric vehicles (EVs). All these applications are designed at different voltages to meet their specific requirements individually because of the lack of standardization. Thus, the factors influencing the DC voltages and system operation needed to be surveyed and analyzed, which include voltage standards, architecture for existing and emerging applications, topologies and control strategies of power electronic interfaces, fault diagnosis and design of the protection system, optimal economical operation, and system reliability.
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Table of Contents
Cover
Title Page
Copyright
Preface
1 On the DC Microgrids Protection Challenges, Schemes, and Devices – A Review
1.1 Introduction
1.2 Fault Characteristics and Analysis in DC Microgrid
1.3 DC Microgrid Protection Challenges
1.4 DC Microgrid Protection Schemes
1.5 DC Microgrid Protective Devices (PDs)
1.6 Conclusions
References
2 Control Strategies for DC Microgrids
2.1 Introduction: The Concept of Microgrids
2.2 Introduction: The Concept of Control Strategies
2.3 Control Strategies for DGs in DC MGs
2.4 Conclusions and Future Scopes
References
3 Protection Issues in DC Microgrids
3.1 Introduction
3.2 Fault Detection in DC MGs
3.3 Fault Location
3.4 Islanding Detection (ID)
3.5 Protection Coordination Strategy
3.6 Conclusion and Future Research Scopes
References
4 Dynamic Energy Management System of Microgrid Using AI Techniques: A Comprehensive & Comparative Study
Nomenclature
4.1 Introduction
4.2 Problem Statement
4.3 Mathematical Modelling of Microgrid
4.4 Optimization Algorithm
4.5 Results
4.6 Conclusion
References
5 Energy Management Strategies Involving Energy Storage in DC Microgrid
5.1 Introduction
5.2 Literature Review
5.3 Case Study
5.4 Conclusion
References
6 A Systematic Approach for Solar and Hydro Resource Assessment for DC Microgrid Applications
6.1 Introduction
6.2 Methodology
6.2.1 Data Collection
6.3 Result and Discussion
6.4 Conclusion
References
7 Secondary Control Based on the Droop Technique for Power Sharing
7.1 Introduction
7.2 Voltage Deviation and Power Sharing Issues in Droop Technique
7.3 Design and Implementation of the Communication System
7.4 Conclusions
References
8 Dynamic Analysis and Reduced-Order Modeling Techniques for Power Converters in DC Microgrid
8.1 Introduction
8.2 Need of Dynamic Analysis for Power Converters
8.3 Various Modeling Techniques
8.4 Reduce-Order Modeling
8.5 Illustrative Example with the Power Converter
8.6 Controllers for Power Converter
8.7 Conclusion
References
9 Matrix Converter and Its Probable Applications
9.1 Introduction
9.2 Classification of Matrix Converter
9.3 Problems Associated with the MC and the Drives
9.4 Control Techniques
9.5 Basic Components of the Matrix Converter Fed Drive System
9.6 Industrial Applications of Matrix Converter
9.7 Summary
References
10 Multilevel Converters and Applications
10.1 Introduction
10.2 Multilevel Inverters
10.3 Traditional Multilevel Inverter Topologies
10.4 Advent of Active Neutral Point Clamped Converter
10.5 Conclusion
References
11 A Quasi Z-Source (QZS) Network-Based Quadratic Boost Converter Suitable for Photovoltaic-Based DC Microgrids
11.1 Introduction
11.2 Proposed Converter
11.3 Steady-State Analyses
11.4 Comparison with Other Structures
11.5 Converter Analyzes in Discontinuous Conduction Mode (DCM)
11.6 Simulation Results
11.7 Real Voltage Gain and Losses Analyzes
11.8 Dynamic Behavior of the Proposed Converter
11.9 The Maximum Power Point Tracking (MPPT)
11.10 Conclusions
11.11 Appendix
References
12 Research on Protection Strategy Utilizing Full-Scale Transient Fault Information for DC Microgrid Based on Integrated Control and Protection Platform
12.1 Introduction
12.2 Topological Structure and Grounding Model of Studied Microgrid
12.3 Fault Characteristics of DC Microgrid
12.4 DC Microgrid Protection Strategy
12.5 Simulation Verification
12.6 Conclusion
References
13 A Decision Tree-Based Algorithm for Fault Detection and Section Identification of DC Microgrid
Acronyms
Symbols
13.1 Introduction
13.2 DC Test Microgrid System
13.3 Overview of Decision Tree-Based Proposed Scheme
13.4 DC Microgrid Protection Using Decision Tree Classifier
13.5 Performance Evaluation
13.6 Conclusion
References
14 Passive Islanding Detection Method Using Static Transfer Switch for Multi-DGs Microgrid
14.1 Introduction
14.2 Islanding
14.3 Static Transfer Switch (STS)
14.4 Proposed Scheme of Islanding
14.5 Flow Chart
14.6 Simulation Results
14.7 Experimental Results
14.8 Conclusion
References
Index
Also of Interest
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Grounding configurations comparison [11, 30, 71–75].
Table 1.2 DC microgrids protection challenges, problems and solution methodology...
Table 1.3 DC microgrid protection strategies merits and demerits.
Table 1.4 PDs assessment based on Cost, Efficiency, Reliability, and Fast operat...
Chapter 4
Table 4.1 Cost coefficients.
Table 4.2 Power ratings of DGs.
Table 4.3 Optimization results.
Chapter 5
Table 5.1 Classical approach of EMS.
Table 5.2 Meta-Heuristic approach of EMS.
Table 5.3 Artificial intelligence approach of EMS.
Table 5.4 Model predictive, stochastic and robust programming approach of EMS.
Table 5.5 DC microgrid subsystem’s parameters.
Chapter 6
Table 6.1 Classification of hydro power plants in India.
Table 6.2 Training and testing of ANN architecture data accuracy.
Chapter 7
Table 7.1 Comparison between distributed secondary control techniques for DC Mic...
Table 7.2 Parameters used in the Stability Analysis of Hybrid Control (Reprinted...
Table 7.3 Simulation/Experimental Parameters of Hybrid Control (Reprinted with p...
Table 7.4 Parameters used for the stability analysis on unique vs control (Repri...
Table 7.5 Simulation/experimental parameters used in unique vs control. (Reprint...
Chapter 8
Table 8.1 Modeling techniques with example.
Table 8.2 Type of analyses carried out in modeling methods.
Table 8.3 Time-domain characteristics.
Table 8.4 Black box vs. white box modeling approach.
Table 8.5 Example for Leverrier’s algorithm.
Table 8.6 Techniques for model order reduction.
Chapter 9
Table 9.1 Voltage gain in different types of three-to-three phase matrix convert...
Chapter 10
Table 10.1 Switching states of DCMLI.
Table 10.2 Switching states of FCMLI.
Table 10.3 Switching states of seven-switch five-level ANPC inverter.
Table 10.4 Advantages and disadvantages: a comparison of ANPC MLI with conventio...
Chapter 11
Table 11.1 Comparative analyses.
Table 11.2 Values of simulation parameters.
Table 11.3 The parasitic elements of the proposed structure.
Table 11.4 The values of the losses factors for the plot in Figure 11.14.
Table 11.5 RMS value of the converter devices.
Table 11.6 The parameter values of closed-loop simulation.
Chapter 12
Table 12.1 Parameters of major components for the studied microgrid.
Chapter 13
Table 13.1 Details of the data for training and testing of the algorithm.
Table 13.2 Parameters during the training of the classifier.
Table 13.3 Comparison of decision tree-based scheme with SVM and standalone kNN-...
Table 13.4 Comparison of the proposed scheme with other algorithms.
Table 13.5 Response of DT against the variation of fault resistance.
Table 13.6 Performance analysis of section identifier (DT-3 and DT-4) under PG f...
Table 13.7 Performance analysis of section identifier (DT-3 and DT-4) under PP f...
Table 13.8 Comparative analysis of the proposed scheme with other reported techn...
Chapter 14
Table 14.1 Different anti-islanding standards. (* Normal)
Table 14.2 Evaluation of IDM on different parameters
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
Also of Interest
End User License Agreement
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Scrivener Publishing
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Publishers at Scrivener
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DC Microgrids
Advances, Challenges, and Applications
Edited by
Nikita Gupta
Mahajan Sagar Bhaskar
Sanjeevikumar Padmanaban
and
Dhafer Almakhles
This edition first published 2022 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-77716-8
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Cover design by Kris Hackerott
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Preface
The electric grid is on the threshold of a significant change. In the past few years, the picture of the grid has changed dramatically due to the introduction of renewable energy sources, advancements in power electronics, digitalization, and other factors. All these mega-trends are pointing toward a new electrical system based on Direct Current (DC). DC power systems have inherent advantages of no harmonics, no reactive power, and high efficiency, over conventional AC power systems. Hence, DC power systems have become an emerging and promising alternative in various emerging applications, which include distributed energy sources like wind, solar, and Energy Storage Systems (ESS); distribution networks; smart buildings, remote telecom systems; and transport electrification like electric vehicles (EVs) and shipboard. All these applications are designed at different voltages to meet their specific requirements individually because of the lack of standardization. Thus, the factors influencing the DC voltages and system operation needed to be surveyed and analyzed, which include voltage standards, architecture for existing and emerging applications, topologies and control strategies of power electronic interfaces, fault diagnosis and design of the protection system, optimal economical operation, and system reliability.
This groundbreaking new volume presents these topics and trends of DC microgrids, bridging the research gap on DC microgrid architectures, control, and protection challenges to enable wide-scale implementation of energy-efficient DC microgrids. Whether for the veteran engineer or the student, this is a must-have for any library. This book also presents a detailed analysis of DC electrical architectures and power management methods with their technological aspects. Since the late twentieth century, power electronics technology has progressively gained traction. This technology is now practically allowing DC to reclaim ground from AC power. Of course, standard inertia is a huge roadblock, and it may be a long time before DC overtakes AC, but it will happen eventually. DC power distribution is expected to play a vital role in the optimal use of renewable resources as well as the enhancement of electrical system resilience. DC will cause higher-performing smart grids that are more stable and efficient throughout their life cycle while conserving energy and raw materials. In this book, at various stages of power conversion, the authors give a variety of energy models, management tactics, and control rules. They concentrate on renewable energy conversion like solar, energy storage systems, backup generators, and, of course, smart microgrids. Furthermore, the numerous experimental results and associated simulations contribute significantly to the book’s high quality.
Chapter 1 discusses DC microgrid protection challenges, fault detection methods, and design criteria for an efficient protective system. DC micro-grid protection strategies and both line-to-ground and line-to-line faults, besides their impacts, are reviewed. Also, DC fault current interrupting devices are presented. Chapter 2 provides a comprehensive overview of different existing control schemes for DC microgrids, along with the motivation and challenges behind them. It covers the basic and multi-level control schemes in detail, along with the converter control scheme for solar, wind, battery, and fuel cell-based systems. Chapter 3 describes different basic fault detection, location, and islanding detection methods for DC microgrids, along with the advantages and disadvantages of the schemes. Chapter 4 proposes an optimized energy management system for a micro-grid consisting of solar and wind generation units, an energy storage system, and a diesel generator. An optimization model for the energy management system was developed to minimize the overall operating and maintenance costs for the real power flow in the microgrid. The optimization was done using genetic algorithms and pattern search algorithms and a comparison is made based on the total operating and maintenance costs for real power flow. Chapter 5 presents the literature review of various energy management strategies involving energy storage in a microgrid, and a case study is presented by optimizing the involved objective functions using the linear programming method for a residential microgrid. The optimal operating mode is decided hourly to get the optimal cost. Chapter 6 presents the design of a hybrid renewable energy system using parameters such as available solar radiation, available hydro potential, demand assessment, etc., for DC microgrid application. The present study developed a systematic approach for solar and hydro potential assessment for DC microgrid applications. Chapter 7 details the influence of line equivalent resistances on power-sharing and on average DC bus voltage errors caused by droop techniques, reviewing the secondary control methods proposed in the literature to correct such errors. Moreover, two secondary control techniques based on distributed control, proposed by the authors, are highlighted, as well as the design of the communication system. Chapter 8 discusses the improvement of power electronic interface performance using dynamic analysis. An improved pole clustering method is proposed for dynamic analysis of quadratic boost converters with switched inductor cells only to unravel the resolvent matrix and prove the methodology’s easiness. Additionally, a view of various controllers applied to DC-DC converters is given for researchers to have as a guide for their analytical study of mathematical modelling. Chapter 9 presents a detailed study of the matrix converter. The control of a multiphase matrix converter for multiphase drive applications is discussed to make the system more reliable. A brief description of each part of the drive system fed from the matrix converter is presented. Moreover, several issues associated with the matrix converter, such as modulation, control, and mitigating effects of non-idealities, are discussed. The different applications, especially in more electric aircraft applications, wind power conversion, and applications in electrical drives, are also discussed. Chapter 10 proposes an Active Neutral-Point Clamped Multilevel Inverter that has the advantages of both flying capacitors and neutral point-clamped multilevel inverters, such as flexibility of switching redundancy and robustness to produce multilevel voltages. The operation of the proposed Active Neutral-Point clamped multilevel inverter is explained in detail. Chapter 11 presents a quasi-Z-source network-based DC-DC converter with a quadratic voltage conversion ratio that applies to photovoltaic systems and DC microgrids. In Chapter 12, a typical multiterminal VSC DC microgrid for the study is proposed, with careful consideration of topology design and grounding mode selection. Then the DC fault characteristics of a multi-terminal DC microgrid are analyzed, including DC unipolar fault and DC bipolar fault, and are used as the theoretical basis for protection configuration. Integrated control and protection platform are introduced, on which a layered setting of protection action delays and some supplementary control measures, such as de-blocking the VSCs and re-closing the DC breakers in the non-fault area, can be implemented. In Chapter 13, a decision tree-based protection scheme is designed for fault detection/classification and faulty section identification of DC microgrids during a faulty condition. The simple structure of the decision tree allows for achieving high accuracy in executing the protection task. The proposed algorithm is based on the acquisition of the voltage and current signals from the relaying buses. In Chapter 14, various passive and active islanding methods are discussed along with their merits and demerits, and individual lower-order harmonics are investigated along with the static transfer switch, which, as a new effective passive islanding method, is proposed for the inverter centered utility connected system.
Nikita Gupta, Mahajan Sagar Bhaskar,Sanjeevikumar Padmanaban and Dhafer AlmakhlesEditors
1On the DC Microgrids Protection Challenges, Schemes, and Devices – A Review
Mohammed H. Ibrahim*, Ebrahim A. Badran and Mansour H. Abdel-Rahman
Electrical Engineering Department, Faculty of Engineering, Mansoura University, Mansoura, Egypt
Abstract
Expansion in using renewable energy sources has demonstrated a promising and growing penetration of distributed generations into utility networks, causing safety, reliability, and quality challenges which have led to a renewed interest in DC power systems. DC microgrids are an enticing solution for these challenges owing to the use of power electronic devices. Also, it is a relevant approach to conjugate distributed energy systems with power grids, furthermore, its excellent performance and operational capability in grid-connected and islanded modes. Protecting DC microgrids from various faults is a major challenge because of the essence of DC power networks, like enormous DC capacitors, small impedance of DC cables, lack of natural zero-crossing points, high transient current, and voltage. Converters limited fault current short lines and convenient grounding scheme are other protection issues of DC microgrids. This paper discusses DC microgrid protection challenges, fault detection methods, and design criteria for an efficient protective system. DC microgrid protection strategies and both line to ground and line to line faults besides their impacts are reviewed. Also, DC fault current interrupting devices are presented.
Keywords: DC microgrids, protection challenges, protection schemes, DC fault, grounding, and protection devices
1.1 Introduction
The enlargement of using Renewable Energy Sources (RESs) and the speed of power electronics technology evolution have led to an increase in the number of Distributed Generators (DGs). DGs also have a few negative effects on utility grids, partially because of the periodic presence of most RESs. The critical challenges include the rise of voltage, protection coordination, power quality and stability of grid, and these must be solved. Hence, dependable, configurable, and smart energy distribution systems are required. Thus, microgrids have arisen and been an enticing arrangement for the implementation of renewable-based DGs [1, 2]. The number of DC-powered devices and applications has grown, including fuel cells, photovoltaic panels, storage batteries, electrical vehicles (EVs), and DC loads which have made use of DC microgrids inevitable, in addition to increasing the efficiency due to a decrease in the stages of power transfer [3]. DC microgrids have several connections linked to the DC bus that can be interfaced by individual systems such as loads, sources, and bi-directional devices using power converters as shown in Figure 1.1. Power converters are sometimes Voltage-Source Converters (VSCs) or Line Commutated Converters (LCCs). The thyristors in LCCs have high power capacity and voltage, but the control performance is lower compared to Isolated Gate Bipolar Transistors (IGBTs) in VSCs. One of several inherent problems with VSC-based DC microgrid protection is the quick observation of fault current and rapidly interrupted it because the current fault that withstands VSC ratings is twice the converter load capacity current [4]. DC microgrids are categorized into two voltage-based levels as follows [5]:
Medium voltage DC (MVDC) microgrid and,
Low voltage DC (LVDC) microgrid.
LVDC microgrids are connected with AC network through converters, and in this scenario the power stream is bi-directional, so various protection strategies are obligatory for the DC microgrids [6]. Also, as a result of the variances between the protection approaches of AC and DC microgrids, the fault position, classifying and detecting are protection concerns. Furthermore, DC microgrids normally have many load converters, which act as Constant Power Loads (CPLs) [7]. CPLs have an unfavorable impact on the protection system after a voltage decrease, and this is due to their negative incremental impedance [8]. In DC microgrids, there are challenges regarding the continuity of their operation efficiently considering some unusual conditions, such as current of short circuit in the DC bus and instability due to transient; therefore a compatible protection system is necessary for more reliability and efficacy of operation. There are two sorts of faults in DC systems:
Figure 1.1 Unipolar DC microgrid architecture diagram.
Line to Line (L-L) fault is known as the path between positive and negative poles formed in the line section. DC voltage across the fault location is reduced to zero, leading to high, fast-rising fault current [4, 9].
Line to Ground (L-G) fault, which is the most common type of fault in industrialized distribution systems [10, 11] and is induced by a path connecting either the positive or negative poles to ground. Relying on the grounding layout and the type of grounding, the fault impedance can be either high or low [12].
In this paper, the DC microgrids protection challenges are introduced. Also, the protection strategies and schemes are discussed. Furthermore, the protective devices in DC microgrids are explained.
1.2 Fault Characteristics and Analysis in DC Microgrid
DC microgrid short-circuit fault can be either bus or line fault based on location of the fault. Once fault location is closer to power sources, this will be more perilous, hence the bus fault has major consequences for the whole system [13–15]. DC bus faults will lead to an enormous current flow in the line depending on the fault impedance and the system configuration, thus the voltage of DC link will decrease to low level, maybe to zero, the terminal capacitors will discharge through the fault. Moreover, the fault current will increase from the AC side. So, the protection units have to spot faults and send a trip signal to the appropriate breaker and converter. If the fault is not isolated rapidly, overcurrent might spoil the reverse diodes and probably all DC systems. The key factors affecting the fault characteristics are microgrid topology, fault impedance, grounding layout, DG interfacing converters, DG source types, fault type and its location [16–18]. The pole to pole fault in DC bus is illustrated in Figure 1.2 [4]. The fault current consists of two parts [4, 14]:
Transient part represented by (I
c
) from bus capacitors and cable discharge of converters
Steady-state part represented by (I
s
) from power resources
Figure 1.2 (a) Pole - Pole fault in a bipolar DC bus, (b) an equivalent circuit model, (c) Typical fault current waveform.
Equation (1.1) shows the mathematical expression of fault current whenever the equivalent resistance in the fault circuit is minimal enough to trigger an oscillation [4]:
Where is natural frequency and is the damping factor of fault current and [14]. Es is the line voltage, R & L are the equivalent system resistance and inductance, C is the equivalent capacitance in the fault path.
If the fault resistance or bus capacitance is too high, as in a line-toground fault with slightly higher ground resistance, the fault current can be modelled as first-order damping with no oscillation [4]. Short-circuit currents calculation in a DC network is very important to design proper protection strategy. So, it is reasonable and interesting to study short-circuit current in a DC network and how it can be calculated. It should also be observed that when calculating the current of short-circuit, the current load is not taken into account. Figure 1.3 illustrates the International Electrotechnical Commission (IEC) standard approximation function for short-circuit fault current that covers the variations of a short-circuit fault current according to different sources, and the equations controlling the function are [4, 14, 19]:
Figure 1.3 Short-circuit fault current in DC system standard approximation.
Where,
where if (t) is the total fault current, i1 (t) and i2 (t) are the partial fault currents; ip is the peak short-circuit current; Ik is the quasi steady-state short-circuit current and it is the value of the short-circuit current 1 second after the beginning of short circuit, tp is the time to peak τ1 is the rise-time constant, τ2 is the decay-time constant and TK is short circuit duration time [19, 20]. Both currents i1 (t) and i2 (t) determine the maximum short-circuit current that rules the rating of electrical equipment and the minimum short-circuit current that can be taken as the basis for fuse and protection device ratings, respectively [4, 12].
In DC microgrids, the DC fault might take place either in the DC bus or cables that connect the microgrid apparatuses. DC bus, and DC inter-connections are proposed to perform as a single node of power interface between DG units, Energy Storage Systems (ESSs) and loads. On the viewpoint of protection, the drawback is that a fault on a DC bus or DC cable link affects DG units, ESSs, and loads, both of which may add and participate to the fault current. Hence, if the protection system design is insufficient, a single fault anywhere within the system may have unrecoverable impacts. VSCs play a major role in DC microgrids, where the crossing point to the AC side is via the inductor and to the DC side via the capacitor. During fault condition and due to the VSC design, the DC side capacitor discharges through the DC network, then the fault current involvement from the interfaced converter sources forms the second part of the response capacitor discharge and would result in high current amplitude that could affect the faulty components and other components. If the current fault is part of the protection plan, the excess maximum value of fault current must be taken into consideration on all system and component design procedures. The DC faulted current has three phases during a VSC fault: capacitor discharge, diode freewheeling, and grid-side current feeding phase. The phases were analyzed and characterized mathematically in [14].
1.3 DC Microgrid Protection Challenges
Protecting the DC microgrids with RES from different faults is a major obstacle. Therefore, accurate diagnosis (detect and locate) of the fault leads to fast restoration and recovery of the system and moreover decreases interval time of power outage [21]. The main obstacles associated with DC microgrid protection challenges are presented in the following subsections.
1.3.1 Low Inductance of DC System
During faults in AC power systems, the fault current is limited by fault impedance which has two components: reactance and resistance. The reactance value of fault impedance is generally larger than resistance value. Unlike DC systems, the reactance value is insignificant relative to the resistance of system. Subsequently, not only is the maximum value of DC fault currents greater than the maximum value of fault currents in equivalents AC networks but also, the lower value of the fault impedance results in a higher rate of DC fault current change [22]. The fault impedance is primarily resistive in nature which contributes to a high peak in fault currents [1, 23].
1.3.2 Fast Rise Rate of DC Fault Current
As DC faults generate high-rise currents, faults in VSC-based DC systems evolve faster than AC systems. In contrast, the withstand rate of semiconductor devices used in VSCs is less than that of AC power generators. As a result, the protection systems in DC systems must run comparatively faster to avoid any damage to the converter’s semiconductor. Fast rate of rise in DC fault current such AC protection devices can’t handle with it [23].
1.3.3 Difficulties of Overcurrent (O/C) Relays Coordination
Owing to the line reactance small value and the high increasing rate of DC fault currents, harmony and discrimination between O/C relays in DC networks is a difficult issue. The DC fault currents rate of increase is quite higher than AC fault currents. So, post-fault, all in series O/C relays simultaneously measure a high ratio of fault current and setting values. This can result in a lack of protection coordination. Additionally, O/C relays must be capable of detecting both positive pole to negative pole (P-P) and pole to ground (P-G) faults efficiently; however, there is a substantial difference in the current magnitudes of these two forms of fault [24]. O/C relays face the challenges of securing and protecting DC systems in a coordinated method which may reduce selectivity. Even though definite time O/C relays may be used as a choice to obtain selectivity in DC networks, coordination of certain time relays can lead to a long time to clear the fault in bulky DC systems with many radial lines [25].
1.3.4 Fault Detection and Location
The key criteria for effective locating and detecting faults in a DC microgrid are reliability, accuracy, functionality during system configuration modifications, and certainly the cost [26]. Absence of frequency and phasor data makes it hard to detect and locate faults in DC systems. Moreover, the short lines of DC microgrids, and small line resistance. Hence, the fault locating error in a DC microgrid is greater than in AC power networks. In [27], the scheme of protection for a DC Ring microgrid was suggested for detecting and locating the fault by using a number of line segments and the Intelligent Electronic Device (IED) at both ends of the line for controlling Solid-State Circuit Breakers (SSCBs). The fault location was recognized by approximating the fault inductance on the onset of fault by the least-square estimation technique. The proposal did not study pole to pole fault. In [14], a DC bus microgrid fault protection method including backup protection was proposed for detecting and isolating the fault without de-energizing the entire system based on use of a ring bus with overlapping nodes and links controlled by IEDs.
In [28], the protection method for fault detection and location configurations for loop type LVDC microgrids systems was proposed based on Multi-Criterion System (MCS) and Neural Network (NN).
In [29], during short-circuit fault, the transient characteristics of a VSC-based DC microgrid must be accurately determined in order to achieve fault detection, parameter and protection configuration. So, a transient modeling approach based on the VSC- DC microgrid was proposed by analyzing both DC bus voltage and active power control mode. Then, the faulted DC microgrid was modeled containing the VSC model and DC line model, and the calculation terms of state variables in the DC microgrid were provided. Following, the accuracy of the suggested modeling methodology was proved by comparing it with the electromagnetic transient (EMT) simulation by MATLAB/Simulink.
In [30], a protection strategy for a loop-type DC bus microgrid was implemented in order to locate the fault and isolate the faulted section so that the healthy part of the system could continue to operate functionally. One master controller, two slave controllers, and freewheeling branches between each line and ground formed the protection scheme. Slave controllers measure the current at each end of the bus section that connects two components and send it to the master controller.
In [31], a protection technique for the smart DC microgrid with ring configuration was introduced based on a parameter estimation. The method used the least squares-based method (LS) for predicting of observed inductance at each IED during fault from which forward and reverse faults were distinguished. By using the fault data path of both ends of the line section in a loop network, internal and external faults were identified to protect the network. It was mentioned that using directional information for local faults, every IED recognized any internal faults accurately.
In [32], a centralized protection system was employed to cover DC bus faults, short circuit, open circuit line faults and overload conditions based on impedance slope for 96 V DC ring microgrid with solar PV arrays, energy storage system and DC loads. The protection systems were developed by accumulative sum average techniques. The slopes were observed and noted at both ends of every line during the fault. The forward and reverse faults were classified and if the relevant slopes were observed at both ends of the faulted line. The slopes were either negative or positive based on bus fault and line fault section, respectively. SSCBs were used to isolate the faulty part. The research didn’t take into consideration pole to pole fault.
In [33], a ring configuration DC microgrid protection scheme was introduced to calculate the fault distance and resistance. The proposal used two localized Protection Detection (PD) at one place, which used fault current deviation equations based on the nature of DC current to determine the location of the fault across two stages. Firstly, the fault resistance value was determined by an equation based on the fault current and voltage at the PDs place. Secondly, the fault resistance value was obtained, the fault distance was computed also by using voltage and current equations based on the PDs measured values.
In [34], a non-unit-protection scheme was proposed for mesh-type bipolar LVDC microgrid with different RES. The proposed approach detects a fault by comparing the calculated values of fault current and P-P voltage to pre-defined thresholds and then identifies between internal and external faults relied line parameter estimation.
In [35], the impedance-based fault location technique, based on a digital Lock-In Amplifier (LIA) for DC microgrids was presented which allowed the detection of the impedance by extracting the distances relied on the wiring characteristics of the system. The LIA detected both the magnitude and phase of the impedance (resistive and inductive components) and allowed two distances to be estimated: one based on resistance and the other based on inductance.
In [36], in MVDC microgrids, an ultra-high-speed technique was proposed for detecting, classifying, and locating various fault types based on the polarity and wave shape properties of Traveling Waves (TWs) without any communication. The fault launched voltage and current TWs which propagated in both directions from the fault location towards the line terminals. The current TW polarity was used to recognize the fault direction.
In [37], in MVDC shipboard power systems, a graph traversal-based approach was introduced for automating system-wide programming of differential fault protection. The Percentage Differential Protection (PDP) scheme based on KCL was used for fault detection and identification. It was mentioned that if the PDP rules detected a fault, a minimum set of devices capable of isolating the fault was determined. As disconnect switches (DSs) only open when there is no or insignificant current flowing through them, the current flow through all DSs in the isolation set had to stop.
1.3.5 Arcing Fault Detection and Clearing
The periodic zero-crossing is implicit in AC systems as the polarity of voltage and current alternates many times per second depending on operating frequency. However, in DC networks, the lack of natural zero-crossing point is making self-sustaining arcs during normal switching or fault clearing. The interruption of DC current is a main problem which not only causes a serious risk for personnel safety, but also results in the contact corrosion of CBs. To successfully extinguish an arc in DC systems, the electrical contacts must move not only further away from each other but also faster than in AC systems [38]. It is necessary to know the properties and nature of the arc to handle it well and know how to extinguish it quickly. The channel of arc is related with high impedance and consequently reasons lower fault current than nominal current and increases considerable problems to conventional arc fault interrupter and large number of the DC protection schemes [39].
Arc faults are classified into two categories, series and parallel. Series arc faults frequently happen due to loose electrical connections. The parallel arc faults across transmission lines or within loads oftentimes incite a rapid increase in current and activate overcurrent circuit breakers. Two models were introduced in [40] to describe the dynamics of the evaluated DC network. The first model was large-signal analysis which was tested for arc fault boundary conditions and also for system response. The second model was small-signal analysis which calculated the noise propagation within the network. Series arc fault was characterized as an accidental power discharge between two conductors in series with the circuit [41]. When the air in the gap becomes ionized by the great relative charge on either side of the conductors, it permits the current to stream by producing heat and can ultimately cause fires. Therefore, in a series arc fault, a resistance was added in series with the line or circuit. Series arc faults detection and localization in DC microgrid were proposed using Kalman Filter (KF) and Adaptive Kalman Filter (AKF) algorithms-based admittance estimation in [41]. The KF/AKF algorithms were tested via three cases: nominal operation, series arc fault on specified line followed by another series arc fault on another line and disconnection of third line with various times, and series arc fault on identified line with change in load tracked by another series arc fault on third line.
In [42], a model of a series fault for transient simulations of low-voltage DC microgrids was proposed for electromagnetic transient simulations of series DC arc faults. Three types of series faults were investigated:
Constant-speed gap (conductors tearing longitudinally at a constant speed),
Fixed-distance gap (the electrodes still stationary after reaching a preset distance), and
Accelerated gap (the separation of two conductor sections at accelerated rate).
The DC microgrid under study didn’t contain complex parameters like constant power load (CPL), renewable energy sources (RES) or parallel inverters. In [43], a method of series fault detection algorithm in LVDC microgrid was proposed by detecting the occasional discharges at the beginning of the fault due to the high input capacity of the interface. The open-circuit performances and high current spikes were used for proposed identification algorithm by calculating the cross-correlation of the current signal with a pre-determined signal to identify the sparks. Experiments were planned and performed on a DC arc test platform with a buck converter based on constant power load. The outstanding features of spark due to DC arc fault are mentioned as following:
Sparks are precedent during open circuit conditions.
A current spike was usually observed at the beginning, or sometimes in the middle, of the discharging process.
The sparks were typically repetitive. The interval between discharges corresponds to the time constant of the input capacitor of the power electronics interface.
In [44], a statistical analysis approach was suggested for a fault detection technique that was suited for suppressing the effect of interfacing device operation, power circuits for system elements. Normal and arc signals were evaluated using Fast Fourier Transform (FFT). The presence of arc fault leads to a transition in the frequency spectrum and the arcing signal simply raises the spectral power density of the current. The probability curves of all operating conditions as a function of threshold values have been plotted. There was a straightforward trade-off between false-positive and false-negative. False-positive was identified as a detector malfunction that decides the presence of arc in non-arcing state, whereas false-negative was defined as a failure of arc detection. Thus, the probability of false-positive increases as one attempts to set a lower threshold to obtain a greater probability of effective identification.
In [45], detection of load voltage drop during series arc beginning was proposed as unified arc fault detection in low voltage DC microgrid. The reduction in the load voltage associated with the series arc activation was directly based on the electrode material and was observed by the load side power electronic converter, which was then acted similarly to disconnect the load from the grid by reducing the load current to zero and thereby the arc is extinguished easily and quickly. DC systems require the use of DC-DC converters in the distribution system and for this the high-frequency noise from DC-DC converter switching and other electromagnetic interference obscure the arc signature, allowing an arc to initiate and proceed unnoticed. During arc fault, switching harmonics are present, signal-to-noise ratio is low, and arc signal is not a periodic signal, so the time domain or frequency domain analysis using a Fourier transform (FT) do not work well. Therefore, in [46], Wavelet Transformation (WT) was proposed to analyze and detect arc fault and arc flash in DC microgrid systems by simulation synthetic waveforms generated in MATLAB/Simulink and then experimentally use arc waveforms measured from photovoltaic (PV) modules and an experimental arc generator.
In [47], a steady-state model of DC arc was proposed for studying series the impact of arc fault on a ring type DC microgrid. The arc impedance was modeled using macroscopic V-I equation to make the decision on DC arc resistance with a given gap length (a non-linear resistor) and arc noise was modelled by using the Gaussian distribution fitting of filtered arc current using a 1.5 - 45 kHz bandpass filter. The DC arc fault was characterized as a 10 Ω resistor in parallel with a controlled current source generating Gaussian distribution noise. The research did not take into consideration the transients at the initial stage of arc development; also, the impact of DC arc fault was studied only with resistive loads.
