Abstract: With high penetration of distributed energy resources, fault management strategy is of great importance for the distribution network operation. The objective of this paper is to propose a current and voltage limiting strategy to enhance fault ride-through (FRT) capability of inverter-based islanded microgrids (MGs) in which the effects of inverter control system and inverter topology (four/three-wire) are considered. A three-phase voltage-sourced inverter with multi-loop control system implemented in synchronous, stationary, and natural reference frames is employed in this paper for both four- and three-wire configurations. The proposed strategy provides high voltage and current quality during overcurrent conditions, which is necessary for sensitive loads. Several time-domain simulation studies are conducted to investigate the FRT capability of the proposed strategy against both asymmetrical and symmetrical faults. Moreover, the proposed method is tested on the CIGRE benchmark MG to demonstrate the effectiveness of the proposed limiting strategy.

Abstract: In this paper, a fault-tolerant control is proposed for dual three-phase permanent-magnet synchronous machine (PMSM) drives under open-phase faults. The object of the proposed fault-tolerant control is to maximize the torque capacity while the overcurrent protection is taken into account. Hence, the proposed fault-tolerant control is named as torque mode in this paper. Another existed fault-tolerant control for dual three-phase PMSM drives under open-phase faults is loss mode, in which the system copper loss is minimized. Torque mode is compared with loss mode, and two important conclusions are given: 1) loss mode can reduce more copper loss; and 2) torque mode can output more torque. The performances of torque mode and loss mode are verified by experimental results.

Abstract: For doubly fed induction generator (DFIG)-based wind turbines, the rotor side of DFIG is prone to suffering from overcurrent during grid faults, due to large electromotive force (EMF) induced in the rotor circuit. To solve this problem, this paper proposes an inductance-emulating control strategy for DFIG-based wind turbine to suppress the postfault rotor current, thereby enhancing its low-voltage ride through capability. Under the proposed control strategy, once the grid fault is detected, the rotor side converter (RSC) is controlled to emulate an inductance. Furthermore, with proper inductance value, both the required rotor voltage and postfault rotor current can be reduced within the permissible ranges of RSC, thus the controllability of control system can be maintained during transient process. Moreover, the oscillation of electromagnetic torque can be effectively suppressed during transient state of both grid fault and fault recovery. Finally, the simulation and experimental results are presented to demonstrate the effectiveness of the proposed method.

Abstract: Detection of high-resistance faults on meshed low-voltage dc grids poses a challenge, as such faults have very low fault current magnitudes. This paper proposes a hybrid passive-overcurrent relay to overcome this problem. The proposed relay consists of one current and one voltage transducer, as well as two passive elements: 1) an inductor; and 2) a capacitor. For bolted and relatively low-resistance faults, the relay uses a simple overcurrent function to detect the resultant high fault current magnitudes within 2 ms. On the other hand, for relatively high-resistance faults, a real-time discrete wavelet transform is used to detect the voltage transients generated by the relay passive elements in less than 5 ms. Furthermore, the proposed relay is inherently capable of identifying the type of fault. The proposed approach relies on local-bus measurements to detect and classify various types of faults with resistance up to 200 ohms. Analytical modeling proves that the proposed approach is system independent. Testing the hybrid passive-overcurrent relay on a ±750 V meshed TN-S dc grid reveals that the proposed relay is fast, sensitive, and selective under various conditions.

Abstract: Distributed generation (DG) brought new challenges for protection engineers since standard relay settings of traditional system may no longer function properly under increasing presence of DG. The extreme case is coordination loss between primary and backup relays. The directional overcurrent relay (DOCR), which is the most implemented protective device in the electrical network, also suffers performance degradation in the presence of DG. Therefore, this paper proposes the mitigation of DG impact on DOCR coordination employing adaptive protection scheme (APS) using differential evolution algorithm while improving overall sensitivity of relays. The impacts of DG prior and after the application of APS are presented based on interconnected 6 bus and IEEE 14 bus system. As a consequence, general sensitivity improvement and mitigation scheme is proposed.

Abstract: Display Omitted We solved the Directional Over Current Relays (DOCRs) optimal coordination problem.We used the Electromagnetic Field Optimization Algorithm (EFO).We improved the EFO algorithm for the optimal coordination of DOCRs.The proposed algorithm is better than other optimization algorithms. The optimal coordination of Directional Overcurrent Relays (DOCRs) is of paramount importance for power systems protection. The optimization model of this problem is non-linear and highly constrained. The main objective of this paper is to develop a modified version of the Electromagnetic Field Optimization (EFO) algorithm referred to as MEFO for the optimal coordination of DOCRs. The EFO is inspired by the behaviour of particles of electromagnets with different polarities where attractionrepulsion forces among these electromagnets lead particles toward global minima. It uses also the golden ratio. The proposed algorithm has been applied to three test systems including the 8-bus, the 9-bus and the 15-bus test systems. Furthermore, the results obtained using the proposed MEFO are compared with those obtained using the traditional EFO and a number of well-known algorithms. The obtained results show the effectiveness of the proposed MEFO to minimize the relay operating time for the optimal coordination of DOCRs.

Abstract: One major concern in using over current relays (OCRs) is their coordination in power system protection to decrease the pressure of electrical equipment and avoid mal-trip in the primary and backup relays. The issue has recently become more challenging with the increasing use of distributed generation (DG). This study proposes the use of firefly algorithm (FA) to achieve an optimised coordination of OCRs in power system protection when DG is present in the system. Typically, the FA is done by selecting plug setting and time setting multiplier parameters. However, the algorithm associated with such method requires a long convergence time and has the risk of getting trapped in local optima. Therefore, an adaptive modified FA (AMFA) has been developed to overcome these issues and obtain the optimised coordination of OCRs. The AMFA is tested in five case studies of a power system with DG whose results are then compared with those obtained from the conventional and FA methods. The results demonstrate that the proposed AMFA is able to achieve the optimised coordination of OCRs in all test cases with significant improvement in time reduction reaching 40.446%.

Abstract: Increasing prevalence of dc sources and loads has resulted in dc distribution being reconsidered at a microgrid level. However, in comparison to ac systems, the lack of a natural zero crossing has traditionally meant that protecting dc systems is inherently more difficult—this protection issue is compounded when attempting to diagnose and isolate fault conditions. One such condition is the series arc fault, which poses significant protection issues as their presence negates the logic of overcurrent protection philosophies. This paper proposes the IntelArc system to accurately diagnose series arc faults in dc systems. IntelArc combines time–frequency and time-domain extracted features with hidden Markov models (HMMs) to discriminate between nominal transient behavior and arc fault behavior across a variety of operating conditions. Preliminary testing of the system is outlined with results showing that the system has the potential for accurate, generalized diagnosis of series arc faults in dc systems.

Abstract: Grid faults constitute the main challenge for distributed generation, especially permanent magnet synchronous generator (PMSG)-based wind turbines connected to the grid via back-to-back (BTB) converters. On the other hand, grid codes enforced wind turbines to aid network and accomplish low-voltage ride-through (LVRT) requirements during grid faults. DC-link overvoltage suppression without external devices and reactive current injection to the grid in symmetrical and asymmetrical faults were the main required subjects in different grid codes. Reactive current injection may lead to overcurrent in one or two phases during asymmetrical faults. This paper presents solutions to accomplish LVRT requirements during grid faults, including dc-link overvoltage suppression by improving BTB converter's controllers and the design of active power limitation to maintain the peak current of the grid side inverter in a safe range during different asymmetrical grid faults. In fact, by controlling generation of PMSG active power directly with grid side inverter, the peak current remained in the safe limit and required reactive current, including negative-sequence component can be injected to the grid. Furthermore, the grid side active power oscillation and dc-link voltage ripple can be suppressed by using the proposed controller. Simulation results showed the validity and efficiently of the proposed control approach in different conditions.

Abstract: The coordination of directional overcurrent relays (DOCRs) in meshed power grids with multiple sources is a constrained optimization problem, which has been stated in recent literature as linear (LP), nonlinear (NLP), and mixed-integer nonlinear programming (MINLP) problem. In this paper, the DOCR coordination NLP problem is reformulated as an equivalent quadratically constrained quadratic programming (QCQP) model, leading to significant reduction of problem complexity. Another contribution of this work is the systematic problem statement using graph theory concepts. The proposed method is applied to three different meshed power systems, employing state-of-the-art optimization software. Simulation results demonstrate the efficacy and superiority of the proposed QCQP model over the prevailing NLP approach.

Abstract: The protection system has been facing new challenges with the distribution systems with distributed generation (DG), which may be successfully accomplished through the combination of the existing protections, such as overcurrent protection, with modern protections based on emerging technologies. By using conventional and modern protection trends, the aim of this paper is to propose the overcurrent protection based on the boundary discrete wavelet transform. In real time, this transformation decomposes a signal in the boundary scaling and wavelet coefficient energy. Following the conservative trends of power system protection, the instantaneous and time-delay overcurrent elements (phase and neutral units) are reproduced in the wavelet domain by using the boundary scaling coefficient energy, providing similar performance in the fault detection, a faster trip, and less computational burden than the conventional overcurrent protection. Following the modern protection trends, the boundary wavelet coefficient energy provides real-time detection of the fault inception time in order to reduce the relay operating time and minimize the harmful effects of the faults. A distribution system with DG was modeled in a real-time digital simulator, and wavelet-based overcurrent relays were evaluated and compared to conventional overcurrent relays based on the Fourier transform in real-time simulations, and promising results were obtained.

Abstract: The traditional overcurrent protection is not suitable for the microgrid. One of the main reasons is that the fault response of the inverter-interfaced distributed generator (IIDG) is very different from that of the rotational generator. This paper develops fault models of IIDGs within a low-voltage microgrid, including active/reactive power (PQ)-controlled IIDGs and voltage-controlled IIDGs. Considering that the control strategy of the PQ-controlled IIDG varies, this paper introduces adjustable parameters into the model to reflect its fault characteristics as comprehensive as possible. Voltage-controlled IIDG is of great significance on maintaining a stable microgrid. The model of voltage-controlled IIDG proposed in this paper keeps its voltage-source nature while accomplishing current limiting. The performance of the proposed IIDG fault models has been tested by Matlab simulations. The achievements of this paper are expected to have a positive impact on the development of microgrid fault analysis and protection.

Abstract: Utilities have installed distributed generators to improve reliability of the power distribution system. Distributed generators at various sites provide additional circuit paths in case energy provision to the end user is interrupted by a blackout or equipment failure. When the microgrid is working on island and grid-connected modes, overcurrent protections in power lines must be adjusted to different distributed generator circuit paths and feeder fuses in the microgrid. This study presents an adaptive overcurrent protection that integrates technical and economic advantages of fuses and relays in a microgrid with distributed generators. This fuse relay adaptive overcurrent protection (FRAOP) scheme protects power lines and feeders by grouping identical inverse time overcurrent settings of relays, and logic gates of relay's breakers. Selectivity, reliability, and speed of the FRAOP was verified by a real-time simulator with relays in-the-loop.

Abstract: Due to the simplicity and high reliability, brushless dc (BLDC) motors are widely used in space application. High-reliability levels are the vital aspect for ensuring the long-term stable operation of the BLDC motor system, which is used in aerospace applications. The fault-tolerant control of the BLDC motor is of great importance for its continuous operating capacity even under the faulty situation. This paper proposes a fault-tolerant topology composed of an additional phase leg and a fault-protective circuit for the high-speed low-inductance BLDC motor. Based on the analysis of the overcurrent and overvoltage phenomenon after the switch faults, a novel fault isolation and system reconfiguration method is presented. The method can achieve safe isolation and reconfiguration to avoid the secondary fault caused by direct switch of the redundant switch and the faulty switch after the fault-diagnosis process. Both simulation and experimental results confirm the feasibility and effectiveness of the proposed method.

Abstract: This study presents a control strategy for a microgrid that consists of multiple distributed generators (DGs), for both grid-connected and islanded modes of operation, where every DG is interfaced to the main grid via a voltage sourced inverter (VSI). finite control set model predictive control (FCS-MPC) is used as the primary controller to regulate the output power of each DG (in the grid-connected mode) or the voltage of the point of DG coupling (in the islanded mode of operation). In the grid-connected mode, direct power model predictive control (DPMPC) is implemented to manage the power flow between each DG and the main grid. In the islanded mode, voltage model predictive control (VMPC), as the primary control, and droop control, as the secondary control, are employed to control the output voltage of each DG and system frequency. The controller is equipped with a supplementary current limiting technique to limit the output current of each DG in case of overcurrent scenarios. The control approach also enables a smooth transition between the two modes. The performance of the control strategy is investigated and verified using PSCAD/EMTDC software platform.

Abstract: The large number of switching elements in the modular multilevel converter (MMC) is a challenging problem when modeling the MMC-HVDC systems for the computation of electromagnetic transients. The modeling complexity increases even further when a multiterminal (MT) MMC-HVDC system is used to integrate offshore wind farms (OWFs) with power-electronics-based wind energy converters, such as doubly fed induction generators (DFIGs). This paper compares modeling accuracy and computational performances for various combinations of MMC and OWF models. Onshore and offshore ac fault simulations are performed for an OWF system composed of DFIG-type wind turbines and connected to a practical ac grid through an MT MMC-HVDC system. The OWF system model includes the detailed representation of the offshore collector grid and the associated overcurrent protection. The offshore MMC controls include an offshore fault current limiter and fast OWF power generation reduction-based fault-ride-through function.

Abstract: Directional overcurrent relays are often used in the protection of electrical power transmission and distribution systems These devices are installed along the network, and they must be coordinated to operate as fast as possible and in the adequate sequence However, the coordination of relays is a complex problem, due to its discrete non-linear nature and its hard constraint structure A matheuristic algorithm to provide coordination of directional overcurrent is proposed in this work The algorithm combines a differential evolution strategy and linear programming formulations In addition, two local search procedures are proposed to improve convergence and to handle continuous and discrete relay setups Results for five instances show that the proposed method is able to obtain identical or better solutions than those already reported in the literature with considerably lower computational cost Furthermore, the algorithm can directly handle robust coordination by considering possible changes in the systems conditions

Abstract: This paper provides results and conclusions on a survey on the implementation and reliability aspects of CubeSat bus interfaces, with an emphasis on the data bus and power distribution. It provides recommendations for a future CubeSat bus standard. The survey is based on a literature study and a questionnaire representing 60 launched CubeSats and 44 to be launched CubeSats. It is found that the bus interfaces are not the main driver for mission failures. However, it is concluded that the Inter Integrated Circuit (I2C) data bus, as implemented in a great majority of the CubeSats, caused some catastrophic satellite failures and a vast amount of bus lockups. The power distribution may lead to catastrophic failures if the power lines are not protected against overcurrent. A connector and wiring standard widely implemented in CubeSats is based on the PC/104 standard. Most participants find the 104 pin connector of this standard too large. For a future CubeSat bus interface standard, it is recommended to implement a reliable data bus, a power distribution with overcurrent protection and a wiring harness with smaller connectors compared with PC/104.

Abstract: One main challenge in the practical implementation of a microgrid is the design of an adequate protection scheme in both grid connected and islanded modes. Conventional overcurrent protection schemes face selectivity and sensitivity issues during grid and microgrid faults since the fault current level is different in both cases for the same relay. Various approaches have been implemented in the past to deal with this problem, yet the most promising ones are the implementation of adaptive protection techniques abiding by the IEC 61850 communication standard. This paper presents a critical review of existing adaptive protection schemes, the technical challenges for the use of classical protection techniques and the need for an adaptive, smart protection system. However, the risk of communication link failures and cyber security threats still remain a challenge in implementing a reliable adaptive protection scheme. A contingency is needed where a communication issue prevents the relay from adjusting to a lower current level during islanded mode. An adaptive protection scheme is proposed that utilizes energy storage (ES) and hybrid ES (HESS) already available in the network as a mechanism to source the higher fault current. Four common grid ES and HESS are reviewed for their suitability in feeding the fault while some solutions are proposed.

Abstract: In this paper, a comprehensive protection strategy is proposed for insuring dependable and secure operation of an islanded microgrid system. This is implemented using microprocessor-based relays to prevent unnecessary loss of critical loads and distributed generators (DGs). Several improvements are proposed to clear the way for plug-and-play of DGs. Furthermore, recommendations are presented to tackle the elusive high-impedance fault problem, commonly encountered in distribution systems. An optimal directional overcurrent bus protection is also presented. Several case studies and analyses are carried out to demonstrate the proposed protection strategy. Results from simulation using MATLAB/Simulink are also shown.

Abstract: Foreword.Symbols and Indices.Terms and Definitions. General Information About IEC 60 909. The Significance of IEC 60 909. Supply Networks. Network Types for the Calculation of Short Circuit Currents. Systems up to 1 kV. Neutral Point Treatment in Three--phase Networks. Impedances of Three--phase Operational Equipment. Impedance Corrections. The Method of Symmetrical Components. Calculation of Short Circuit Currents. Motors in three--phase Networks. Mechanical and Thermal Short Circuit Strength. Calculations for Short Circuit Strength. Equipment for Overcurrent Protection. Short Circuit Currents in DC Systems. Programs for the Calculation of Short Circuit Currents. Examples: Calculation of Short Circuit Currents. Appendices: Calculation Tools for Electrical Engineering. The Elaplan Program.The KUBS plus Program.Index.

Abstract: Overcurrent relays are the most economical protective relays that are used as main/backup protection in distribution networks and as backup protection in transmission and subtransmission networks. The most effective overcurrent relays have inverse time-current characteristics such as standard (normal) inverse (SI) characteristic, very inverse (VI) characteristic, and extremely inverse (EI) characteristic. Lately, the use of numerical protective relays is considered due to their programmable functionality, greater flexibility, higher speed, and accuracy. Numerical overcurrent relays usually allow the users to define and setup arbitrary time-current characteristics. This paper aims to develop a new time-current characteristic to be used in numerical overcurrent relays to overcome some drawbacks of the conventional characteristics. This characteristic curve is constructed by connecting successive straight lines and can easily be tabulated to write to the numerical relays.

Abstract: This paper proposes a model predictive direct power control (MPDPC) approach with reduced switching frequency for grid-connected voltage-sourced converters (VSC). This approach is proposed for both a real/reactive power controller (with a fixed input voltage) and a power port controller (with a variable input voltage). It uses two different cost functions: one for the steady state and one for transients. Utilizing these two cost functions leads to having a low switching frequency in the steady state and a fast response during transients. The proposed MPDPC also provides overcurrent protection of the VSC. In applying the proposed MPDPC to a power port, drawbacks of conventional voltage controllers are addressed by employing power estimation and set point modulation in the controller. Simulation and experimental results validate the proposed MPDPC and the voltage controller.

Abstract: This paper presents a modified power control to integrate inverter-based distributed generation (DG) in distribution networks. This control is designed for mitigating the maloperation of the overcurrent protection coordination, due to the DG current contribution during the fault. The proposed power control introduces a new approach to perform the inverter control by using new orthogonal components of the grid voltages that provide a modified controlled power. Using the proposed methodology, the injected current to the grid during faults is limited to the DG-rated level. By controlling the reference power based on a proposed function of the per-unit terminal voltage, the injected current is limited to a desired value. The results of the simulated system corroborate the control capability to completely limit the inverter-based DG effects on protection coordination in the distribution level.

Abstract: A fast overcurrent protection scheme was developed for GaN gate injection transistors (GITs), harnessing the relationship between the externally measured vgs and id in steady-state operation. This relationship has been characterized in both static and dynamic testing over a wide range of operating conditions, and a circuit has been constructed to implement this control scheme. The circuit uses analog components to integrate the protection feature into a commercially available GIT gate driver. The scheme was experimentally verified in a double pulse test setup for experimental verification, and its total fault response time was recorded at less than 70 ns, with 400 V dc bus and a 30 A threshold. Compared with conventional desaturation protection, which detects faults based on drain voltage rather than gate voltage, the proposed scheme offers benefits in terms of speed, temperature invariance, flexibility in threshold selection, and minimal impact on the GIT's normal switching behavior.