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Flexible DC distribution networks have been widely developed because of their advantages of high efficiency, low cost, and easy access to renewable energy. However, as distribution networks have more ends than the two-terminal transmission line, it is more challenging to protect them and locate their faults. Therefore, this paper proposes a modified T-source circuit breaker to protect against DC faults. Based on the proposed circuit breaker, an integral fault location method using double-terminal asynchronous data is proposed. The accuracy of the algorithm is not affected by the fault resistance. There are no differential terms in the algorithm, which greatly reduces the sensitivity to noise. The integral algorithm can remain steady when the voltage and current are zero in oscillation and attenuation periods. This constitutes the main contribution of this paper. Finally, it is verified by simulation and experiment that the proposed circuit breaker can isolate the power source within a short time in the typical DC distribution network. Under a noise of 30 dB for simulation, the location error of the proposed algorithm is <0.4% when the fault resistance is 0.5 Ω–500 Ω. [ABSTRACT FROM AUTHOR]

Closed-form determination of fault current limiter (FCL) impedance for symmetrical faults was reported previously using a mathematically rigorous method. In this paper, extensions of the method to asymmetrical faults are presented. Single line-to-ground faults and line-to-line faults are studied. As shown in this paper, although the material corresponding to asymmetric faults use the same basic concepts described before, the different nature of asymmetrical faults and application of symmetrical components theory lead to a different formulation. Also, the graphical representation of asymmetrical faults is as contour plots instead of the circle diagrams for three-phase faults. A three-bus and the IEEE 39-bus systems are used to illustrate the virtues of the method for the proper selection of FCLs impedance. Results demonstrate that the FCL designed, as shown in this paper, works properly for asymmetrical faults. The new formulation and graphical representation for asymmetrical faults are compatible with traditional power system short-circuit analysis software. An example with both symmetrical and asymmetrical faults is presented to illustrate how a full FCL impedance design can be achieved with the method. [ABSTRACT FROM AUTHOR]

This paper provides a pragmatic solution to the challenge of testing fault current interruption of high-voltage direct current (HVDC) circuit breakers (CBs). The critical parameters in the design of a test circuit capable of supplying the necessary stresses: current, energy, and voltage (both during and after interruption) are discussed. In addition, a practical implementation of a test circuit based on ac short-circuit generators operated at low power frequency, which is capable of testing the current interruption performance of the proposed technologies of HVDC CBs, is discussed. Tests validating the proposed method and circuit have been conducted on a prototype of an HVDC CB and the test results are presented. Since the performance of some technologies of HVDC CBs can depend on the magnitude of the interrupted current, four test duties are defined and demonstrated in the paper. Moreover, testing of HVDC CBs using ac short-circuit generators poses new challenges, such as the application of dielectric dc stress after current interruption and the protection of both the test-object as well as the test-circuit components when the HVDC CB fails to interrupt. Methods to overcome these challenges are developed and practically demonstrated in a test laboratory. Finally, taking into account the available resources of the author's test laboratory, the capability to test multiple series-connected modules of different technologies of HVDC CBs is verified and example cases are demonstrated. Six short-circuit generators (13 500 MVA @ 50 Hz) and up to ten step-up transformers (up to 550 kV) were actually used. [ABSTRACT FROM AUTHOR]

Conventional half bridge modular multilevel converter (MMC) system will suffer great over-current when there is a dc side fault. Therefore, topology study in sub-modules with dc side fault blocking capability have become one of the hotspots in MMC field. However, fault blocking topologies using conventional IGBT devices must employ more semiconductors than half bridge topology. The extra semiconductor devices employed in the topology will lead to the increase of cost and conduction loss. To reduce the conduction loss of the fault blocking topology, a novel sub-module topology is proposed in this paper. The proposed topology employs reverse blocking devices to form the low conduction loss path. Conduction loss analysis is conducted in this paper to show the priority of the proposed topology. Simulation and experiment results verified the dc fault blocking capability of the proposed topology. [ABSTRACT FROM AUTHOR]

Inverter-Based Resources (IBRs), including Wind turbine generators (WTGs), exhibit substantially different negative-sequence fault current characteristics compared to synchronous generators (SGs). These differences may cause misoperation of customary negative-sequence-based protective elements set under the assumption of a conventional SG dominated power system. The amplitude of the negative-sequence fault current of a WTG is smaller than that of an SG. This may lead to misoperation of the negative-sequence overcurrent elements 50Q/51Q. Moreover, the angular relation of the negative-sequence current and voltage is different under WTGs, which may result in the misoperation of directional negative-sequence overcurrent element 67Q. This paper first studies the key differences between the WTGs and SG by comparing their equivalent negative-sequence impedances with SG's. Then, simulation case studies are presented showing the misoperation of 50Q and 67Q due to wind generation and the corresponding impact on communication-assisted protection and fault identification scheme (FID). The impact on directional element is also experimentally validated in a hardware-in-the-loop real-time simulation set up using a physical relay. Finally, the paper studies the impact of various factors such as WTG type (Type-III/Type-IV) and Type-IV WTG control scheme (coupled/decoupled sequence) to determine the key features that need to be considered in practical protection studies. The objective is to show potential protection misoperation issues, identify the cause, and propose potential solutions. [ABSTRACT FROM AUTHOR]

This two-part paper proposes a revised method for calculating high voltage (HV) cable sheath currents, and for fault location of cross-bonded HV cable systems. The equivalent circuit and the cable impedance per unit length under short-circuit fault conditions are revised from those under non-fault conditions. Part 1 of this paper proposes a revised phasor domain mathematical model for the sheath current calculation under fault conditions, following demonstration that short-circuit faults can cause changes in the equivalent circuit of the cable metal sheath, and cause changes in the cable impedance per unit length. The calculation results, as given in a case study, show that there exists a unique relation between the fault position and the sheath fault currents, making it possible to locate a fault by analyzing the characteristics of the sheath currents during a fault. Part 2 of the paper focuses on the fault location method via sheath current monitoring. [ABSTRACT FROM AUTHOR]

Protection of microgrids at the Point of Interconnection (POI) is a challenging task. Particularly, single-line-to-ground (SLG) faults on interconnection lines are difficult to detect using the fault current detection relays if the microgrid is ungrounded at utility side of its interconnection transformer; such microgrid contributes very low fault current, which disappears once the utility feeder breaker opens. Other common schemes for protection of microgrid interconnection line against SLG faults include Direct Transfer Trip (DTT) and over-voltage relay (59G). The main challenges associated with these methods are installation and maintenance costs of high-speed communication system, sensitivity of relay settings, and delayed fault clearing time due to selectivity requirements. To address such concerns, this paper proposes the use of a distance relay at the microgrid/LV side of the interconnection transformer to provide protection against SLG faults on the interconnection line at the utility/HV side. The designed distance relay executes an enhanced apparent impedance calculation using residual voltage compensation to correctly detect the fault, properly measure the fault location, and timely isolate the fault. In this paper, various scenarios are analyzed using ASPEN and PSCAD simulation tools to demonstrate the effectiveness of the proposed method for protecting the microgrid interconnection line under different fault scenarios. [ABSTRACT FROM AUTHOR]

The short-circuit response of inverter-interfaced distributed generators (IIDGs) is not adequately represented in many conventional protection studies. This paper presents an in-depth analysis of IIDG behavior during grid faults and proposes a more accurate reduced-order parameterized short-circuit current (RPSC) model of inverters. Typical inverter components are thoroughly investigated to identify those that play dominant roles during faults. The paper shows that the current limiter is the dominant factor for the steady-state fault-current of inverter; while the inverter filter along with the severity of the voltage disturbance largely determine the initial transient spike of inverter fault current. The proposed model is low-order and can be used in large scale simulations. The parameters of the proposed RPSC model can be extracted from laboratory experiments without requiring proprietary manufacturer information. The proposed fault-current model is analogous to the well-known synchronous machine model that segregates the inverter fault current into subtransient, transient, and steady-state fault currents. Experimental and simulation tests are presented to validate the model. [ABSTRACT FROM AUTHOR]

High Impedance Faults (HIFs) occur by the contact between an energized conductor and a high impedance surface. Due to the low fault current level, HIFs cannot be detected by conventional protection and there is no fully efficient solution to this problem. HIF detection methods often extract metrics using signal processing techniques, such as Fourier Transform, Wavelet Transform, Stockwell Transform, and Mathematical Morphology. However, these techniques are applied under specific conditions, which hinders comparative and critical analyses among them. Therefore, this paper presents a critical review of HIF detection methods based on the aforementioned techniques, and also shows a detailed investigation of the performance of the metrics commonly used with them. To do this efficiently, the authors proposed a set of assessment indices based on the ratio between the metrics’ characteristics and another one based on the repeating of the metrics features. The proposed indices revealed that some of these metrics fail to distinguish HIF from other typical occurrences in power distribution systems, and their performances are negatively affected by the fault location and by the existence of noise in the measurements. Additionally, the results showed a need to specify system conditions in which any HIF detection technique is valid. [ABSTRACT FROM AUTHOR]

This paper deals with the modeling, hardware results and model validation by measurements of a VSC assisted resonant current (VARC) dc circuit breaker (CB) and the application within a future network by simulation. The newly emerging VARC dc CB can be used as a solution for the protection of offshore multi-terminal HVDC (MTDC) grids. In this paper, the proposed VARC dc CB is modeled in detail in a PSCAD environment, by taking into account dielectric strength of the vacuum gap, high-frequency current quenching ability and parasitic components. The PSCAD-model is then verified by data from the testing of a 27 kV VARC dc CB prototype with maximum current interruption capability of 10 kA. Additionally, the initial transient interruption voltage and current slope at zero-crossing during the interruption are analyzed. With respect to scaling to a higher voltage level, three types of series connected modules are presented and the performances are compared. The performance of the series connected modules is simulated in a model of a 4-terminal HVDC grid. The obtained results validate the VARC dc CB as a promising solution for the dc fault isolation in MTDC grids. [ABSTRACT FROM AUTHOR]

In this part of the paper (Part II), available technical solutions to minimize or prevent wildfires caused by power networks are reviewed in more detail. These methods include fundamental changes to power system equipment, periodic checkups and maintenance, identification of faults via voltage/current waveforms, changes to protection and relaying settings and coordination, and installation of new power system apparatus. The strengths and drawbacks of each method are discussed. The discussion section brainstorms some future possible solutions that smart-grid technologies can enable. The paper concludes that there has neither been enough research in universities nor enough investment by industry to develop viable preventive solutions for wildfires that initiate from the daily operation of electrical power grids. This, in the authors’ opinion, is the crucial time for investment in research and development to tackle the lethal wildfire problem while electric power networks offer “the lowest hanging fruits to pick.” [ABSTRACT FROM AUTHOR]

A DC system saturated core fault current limiter (DCSFCL) was proposed to limit the fast rising trend of fault current in HVDC system. However, traditional method to calculate and design the DCSFCL tend to neglect leakage flux around the permanent magnet (PM), leading to large calculation error and inappropriate design procedure. The novel contribution of this paper is that it proposed a modularization method to analyze the permeance of both PM and adjacent air. Hence, the leakage flux effect is considered properly and a more accurate permeance matrix can be derived in a pure mathematical way. In this case, DCSFCL can be calculated much faster than the FEA simulation program with a promising accuracy regarding fault limiting performance. More importantly, design flow of the DCSFCL and related PM biased devices was given as well. Calculation result shows that inductance value calculated by the proposed method has less than 1% error compared with fine meshed FEA simulation result and about 20% increase in accuracy compared with traditional method that neglects PM leakage flux effect. Prototype of the DCSFCL was also built and experimental tests were carried out to verify the correctness of the proposed method. [ABSTRACT FROM AUTHOR]

The penetration of inverter-based resources is growing rapidly around the globe. During a fault, the behavior of inverter-based resource is very different from a traditional synchronous machine based generation. As the penetration of inverter-based resources increases, there is an urgency for the industry to understand their behavior so that these resources can be accurately modeled and their impact on various protection schemes can be studied in detail. This path begins with standardizing the response of inverter-based resources to a fault. Additionally, this paper discusses issues with modeling inverter-based resources in a short circuit model which is developed for systems dominated by synchronous machines and finally a need for further research to understand impacts on traditional protection schemes as inverter-based resources continue to displace traditional generation. This paper does not offer solutions however, it is intended to encourage further study by industry professionals toward development of solutions. [ABSTRACT FROM AUTHOR]

When a fault occurs on the dc side of the voltage source converter based high voltage direct current (VSC-HVDC) transmission system, the fault current rises quickly due to the low damping of the dc system. Using a dc fault current limiter (DC-FCL) to limit the rise of the fault current can significantly reduce the breaking requirements of the DCCB. A topology of a bidirectional active dc fault current limiter (BA-FCL) based on coupled inductor is proposed in this paper. No expensive full-controlled power electronic devices are used in this topology. Only a few half-controlled power electronic devices are used to achieve active control of the current limiting unit. Compared with many topologies of current limiter at present, the fault current limiting effect of BA-FCL is improved significantly while the cost, volume, topology complexity, and control complexity are reduced. The stress analysis and parameter design of the proposed current limiter are carried out. Finally, the simulation case study and hardware experiment are implemented to verify the good economy and current limiting performance of the proposed topology. [ABSTRACT FROM AUTHOR]

Due to the complex structures and fault conditions, accurate fault sensing of the underground distribution cables is a difficult task. The distribution automation system (DAS) and fault indicator system (FIS) are only applicable to the severe faults. In addition, the current DAS and FIS are all based on the centralized data processing system (CDS), increasing the communication pressure and data processing time of the main station. Considering the advantages of edge computing, this paper investigates using the grounding line currents to sense the fault in distribution cable networks. First, an edge computing based fault sensing system (ECS) is designed. The equivalent time-domain distribution cable models considering multi-conductor mutual couplings are then established in both normal and fault cases. The time-domain features of the cable grounding line currents prior to and after the fault are analyzed to create the fault detection criterion. Combining the network topology, locations of the measurement units (MUs), and direction factor of the ground line current, a space matrix based algorithm is created to achieve fault cable section location at edge. Finally, a distribution cable network is created by PSCAD/EMTDC. Various simulations are conducted to validate the effectiveness and robustness of the algorithm. [ABSTRACT FROM AUTHOR]

The vacuum interrupter-based DC circuit breaker is one of the most important equipments in the DC power system. The dielectric recovery (DR) of vacuum interrupter (VI) is essential for breaking reliability, especially on some occasions with ultra-large fault current such as the outlet of generator. To measure the DR of VI under high arcing current, an experiment circuit is designed and the DR under arcing current from 20 kA to 60 kA is presented in this paper. The influence of arcing current, contact separation, di/dt before current zero and arcing time on the DR is investigated. Particularly, it's found that under a specific arcing current, there exists an optimal arcing time for DR, and it shows a negative correlation with the arcing current. [ABSTRACT FROM AUTHOR]

Most microgrid protection schemes found in published literature suffer from a lack of generality in that they work well for the assumed topology, including type and placement of sources. Other generic protection schemes tend to be too complicated, too expensive, or both. To overcome these drawbacks, a topology-agnostic, scalable, and cost-aware protection based on fundamental principles that work in the presence of high penetration of inverter-based resources (IBRs) is developed and tested in this paper. The protection system also implements stable automatic reconfiguration of the healthy sections of the system after clearance of fault, thus increasing resilience by self-healing. To achieve this ambitious goal, stable inverter models are developed that operate in unbalanced networks in grid-connected and islanded modes, even with 100% IBRs, share power without conflicting controls, and can ride through faults while limiting fault currents. The scheme is tested for primary and backup protection and reconfiguration on the IEEE 123-node feeder in grid-connected and islanded modes with 15 IBRs connected to the system. [ABSTRACT FROM AUTHOR]

The current-limiting inductors (CLIs) are commonly used to suppress DC fault current to safely trip DC circuit breakers in MMC-HVDC systems, but it significantly jeopardizes DC dynamics and causes instantaneous power mismatch between AC and DC sides of islanded rectifier station (RS), exposing MMC internal states to large disturbance. This paper firstly provides a detailed analysis of the influence of CLIs on AC fault, establishes RS mathematical representation including AC side dynamic, and reveals the mechanism of DC voltage oscillation, modulation saturation and PCC waveform distortion. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, but coupled under excessively low submodule capacitor voltage, where AC modulation is saturated and PCC waveforms are distorted. To solve this issue, a novel RS AC fault ride through method for MMC-HVDC system with CLIs is proposed to stabilize RS DC voltage, desaturate AC modulation and decouple fault characteristics by regulating the zero-sequence modulation of circulating current suppression. In addition, a current reference disturbance and an AC energy dissipation device coordinated control are designed, which adaptively recovers the submodule capacitor voltage. Finally, the feasibility of the proposed method is verified by simulation results. [ABSTRACT FROM AUTHOR]

Optimal protection coordination is usually solved for the original network topology with all lines, loads, and generation intact. However, power grids may experience contingencies due to transient events, e.g., generation or line outages. Low fault currents of inverter-interfaced distributed generators (IIDGs) necessitate a sensitive and reliable protection scheme. This paper proposes a protection scheme for islanded microgrids powered by droop-based IIDGs. The protection scheme utilizes virtual impedance-fault current limiters to limit IIDGs fault currents and achieve protection coordination. A two-stage method for optimal protection coordination (OPC) of directional overcurrent relays (DOCRs) is devised. In Stage I, relays short-circuit currents are calculated. Then, constraints on the operation times of primary and backup DOCRs are formulated for the islanded topology and each possible topology following an N-1 contingency. Lastly, in Stage II, the OPC problem is formulated as a constrained nonlinear programming problem and solved to obtain the optimal DOCRs settings. A radial test microgrid that is part of a Canadian urban distribution system is used to ensure the success of the proposed OPC method. [ABSTRACT FROM AUTHOR]

Penetration of DC networks is rapidly increasing in modern power systems. As transient behavior of DC grids is different from that of AC ones, AC grid protection schemes might not be applicable to protect DC networks. Exact fault location is vital for protection of DC networks to reduce repair cost and achieve fast power recovery. This paper proposes a novel topology independent multi-terminal DC scheme to identify fault section and location for DC transmission systems. Voltage and current signals measured at the terminals are analyzed by the least squares error (LSE) algorithm. Then, the proposed scheme uses low frequency components of the signals to locate DC faults offline. Unlike some other methods, it does not require sophisticated sensors and hardware to capture very high frequency content of the signals. Hence, the proposed scheme can be implemented in practice without additional measuring infrastructure. The performance of the proposed scheme is evaluated under various fault conditions for different topologies of DC networks in both overhead lines and cables. Numerous simulations carried out demonstrate that the proposed scheme is quite accurate and provides excellent performance under different fault resistances, sections, and locations. [ABSTRACT FROM AUTHOR]

The missing capability of DC fault current limitation is an inherent disadvantage in the typical half-bridge modular multilevel converter (MMC), which restricts its application in long-distance high voltage direct current (HVDC) systems. The full-bridge submodule (FBSM) and clamped double submodule (CDSM) topologies allow for fault current self-clearing, but also suffer from additional hardware cost and power losses. In this paper, an improved submodule topology utilizing the new reverse-blocking insulated gate bipolar transistor (RB-IGBT) is proposed to improve system economy without compromising its performance. The operating principles and DC fault characteristics of the proposed topology are analyzed in details, including comparison with various topologies in terms of cost, losses, fault current clearing speed and thermal. Compared with the traditional CDSM topology, the proposed RBSM can clear DC faults faster with reduced cost of switching devices. A DC fault protection strategy for MMC-HVDC system is also presented. Simulation results under normal operating, permanent and transient fault conditions are carried out to verify the effectiveness of the proposed topology. [ABSTRACT FROM AUTHOR]

This paper proposes an adaptive protection scheme for distribution systems with distributed generations (DGs). To mitigate the influences on the protection devices by DGs, an online calculation method of fault current under various system operation conditions is presented. By sampling local available measurements in the buses, a proposed optimized estimation method is used to calculate the Thevenin equivalent circuit parameters of the system dynamically. Moreover, the short-circuit current can be calculated accurately to set the overcurrent relay protection value considering the behavior of DGs during the fault. To ensure the accurate operation of the protection devices, a coordinated protection scheme, including primary and backup protection is presented. Performances of the proposed method and scheme have been verified by simulations on a sample distribution system based on PSCAD/EMTDC. The sensitivity and selectivity of the coordination between primary and backup protection are satisfied under different system operation conditions, fault locations, and types. [ABSTRACT FROM AUTHOR]

This paper presents tests carried out at an outdoor experimental facility, described in the companion Part 1 paper, to investigate the performance of practical grounding systems. Here, the results of low-voltage dc, ac, and impulse tests performed on rod and grid electrodes are described, and the measured quantities are compared with computed values obtained from numerical models. Measured ground resistance and impedance at low frequency (including power frequency) showed reasonable agreement with simple standard formulae and computational models, but revealed a significant falloff with current magnitude in the range often used for practical testing of high-voltage grounding systems. This may have implications for the specifications of grounding test equipment and extrapolation of measured ground resistance/impedance at low-voltage/current to values representative of realistic fault currents. A frequency dependence of ground impedance was also measured. Specifically, a fall in impedance over a frequency range up to 100 kHz is not generally accounted for in grounding models. [ABSTRACT FROM AUTHOR]

The existing Electromagnetic Time Reversal (EMTR)-based fault location methods generally leverage the features (e.g., energy) of the signals simulated in the EMTR reversed-time stage (relative to the direct-time stage wherein a fault occurs and its originated transient signals are measured). This paper presents for the first time an integrated time-frequency analysis that identifies the similarity between the signals respectively present in the direct time and in the reversed time. It is demonstrated that the fault current observed at the true fault location exclusively behaves as a quasi-scaled copy of the time-reversed back-injected transient current, such that, the fault location can be identified through a similarity assessment. In this respect, the paper proposes calculating a cross-correlation metric to quantitatively represent the level of similarity between the back-injected transient signal and the fault current signal simulated at the guessed fault locations. The similarity characteristic and the fault location performance of the proposed cross-correlation metric are validated first through simulation case studies based on the IEEE 34-bus test distribution feeder, and then using a pilot test wherein a live medium-voltage radial distribution network was subject to realistic fault occurrences. It is demonstrated that the proposed similarity metric is applicable to locating various types of faults and is robust against uncertainties like fault impedance and fault inception angle. [ABSTRACT FROM AUTHOR]

This two-part paper presents a method to mitigate half-cycle saturation of transformers caused by monopolar operation of neighboring HVDC transmission lines while keeping the ability to detect the zero-sequence fault currents (ZSFC) when ground faults occur. Part I of this paper has presented the mitigation principles and device design of the proposed neutral current blocking switch. In Part II, the performance of the proposed method to permit the circulation of zero-sequence current is investigated. An operation strategy is proposed that simultaneously allows the mitigation of half-cycle saturation and the detection of ZSFC. Simulations on a widely-used 500 kV system show that the proposed mitigation technique, using a sub-synchronous switching frequency (no higher than 30 Hz), can effectively mitigate the half-cycle saturation while allowing the circulation of ZSFC. The novel mitigation method exploits the characteristic differences between half-cycle saturation (dc) and asymmetric faults (ac). The method provides an implementable solution to the dc-bias phenomenon because it delivers concurrently dc-bias mitigation, minimal impact on ground fault detection, and no switching stresses on the power electronic switches. The proposed technique can also be applied to the mitigation of geomagnetically induced currents. [ABSTRACT FROM AUTHOR]

Harmonic blocking is usually applied by converter transformer differential protection to prevent protection mal-operation during an inrush condition, but it may incorrectly block the protection during an internal fault. This paper first makes an in-depth analysis on factors that lead to differential protection failure to trip for converter transformer internal faults. The main factor responsible for the protection failure to trip is half-cycle saturation of the converter transformer core, which results from the dc component in the fault current. This suggests that harmonic blocking may not be suitable for differential protection of converter transformers to identify inrush conditions. In this regard, this paper proposes a novel method using mathematical morphology to effectively discriminate between the conditions caused by inrush and internal faults. This method implements the waveform symmetry criterion and improved morphological gradient criterion to distinguish between these two conditions. The effectiveness of the proposed method is evaluated based on extensive simulation, and the presented results show that the proposed method can ensure operation for all types of internal faults. Additionally, this method improves the accuracy of inrush identification, even in extreme cases, such as sympathetic inrush and inrush with current transformer saturation. [ABSTRACT FROM AUTHOR]

This paper presents a new multilevel cascaded-type dynamic voltage restorer (MCDVR) with a fault current-limiting function. This topology can operate in two operational modes: 1) a compensation mode for voltage fluctuations and unbalances and 2) short-circuit current-limiting mode. The current-limiting function of the MCDVR is performed by activating antiparallel thyristors during the short-circuit fault, and deactivating them during normal operation. The mathematical model of the MCDVR system is also established in this paper. The control scheme design and optimal parameter selection are outlined based on the detailed theoretical analysis of the converter. The transient states of the MCDVR in the compensation mode and current-limiting mode are also analyzed. Simulation results based on the PSCAD/EMTDC software, and experimental results on a laboratory setup help to validate the proposed topology and the theoretical analysis. [ABSTRACT FROM AUTHOR]

The integration of high bandwidth energy-storage systems (ESS) in compact dc electrical power systems can increase the operational capability and overall flexibility of the network. However, the impact of ESSs on the performance of existing dc protection systems is not well understood. This paper identifies the key characteristics of the ESS that determine the extent of the protection blinding effects on slower acting generator systems on the network. It shows that higher fault impedances beyond that of an evaluated critical level will dampen the response of slower-acting generator systems, decreasing the speed of corresponding overcurrent protection operation. The paper demonstrates the limitations of existing protection solutions and identifies more suitable protection approaches to remove/minimize the effects of protection blinding. [ABSTRACT FROM AUTHOR]

The result of single-ended traveling wave fault location method based on wavelet transform will be affected by false wavelet transform modulus maxima (WTMM) points. In this paper, fault distance/impedance information contained in the wave front of the traveling wave is analyzed first. Then, the fitting method based on generalized logistic function is proposed to extract the fault information. Based on the fitting results, a predictor–corrector fault location method is proposed, which includes the following content. First, the fitting coefficients of the zero-mode fault current wave front are used to estimate fault distance and fault impedance interval. Second, the arrival time of the line-mode fault current reflected wave can be calibrated and the distance calculation formula can be selected according to the fault distance estimation interval. Last, the fault distance will be corrected to be within the fault distance estimation interval, if the first reflected wave is undetected. A model of ±400 kV DC transmission project is built in PSCAD/EMTDC. The simulation results show that the fitting coefficients can directly correspond to fault distance/impedance. The proposed method can effectively eliminate the influence of the false WTMM points, and give the accurate fault location results as well as fault impedance interval. [ABSTRACT FROM AUTHOR]

The development of voltage-source-converter (VSC)-based high-voltage direct current (HVDC) systems has increased significantly. However, the implementation of VSC-HVDC systems is limited for lacking fast, low-loss, and reliable circuit breakers. Hybrid dc circuit breakers (DCCBs) may help to improve this situation. In this paper, an efficient modular multilevel converter (MMC) HVDC system model based on the Zhangbei–Beijing \pm \text500\,\textkV bipolar MMC-HVDC transmission (overhead line) project is first established. Then, the system transient characteristics are investigated involving permanent line-to-ground faults (LGFs), permanent short-circuit faults (SCFs), temporary LGFs, and temporary SCFs with either ac circuit breakers (ACCBs) or DCCBs in service. Finally, the performances, including the fault clearance and recovery, comparison of DCCBs with two different breaking methods, and comparison of ACCBs and DCCBs, in the MMC-HVDC system are presented. This paper provides a deep understanding of LGF and SCF in the MMC-HVDC systems, operation of DCCB, and comparison of faults clearance and recovery under these faults conditions using ACCB and DCCB, respectively. Results also benefit the application of the DCCB and the operation strategies in high-voltage and high-power MMC-HVDC systems with overhead lines. [ABSTRACT FROM AUTHOR]

This paper proposes a fault location method based on the adaptive control of neutral-to-earth complex impedance in a resonant grounded system, whose neutral point grounds are via an electromagnetic hybrid Petersen coil (EHPC). The EHPC can be equivalent to a parallel circuit of a negative resistance and an inductance, both of which can be adjusted. When a permanent single line-to-earth fault occurs, the ground-fault current will be compensated to almost zero, and the capacitance to earth and leakage resistance of system can be measured accurately for subsequent fault location. Then, the complex impedance of EHPC is adjusted, and the faulty feeder and its faulted section can be identified online based on the characteristics that the measurements of zero-sequence admittances of points along the faulty feeder change with the system's neutral-to-ground complex impedance. The detection criterion is certain and unique, without comparing all of the feeders. This paper also proposes an adaptive jitter adjustment and decoupling control of neutral-ground complex impedance by EHPC, which is adaptive to the grounding transient resistance and is repeatable, and can ensure that the magnitude of the zero-sequence parameter is large enough to be measured while without arc re-ignition, including in high impedance fault states. Simulations and experiments verified the feasibility of this fault location method and its control strategy. [ABSTRACT FROM PUBLISHER]

Sympathetic interaction between transformers is a quite normal phenomenon in power systems. For the purpose of preventing transformer differential protection relays from malfunction, this paper proposes a morphological method for the identification of sympathetic inrush, which is the first time when mathematical morphology is applied in this field. Since the waveform of differential current is symmetrical in an internal fault case while asymmetrical in a sympathetic inrush case, the proposed method uses a morphological operator to extract the peaks and valleys of the differential current to distinguish sympathetic inrush. Considering the possible current-transformer (CT) saturation conditions, this paper combines a morphological gradient with a weighted mathematical morphological operator to improve the effectiveness of the proposed method. The proposed method is evaluated on data collected from simulation cases established in PSCAD/EMTDC and from laboratory experiments, respectively. Identification results have verified that by comparing with the traditional second harmonic restrain method, the proposed method can distinguish sympathetic inrush from an internal fault current more accurately and more effectively, even when the CT is fully saturated. [ABSTRACT FROM PUBLISHER]

This paper proposes a new fast algorithm to estimate the fundamental phasor of fault current signals, based on discrete Fourier transform (DFT). Since fault currents include decaying dc component (ddc), DFT-based algorithms have inaccuracy in phasor estimation. The proposed algorithm consists of four steps. First, a new auxiliary signal is introduced based on a simple high-frequency modulation of the fault current. Then, DFT of the fault current and sample-summation of both fault current and auxiliary signal are calculated, for each one-cycle-length data stream. Next, DFT estimation error due to the ddc is calculated. Finally, by removing this error, accurate fundamental phasor is obtained. This paper uses an innovative and accurate formulation based on notations of signal processing literature. To validate the proposed algorithm, several computer-simulated signals and an electromagnetic transient program-generated signal are employed. The proposed algorithm is compared with recent phasor estimation methods, using wide variety of test signals. Standard indices of rise time, settling time, and percentage overshoot are used for comparison. These comparisons show that the proposed algorithm has robust performance in off-nominal frequency condition, and presence of harmonics, noise and multiple ddc components. Moreover, it provides faster convergence speed and lower computation burden. [ABSTRACT FROM AUTHOR]

In this paper, the finite-elements method was used to analyze the approximation being typically used in the international standards for the calculation of the touch voltage, appearing at earth faults in power networks. The calculations in this paper were performed on the series of earthing systems applied in practice and the results are compared with the results of the widely accepted approximate method from standards. The origin of discrepancy in the results is analyzed and evaluated. The outcome of this paper is the justification of the application of Thevenin's theorem and the development of the higher accuracy equations for the resistance between feet and remote earth in case of the application of high resistivity additional surface material. [ABSTRACT FROM PUBLISHER]

Phasor-based differential protection is widely used as the main protection function of the power transformer due to its reliability and ability to discriminate internal from external faults and inrush currents. However, these methods present some delays due to phasor convergence during the fault occurrence and can fail during challenging situations, such as transformer energizations with low second-harmonic content and turn-to-turn and turn-to-ground internal faults. This paper proposes a novel time-domain power transformer differential protection based on Clarke and wavelet transforms with only one differential unit and with automatic setting to be used in any power transformer. Considering both actual and simulation data, the performance validation reveals that the proposed method is efficient, ultra-fast, simple, and independent of the fundamental and harmonic components of the differential current. The method was also implemented in a real-time digital simulator to demonstrate its practical feasibility. [ABSTRACT FROM AUTHOR]

Multi-inverter microgrid systems, particularly those with loop topology, offer higher power supply reliability and robustness compared to conventional radial distribution systems. In meshed systems, communication-less protection schemes have proved to be ineffective for multi-inverter microgrids due to bidirectional power flow, and limited and controlled fault currents generated by the voltage source converters interfacing the energy source to the network. This makes communication-assisted line protection schemes preferable for such systems despite the necessity for communication means. While these protection schemes are effective, their reliability depends much on the communication availability. This requires a back-up communication path in case the main one fails bringing up the cost issue that hinders their uptake. This paper proposes a novel and low-cost line protection based on directional blocking strategy that can operate as a main as well as a back-up protection to any protection scheme using communication means between the line terminals. As the main, it requires low-bandwidth communication system. As a back-up, it would share the same communication means with the main one and use those of the healthy lines when the faulted line's fails. Thus saving the cost of back-up communication system. [ABSTRACT FROM AUTHOR]

In this paper, a new dc switchyard structure for a multiterminal dc (MTDC) grid is proposed. Fast fault current breaking capability, which is essential for a dc grid, can be achieved with available HVDC breaker solutions (e.g., ABB's hybrid HVDC breaker). However, the hybrid HVDC breaker concept also opens up possibilities for unconventional (relative to ac) switchyard solutions which may be more cost effective. The main aim of this paper is to expound how the proposed dc switchyard can be built with hybrid HVDC breakers in a configuration where the number of components is much less compared to a conventional switchyard structure. After that, the switchyard operation, fault current handling flexibility, and actions against component failure are described conceptually. Finally, the performance of the proposed switchyard is validated and compared with that of a conventional double breaker double busbar switchyard structure for selected cases through time-domain simulations [ABSTRACT FROM PUBLISHER]

Protection coordination for the microgrid system in both islanded and grid connected modes is considered a challenge for microgrid operation. The variation in short circuit currents in each mode of operation, limitation of protection curves in the relays, and the need for fast and reliable communication are the main challenges tackled in research. Direction overcurrent relays are considered the main protection instrument for the microgrid. However, one protection setting for both (main and backup) operation does not provide a fast-operating time. On the contrary, the dependence on communication to maintain the proper backup protection operation is not a reliable solution for protection system. Therefore, this paper introduces a fast, reliable, and simple protection scheme for secure microgrid operation in both islanded and grid connected modes. In this context, one set of protection settings is optimized and used in both modes of operation. A protection setting consists of two time-dial settings and one pickup current setting for each relay. The proposed design technique minimizes the total operating time for both main and backup relays. As all protection functions are executed in the forward direction, no communication is needed among relays. The proposed scheme is tested and verified on a modified portion of IEEE 30 bus test system equipped with distributed generation units. A comparative analysis is carried out and the results demonstrate superior performance by the proposed scheme in comparison with the dual setting scheme with communication. [ABSTRACT FROM AUTHOR]

For modular multilevel converter (MMC) based DC grids, current-limiting reactors (CLRs) are mainly employed to suppress the fault current and provide boundary effects to detect internal faults. Thus, most existing protection schemes are highly dependent on the larger CLRs to guarantee high selectivity. However, in existing MMC based HVDC projects, the size of CLRs is restrained by the cost, weight, and system stability under normal state. Thus, boundary protections may fail to detect high-resistance faults and pole-to-ground faults under weak boundary conditions. To overcome these shortcomings, this paper proposes a fast and selective DC fault detection scheme independent of boundary components. The propagation characteristics of line-mode backward traveling-waves (TW) are analyzed to identify external and internal faults. The polarities of zero-mode backward TWs are employed to select faulted poles. The directional overcurrent based pilot protection is adopted as a complementary criterion to detect remote faults. The proposed method can be applied in MMC-HVDC systems with small CLRs that cannot provide strong boundary conditions. Besides, the detection speed is fast (less than 1.5ms). Moreover, it is robust to fault resistance and immune to noise. Various simulation results in PSCAD/EMTDC verifies the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

This paper proposes a new method to select single-line-to-ground (SLG) fault feeder in resonant grounding power distribution system based on compensation characteristic of extinction coil in different frequencies. The proposed method is insusceptible to the influence of device polarity reverse installation. Firstly, through analyzing the zero sequence equivalent circuit under a SLG fault, the difference of zero sequence current share in fundamental frequency and second-harmonic frequency is investigated. And then, according to the change of current share (CCS), fault feeder selection criterion and steps are established. Finally, numerous simulation tests demonstrate the effectiveness of the proposed method in identifying the faulty feeder or the faulty bus. Besides, its reliability is also verified by tests with different system parameters and different fault conditions. Specially, device polarity reverse installation tests and comparisons are also implemented, which shows the advantages of the proposed method in dealing with device polarity reverse installation. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) based high voltage direct current (HVDC) transmission system is one of the important development trends of DC power grid. After fault isolation, the traditional reclosing method of MMC-HVDC system is directly reclosing DC circuit breaker (DCCB). Once the fault point still exists, DCCB will be tripped again. This method will make the DC system suffer from the secondary strike of fault in a short time, and seriously endanger the service life of DCCB. In order to overcome the drawbacks of the traditional reclosing schemes, this paper proposes a novel adaptive reclosing scheme based on pulse injection from parallel energy absorption module. When dc fault current is interrupted by DCCB, the fault energy stored in current-limiting reactors (CLR) and dc lines is transferred to energy absorption module automatically. By switching on IGBTs embedded in absorption module for a short time, a voltage pulse is injected directly into the faulty line without interference from CLR. Then the nature of fault is identified and the fault distance can also be calculated in the case of permanent faults. The simulation results verify the effectiveness of the proposed reclosing scheme. [ABSTRACT FROM AUTHOR]

DC transformer is an essential equipment for formation of multi-voltage-level DC grid. This paper proposes a novel non-isolated Boost-type alternate arm DC transformer (Boost-AADCT) for medium- and high-voltage level DC grid application. The Boost-AADCT is comprised of three submodule-cascaded arms, two thyristor-cascaded director switches, and two buffer capacitors. Bidirectional power transmission is achieved by alternatively charging and discharging of the intermediate arm following the Boost principle, while the pulsating power component caused by intermittent transmission is actively smoothened by grid-side buffer subcircuits formed by the buffer capacitors and grid-side arms. The active power buffering method increases the tolerance range of voltage ripple of the buffer capacitor, and the required energy storage capacity is resultantly reduced. Circuit configuration, operation principle, and mathematical model are first introduced. Then, the control strategy is proposed, the main circuit parameters are designed, and bidirectional fault-blocking capability is analyzed. Comparison with other DC-DC converters shows that the Boost-AADCT significantly reduces the requirement of semiconductors and has relatively lower synthetic cost. At last, performance of the Boost-AADCT and the proposed control strategies are evaluated by simulation tests conducted in MATLAB/Simulink environment. [ABSTRACT FROM AUTHOR]

This paper presents the DC fault transient study of a meshed DC grid with high-temperature superconducting (HTS) HVDC cables to integrate offshore wind power. A four-terminal meshed DC grid model with a ±100 kV DC voltage rating is developed. DC circuit breakers (DCCBs) are implemented in the DC grid to deal with DC cable faults. To conduct the fault transient study, the fault scenarios with different fault locations and protection delays are studied. The sensitivity study versus different DC fault locations reveals that the faults on the faulted cable will not cause the quenching of healthy cables. The sensitivity study considering different DC fault protection delays demonstrates that the HTS cables have a current-limiting effect and the long delay of DC fault protection does not result in large fault current and the quenching of healthy cables. Extensive simulations in PSCAD/EMTDC validate the founding. [ABSTRACT FROM AUTHOR]

The single-phase to ground fault (SPGF) is very common in power grids. To decrease the fault’s influence on the power supply’s reliability, the neutral point is usually grounded via the Petersen coil, significantly reducing the outage rate. Nevertheless, this coil would weaken the fault current and make the faulty line identification much more difficult. To address such an issue, an identifying method for the faulty line is proposed in this paper based on the detection of an extremely low frequency current(ELFC), after analyses are made on the electrical signal’s characteristics when the fault exists based on a common physical model of a medium-voltage distribution system. Then, to coordinate with this method, a set of devices are developed to accurately measure the ELFC and ensure feasibility. At last, a simulation and an experiment are conducted, and the results prove this method’s validity in the identification of a faulty line. [ABSTRACT FROM AUTHOR]

Incipient faults in underground cables are often resulted from electric stress and cable aging. If such faults occur as current spikes in short periods, permanent faults may appear. Moreover, incipient faults may disturb electricity transmissions because of detection delays and thus, precise well-timed protection decisions cannot be made. Therefore, one of the most important considerations of utilities in the monitoring process is to recognize this type of faults from other conditions as soon as possible. In this paper, a precise approach based on CUmulative SUM and ADAptive LInear NEuron has been proposed. In addition to its high-speed detection ability, the practical on-line implementation of the proposed approach is simple as well. The simulation results in EMTPWorks environment demonstrate that the proposed approach has a strong capability in distinguishing the cable incipient faults from other similar conditions in distribution systems. [ABSTRACT FROM AUTHOR]

Because of the long distance and huge distributed capacity of half-wavelength AC transmission lines (HWACT), the fault characteristics of HWACT lines differ a lot from those of conventional lines. In this paper, the unique fault characteristics at the measuring points are simulated and the adaptation of conventional fault tripping criteria in HWACT lines is analyzed. It is pointed out that the stability control devices may operate incorrectly when the fault occurs far from the measuring point of the HWACT line. The fault location is determined by the starting time difference of the devices at both sides of the line. With the fault location and distributed element model of HWACT line, the voltage and current of the fault point are calculated and the new fault tripping criteria based on the electrical quantities of both the fault point and the measuring points are proposed. The simulation results show that the new criteria can distinguish the single-phase fault from phase-to-phase fault correctly and reliably. [ABSTRACT FROM AUTHOR]

This paper proposes a fast dc short-circuit fault detection method for HVdc grids, which relies on the voltage across the dc fault-current-limiting reactor. In the proposed approach, primary detection uses only the single-terminal dc reactor voltage information and is shown to be able to identify the faulted line as well as the faulted pole within 1 ms. The backup detection is primarily required for high resistance faults and uses communication between the two sides of the line. Its speed is dependent on the travel time of signals between the two ends, but will operate, as shown in the paper, typically in 2 ms or less. The approach is justified using analytical calculations and exhaustively validated via electromagnetic transient simulation for pole-to-pole and pole-to-ground faults, different fault resistance, fault locations, and dc reactor values. The validation shows that the method is able to identify all genuine dc line faults, but is not prone to misoperation with ac line faults, different loading levels, and so on. [ABSTRACT FROM AUTHOR]

This paper presents a decentralized fault detection technique for power distribution systems with resonant grounding. The aim of this paper is to detect single phase to ground faults and identify the faulty feeder within three cycles of the fault occurrence. In the proposed technique, faults are first detected based on the neutral voltage displacement. The pre-fault and post-fault voltages (phase to ground) are then used to identify the faulty phase. Finally, the faulty feeder (as well as the faulty area of a long feeder) is identified from the relationship between the initial transient of the zero-sequence current and the faulty phase voltage just after the occurrence of faults. A signal processing tool called mathematical morphology is utilized to identify the faulty feeder. To identify the faulty feeder, it is also required to know the fault occurrence time that is estimated using the slope of the neutral voltage. The main feature of the proposed scheme is that this technique only uses voltage and current signals from the corresponding voltage transformers and current transformers. Therefore, it does not require communication among protection devices in the same or different feeders to identify the faulty feeder. The proposed technique also has the ability to distinguish the nature of faults, i.e., whether the faults are permanent or temporary. The effectiveness of the proposed scheme is tested, on an IEEE test system as well as on a practical test system, using MATLAB/SimPowerSystems. Simulation results show that the proposed technique can quickly detect the single phase to ground faults in a resonant grounding power distribution system and identify the faulty feeder. It is also capable of distinguishing faults from other disturbances. Moreover, it works under different compensation levels as well as for different fault inception angles. [ABSTRACT FROM AUTHOR]

This paper presents two approaches where the single-core cable line and an additional conductor laid in parallel can be reduced to a simple equivalent \pi-circuit containing an active element beside the usual passive impedances. The parameters of the equivalent circuit are calculated using a numerical approach where the distributed parameter line was discretized and an analytical approach is based on the wave propagation theory. A series of tests has been performed to determine error levels in both cases showing that they are less than 1%. With numerical examples, we show that the cable equivalent circuit offers a simple way to calculate voltages in complex grounding systems. The results are in good agreement with the previously published calculated and/or measured values. Both approaches offer the possibility of calculating voltage and current distribution along the cable line which gives insight into the significance of each cable component in the complex grounding system. [ABSTRACT FROM PUBLISHER]

This paper presents an advanced fault control for transformerless modular multilevel converters (MMCs) in AC/DC hybrid distribution networks (HDNs) under the single-phase grounding (SPG) fault. The characteristics of voltages and transmission paths of zero sequence currents in AC networks with different voltage levels and neutral grounding types under SPG faults are analyzed. According to the analysis, the SPG fault results in two issues: 1) the voltage imbalances in nonfaulted networks, which have negative influence on the operation of nonfaulted networks; 2) the overcurrent of MMC-based terminals results in the shutdown of MMCs instead of fault ride-through. To solve these two issues, two types of zero sequence fault control are proposed, which are suitable for different fault ride-through abilities of transformerless MMCs in AC/DC HDNs. The proposed fault control is implemented by an advanced zero sequence current controller (ZSCC). Furthermore, to reduce the effect of fault control on the sensitivity of current differential protection based on the zero sequence fault current, an improved set of current reference value is addressed. Finally, the theoretical analysis and proposed fault control are verified through simulations performed in MATLAB/Simulink. [ABSTRACT FROM AUTHOR]