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Cable fault location is generally much more difficult if cable defect has not reached the level of serious breakdown. Traditional methods such as time domain reflectometry (TDR) and partial discharge detection cannot solve this problem well due to their low sensitivity. This paper proposes a new non-destructive detection method based on the impedance spectroscopy (IS), which can effectively locate soft faults in the cable. In this paper, a numerical model for cable with distributed parameters is established firstly. Based on this model, cable faults are extracted from the imaginary part of impedance spectrum with Discrete Fourier Transform (DFT). The Chebyshev window is introduced to suppress interference and enhance signal-to-noise ratio, which improves the sensitivity and accuracy of fault location significantly. Experiments and simulations show that this method can well locate multi-segment defects in cables with good accuracy. The window function successfully amplifies the normalized amplitude of faults and improves the positioning accuracy. Compared with other method, this method reduces distortion points effectively. The fault location error is within 0.53%, which has great potential for field application. [ABSTRACT FROM AUTHOR]

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]

The commutation failure (CF) is the most common fault in line-commutated high voltage direct current (LCC-HVDC) systems that may lead to the transient overvoltage in the sending-side system. In the worst condition, the CF may lead to large-scale wind turbine tripping. To resolve this problem, the mathematical relationship between the reactive power consumed by the rectifier and DC voltage, DC current is derived. Then, a transient overvoltage calculation method is proposed in this paper. Furthermore, the mechanism of transient overvoltage caused by the CF is analyzed; it is revealed that the reason for three times transient overvoltage is the rapid decrease of the reactive power consumed by the rectifier during the CF and the recovery period from the CF. This paper proposes a constant reactive power control (CRPC) to inhibit transient overvoltage of the sending-side AC system. The proposed CRPC can increase the reactive power consumed by the rectifier, reduce the exchange reactive power between AC and DC systems, and suppress the transient overvoltage. A simulation model in PSCAD serves to verify the proposed CRPC on the transient overvoltage suppression in the situation of different fault types, fault duration, fault severity and short circuit ratio (SCR). [ABSTRACT FROM AUTHOR]

Inverter-based resources respond differently than synchronous generation sources during power system faults, challenging line protection reliability. To understand the reliability challenges, Sandia National Laboratories and North American Electric Reliability Corporation (NERC) led a collaborative study with four inverter manufacturers (mix of Type 3 Wind, Type 4 Wind, and PV Solar) and two North American protective relay manufacturers. As part of this study, an electromagnetic transients program (EMTP) with “real-code” models representing inverter firmware was used to investigate the protective relay response to faults on the modeled power system with the different inverter types. The currents and voltages obtained from the study were then played back through line relay hardware with settings typical of systems with conventional generation. The protective relays exhibited the expected reliability challenges but provided insight into the behavior of inverter-based resources during power system faults. In this paper, we share some of the challenging cases with the responses of the different line protection schemes. Using insight from the various event records, Part II of this paper shows how relay settings can be modified to obtain a substantial improvement in reliability. [ABSTRACT FROM AUTHOR]

Weather-related or other types of faults can negatively impact the continuity of the electricity delivery to the consumers and, thus, lead to poorer reliability scores. 90% of customer outage-minutes are due to events which affect local distribution systems. The radial configuration is simple and has a low cost design; however, when a fault occurs it often causes power outages beyond the location of the fault itself. Therefore, it is beneficial for utilities to automatically isolate the fault and, thus, restore power to as many consumers as possible. The benefits of fault isolation and service restoration include, but are not limited to: enhanced reliability and resiliency, reduced loss of revenue to the utility, and enhanced comfort and satisfaction for the utility’s consumers. This paper provides an algorithm to quickly isolate a fault and automatically restore power to as many nodes of the distribution system as possible by opening and closing switches already placed within the system. The expert model and centralized algorithm that are proposed in this paper guarantee an optimal solution and a short run-time regardless of the size of the distribution system. A mathematical proof of the optimal nature of the solution is provided. In addition, the model is validated in practice, solving two different-sized systems. [ABSTRACT FROM AUTHOR]

This paper presents a novel methodology to estimate the saturation curve parameters of single-phase transformers, using minimum information from inrush and no-load tests. Only the value of the first current peak and the decay time are used from a unique inrush test, and only the third and fifth current harmonics from a unique no-load test. The proposed methodology is based on the existence of a signature for each transformer which can be easily obtained and characterize all possible inrush currents. This is the main contribution of the paper. Different transformers with different parameters have been tested in the laboratory. Experiments and simulations are carried out to validate the methodology. First, the respective no-load and inrush tests are made. Second, the parameters of each transformer are estimated. Third, the performance and accuracy of the proposed methodology are verified by a close agreement between the measured and the estimated saturation curves and between the simulated and the recorded inrush waveforms. Some remarkable advantages of the proposed methodology are: (a) considers the deep saturation, (b) avoids the need to know the residual flux (or to demagnetize the transformer) and the switching angle from the inrush test, and (c) it is computationally straightforward. [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]

DC-DC converters are needed for the future development of high voltage direct current (HVDC) grids, as they allow to interconnect lines with different voltages and topologies. The dc-dc converters can increase the grid controllability adding power flow control, voltage regulation and/or fault blocking capability. The dc modular multilevel converter (dc-MMC) is a non-isolated solution proposed to interconnect HVDC systems with the same line topology. This paper proposes a new dc-MMC with a control strategy, which allows the converter to interconnect different line topologies (e.g., rigid bipole connected to a symmetric monopole). The paper presents the different line topologies in HVDC installations. Then, a mathematical model with a variable transformation is proposed for the new dc-dc converter. A control structure is proposed and implemented in Matlab/Simulink using an average arm model and simplified dc grids. The results validate the control in normal operation, fault blocking capability and post-fault scenario (degraded mode). [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]

Aiming at the fault traveling wave on the LCC-MMC HVDC transmission line, this paper analyzed its reflection and refraction characteristics at the DC buses and the fault point, plotted its propagation grid diagrams, obtained the relationship between the polarity of fault traveling wave and the boundary elements, and drew the conclusion that when the impedance of DC filter is far less than the wave impedance of transmission line, the existing fault location methods that need to distinguish these two waves according to their polarities are not applicable to the LCC-MMC HVDC system. On these bases, this paper proposed a modal traveling wave arrival time difference-based location principle for pole-to-ground fault, and a fault traveling wave propagation path ratio-based location principle for pole-to-pole fault, these principles neither need to identify the wave polarity, nor need the wave velocity value. In order to better extract the wave heads and their arrival time, this paper pre-treated the sampling data to make sure all the wave heads are extracted from the same frequency band, then the wave heads and their arrival time can be extracted with the empirical mode decomposition. The simulation results verified the effectiveness of the proposed fault location principle. [ABSTRACT FROM AUTHOR]

This paper presents an optimization framework for sequential reconfiguration using an assortment of switching devices and repair process in distribution system restoration. Compared to existing studies, this paper considers types, capabilities and operational limits of different switching devices, making it applicable in practice. We develop a novel multi-phase method to find the optimal sequential operation of various switching devices and repair faulted areas. We consider circuit breakers, reclosers, sectionalizers, load breaker switches, and fuses. The switching operation problem is decomposed into two mixed-integer linear programming (MILP) subproblems. The first subproblem determines the optimal network topology and estimates the number of steps to reach that topology, while the second subproblem generates a sequence of switching operations to coordinate the switches. For repairing the faults, we design an MILP model that dispatches repair crews to clear faults and replace melted fuses. After clearing a fault, we update the topology of the network by generating a new sequence of switching operations, and the process continues until all faults are cleared. To improve the computational efficiency, a network reduction algorithm is developed to group line sections, such that only switchable sections are present in the reduced network. The proposed method is validated on the IEEE 123-bus and 8500-bus systems. [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]

HVDC grid protection with adequate speed and reliability is required to minimise the impact of DC faults. In particular, fast breaker failure backup protection algorithms are needed to meet the expected reliability requirements of HVDC grids. In this paper, existing breaker failure backup protection algorithms are shown inadequate to detect partial failures like a single module failure of breakers with a modular structure. This paper proposes a backup protection algorithm which rapidly detects a DC breaker failure based on estimating the counter-voltage created by the energy absorption branch during an interruption. The performance of the proposed algorithm is evaluated using a four-terminal test network with both hybrid and mechanical DC breaker technologies. The simulation results show that the proposed algorithm is able to quickly detect both complete and partial failures of the two breaker technologies even considering measurement errors, noise and ageing of the energy absorption components. [ABSTRACT FROM AUTHOR]

The inrush current generated by power transformer energization (TE) causes voltage sag of the nearby bus, the typical residual voltage of the voltage sag caused by transformer energization (VSTE) is about 0.7 p.u.--0.9 p.u. and lasts for a longer duration, which may lead to the sensitive equipment trips. This paper proposes an estimation method of voltage sag frequency considering VSTE. Firstly, based on the maximum entropy method, the residual flux distribution is calculated by considering the relationship between the residual flux and closing angle at the moment of TE. The voltage sag waveform caused by TE is calculated based on the estimation of the probability distribution of residual flux, to evaluate VSTE objectively and comprehensively. Secondly, this paper proposes a non-rectangular voltage sag equivalent frequency statistical method based on multiple magnitude-duration (MMD) function, to avoid the overestimation of voltage sag frequency by the traditional methods. Furthermore, this paper proposes an assessment algorithm of voltage sag frequency. Finally, the simulation of IEEE-30 test system verifies the necessity and correctness of the proposed method. [ABSTRACT FROM AUTHOR]

The Ultra High Voltage (UHV) converter transformer may generate serious inrush current during closing. It will cause commutation voltage distortion and easily result in commutation failure of converters on the same commutation bus. Due to the change of flux amplitude and phase caused by closing resistance in the Ultra High Voltage Direct Current (UHVDC) system, it is difficult to determine the closing time of the converter transformer when considering the influence of residual flux. Therefore, this paper first analyses the existing closing resistance strategy, and constructs the magnetic bias function according to the influence characteristics of closing resistance on the flux and phase. Then an inrush current suppression strategy based on magnetic bias simulation is proposed for star connection and delta connection of UHV converter transformers, which can accurately calculate the ideal closing time of mutual cancellation between the residual flux and bias flux during closing. Finally, the no-load closing model of the UHV converter transformer is established by PSCAD/EMTDC for simulation analysis of the feasibility, real-time verification is carried out using the RTDS closed-loop experimental platform. The results show that the proposed inrush current suppression strategy can effectively suppress the inrush current under different residual flux conditions. [ABSTRACT FROM AUTHOR]

DC fault protection is the bottleneck that restricts the development of modular multilevel converter based high voltage direct current (MMC-HVDC) system. Superconducting fault current limiters (SFCL) is an ideal protection device that can significantly limit short circuit current and enhance the fault tolerance ability of MMC-HVDC system. At present, several topologies of SFCLs have been proposed, but there is no systematical study of their current-limiting characteristics in MMC-HVDC. This paper studies the development process of DC short-circuit faults in MMC-HVDC system, and analyzes its demand for fault current limiting. Then, resistive type, saturated iron core type and hybrid type SFCL are modeled and their limiting effect of critical fault current under DC pole-to-pole fault is tested. By comparison, the current-limiting characteristics of three types of SFCL are concluded. Results show that resistive SFCL has best performance in steady state, while saturated iron core SFCL has best performance in transient state. The current-limiting characteristic of hybrid type SFCL is between the other two types. [ABSTRACT FROM AUTHOR]

Currently produced insulating oils tend to generate gases at relatively low temperatures (90–200 °C). This phenomenon, defined by CIGRE as stray gassing (SG), can hinder the diagnostics of oil-filled electrical equipment based on the DGA method, because high amounts of hydrogen H2 and saturated hydrocarbons can sometimes occur in service without an actual fault. The aim and contribution of this article is to present how to distinguish such cases from electrical partial discharges or hot spots in paper, which may be risky for the safe operation of equipment and demand further investigation. Very high levels of H2 and high levels of C2H6 are observed recently in some families of HV transformers in Poland. This article presents DGA results of oil samples taken from these transformers together with results of laboratory tests of stray gassing of these oils performed according to CIGRE procedure. By using Duval Pentagon 2 it is possible to compare stray gassing patterns in the laboratory and in real transformers and evaluate the temperature range of possible defects. Stray gassing/overheating of oil in transformers, within this temperature range and whatever the possible other causes, is not a concern for their safe operation, according to observations published by CIGRE. [ABSTRACT FROM AUTHOR]

Although FRA method is very popular and well-known for its accuracy in fault-detection in three-phase transformer, it has not become an adept so far in fault localization. Fault-localization in transformer winding is undoubtedly an important aspect in fault diagnosis and providing a solution to any extent for this problem would be certainly a worthwhile contribution. Towards this approach, even identifying the faulty winding could be a substantial contribution to enhance the diagnostic performance of FRA method. To this context, this paper presents an innovative FRA measurement technique to identify the faulty phase (whether the middle or either of the two outer phases) in ‘delta-connected’ HV windings. Proposed technique imposes an apparent ‘second-delta’ from the terminals together with the presence of original ‘delta’ inside and measures the two desired output current responses simultaneously for an excitation to the middle phase. A mere observation of symmetry or asymmetry between the two responses determines whether the fault exists in the middle phase or in any of the two outer phase windings. Proposed technique is first evaluated by the simulation results followed by experimental measurements on actual three-phase transformer windings. Results reported in this paper are also found very promising and encouraging. [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]

Inverter-based resources respond differently than synchronous generation sources during power system faults, challenging line protection reliability. Part I of this paper presented cases that challenge line protection schemes. In this paper (Part II), we provide solutions that can be applied to gain a substantial improvement in reliability. [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]

Hybrid high voltage direct current systems utilized for bulk-power transmission have recently attracted more attention. The overhead lines (OHLs) are mainly used for the implementation of such systems due to their outstanding economic advantages. However, OHLs are vulnerable to temporary faults, jeopardizing the safe and reliable operation of the network. The resiliency to the line faults is of considerable significance to the power transmission. This paper proposes a fault property identification strategy based on the Pearson correlation coefficient for the LCC-MMC hybrid HVDC system which can differentiate temporary and permanent faults. To achieve this, a voltage signal is injected into the faulty line by decreasing the firing angles of the LCC-based rectifier. Based on the established OHLs model and the injected signal, an estimated backward travelling wave can be accurately calculated. The fault property is identified by comparing the wave difference of the real backward travelling wave and the estimated backward travelling wave. The proposed strategy has a high accuracy since all influencing factors including the fault resistance, fault distance, and line parameters have been taken into account. Finally, to verify the efficacy of the proposed algorithm, several simulations have been performed using PSCAD/EMTDC. The simulation results show that the proposed algorithm can effectively distinguish the fault property. [ABSTRACT FROM AUTHOR]

Stray fluxes passing through the current transformer (CT) core produced by adjacent bus-bars and conductors or eccentric installation of primary conductors results in asymmetric magnetic flux distribution along the core. This may lead to exceeding the magnetic flux from its appropriate saturation level in some parts of the core, which is called partial saturation (PS) phenomenon. Under such a condition, CT error increases, and some relays maloperations may take place. This paper proposes a novel analytical approach to design flux equalizing winding (EW) for resolving such a problem. Moreover, PS occurrence in a core balance CT (CBCT) is analyzed using the finite element method (FEM), and its negative influence on a sensitive earth fault (SEF) relay is scrutinized as a case study. Simulation studies reveal that the proposed approach can prevent PS-related undesirable effects in a real power system. [ABSTRACT FROM AUTHOR]

This paper proposes an approach to mitigate adverse effects of the fault management of closed/ring grids with high shares of distributed generation (DG). The approach refers to conventional distance protection relays in particular. The rising share of DG, especially in distribution systems, can lead to overloadings and intolerable voltages. Closed-ring structures increase the grid capacity for DG due to more balanced load and voltages profiles. However, malfunctions of protection relays may occur because fault locating and selective fault clearing can be disturbed. This paper presents a distance protection algorithm for feeder relays that complies with the requirements of closed/ring grids with inter/infeeds. The proposed current comparison method (CCM) bases on the comparison of negative/sequence currents. It needs no voltage measurement and remains unaffected from inter/infeeds and fault resistances. Additionally, for relays located at the ring coupler, where the ring can be opened, a coupling relay method (CRM) is proposed. It locates unsymmetrical faults in closed/ring grids with inter/infeeds and enables fast and selective opening of the ring. This helps maintaining reliable power supply for customers in closed rings. Finally, the paper presents a grading strategy reducing the average fault clearing time. Lab tests prove the capability of both proposed algorithms. [ABSTRACT FROM AUTHOR]

When a single-phase-to-ground (SPG) fault occurs in distribution networks, the existing faulty feeder detection techniques do not present reliable detection results. This paper introduces a novel faulty feeder detection method based on transient energy and cosine similarity. First, the fault characteristics of transient zero-sequence current (TZSC) are analyzed. It is found that the fault transient characteristics of transient components decrease as the grounding resistance increases. Second, to fully maintain the fault transient features, a fundamental frequency component (FFC) shifting method is introduced to remove FFCs. Next, the transient currents without containing FFCs are utilized to generate two fault indicators: transient energy within the first half power cycle and cosine similarity after the first half power cycle. After that step, to effectively merge the two fault indicators, the Laplace distribution is utilized. The fault possibility of each feeder can be quantitatively estimated by calculating the cumulative density function (CDF). Finally, the feeder with the maximum fault possibility is determined as the faulty feeder. The effectiveness and applicability of the proposed detection method are verified in the radial distribution network and the modified IEEE 34-bus distribution network. The results of the field test also indicate the feasibility of the proposed SPG detection method. [ABSTRACT FROM AUTHOR]

Fault response of an inverter based resource (IBR) is dictated by its control scheme, making it markedly different from the response of a synchronous source. This has led to misoperation of distance relays when fed by IBRs. Published literature is mostly focused on changing the control design of inverters, but this approach is not practical, as these controls are proprietary and beyond the reach of utilities. To resolve this issue, this paper proposes a method for a source-agnostic distance relay that works in time-domain. Using only single ended measurements, the relay detects, classifies and locates any fault on the line it monitors. The method does not require any polarization technique. It is shown to work for unbalanced lines of any length, for transmission or distribution system with any number (even 100%) of IBRs, is immune to load encroachment, decaying dc offset and prefault currents. With very limited communication, it is also shown to be immune to fault resistance. [ABSTRACT FROM AUTHOR]

Multiphase drives with fault-tolerant capability are favored in high-reliability applications. In most fault-tolerant multiphase drives, the fault types and locations must be known prior to the fault-tolerant control. Therefore, fault diagnosis is an indispensable procedure. This paper proposes a generalized method to detect and identify the open-circuit faults (including open-switch and open-phase faults) in star-connected symmetrical multiphase drives with different phase numbers. The selected fault diagnosis signals are calculated solely from simple arithmetical operations of measured phase currents, resulting in easy real-time implementation. As fault detection signals are derived from the physical constraint imposed by the neutral point, the proposed method is naturally robust and not sensitive to operating points, machine transients and harmonics in stator currents. Therefore, a relatively small fault detection threshold can be used and fast detection can also be achieved. After the fault has been detected, the fault type is further identified according to the polarity of the integration of phase currents. Experimental results on nine-phase and five-phase induction machine drives verify the generality, fast diagnosis speed and robustness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper analyzes magnet defect fault signature in the surface-mounted permanent magnet motor (SPMM) using nonlinear analytical model considering the influence of magnet shape and magnetization direction. Based on the surface current method and conformal mapping technique, the magnet defect equivalent current and equivalent nonlinearity current are proposed to represent the magnet defect. Accordingly, the performance of SPMM can be obtained. The proposed model has great potential to investigate SPMM with non-uniform magnet defect and gives theoretical basis for fault diagnosis. The extra sensing coil is added to measure the voltage waveforms, which can also be predicted using the nonlinear analytical model and shows the influence of magnet defect fault. Finally, the finite element analysis and experimental results verify the high accuracy of nonlinear analytical model. [ABSTRACT FROM AUTHOR]

Mal-operation of distance protection relays during power swing (PS) condition can endanger the power system stability. Therefore, it is crucial to distinguish between short-circuit faults and PS by a power swing blocking (PSB) function to prevent undesired tripping of distance relay. This paper develops a novel and straightforward PSB method based on the rate of impedance angle change (RIAC). Not only can the proposed approach fairly discriminate both symmetrical and asymmetrical faults from PS, but also it can appropriately preserve protection scheme security during PS under single-pole tripping (SPT) on a transmission line. Furthermore, there is no need to perform extensive dynamic stability studies for setting the PSB function parameters. Unlike the rate of impedance change approaches utilized two blinders, the developed approach only needs an inner blinder. Thus, it can be used for networks running near their operational limits. The proposed method is evaluated through a modified IEEE-PSRC benchmark system in PSCAD/EMTC environment. The obtained results verify superior performance of the proposed method to traditional impedance rate of change method under different scenarios. [ABSTRACT FROM AUTHOR]

Although the harmonics interaction plays a vital role in causing commutation failure (CF) in an ultra HVDC (UHVDC) system with hierarchical connection, it has not been sufficiently considered in previous work. In this paper, the electric interaction among the inverter valves of the UHVDC system with hierarchical connection during receiving system fault transient process is demonstrated. Moreover, the mechanism of CFs in UHVDC system with hierarchical connection is also investigated via the analysis of harmonics interaction. After the AC fault occurs, the appearance processes of the local commutation failure (LCF) and concurrent commutation failure (CCF) are divided into several stages and interpreted by schematic diagrams. And the characteristics of each stage are also concluded. In addition, a coordinated CF prevention scheme based on harmonics and DC current detection is proposed to mitigate the CFs in UHVDC system with hierarchical connection. Finally, the effectiveness of the proposed scheme is verified via a $\pm 1100$ kV UHVDC transmission system under various short circuit ratio values and AC-side coupling strengths. [ABSTRACT FROM AUTHOR]

Accurate fault location for underground cables in MMC-HVDC grids reduces the time finding faults as well as the system operational costs. For MMC-HVDC grids, the data window during faults is usually short. During such severe transients, the frequency dependent line parameters of the underground cables have strong influence on the fault location accuracy. In this paper, a novel fault location algorithm for underground cables in MMC-HVDC grids is proposed. First, a multi-layer model for the underground cable using partial differential equations is proposed, which considers the distributed and frequency dependent line parameters in time domain. Afterwards, the numerical finite difference solution of the multi-layer model is proposed to obtain the voltage distributions along the entire cable. Finally, the voltage method is applied to locate faults. Numerical experiments show that the multi-layer model accurately considers the frequency dependency of the cable parameters, and the solution of the model obtains accurate voltage distribution even during system transient. Extensive test cases prove that the proposed algorithm locates faults with less errors than the existing algorithm. The method is also compatible with a relatively low sampling rate (20 kHz) and a short data window (5 ms). [ABSTRACT FROM AUTHOR]

To improve the reliability of power supply, reclosing schemes are required after transient faults, which commonly occur in overhead line based high voltage DC (HVDC) systems. However, in the event of permanent faults, the auto-reclosing scheme may cause a severe strike. To avoid the severe impacts caused by permanent faults, the fault type should be discriminated before activating the reclosing scheme. Therefore, an adaptive reclosing scheme based on phase characteristics is proposed in this paper. Firstly, the modulation of a periodic voltage by actively controlling the hybrid DC circuit breaker (DCCB) is introduced. Then, a cascaded π equivalent model and its decoupling algorithm are presented to analyze the frequency-domain characteristics of the measured impedance of the coupled overhead lines. From the frequency-domain characteristics, the frequency of the periodic detecting voltage is determined to analyze the phase features of the measured impedance at primary frequency. The permanent or transient faults can thus be accurately identified by using these different phase characteristics, with negligible influence on the healthy lines. In addition, the proposed scheme is robust to various fault resistances, leading to improved reliability. The effectiveness of the proposed scheme is verified in PSCAD/EMTDC. [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]

Contemporary and emergent automotive systems are heavily populated by complex integrated electronics. The number of safety-critical devices used in advanced driver-assistance systems or autonomous vehicles is growing with high-end models containing hundreds of embedded microcontrollers. Achieving functionally safe automotive electronics requires test solutions that might be costly to engineer. Therefore, to address challenges posed by high-quality and long-term reliability requirements, this article presents low-cost test pattern generation schemes for a scan-based hybrid logic BIST of automotive ICs. It may allow one to optimize test coverage and test time during in-system test applications. The first presented technique deploys a seed-flipping PRPG to periodically complement PRPG stages in a methodical tree-traversal manner. The second scheme is based on a seed-sorting approach that allows additional tradeoffs between test data volume and test coverage. As shown in this article, the proposed schemes can be easily integrated with a test compression environment and deployed in different modes of in-system testing, such as key-off, key-on, and periodic (incremental) online tests. Experimental results obtained for automotive designs and reported herein show improvements in test quality over conventional logic BIST schemes. [ABSTRACT FROM AUTHOR]

The difficulty in acquiring fault label data is a major obstacle to the application of data-driven fault isolation in DC microgrids. To remove this barrier, this paper introduces an approach of generating synthetic data with the line currents measured during normal operation as the substitute for fault label data in training an ensemble model, which is aimed to isolate line short-circuit faults in DC microgrids. Based on a high-frequency model of DC microgrids, we prove that the line currents during the closing of unloaded DC lines have similar high-frequency features as those during a line short-circuit fault. On this basis, the synthetic data are obtained through performing discrete wavelet packet transform on the line currents measured during the circuit breaker operation. With normal operating data and synthetic data, an ensemble model is trained as the fault classifier. In the verification tests of different fault scenarios in a three-terminal DC microgrid model, the detection rates of the proposed ensemble model are over 90% while the false positive rates are below 0.5%. These results prove the effectiveness of using synthetic data as the fault labels in the ensemble learning-based fault isolation in DC microgrids. [ABSTRACT FROM AUTHOR]

This paper presents a novel two-terminal fault location algorithm without using line parameters. The algorithm is formulated using unsynchronized voltage and current phasors of the fundamental frequency at both ends. The novelty of the proposed method lies in the fact that it takes into account different angles of synchronism mismatch in pre-fault and fault conditions while considering distributed parameter line model and its solution is in closed form. Simulation results and performance comparison with existing methodology are presented under different fault conditions. The simulations were performed on EMTP-RV platform. The algorithm has also been tested with practical data of Power Grid Corporation of India Limited (PGCIL). The test result shows the validity and accuracy of the developed fault location algorithm. [ABSTRACT FROM AUTHOR]

The paper presents the results of experimental registrations and numerical simulations of the transient overvoltages caused by single phase intermittent arc earth fault in 35 kV network with isolated neutral. Experimental laboratory investigation is performed in order to implement appropriate arc fault model for arcs with low and discontinuous current. Single phase to earth arc fault is created in field in 35 kV network. Numerical model of the system is created in EMTP-ATP software. Overvoltage waveshapes and overvoltage factors are calculated and compared to measured values. Good agreement between the measured and calculated results is achieved. [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]

Accurate faulty phase detection under single-line-to-ground (SLG) fault is the basis of the emerging voltage-based fault arc suppression technology. Most conventional faulty phase recognition criteria assume the distributed line-to-ground parameters are symmetrical, therefore suffering invalidity in actual distribution networks especially under high-resistance ground-fault conditions. This paper firstly analyzes the magnitude and phase angle variation models of the phase-to-ground voltages before and after the SLG fault. Considering the asymmetrical distributed parameters and the line impedance, the specific phase-to-ground voltage variation rules corresponding to SLG fault on each phase are then discussed. Based on the voltage variation rules, a robust and practical method for faulty phase identification is proposed, which does not need the distributed parameters. Finally, the correctness of the variation rules and the availability of the proposed method is verified by simulation. Comparative study shows the proposed method has better accuracy and applicability with the conventional methods when the asymmetry and ground-fault resistance are high. [ABSTRACT FROM AUTHOR]

The high temperature superconducting (HTS) cable has shown promising prospects in megacities for the advantages of high transmission power density and low loss. However, when HTS cable is working on grid, various kinds of faults may cause irreversible damage, resulting in the failure of reclosing. In this paper, the resistive superconducting fault current limiter (R-SFCL) is introduced into the power system to cooperate with the HTS cable, so as to reduce the potential damage caused by fault current. Firstly, the equivalent circuit models are established, and then transient response of HTS cables under fault condition is analyzed, the recovery time of HTS tapes are used to define the design of R-SFCL. Finally, the operating reliability of HTS cable has been evaluated and measures about adaptive reclosing times were proposed under different fault currents. [ABSTRACT FROM AUTHOR]

Currently, HVDC is in the spotlight for long-distance transmission and power connection between countries. Among them, MMC-HVDC is a new conversion technology that can solve the problems of switching loss and harmonics. MMC consists of many submodules connected in series. And the one submodule is composed of IGBTs, Diode, and Capacitor. At the transient state, the charged capacitor is discharged and added to the fault current. It will generate tens of times the surge current during the delays all IGBTs turn-off. This causes semiconductor device destruction. To solve this problem, we applied SFCL with fast response characteristics to the MMC. The response speed of the resistive SFCL is faster than that of the MMC system protection device, which can effectively limit the capacitor discharge current before the protection device operates. In this paper, AC grid, MMC-HVDC, DC cable and SFCL were modeled using PSCAD/EMTDC. After that, we confirmed the capacitor discharge current, current limiting rate, and interruption characteristics through DC line short circuit failure simulation. As a result, SFCL effectively limits the capacitor discharge current and continuously lowers the level of the fault current. [ABSTRACT FROM AUTHOR]

Reliable voltage measurement is a pre-requisite for correct operation of a relay's voltage-dependent functions. The secondary circuit of a relay's voltage transformer (VT) is usually fuse-protected. The blowing of a VT's three-phase fuses prevents correct voltage measurement, commonly referred to as the loss-of-voltage (LOV) condition. Relays are equipped with different methods to detect the LOV conditions and avoid confusing them with grid faults. However, this paper shows that existing LOV detection methods are prone to misoperation in the presence of inverter-based resources (IBRs). The paper's findings are corroborated by hardware-in-the-loop testing of relays with the state-of-the-art LOV detection methods. This paper also develops a new LOV detection method for transmission system relays that addresses the unveiled problem. The proposed method considers the fault behavior of IBRs and detects LOVs by examining certain conditions in three stages, namely phase-voltage-check, phase-current-check, and current-transients-check. Simulation studies are used to verify the dependable and secure operation of this new method for different LOVs, faults, IBRs, and VT configurations. The obtained results demonstrate that this method performs successfully regardless of the type of generation units. Moreover, it is sufficiently fast to prevent relay misoperation during LOV conditions and does not require extra hardware. [ABSTRACT FROM AUTHOR]

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]

In this paper, novel neutral reactor structures are proposed to mitigate the secondary arc current (SAC) in non-conventional Multi-circuit Multi-voltage Transmission lines (MCMVTLs). The parameters of the reactors in the structure are optimized using the Non-dominated Sorting Genetic Algorithm II (NSGA-II). The optimization algorithm, coded in Python, regulates the multiple runs of an Electromagnetic Transients (EMT) simulation program to minimize the secondary arc current. The optimization trials are run in parallel on a 64 core computer to minimize the solution time. A detailed arc model was tested on the EMT program (PSCAD). Using this model and optimized parameters in the proposed neutral reactor schemes, the SAC is indeed extinguished in acceptable times. The SAC and TRV for neutral reactors with optimized parameters are up-to 68 and 77%, respectively, lower than the ones with parameters calculated by the conventional approach which ignores inter-circuit coupling. Meanwhile, the extinction times are up-to 46% faster. As a result, single-phase auto-reclosing in MCMVTLs is viable. [ABSTRACT FROM AUTHOR]

The inevitable voltage sag events cause losses to distributed generation (DG) and sensitive users. Reasonable planning for both of them helps to mitigate the impact of voltage sag. In this paper, a joint optimal planning method is proposed for DG and large sensitive user (LSU) considering the influence of voltage sag. Firstly, the new models of accelerated trip and chain trip for DG under voltage sag are established, to characterize the influence of DG tripping on other DG and sag characteristics during the sag event accurately. Secondly, a new voltage sag frequency assessment method is proposed, which considers models of accelerated trip and chain trip for DG, the tripping characteristics of distance protection (DP), to quantify the voltage sag severity accurately of DGs and LSUs in the gird which is with DP. Lastly, a joint optimal planning model is established considering the losses of DG and LSU under voltage sag, to support the planning of DG and LSU in different scenarios. The method is applied to IEEE RTS-30 for verification, proving that the method can give a direction of the most beneficial solution for the planner, to reduce the losses of voltage sag to both the power system and users. [ABSTRACT FROM AUTHOR]

This paper presents a new Magnetic Equivalent Circuit (MEC) to model the Variable Reluctance Resolver (VR-Resolver) and study the effect of different windings configurations on its performance. Therefore, VR-Resolver with non-overlapping (constant turn) and overlapping (variable turn) windings are modeled to investigate their effect on the accuracy of obtained results. The proposed method gives the possibility to model both healthy and faulty conditions by a unique model to analyze faults effects on the position error. For this purpose, the inter-turn fault is considered on the excitation winding, and both dynamic and static eccentric rotors (DE and SE) are studied. It is shown that the variable turn VR-Resolver has more accuracy and lower sensitivity to eccentricity fault, while the constant turn type is less sensitive to inter-turn fault. However, the latter may be preferred in some applications thanks to its simpler windings and lower manufacturing costs. Finally, the MEC results are proved by Finite Element Method (FEM) and experimental results. Generally, the model flexibility and shorter processing time can be considered as the proposed MEC advantages over FEM. [ABSTRACT FROM AUTHOR]

Winding faults are the most commonly occurring type of fault in offshore doubly-fed generator units. Such faults are difficult to detect and repair, and therefore higher fault identification accuracy is required. Most of the existing research analyzes stator and rotor winding faults separately, but due to the coupling characteristics between windings, it is necessary to analyze doubly-fed induction generator (DFIG) winding faults in a unified way. This paper proposes a general fault diagnosis method that can be applied to both the stator and rotor winding faults of DFIGs. First, a general mathematical model of a windings short circuit of a doubly-fed generator is established. Then, according to the mechanism of a turn-to-turn short circuit fault of a DFIG and the expression of a bilateral flux linkage between stator winding and rotor winding, a new fault feature-bilateral flux linkage difference vector (FLDV) is obtained by mathematical derivation. Finally, the recommended value for an experimental motor of feature quantity is set. The proposed method for inter-turn fault (ITF) diagnosis of stator and rotor windings provides technical support for the sustainable development of offshore wind power. A simulation and experiment prove that FLDV has high sensitivity and high reliability for windings ITF diagnosis. [ABSTRACT FROM AUTHOR]

Autotransformer (AT) traction network (ATN) expands widely with the rapid development of high-speed railway (HSR). The fault occurrence in ATN will affect the safety and reliability of power supply for electric multiple units (EMUs). However, due to the nonlinear relationship between measured impedance and distance, the accuracy and efficiency of the fault location for ATN is facing serious challenges, especially with the diverse fault types and operation modes. In this paper, the general circuit topology of subsections is given and the characteristics of voltage distribution are analyzed according to the impedance matrix of ATN. Through the decoupling method, a comprehensive equivalent model adopting composite impedance is established, which can illustrate a series of characteristics for all fault types. Furthermore, the fault-location method is proposed to express the mapping between the fault position and the relevant electrical quantities for the subsection, and then a reasonable compensation for the effect of mutual impedance between faulty and non-faulty subsections is provided to improve the accuracy. Consequently, the multi-terminals synchronous information is employed for unified fault-location for ATN. Case studies including simulation and field testing have verified that the method is accurate and quite robust to various fault scenarios. [ABSTRACT FROM AUTHOR]

Offshore wind farms (OWF) are being rapidly integrated to the power systems, which can be primarily attributed to saturation of the mainland onshore area, and high wind speed in offshore sites. However, to maintain stable grid operation, OWFs are required to comply with Low Voltage Ride Through (LVRT) capability. OWF during LVRT period should not only stay grid connected, but also support the grid during the disturbance. In this paper, a new communication-independent LVRT control scheme for HVDC connected OWF has been proposed. Under the proposed strategy, OWFs not only successfully ride through a fault induced voltage dip, but also comply with active power recovery ramp rate during the fault recovery period. Besides complying with LVRT requirement and ramp rate limit, the proposed strategy improves voltage dip induced frequency (VDIF) issue by increasing active power injection from the OWF during LVRT period. The proposed LVRT strategy also exploits the stored energy in HVDC link capacitors in order to reduce the impact of VDIF issue. The proposed and the conventional LVRT strategies for HVDC connected OWF have been implemented and evaluated on IEEE 39 bus benchmark system. [ABSTRACT FROM AUTHOR]