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A hybrid sliding mode controller is proposed for rapid earth fault current limiters (REFCLs) used in resonant grounded power distribution networks to self‐extinguish powerline bushfires where the concepts of the proportional integral (PI) action and nonsingular fast terminal sliding surface (NFTSS) are combined together to utilize the benefits of both schemes. A hybrid sliding surface is designed for obtaining the control input so that the fault current compensation is achieved by the residual current compensation (RCC) inverter used within a REFCL to a value that the fire does not ignite. The inclusion of an integral action along with the NFTSS helps to significantly reduce the chattering effect. Furthermore, a continuous function is introduced within the proposed scheme to replace the discontinuous signum function. The switches for the RCC inverter are controlled by deriving control actions from the Lyapunov stability theory applied on the hybrid PI‐NFTSS. The performance of the proposed controller is compared with a nonsingular fast terminal sliding mode controller. For this reason, this paper also includes the design process of nonsingular fast terminal sliding mode controller which is designed using only the NFTSS. Simulations under different scenarios having different fault resistances clearly depict the superiority of the proposed scheme over the nonsingular fast terminal sliding mode controller and an existing integral sliding mode controller in terms of maintaining transient responses. [ABSTRACT FROM AUTHOR]

Summary: This article presents an adaptive voltage sag compensation scheme by using interline bridge‐type fault current limiter (IBFCL) in radial distribution network to protect customers connected to the point of common coupling (PCC). Regarding voltage sag depth, the IBFCL provides a variable resistor in faulted feeder to compensate the PCC voltage at the acceptable level under whole voltage sag condition. To achieve this, a new control system based on the deviation of voltage sag at PCC bus is designed and implemented to the IBFCL. To check the effectiveness of the IBFCL for voltage sag compensation, extensive time domain simulations are performed under both symmetrical and asymmetrical fault conditions on the distribution system in professional's simulation tool for analyzing power systems transients (PSCAD)/electromagnetic transients including DC (EMTDC) environment. Also, its performance and control system are compared to the conventional BFCL and transformer‐type solid‐state fault current limiter (TSSFCL). The simulation results show that by using only one limiting resistor in the IBFCL used in distribution systems including multifeeders connected to the PCC, in addition to reducing the cost of limiting the fault current, the PCC bus voltage has effectively compensated the PCC bus. Also, comparison analysis with the conventional BFCL and TSSFCL shows that it outperforms them in terms of fault detection, voltage recovery time, and fault current limitation. [ABSTRACT FROM AUTHOR]

Summary: The fault current level in distribution systems has increased in the last years mainly due to the energy consumption is in the limit of the production capacity and also the insertion of distributed generation, since the short circuits happen very closer to the generators now. In addition to this, most of the power substations were designed decades ago, so their equipment is overcome, and they may be damaged in a fault occurrence. To effectively protect the settled equipment or to avoid high investment costs in replacing them, one possible solution is to install a solid‐state fault current limiter (SSFCL), which may reduce the fault current to a value under the supported level. In this context, this work proposes the development of two kinds of SSFCLs: series‐switched reactor and resonant circuit. These devices were designed and compared experimentally in bench prototypes. The measurements showed that both topologies could reduce the fault current from 600 APEAK to 75 APEAK. Both SSFCL‐tested configurations were implemented using silicon controlled rectifier or insulated gate bipolar transistor. The results present the comparison between them, showing the impact in the first semi‐cycle current profile, depending on the type of semiconductor switch adopted. This article also proposes a fault detection algorithm evaluated through digital implementation in a microcontroller. The algorithm detects the fault by the instantaneous current value and its derivative, using a control strategy that avoids false detection due to measurement noises. The theoretical analysis and results are useful contributions to support the SSFCL topology choice regarding its application. [ABSTRACT FROM AUTHOR]

The increasing permeation of the distributed generators in the power system brings great challenges for fault diagnosis, especially for the distribution networks with ungrounded neutral or grounded by Peterson coil as the fault current is limited and easily affected by the noises and interferences. A single phase-to-ground fault section identification method is proposed based on feature extraction of the synchronous waveforms and the calculation of the eigenvalues for the time-sequenced features. First, several fault features are defined and extracted from the synchronous current waveforms obtained by the fault recorders. Then, the topology related fault feature matrix is constructed using the time-series features obtained from different measurement sites, and the eigenvalues of the matrix are calculated based on the random matrix theory. Lastly, using the distribution characteristics of the eigenvalues, improved K-means clustering algorithm is utilised in classifying the fault cases and identifying the faulty sections. The effectiveness of the proposed scheme is verified by IEEE 34 nodes test system and a multi-feeder distribution network. [ABSTRACT FROM AUTHOR]

Types I, III and IV wind distributed generators show different and wide range of current sharing capacity during the occurrence of faults in the distribution feeders. In a hybrid microgrid consisting of any of the above mentioned types of wind distributed generators and photovoltaic distributed generators, the fault current in a feeder shows different behaviour which changes as per the type of distributed generators, grid/islanded connection of microgrid operation, distance of the fault from the point of common coupling and nature of the loads in the microgrid system. The novel contribution of this paper is the implementation of a fuzzy logic based adaptive protection scheme through analysis of the q0 components of fault current which are used to detect the low X/R ratio of distributed generators, modes of operation, transient reactance during the series and shunt faults in types I, III and IV wind turbines and PV integrated hybrid microgrid. Considering q0 components and transient reactance, a new relation between the relay current settings and modes of operation has been identified for adaptive relaying. The effectiveness of the proposed adaptive protection scheme for the hybrid microgrid is verified through a simulation case study using Matlab -Simulink software. [ABSTRACT FROM AUTHOR]

The adoption of low-voltage DC microgrid at a large scale is hindered by the lack of an effective protection scheme. This work proposes a dedicated protection scheme based on multi-threshold current values, which enhances the security of the system by avoiding the risk of false tripping. This is achieved in a few milliseconds without losing the deadline of critical fault clearing time post, which the voltage sources converter interfaced power sources may get disrupted leading to de-energisation of the system. The algorithm relies upon the transient response of the series resistor–inductor–capacitor (R–L–C) equivalent circuit having initially charged filter capacitors. The threshold values of the fault current for each protection device at every network node are set locally by considering the weakest fault criterion. The extensive simulation study has been performed on MATLAB/Simulink for validating the reliability of the proposed algorithm under heavy load transient, at different fault locations, and with high fault resistance. [ABSTRACT FROM AUTHOR]

Exploiting series compensated transmission lines (SCTLs) to transfer bulk power produced by power electronics‐interfaced renewable energy power plants (PE‐REPPs) to the load canters has been accepted as an economical solution as well as the practical approach. However, SCTLs impose some difficulties on protective relays, especially conventional directional relaying which mainly employ phase angle difference between voltage and current signals to discriminate between forward and reverse faults and can jeopardize their operation, leading to a false trip and unjustified isolation of REPPs. To solve this problem, an efficient current‐based directional relaying scheme is presented for the protection of SCTLs. In SCTLs due to the presence of series capacitance in the fault loop, voltage inversion (VI) and current inversion (CI) phenomena occur. The proposed directional relaying scheme can tackle CI and VI problems. Since the proposed method merely uses the phase angle of fault current, it can retain its proper operation during close‐in faults where conventional directional relaying lose their proposer performance due to CCVT transients as well. Therefore, the resilience offered by SCTLs during the integration of REPPs will be attained. The performance of the proposed relaying scheme is evaluated under various operating conditions, and obtained results prove its efficiency. [ABSTRACT FROM AUTHOR]

Summary: This paper proposes a new concept for limiting fault current in distribution systems with interline bridge‐type fault current limiter (IBFCL). It includes two or more BFCLs connected to only a common limiting resistor in DC link, which result in considerable cost saving. This configuration is inserted in series with feeders connected to a point common coupling (PCC). At first, the principle and characteristics of IBFCL are described. Then, a theoretical analysis of limiting performance and parameter design of the IBFCL is explained. Finally, the performance of the IBFCL for limiting fault current is validated on the distribution system comprising of parallel feeders connected to PCC through electromagnetic simulations by PSCAD/EMTDC under symmetrical and asymmetrical faults as well as different load current conditions. Simulation results show that the IBFCL provides a cost‐effective approach for limiting fault current in distribution system. [ABSTRACT FROM AUTHOR]

The penetration of distributed generators (DGs) of large capacities in micro‐grid introduces a considerable difference of current seen by the relay, during faults in its primary and backup region. This leads to several issues with the protection system such as relay miscoordination, delay in relay operation or failure in the relay operation (blinding). This study discusses a methodology to solve such issues by identifying the current status of micro‐grid configuration. The proposed method performs well by maintaining the coordination time interval (CTI) between the relays, irrespective of the source strength (weak or strong), the mode of operation of the micro‐grid (islanded or grid connected) or the micro‐grid configuration (radial or looped). The method proposes three curves with different settings in a single relay. A suitable algorithm has been developed to calculate the relay settings for all the three curves of each relay, well in advance and this program is made to run in the central master controller (MC). Thereafter, MC sends the settings of all relays through a reliable communication channel. The proposed relaying scheme provides desirable CTI margin as compared to conventional over current relay. The simulation results of the novel protection scheme are validated through hardware emulation. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) has great prospects in voltage source converter‐based high‐voltage direct current (HVDC) transmission. Although the typical half‐bridge MMC can bring huge economic benefits, it is unable to clear dc fault. An MMC based on arm transfer (AT‐MMC) and corresponding timing control are presented to effectively solve this drawback. Compared with the half‐bridge MMC, the AT‐MMC adds the arm‐transferring branch to the each phase unit of the converter and adds the current‐breaking branch and the energy‐absorbing branch to the dc line. Through these branches cooperating with each other, the fault current is cleared. Referring to the half‐bridge MMC with the same voltage grade, the AT‐MMC has a good economy and its additional device cost is low and additional conduction loss is very small. A model of 51‐level MMC–HVDC system is built in RT‐LAB OP5600 to verify the effectiveness of the proposed solution. [ABSTRACT FROM AUTHOR]

This study presents a study on the time when the short-circuit fault current in an MV distribution network will cross the zero point in the presence of DC offset affected by the local synchronous and/or induction machines. The motivation of the study is to examine using thyristors to bypass a series compensator in the network while the subsequent turning-off of the thyristors depends on the zero-crossing of the current. One of the main uncertainties is the DC offset in the fault current due to the machines connected locally, which may defer the thyristor turning-off. This possibility has been examined in this study and the results demonstrate the parameters of the machines that have most influence on the time when the fault current will pass the zero point. It is shown that machines with higher unit ratings usually cause more delays in comparison with smaller units. [ABSTRACT FROM AUTHOR]

As vectors' similarity is defined on the basis of the hard boundary of Heaviside function in multiscale multivariate sample entropy, the two families of statistics show high sensitivity to the similarity tolerance. Multiscale multivariate fuzzy entropy (MMFE), which applied fuzzy membership function, overcomes the problem. Hence, the visualisation inrush current discrimination algorithm based on MMFE is proposed. The proposed algorithm divides the MMFE‐τ plane into two parts: active and braking zones. The ratio of the current entropy area to the given braking zone area is designated as the entropy area ratio. The magnetising inrush and fault current of the transformer are identified on the basis of the numerical value comparison between the entropy area ratio and the fixed value of 1. Through the ATP/EMTP platform and dynamic testing system, the transformer currents under different operating conditions are analysed. Results demonstrate that the proposed algorithm can effectively distinguish the magnetising inrush from the fault current of the transformer and is superior to other methods. [ABSTRACT FROM AUTHOR]

As a key element of the DC grid, the technology of DC circuit breakers (DCCBs) still have difficulty to be widely used due to its large cost and volume. This paper studies the basic principle and its realisation methods for DCCBs. First, two basic ways of breaking the fault current are discussed, including the way by inserting a metal oxide varistor (MOV) to force the fault current to zero and the way by inserting capacitor to change the nature of the fault current. Second, a new approach to realise the DCCB has been proposed based on the inserting capacitor way. According to this new approach, three topologies of DCCB are proposed, including the single-branch DCCB with inserted capacitor (SB-DCCB-C), the double-branch DCCB with inserted capacitor (DB-DCCB-C) and the triple-branch DCCB with inserted capacitor (TB-DCCB-C). All these three DCCBs are proved to be able to deal with DC faults effectively according to the simulation results in PSCAD/EMTDC. Finally, the economy of four circuit breaker structures are compared. [ABSTRACT FROM AUTHOR]

Traditional directional relays of AC transmission lines are usually based on 50 Hz information in frequency domain in China. However, this conventional approach cannot provide the correct fault direction for the lines connected to wind farm because of the fault characteristics of the wind power system, that is, frequency deviation, variation of system impedance, weak feed feature, and so on. Then, the conventional approach of finding the phase angle between the fault current and a voltage signal will not identify the correct fault direction. To solve this problem, this paper proposes a novel fault direction identification method in time domain. Based on the RL model of transmission lines, opposite-side voltages of the line can be derived with the measuring current and voltage at wind farm terminal after the fault. Then, the adjusted cosine similarity between the calculation voltage based on the RL model and the memory voltage can be acquired. If the correlation between the calculation voltage and the memory voltage is close to 1, it is a reverse fault. Otherwise, it is a forward fault. Finally, the proposed novel fault direction identification criterion is verified by the PSCAD/EMTDC-based simulation results. [ABSTRACT FROM AUTHOR]

A wavelet-based electromagnetic time reversal method for locating faults in voltage source converter-high-voltage direct current (VSC-HVDC) transmission lines is proposed in this study. This principle, firstly, is to use wavelet decomposition to filter the 1-mode fault currents, which are recorded at ends of the transmission line. Secondly, the filtered currents are mirrored on time axes, which called time reversal. Thirdly, the time-reversed currents are set as current sources at ends of the lossless mirror lines. Through assuming a fault at each point of line, the assumed fault currents are calculated, and the RMS of fault current at actual fault location is maximal according to a strict scientific proof. Theoretically, the Wavelet-based Electromagnetic Time Reversal (WEMTR) method is robust against fault types and resistance. A multilevel converter-HVDC system is established in Power Systems Computer-Aided Design (PSCAD)/Electromagnetic Transients including DC (EMTDC) to verify the feasibility of the proposed method, and the transmission line is simulated by frequency independent phase model. The results show that, the fault location can be calculated exactly without high sampling rates. [ABSTRACT FROM AUTHOR]

The dc transmission line protection plays a crucial role in the secure operation of a multi-terminal modular-multilevel-converter-based high-voltage direct current (MMC–MTDC) system, which is increasingly adopted as a promising option in the power transmission fields. However, the MTDC is the most vulnerable to a dc fault, which paralyses all of the MMCs until the dc fault is cleared out. It is a huge challenge that how to locate and isolate dc fault current and ensure the continuous operation of the healthy part of the MMC–MTDC system under dc faults. This study proposes a least-squares-based ɛ-support vector regression scheme, which captures fault features via the Hilbert–Huang transform. Fault features are used as the inputs of ɛ-support vector regression to obtain fault distance. Then, least-squares method is utilised to optimise the parameters of the model so that it can meet the demand on fault location for MMC–MTDC transmission lines. In the end, the simulation model of the multi-terminal MMC–HVDC system using PSCAD/EMTDC is built to demonstrate that the proposed method has well reliability and accuracy for fault location. [ABSTRACT FROM AUTHOR]

The DC microgrid has become a typical distribution network due to its excellent performances. However, a well-designed protection scheme still remains to be a challenge for DC microgrids. This paper proposes a fault distance estimation-based protection scheme for DC loop-type microgrids relying on two-terminal electrical quantities. Different from the traditional methods, a small inductor is implemented at each head of the line instead of obtaining the derivative of fault current using current difference value. Thus, the derivative of currents can be calculated accurately with the measured voltages on the small inductors during faults. The presented protection method is able to realise fault location and fault isolation quickly and accurately. Furthermore, it is immune to fault resistances. The presented protection scheme has been tested and verified through PSCAD/EMTDC simulations. The results prove that the new protection scheme possesses the excellent performance. [ABSTRACT FROM AUTHOR]

The 110 V lead-acid battery pack is widely used as the DC power supply for the load control of power plants. If the low-voltage DC circuit breaker (LVDCCB) at the DC side of its charging device cannot operate in time when the fault current is flowing in charging loop, there will be of harmfulness to the safety and the durability of the battery pack. The plasma theory is applied to analyse the arc mechanism based on the voltage, the current and the power waveform of LVDCCB when it breaking. Then, an arc extinction is developed by assembling an additional device to LVDCCB. The device, connected to the contacts of LVDCCB, includes an insulated-gate bipolar transistor series branch, an energy-absorbing branch, a transfer switches branch, and a drive circuit. The test shows that the break time of LVDCCB, assembled with the additional device, is significantly shortened than that of the original. [ABSTRACT FROM AUTHOR]

High-voltage direct current (HVDC) grids have been assumed as a promising technology for its advantages in accommodation and integration of large-scale renewable energy. However, the fault current increases dramatically once the fault occurs in HVDC grids, which makes great difficulty in fault clearance. Therefore, it is necessary to take some measures to limit the fault current. A bidirectional solid current-limiter (BSCL) topology is proposed. The BSCL can improve the performance of inductors to limit DC fault currents through the high-frequency switching between two current-limiting branches. Besides, the BSCL can be installed at both ends of a DC transmission line or between a converter and a DC bus. The feasibility of the proposed topology is demonstrated in power systems computer-aided design/electro-magnetic transient design and control. Simulation results indicate that the BSCL can effectively limit the rising speed and the amplitude of short-circuit currents, decrease the maximum breaking currents of DC breakers and improve the breaking speed of DC breakers. [ABSTRACT FROM AUTHOR]

With the advantage of saving resources and high utilisation, DC transmission becomes the vital part of the sustainable development. However, due to the control strategies of grid-side inverter, the fault characteristics of the grid will change dramatically, which influences the reliability of grid protection. By establishing the model in PSCAD, the fault characteristics of the inverter was obtained and the effects on current protection was analysed here. The results reveal that the magnitude of fault current was directly influenced because of being connected with the inverter. The grid inverter can make a difference in selectivity and reliability of current protection. In order to solve this problem, the paper constructs the equivalent model of the inverter and attains the unique feature that the fault component impedance measured in the location of protection decreases with the increasing distance of fault location. According to the characteristics, a targeted fault component impedance protection is designed. The feasibility and effectiveness of this protection is verified by simulation in PSCAD model. [ABSTRACT FROM AUTHOR]

The presence of distributed generation (DG) in a power distribution network can cause difficulties in managing the voltage, power flow and fault level as the network was designed for supplying passive loads. This study focuses on the control of a back-to-back (B2B) voltage source converter operated as a synchronverter and a soft open point (SOP) between adjacent feeders to mitigate the voltage fluctuation caused by DG and to increase the reliability of the supply without increasing the fault level. Better reliability is achieved as the B2B converter can continue to supply the loads when a loss of mains occurs on one side. The proposed control of the two DC–AC converters, which also needs to regulate the DC link voltage and adapts the synchronverter algorithm to the new application requirements, so that events will cause minimum disruption. Simulation shows that the B2B-synchronverter is capable of regulating the load voltage under normal mains connected condition, and also limiting the fault current and supplying the loads within its rating during islanding, providing seamless transition to the islanded mode. [ABSTRACT FROM AUTHOR]

The dynamic behaviour of microgrid system under faulty conditions makes adaptive protection a general necessity for reliable microgrid operation. In design of adaptive protection, the grid‐connected and islanded modes have immense importance including grid‐connected mode without distributed generators (DGs) in microgrid. In this study, a new adaptive protection scheme is proposed based on the above‐mentioned modes of microgrid operation. The proposed method considers nature of DGs connected, fault location detection and fault nature identification based on quadrature and zero sequence components of fault current considering impact of X/R ratio of DGs. The proposed methodologies for adaptive protection schemes are verified in Matlab‐Simulink environment and the results are found to be satisfactory while various faults are simulated at different nodes of the microgrid model. At the time of verification of effectiveness of the proposed methodologies, the time derivative of quadrature and zero‐axis components of fault current are considered sufficient to instantaneously detect the fault location and fault nature in microgrid system. [ABSTRACT FROM AUTHOR]

Let‐through energy (LTE) refers to the I2t or Joule energy that a conductor is exposed to during a fault on the feeder. This energy is influenced by the magnitude of the fault current and time it takes for the protection system to clear the fault. If the LTE exceeds the conductor thermal energy limit, the conductor will get damaged. This concept of LTE evaluation is applied to the inverse definite minimum time (IDMT) current based back‐up protection elements on a multisource high‐voltage feeder in a hypothetical and actual network. Another method to calculate the relay operating time for IDMT relays was developed based on an average disk speed of electromechanical over‐current relay and the proportionality of its speed to the magnitude of the fault current. This method was incorporated into a software application to generate results. These results allow the user to evaluate the conductor LTE exposure, total fault time exposure, the effect of instantaneous fault clearing and the application of auto‐reclose cycles. An energy‐area evaluation was applied to quantify and evaluate small protection settings changes. The conclusion is that LTE analysis on back‐up protection should be considered for high‐voltage feeders to ensure that the conductors are protected. [ABSTRACT FROM AUTHOR]

In this study, discrete wavelet transform (DWT) and deep neural network (DNN) are utilised for fault location in a series‐compensated three‐terminal transmission line. The series compensation causes challenges in fault location schemes of the three‐terminal transmission lines. The presented fault location method has been extensively tested using the SIMULINK model of a three‐terminal transmission line. Features extracted from synchronous measurements of fault currents at the three terminals using DWT are fed to the DNN. Faulted section determination and fault distance calculation are carried out using a single intelligent network simultaneously. Faulted section is determined with 100% accuracy, and the efficiency of algorithm is validated for symmetrical and unsymmetrical faults, and different values of fault resistance, inception angle, and location. The accuracy of the algorithm is acceptable for large fault resistances (above 100 Ω) and fault inception angles near zero. Total mean error for test data is 0.0458% which is much improved with respect to other similar works. [ABSTRACT FROM AUTHOR]

Quick fault detection and isolation of faulty section are desired in DC microgrid due to the presence of power electronic converters and low cable impedances. Owing to need of fast disconnection, limited time and data are available for online fault distance estimation. Some of the existing techniques consider source capacitors connected at only one end of the cable; therefore, assume that the fault current is contributed by only one end of the cable. This may not be true in the case of multi‐source DC microgrids, where fault current would be supplied from both the ends. Further, existing communication‐based techniques require either data synchronisation or fast communication network. To address these issues, this study proposes an online fault location method for multi‐source DC microgrid without using communication. The mathematical model of faulted cable section connected to sources at both the ends is derived. This model is used along with the measurements to determine the fault distance. The model consistency with the measurements is quantified using the confidence level based on the residual analysis. A ring‐type multi‐source DC microgrid system is considered and simulated on real‐time digital simulator to demonstrate the effectiveness of the proposed algorithm. [ABSTRACT FROM AUTHOR]

The need for enhancing grid resilience has led to wider acceptance of photo voltaic (PV) integrated microgrid. The variation in fault current during grid connected and islanding operation makes the microgrid protection task challenging. A protection scheme based on ensemble of classifiers has been devised by exploiting the effectiveness of a classifier set coupled with voting strategy. Unlike the existing classifier‐based approaches involving single classifier, the ensemble‐based approach is insensitive to the biasness of individual classifier and dimension/size of dataset. The proposed scheme is formulated to simultaneously perform the tasks of mode detection, fault detection/classification, section identification and location. The scheme is able to discriminate between faults and power quality disturbances, which avoids unintended false tripping. Along with accuracy, the reliability assessment of proposed scheme has been carried out using two indices, i.e. dependability and security for different faults, operating mode and contingencies. For evaluating the reliability of the fault locator, a stochastic approach (Monte‐Carlo simulation) has been adopted. The simulation results justify the applicability of the proposed ensemble classifier‐based scheme for complete protection and reliable operation of microgrids with PV penetration. The scheme also has been validated for real‐time settings using hardware in loop simulations. [ABSTRACT FROM AUTHOR]

High‐impedance faults (HIFs) in electrical power distribution systems produce a very random, non‐linear and low‐magnitude fault current. The conventional overcurrent (OC) relaying‐based distribution system protection schemes find difficulty in detecting such low‐current HIFs. In this study, a simple two criteria‐based protection scheme is proposed for detection and isolation of HIFs in multi‐feeder radial distribution systems. It utilises one‐cycle sum of superimposed components of residual voltage for HIF detection and the maximum value of one‐cycle sum of superimposed components of negative‐sequence current for faulted feeder identification. The performance of the proposed scheme is evaluated for a wide variety of possible test cases by generating data through power systems computer‐aided design/electro‐magnetic transient design and control software. Results clearly show that the proposed scheme can assist conventional OC relay for detection and isolation of HIFs in distribution systems with any grounding connections in a more reliable and faster way. [ABSTRACT FROM AUTHOR]

Modular multilevel converter HVDC (MMC‐HVDC) systems have been commissioned in China, America and Europe. With the demand of long‐distance and large‐capacity power transmission, the MMC‐HVDC systems using overhead lines get more attention. When a single pole‐to‐ground permanent fault occurs, the fault current is very small, therefore if the fault location is determined, the fault can be cleared and the HVDC system can revert to normal operation without interruption. MMC‐HVDC can quickly control the work state of sub‐modules (SMs), therefore, it is possible to generate travelling wave pulses propagating to the line by controlling SM, without the need of additional equipment. So in this study, a fault location method for MMC‐HVDC using active pulse is proposed. Meanwhile dichotomising recursion singular value decomposition is used to detect the arrival time of active pulse. The proposed method does not require additional equipment and the method is not influenced by transition resistance. And fault location can be repeated since the pulse can be sent repeatedly. Computer simulations are performed and the results show that the proposed method can locate the grounding point of MMC‐HVDC reliably and with increased accuracy relative to existing methods. [ABSTRACT FROM AUTHOR]

Conducted experiment proves that third harmonic (TH) component of salient-pole synchronous generator behaves differently compared with fundamental frequency component in terms of generated voltage. Fundamental frequency-generated voltage depends on the excitation system, and the generated voltages before and during fault are almost the same. However, TH-generated voltage is independent of the excitation system. In other words, variation of the excitation current does not affect the generation of TH voltage as much as it affects the generation of fundamental frequency voltage. On top of that, the generation of TH voltage before and during fault varies significantly. Hence, a typical method to solve fault analysis for 3-phase-to-ground fault is not applicable in TH domain. Therefore, in this article, a method to find TH-generated voltage is proposed. The TH-generated voltage during 3-phase-to-ground fault is presented in terms of percentage called the harmonic fundamental ratio, which is the ratio of TH zero-sequence-generated voltage to fundamental frequency-positive sequence-generated voltage during fault. The harmonic fundamental ratio is obtained by short circuiting the generator terminal prior to the application in the system. It turns out that the generated TH voltage by synchronous generator depends on the zero-sequence TH impedance of the TH fault current path. Lastly, the proposed method to obtain TH-generated voltage by salient-pole synchronous generator is validated by comparing the calculated TH fault current with the actual TH fault current obtained from experiment. [ABSTRACT FROM AUTHOR]

Limiting the fault current passing through substation components can affect their failure rate reduction. However, evaluating the exact value of the failure rate reduction is complicated. This complexity is because the fault current reduction for each component varies as fault current limiter (FCL) is installed on different substation locations. In this paper, an approach is proposed to calculate the components failure rate after installation of FCL in the substation. In this approach, an equation is derived using 3 factors consisting of the effect of fault current on failure rate, the value of fault current reduction due to FCL location, and the probability of FCL stuck. The reliability indices are calculated for 3 types of substation configurations, considering different values of active and passive FCL failure rates. In addition, the results obtained for various locations of FCL in the substation are compared to that of the case without FCL. [ABSTRACT FROM AUTHOR]

In power grids, directional relays are used in situations that direction of fault current is not fixed. Distribution networks including distributed generation (DG) resources and transmission networks are some of these situations. Directional relays as key elements of distance and over current relays provide selectivity for these protections to improve their performance and power systems stability. In this study, a directional algorithm is proposed, which is able to discriminate fault direction just by sign of imaginary part of post‐fault current phasor. To evaluate the algorithm, plenty of simulations and experiments are carried out in different fault conditions. Moreover, impact of power flow direction at normal operation of the power system is considered in the evaluation of the algorithm. Also, its performance is compared with some other directional algorithms. The results confirm that the proposed algorithm is able to detect fault direction with high degree of accuracy. Moreover, simplicity of the algorithm represents its competence for practical implementation in transmission and distribution networks. [ABSTRACT FROM AUTHOR]

This study proposes a zone partitioning protection strategy for DC systems incorporating offshore wind farm. Regarding a DC system which consists of multiple local zones with radial topology, a multi‐port DC hub is introduced to serve as firewall between different zones. By blocking the converter with DC fault blocking capability in the corresponding DC hub port, the fault can be prevented from spreading among zones. On the other hand, an inter‐zone protection centre is designed and built in the DC hub to implement the continuous operation under the situation of any port outage. Furthermore, in every local zone, the hybrid DC circuit breaker is installed in each branch at the side near the star point in order to isolate the faulty branch quickly and selectively via installing the smoothing reactor at the other side to restrict the fault current. Especially, a zone protection centre is designed and built in the star point of each zone to implement the fault detection and fault isolation. A detailed model of the DC system with the proposed protection strategy is built under PSCAD/EMTDCTM environment. Simulation results under different operating conditions demonstrate the feasibility and validity of the proposed strategy. [ABSTRACT FROM AUTHOR]

Dynamic resistance measurement (DRM) was proved to be an important method for condition assessment of contact sets of SF6 circuit breakers (CBs). The erosion of contact sets due to arc energy is a main factor that influences the electrical lifetime of the extinguishing chamber. In this study, a high‐voltage CB synthetic circuit was used to produce fault current for electrical erosion tests on the contact sets. The contact sets were close to the end of their service life after a series of erosion tests. DRMs with DC amperages of 0.5, 1.0, and 1.5 kA were conducted on the two tested breakers in the interval of each erosion test. Five essential diagnostic parameters were extracted from the contact resistance curves as a function of contact travel. The variation of these parameters during the whole erosion tests were investigated to find an effective condition assessment method for contact sets. From the power balance at the contact surface, the material erosion rate due to metal evaporation for each erosion test was calculated and in good agreement to the experimental results. Finally, the contact length and contact resistance were employed as diagnostic parameters for condition assessment of contact sets. [ABSTRACT FROM AUTHOR]

The half‐bridge modular multilevel converter (HB‐MMC) is one of the most promising topologies for high‐voltage direct current (HVDC) systems. Unlike full‐bridge MMC (FB‐MMC) and clamp double sub‐module MMC (CDSM‐MMC), HB‐MMCs are defenceless against DC side faults. Different types of DC circuit breakers (DC CBs) assisted with arrester banks/damping resistors such as solid‐state CB and hybrid DC CBs can be used to interrupt the DC fault current. Arrester banks are used to protect against overvoltage after interrupting the fault current and in turn to demagnetise the circuit inductors. The main disadvantage of the arresters is that they forcibly break apart when they are overloaded. In this study, an arrester‐less fault current limiter is proposed for HB‐MMC configuration, which provides an opposing voltage during the DC side fault. This is achieved by inserting external pre‐charged capacitors in the DC current path during the fault. The selection of these external capacitors, their suitable initial voltages, and their charging stations are also presented in this study. A comparison between the proposed scheme and the CDSM‐MMC has been held as well. The proposed protection scheme is assessed using a simulation study for a point‐to‐point nine‐level HB‐MMC‐based HVDC system. [ABSTRACT FROM AUTHOR]

Power system security concerns due to significant wind generation could be twofold: first, the frequency response and second, the short‐circuit performance. Continuous development of wind energy may cause economic replacement and retirement of existing synchronous generators. Modern wind turbine generators do not inherently offer inertia and governor response after a disturbance, which may result in declining frequency response behaviour. Besides, they have limited fault current contribution, which may cause unacceptable short‐circuit ratio at the grid connection point of those wind power plants. Traditionally, frequency response and short‐circuit performance are individually improved. However, both of them are concurrently affected by high wind penetration. Hence, a common approach to simultaneously enhance both indices is essential. Therefore, this study introduces an idea of operating some of the retired synchronous generators in the synchronous condenser mode, which is termed as 'post‐retirement scheme (PRS)'. Such a second use of the retired synchronous generators can jointly upgrade frequency response and short‐circuit strength during high wind generation. This study also proposes a methodology to evaluate when and how much PRS should be deployed for ensuring an adequate security performance in a wind dominated power system. [ABSTRACT FROM AUTHOR]

In a multi‐terminal high‐voltage direct current (HVDC) system, DC circuit breakers (DCCBs) are conventionally connected in a star configuration to enable isolation of a DC fault from the healthy system parts. However, a star connection of DCCBs has disadvantages in terms of loss, capacity, reliability and so on. By rearranging the star connection, a novel delta configuration of DCCBs is proposed. The total loss of the proposed delta configuration is only 33.3% of that of star configuration, yielding a high efficiency. Moreover, any DC fault current is shared between two DCCBs instead of one DCCB in the faulty branch suffering the fault current. Thus, DCCB capacities in the proposed delta configuration are only half of those in a star arrangement. Additionally, in the case of one or two DCCBs out of order, the power can still be transferred among three or two terminals, thereby affording high supply security of all HVDC links. On the basis of the DCCB delta configuration, two novel DC fault protection structures with external and internal DC inductances are proposed. Their characteristics are discussed and it is shown a DC fault can be isolated using slow DCCBs. The results demonstrate DC fault tolerant operation is achieved by using the proposed DC fault protection structures with delta configuration. [ABSTRACT FROM AUTHOR]

The main reasons for installation of power quality meters in distribution transformer substations are power quality monitoring and global evolution of electrical network towards the 'smart grid'. In case that all measurements from the meters are properly synchronised and centralised, new possibilities of control or evaluation of the network are enabled. This contribution proposes the possibility for an earth fault localisation with the aid of synchronised data recorded on the low‐voltage side of the medium voltage/low voltage transformers in compensated neutral distribution networks which are equipped with auxiliary resistor for short‐time increasing of the active part of the fault current. The described method uses voltage sags evoked by connecting of the auxiliary resistor for locating the faulty section. The proposed method is tested with the help of numerical model which presents a part of the distribution network. [ABSTRACT FROM AUTHOR]

Back‐to‐back starting is a widely accepted starting method for pumped storage units. A back‐to‐back starting model of pumped storage units was built using the ElectroMagnetic Transients Program including DC (EMTDC) software package. The short‐circuit fault during back‐to‐back starting was investigated for the first time. The starting process was simulated and compared with real data from a pumped storage power plant in northeast China. The rotor speed and the effect of resistance during fault period were also discussed. The results showed that the fault current has significant magnitude and low‐frequency characteristic. Recommendations were provided to realise secure and reliable fault clearance during back‐to‐back starting based on the fault characteristics and real de‐excitation test results of the pumped storage power plant. [ABSTRACT FROM AUTHOR]

A three-in-one HTS cable has been developed. Its cable core is composed of a conductor and a shield wound with BSCCO and electrical insulation of the PPLP. The three cable cores are covered in thermal insulated stainless corrugated pipes. The BSCCO wires represent a dramatic innovation by virtue of a newly developed process named Controlled Over-Pressure (CT-OP). As a new cable technology, a 350-m thermal insulation pipe was tested for 7 months to find that the life of the vacuum intensity is more than 10 years. The HTS cable with tension members was designed for installation into an underground duct. In addition, a fault current of 23 kA, 0.63 s, was applied to a sample cable with no damage to the BSCCO wires or the cable insulation. These technologies will be applied to the Albany project in the USA. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 162(1): 15–24, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20577 [ABSTRACT FROM AUTHOR]

A novel phasor estimation algorithm is proposed, by means of an innovative modification of the traditional full cycle discrete Fourier transformer (FCDFT) algorithm. The main idea is to combine the FCDFT outputs for even and odd sample‐setsobtained for one cycle data window decimated by two and by four. Its performance was compared with the ones of other full cycle algorithms reported in the late literature on the subject. The obtained results reveal the proposed algorithm leads to the best performance, even considering both the decaying DC component of the fault current and the one caused by the current transformer secondary circuit. [ABSTRACT FROM AUTHOR]