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The simulation of electric field transients in HVDC cables can be a fundamental tool to support insulation system design when operation dynamics, which includes voltage polarity inversions and load variation, becomes significant during the expected cable life. The field behavior during voltage and temperature transients is associated with intrinsic and extrinsic ageing processes that can affect the cable design life. Traditional models employed for electric field simulations consider only the non–linear, steady–state dependence of conductivity on temperature and field magnitude, but they neglect the transient behavior of permittivity and conductivity, that is, polarization and charge trapping phenomena. These processes impact on the duration of the field transient and thus ageing. This paper discusses the numerical implementation of an electro–quasistatic model that can be used to perform transient simulations of electric fields in HVDC cables insulation. With respect to the model traditionally employed, the presented one is extended with Debye's equations for dielectric polarization processes. The model is implemented numerically using finite differences, or the Occhini equivalent circuit: this paper demonstrates that the two approaches are equivalent. Numerical simulations show that, during transients, polarization processes slow down the electric field dynamics and reduce its maximum intensity. [ABSTRACT FROM AUTHOR]

The Rogowski coil transducer's integrator will amplify the transmission error, which may lead to mal-operation of the relay. In this paper, a novel method of line relay based on the differential output of Rogowski coil is proposed. Besides, an improved fast distance relay based on the differential output of Rogowski coil is presented. Based on the equal transfer process of transmission lines(ETPTL), the voltage signal should be processed by the virtual Rogowski coil to solve the inconsistent transfer problem between voltage and current. Furthermore, this paper also presents the reconstruction method of voltage at the fault point and the least-square algorithm to solve the R-L differential equation, so as to improve the algorithm accuracy. Furthermore, the equal transfer process error of the virtual Rogowski coil is analyzed in the paper, and the design principle of low pass filter is also proposed. Simulation results and experiment results demonstrate that the improved fast distance relay scheme has high operation speed, which is signed prior to other distance relay schemes. The proposed method also provides a valuable reference for other types of transmission line relay based on Rogowski coil transducer. [ABSTRACT FROM AUTHOR]

The decarbonization plans are leading to the decommissioning of conventional power plants, resulting in a deep transformation of power systems. Balearic Islands are not an exception and, in order to compensate this generation reduction and increase the security of supply, a second HVDC link is planned to interconnect the Balearic power system to the Spanish mainland. In this application paper, Modular Multilevel Converters (MMCs) have been assumed as the preferred technology for this HVDC link. A transient analysis has been presented to evaluate the system performance after the introduction of this new HVDC link. Grid-following and grid-forming control strategies have been implemented, comparing their impact on the system performance and stability. The transient events considered are the disconnections of the main synchronous generators of the islands and AC faults in several locations of the Balearic power system. A final case based on a penetration of 100% of converter-based resources has been also simulated to study the islanded operation of the MMC-HVDC link. [ABSTRACT FROM AUTHOR]

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

Accurate knowledge of the natural frequencies and shapes of corresponding standing waves are essential for gaining deeper insight into the nature of response of coils to impulse excitations. Most of the previous analytical studies on coils assumed shape of standing waves as sinusoidal but numerical circuit analysis and measurements suggest otherwise. Hence, this paper revisits the classical standing wave phenomenon in coils to ascertain reasons for this discrepancy and thereafter extends it by analytically deriving the exact mode shape of standing waves for both neutral open/short conditions. For this, the coil is modeled as a distributed network of elemental inductances and capacitances while spatial variation of mutual inductance between turns is described by an exponential function. Initially, an elegant derivation of the governing partial differential equation for surge distribution is presented which is then analytically solved, perhaps for the first time, by the variable-separable method to find the complete solution (sum of time and spatial terms). Hyperbolic terms in spatial part of solution have always been neglected but are included here, thus, yielding the exact mode shapes. Voltage standing waves gotten from analytical solution are plotted and compared with simulation results on a 100-section ladder network. The same is measured on a large-sized single layer coil. So, it emerges that, even in single layer coils, shape of standing waves deviates considerably from being sinusoidal and this deviation depends on spatial variation of mutual inductance, capacitive coupling, and order of standing waves. [ABSTRACT FROM AUTHOR]

This paper addresses procedures for power cable time-dependent rating calculations. Such calculations are required to obtain cable emergency ratings and also to observe the conductor temperature variations with time when subjected to variable loading. The calculation procedure is described in the IEC Standards 60853-1 and 2. A real difficulty in applying those procedures lies in an ability to take into account variation of the electrical resistances of the cable metallic components with temperature. The standard gives an approximate equation, developed by Goldenberg. However, this equation is applicable to a single current step applied to the cable. The paper shows that it cannot be easily applied to multiple current steps and proposes a new approximate algorithm providing an analytical solution to this problem. A comparison with a complex iterative procedure is also offered. [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]

This paper summarizes the work performed by the Task Force —within Performance Characteristic Subcommittee of IEEE PES Transformers Committee— to investigate potential revisions to IEEE Std C57.142-2010. Although the scope of this document does not implicitly exclude HV and EHV systems, the existing document primarily includes examples and information on distribution voltage systems. A Task Force was formed to investigate the need to include information about power system transient events on HV systems more explicitly in this existing document. The Task Force was charged with the following tasks:-Establish system voltages encompassed by C57-142-2010 Guide. -Gather field data, reports, and literature on HV and EHV failures related to substation transients and transformer interaction. -Solicit input from the other technical committees on this subject. -Review IEC and CIGRE work in this area. -Recommend course of action on revision of the present Guide. -Recommend high level changes to the Guide. -Prepare final paper for the Subcommittee. In the course of this work several failure scenarios were identified. These are categorized and the failures are described in this paper. The final recommendations to the Subcommittee are summarized and recommendations are presented for the course of action on the revision/addition of HV and EHV material to the current C57.142 Guide. [ABSTRACT FROM AUTHOR]

Commutation failures (CFs) including repetitive CFs (RCFs) are frequently occurring faults in line commutated converter based high voltage direct current (LCC-HVDC) systems. Consequently, sending end system will suffer transient voltage disturbances (TVD) with a characteristic of “first reduce then rise and may repeat”. Due to the complex interaction process of TVD, the electromagnetic process behind this phenomenon cannot be easily explained, which imposes great challenges on the development of mitigation strategies. Given this, this paper provides a deep insight into the mechanism and the impact factors of the TVD caused by CFs and RCFs. The numerical relationship between the transmitting power and the TVD is derived. Research results revealed that the transient process after fault can be segmented into five stages. In each stage, the control interactions among HVDC controllers and power interactions at the PCC are analyzed. Besides, we discussed the TVD under RCFs in detail. Several indices are further proposed to facilitate the characterization of the TVD, and main control loops of HVDC that have impacts on the characteristics of TVD are thereby studied. Finally, the mathematical relationship between wind farm power and the TVD is derived to quantify the impact of wind farms on the TVD. [ABSTRACT FROM AUTHOR]

The installation of phasor measurement units (PMUs) in contemporary electrical networks provides enhanced monitoring and control capabilities of the entire system. However, the placement of additional PMU devices is constrained by the relatively high cost and complicated communication infrastructure. Consequently, optimizing the allocation of PMU units is required to achieve complete visibility of the power system operation while minimizing the associated cost. This paper presents a coherent approach for solving the optimal PMU placement (OPP) in order to reduce the total number of PMUs that is required to completely observe the network. Furthermore, the proposed formulation of the OPP allocation problem considers several objectives such as cost minimization, redundancy and efficiency maximization in addition to various constraints like incorporation of zero-injection buses, single PMU failure, single line outage and consideration of flow measurements. Moreover, a two-stage approach is proposed to ensure the numerical observability of the obtained PMU placement. The proposed framework relies on solving a mixed integer linear programming problem for the OPP placement problem based on conventional measurements. The rank of the gain matrix is thereafter computed to check if the solution is numerically observable. In contrast to reported studies in the literature, transient stability enhancement is augmented in the formulation of the optimization problem such that the redundancy of predetermined buses is guaranteed without increasing the total number of installed PMU units. A novel spectral cluster-based online coherency grouping is employed to identify buses which possess the ideal characteristics in terms of transient stability support. [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]

Wind turbines (WTs) can beseriously damaged by lightning strikes and they can be struck by a significant number of flashes. This should be taken into account when the WT lightning protection system is designed. Moreover, WTs represent a path for the lightning current that can modify the well-known effects of the lightning discharge in terms of radiated electromagnetic fields, which are a source of damage and interference for nearby structures and systems. In this paper, a WT struck by a lightning discharge is analyzed with a full-wave modelling approach, taking into account the details of the WT and its interactions with the lightning channel. The effects of first and subsequent return strokes are analyzed as well as that of the rotation angle of the struck blade. Results show that the lightning current along the WT is mainly affected by the ground reflection and by the reflection between the struck blade and the channel. The computed electromagnetic fields show that, for subsequent return strokes, the presence of a WT almost doubles their magnitude with respect to a lightning striking the ground. Such enhancement is emphasized when the inclined struck blade is considered. [ABSTRACT FROM AUTHOR]

This paper presents frequency domain (FD) Norton-type equivalents of single- and three-phase photovoltaic (PV) generators, aimed at transient calculations. The proposed equivalents can be interfaced with a general frequency-dependent network and/or nonlinear loads and include voltage/current sources representing initial conditions aimed at partitioned-time transient simulation. The transient solution is performed via the numerical Laplace transform (NLT) in a sequential scheme by using time sub-windows with corresponding initial conditions and using reduced number of samples. The proposed approach overcomes the issue of computationally heavy simulations as in classical implementations of NLT. Two case studies involving PV generator and nonlinear load are presented to illustrate the performance of the proposed FD PV models. [ABSTRACT FROM AUTHOR]

The classical electromagnetic time reversal (EMTR) fault location method in power systems requires multiple simula-tions in the backward step, assuming different guessed fault loca-tions along the line. This process can be time consuming especially when a high location accuracy is desired, as it may require a significant number of guessed fault locations. To cope with this issue, the concept of EMTR in mismatched media has recently been introduced, allowing reducing the backward simulations to a single run and, thus, substantially improving the computation efficiency of the EMTR-based fault location technique. In this paper, we present a detailed study of the mismatched-media-based mirrored minimum energy property. This property has been applied in a few recent studies but never been theoretically studied and rigorously demonstrated. First, we infer a transfer function that relates the fault source to the voltage along the line resulting from back-injecting the time-reversed transients measured at a given observation point. We present a theorem according to which, at the fault switching frequency and its odd harmonics, the mirror-image point of the fault location with respect to the line center corresponds to a local minimum of the squared modulus of the transfer function. Then, it is proved that the mirrored minimum energy property is a corollary of this theorem. Based on these theoretical findings, we propose an algorithm that utilizes the reversed-time voltage energy as a fault location metric in the frequency domain. We further advance a data-driven strategy to maximize the computation efficiency of the fault location procedure. The applicability and robustness of the proposed frequency-domain fault location metric are numerically and experimentally validated. [ABSTRACT FROM AUTHOR]

In this paper, an improved dynamic state estimation (DSE) based protection (DSEBP) scheme is proposed for transmission lines in MMC-HVDC grids. Firstly, a dynamic model of the protection zone (DC line of interest) is constructed using differential and algebraic equations. The impact of frequency dependent line parameters is mitigated through the utilization of the “line mode” transmission line model. Then, a novel Runge-Kutta (RK) discretization scheme is proposed to consider the dynamics of the control variables during electromagnetic transients, yielding reduced discretization error as compared to the conventional RK scheme. Finally, the batch mode regression framework based DSE is developed to formulate the protection logic considering errors of both measurements and historical estimates. Numerical experiments have shown that the proposed DSEBP scheme can dependably detect and trip internal faults, and securely ignore the external faults. The proposed scheme achieves improved reliability and operational speed as compared with the existing DSEBP schemes. The proposed scheme only requires a typical sampling rate of 10k samples per second, has the robustness to measurement noises, and the calculation burden is low enough for practical implementations. [ABSTRACT FROM AUTHOR]

In this paper, the validity of two extended transmission line approaches proposed for calculating the ground-return impedance and admittance of an underground insulated cable is evaluated taking as reference a full-wave approach based on the 3-D finite-difference time-domain (FDTD) method. The transient responses of an underground cable with realistic radius are calculated for a wide range of soil resistivities and cable lengths. It is shown that the extended transmission line approaches lead to transient waveforms in good agreement with the FDTD method even for relatively short cables and high soil resistivity. The limitations of a traditional formulation that neglects the influence of the ground-return admittance on the calculation of the cable parameters are also demonstrated. Overall, the results confirm the accuracy of the newly-proposed expressions for the calculation of underground cable parameters. [ABSTRACT FROM AUTHOR]

Different from typical circuit breakers, breakers used to switch AC filters in convertor stations have to operate frequently according to the real-time operation mode and daily load changes. Therefore, the performance of filter breakers decreases very quickly, which leads to frequent accidents. Both preinsertion resistors and controlled switching devices can suppress switching transients. In this paper, a comparative analysis of whether a controlled switching device and preinsertion resistor should be adopted is carried out using PSCAD. The transient is calculated as the combined result of the switching angle and preinsertion resistor value, and the optimal configurations are proposed. After determining the resistor value and whether a controlled switching device should be included, the reference voltage of the arrester should be reevaluated. [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]

Renewable energy sources will be more vigorously deployed under the global trend of carbon emission reduction. The connection of numerous renewable energy sources poses an increasingly critical challenge to power quality (PQ) issues in power systems. Time-frequency analysis (TFA) is a foundational technique for real-time monitoring and disturbance detection for power signals. This paper develops an improved S-transform (IST) to accurately detect the disturbances such as oscillatory transient, time-varying harmonics and interharmonics, flicker, swell, sag, interrupt, phase jump, and frequency variation. The proposed IST features the exploration of a designed Gaussian window as the kernel function, whose shape and frequency spectrum can be controlled using a standard deviation based detection frequency parameter. This ensures that the detection requirements at different detection frequencies can be easily met. The IST can be realized by fast Fourier transform (FFT) and its inverse, which ensures that it can be implemented quickly. The IST can accurately detect the amplitude and phase information of fundamental signal, which is beneficial to determine the start and end time, and the intensity of disturbance. With the increase of detection frequency, IST also has excellent energy concentration performance. Simulation and experimental results validated the effectiveness and feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

Expansion of power grids has resulted in the construction of multiple transmission lines within constrained spaces inevitably making them nonuniform in nature. Existing transmission line models available in electromagnetic transient (EMT) simulators are based on classical multiconductor transmission line (MTL) theory with the assumption that the transmission lines are infinitely long and have uniform cross-sectional dimensions. This paper develops a time-domain model, namely dispersive scattered field transmission line (DSFTL) model, for multiconductor dispersive nonuniform overhead transmission lines above lossy, frequency-dependent ground. The proposed model which consists of closed-form equations in the time-domain has been implemented using a modified finite-difference time-domain (MFDTD) algorithm and integrated into an EMT simulator (PSCAD/EMTDC). Results have been compared with and verified by those obtained using a full-wave approach and measured data available in the literature. Simulations of nonuniform structures that include nonlinear components (such as breakers) have also been carried out under fault conditions. [ABSTRACT FROM AUTHOR]

Lightning strikes in substations may result in extremely uneven ground potential distribution, which will threaten the safety and stability of equipment in substation. However, the main parameters for evaluating the performance of grounding grids are grounding resistance and impulse impedance, and little attention is paid to the distribution of the transient ground potential differences. In addition, when measuring the impulse impedance of a grounding grid, the length of the current leads should be very large, which requires a high-capacity impulse generator to suppress the influence of lead inductance on current waveform. In this paper, a test method is proposed for the transient characteristics of grounding grid with the current-return point located in the same grid. The feasibility of the method is illustrated by analyzing the ground impedance between the current injection point and the return point, longitudinal current of the grounding conductor, transient ground potential rise and transient ground potential difference. The effects of parameters such as soil resistivity, lightning waveform, size of the grounding grid, spacing and materials of the grid conductor are discussed to demonstrate that the method can be applied to most substations. Finally, the practicality of the method is verified by experiments in a 220 kVsubstation. [ABSTRACT FROM AUTHOR]

This paper proposes a traveling wave based scheme for wye-connected shunt capacitor bank protection. The proposed method provides high sensitivity and fast response, assisting in mitigating capacitor damage during internal cascading failure. The available protection schemes for capacitor bank use RMS values of voltage and current phasors for trip decision. Threshold selection is required in these methods which needs accurate estimation of equivalent capacitance of the bank. In addition, phasor being estimated from one-cycle running data window, such a method is slow. The proposed traveling wave based method identifies an internal fault in case of opposite polarities of traveling waves seen in the terminal voltage and current signals of the faulted phase. If the mid-point tapping is available in capacitor bank, the faulted half can be identified using the traveling wave of tap voltage. The magnitude of the traveling wave in terminal voltage reveals the number of failed capacitor elements. Such information assists in expediting maintenance. The performance of the proposed method is tested using PSCAD/EMTDC simulation data for various situations, including internal short circuit, external, and identical faults in the shunt capacitor bank. Comparative analysis highlights its higher sensitivity and better accuracy in comparison to available protection methods. [ABSTRACT FROM AUTHOR]

DC cables are supposed to withstand a number of operations involving supply-voltage peak-value time variations, such as energization, switch off and polarity inversion. It is known that voltage-supply time variations cause electric field transients in cable insulation that can last for a long time before steady-state conditions are achieved, and during this time the electric field distribution is not only driven by electrical conductivity, as in steady state, but also by permittivity, as in AC. This paper shows that highly energetic degradation phenomena, that is, partial discharges, PD, can occur during voltage transients that a DC cable has to withstand during life. This results in a cumulative damage which can cause premature insulation breakdown, at times well shorter than those expected based on a cable design which is PD free under steady-state conditions. Quantification of the damage caused by PD, based on aging and residual life models, is proposed in the paper, and it is shown that damage can be significant enough to pose a threat for cable life when operation transient rate is e.g., a few per day, and that the faster the slew rate of the supply voltage variation, the highest the cumulative damage due to PD. [ABSTRACT FROM AUTHOR]

In this paper, a sequential ensemble of intelligence-based methods is used for fault detection and classification in microgrids. The proposed scheme is recommended because of the impracticality of conventional fault detection and protection due to microgrid dynamic behavior and dependency of traditional methods on faults current level or impedance. These methods use the collective decision of learners to increase their accuracy. This is done by distributing the knowledge among the classifiers. The proposed ensemble method is called Brownboost. Its main advantage over its counterparts is that it uses a nonconvex optimization method. This makes it robust to overfitting and applicable and practical for a real-world noisy or misclassified data. In addition, a signal processing method called the Hilbert-Huang transform was chosen for feature extraction from signals transient behavior to reduce the noise sensitivity. In this method, current difference of the both ends of line is selected to decompose various types of adaptive basis signal processing method. The results are validated in IEC test microgrid with various noise penetration level and synchronization delays. Compared to traditional strong classifiers, this method is easy to tune and program and is robust to overfitting. Moreover, it can be justified for real-world imperfect data. [ABSTRACT FROM AUTHOR]

This paper introduces a novel frequency domain technique to globally evaluate the accuracy of electro-magnetic transient simulations. It is shown that simulation accuracy at low frequencies can sometimes be poorer than at high frequency. A modified approach which quantifies accuracy from a driving point as a function of frequency is also introduced that uses the Bilinear Transformation and Norton equivalents, to produce a “simulation accuracy spectrum”. This approach can be applied to large systems without explicitly forming state space equations. It also permits the accuracy analysis of networks with distributed components such as frequency dependent transmission lines. Two examples are used to verify the proposed technique, a small network with a frequency dependent transmission line modeled by the universal line model; and the IEEE 39 bus system connected with an LCC-HVdc system. [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 hybrid LCC/VSC cascaded HVDC (HC-HVDC) scheme, in which the LCC is adopted as rectifier, and one LCC in series with multiple paralleled VSC based MMCs is adopted as inverter, combines the advantages of both LCC and VSC-MMC technologies. Presently, a HC-HVDC project is planned to deliver the hydro power from Baihetan to Jiangsu in China. One of the major challenges is to suppress the DC overcurrent issue of VSC-MMCs especially under commutation failure condition of LCC at inverter side. Based on Baihetan-Jiangsu project under construction, the model of HC-HVDC system is developed in PSCAD/EMTDC. In the system, the rectifier employs a dual 12-pulse LCC, while the inverter adopts a hybrid scheme, i.e., one LCC converter in series with three paralleled VSC-MMC converters. This paper addresses the main cause and feature of the DC overcurrent, then proposes an overcurrent suppression control method for HC-HVDC system based on fuzzy clustering and identification approach. And the system transient performances with the proposed control approach are evaluated under AC fault conditions. The results show that the proposed approach can not only suppress the DC overcurrent following the commutation failure to a certain extent, but also improve the post-fault recovery performance of the overall system. [ABSTRACT FROM AUTHOR]

In this paper, a new pilot protection scheme for AC line connected to LCC-HVDC inverter based on composite-mode model is proposed. First, taking commutation failure into consideration, 1-mode and 2-mode equivalent instantaneous models of inverter in different operating conditions are built. Second, according to the equivalent instantaneous inductances of inverter in different combinations of operating conditions, the equivalent models of inverter are summarized into 4 operation states. Then, the 1-mode and 2-mode equivalent models are combined to establish the composite-mode model of AC/DC hybrid system. And then, using multi-time section information, the instantaneous inductances of backside composite-mode model are calculated. Thus, protection criterion is constructed on the difference between composite-mode models of in-zone faults and that of out-of-zone faults for fault locating. Finally, RT-LAB simulation tests show that, the proposed method offers high sensitivity and reliability, unaffected by commutation failure. [ABSTRACT FROM AUTHOR]

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

In offshore wind farms, reignition overvoltages are usually caused by switching-off operations of vacuum circuit breakers, which degrade the insulation of power equipment, especially transformers. This paper proposes a new quantitative evaluation method of overvoltage suppression measures comprehensively considering three main indicators, which include the amplitude, steepness and reignition number. Based on the quantitative evaluation results, an improved suppression measure against high-steepness reignition overvoltages is developed. Firstly, high-frequency transient models of main components of a typical wind farm are developed in PSCAD/EMTDC. Then the stochasticity of opening instants is considered, and three main indicators are extracted for comprehensively analyzing the transient characteristics of reignition overvoltages. Subsequently, a quantitative evaluation is performed to analyze the effectiveness of different suppression measures, which involves surge arresters, RC snubbers and smart chokes with different properties and installation positions. Results show that a new combination of a surge arrester and a smart choke is capable of providing sufficient protection against a high-amplitude, high-steepness and severe-reignitions overvoltage. This research can provide a new overvoltage suppression solution for the insulation coordination of medium-voltage systems in offshore wind farms. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) are vulnerable to DC short-circuit faults. Analytical estimation of the AC and DC short-circuit currents in the AC in-feed steady-state stage is essential to reveal fault mechanisms and design protections. However, the nonlinearity of the MMC has brought significant difficulties to the calculation. This paper proposes a novel analytical calculation method for the steady-state AC and DC short-circuit currents. First, various working modes of the MMC in the AC in-feed stage are specified, and the relations between the arm current and the short-circuit current in each mode are studied. Subsequently, the arm current is divided into multiple sub-intervals, and the analytical expressions of the steady-state AC and DC fault current are deduced by solving the arm current in the sub-intervals one by one. Moreover, the derived expressions are further optimized considering the resistive-inductive (R/X) characteristic of AC grids, making them applicable for various scenarios. These analytical expressions intuitively show the relations between the system parameters and the short-circuit currents, thus can guide the rating and tuning of protection devices and power equipment. Finally, the effectiveness of the derived expressions is verified, and the influences of the system parameters on the fault currents are discussed. [ABSTRACT FROM AUTHOR]

Prolonged voltage level reduction caused by the fault induced delayed voltage recovery (FIDVR) event may lead to a decrease in distance relays security. Therefore, some distance relays are prone to mal-operation under severely prolonged depressed-voltage conditions, and their security should be considered in the FIDVR event studies. The present paper investigates the possible operation of distance relay during the FIDVR event and proposes an auxiliary protection algorithm to effectively prevent its mal-operation during short-term dynamic voltage transient. The proposed algorithm estimates the critical time and voltage level at which the impedance trajectory arrives in Zone 3 of the critical relays using the rate of change of the impedance trajectory, relay margin index, and time-variant NERC/WECC severity indices. Then, the critical voltage is compared with the expected voltage level at the estimated critical time calculated by piecewise cubic spline interpolation (PCSI) method and three consecutive voltage phasors to decide about the block of relays which entering the impedance trajectory to their Zone 3 is probable during the FIDVR event. The efficacy of the proposed method is demonstrated by simulating some short-term voltage transient scenarios on the IEEE 39-bus test system. [ABSTRACT FROM AUTHOR]

The impulse grounding performance of distribution lines tower is the key factor to lightning protection. Few field high-current impulse test researches are conducted on distribution line tower. In this paper, we used a mobile high-current impulse generator to conduct test on a 10 kV distribution line tower. The peak value of the current injection is 6 kA to 16 kA. The impulse impedance and the surface potential distribution are measured and analyzed. Based on the CDEGS software, an equivalent modeling of the experiment was carried out. The influence of soil ionization was taken into account by the method of changing the conductor radius through two rounds of calculation. The simulation and measured results were basically consistent. Experiments have shown that the soil ionization effect has an important impact on lightning performance of distribution line tower. The graphite earthing modules (GEMs) and pulling wires can significantly reduce the impulse impedance of the tower. The potential rise of the earth surface at a distance of 30 m from the tower can reach 1 kV at peak, under an injection of 7.15 kA. [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]

Although the underlying phenomena when grounding electrodes discharge lightning currents are very complex, it is possible to derive simple formulas of impulse characteristics that allow for important engineering estimates. The interest in simple formulas was recently renewed, resulting in many contributions in the literature. In this paper, a parametric analysis is performed by a simulation of time-domain responses, with parameters in broad ranges. From this study, new formulas of impulse characteristics have been derived, including the expressions for ranges of parameters when grounding electrodes exhibit dominant inductive, resistive, or ionization effects. New formulas are compared with the existing ones, with experimental results, and their accuracy has been investigated. [ABSTRACT FROM AUTHOR]

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

This paper presents an analysis of resonant faults and their transient responses on half-wavelength lines. This type of transmission line has been studied for many years in countries that need alternatives for bulk power transmission over very long distances. An important issue for this technology is the occurrence of critical faults, which produce peculiar behavior in the main electrical entities and can jeopardize line insulation and compromise system stability. Here, the main factors influencing the severity of faults are analyzed: the type of fault and fault location. This paper shows that faults involving positive and negative sequences are the most critical when applied at specific regions and produce high voltage and power with a high rate of rise. The necessity for a mitigation method that removes this severe resonant fault condition until the operation of a protection system is discussed, and such a method is established. A method using two spark gaps with a new and improved location criterion is proposed, tested and evaluated, providing promising results for controlling voltage and power at terminal substations. Digital simulations are performed with PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper proposes a set of fitting features that enables identifying propagation function of cables accurately in the form of multi-delay rational transfer functions while maintaining reduced order of approximation. Frequency domain partitioning and adaptive weighting techniques are applied directly in phase domain for the evaluation of poles and residues simultaneously to ensure the precision of fitting of all entries including the low-magnitude off-diagonal elements. The objective is not only to obtain a precise fitting in phase domain but also to account for intrinsic modal decomposition so that integration errors in time domain are not magnified. The order of approximation is considerably reduced by post-processing the fitting using the balanced realization technique. When the proposed fitting approach is combined with a precise integration technique in time domain, it leads to accurate evaluation of transients as demonstrated in this paper, eliminating the spurious oscillations or numerical instabilities that may be encountered in the universal line model regardless of the integration or interpolation technique. [ABSTRACT FROM AUTHOR]

The possibility of operating distributed generation (DG) in islanded mode brings several benefits but still faces some problems, especially related to the transition from a grid-connected to an islanded operating condition. When connected to the main grid, DG units operate in PQ mode. During this transition, their controls must be changed to regulate frequency and voltage. The delay for islanding detection, associated with abrupt changes in the control references, may result in large and detrimental transients. This paper proposes a new control structure for a distributed synchronous generator (DSG) to allow a smooth transition from the grid-connected to the islanded operation mode. The proposed control structure uses a compensation loop that was conceived based on the main ideas of the anti-windup bumpless transfer (AWBT) theory (although it is not, strictly speaking, an AWBT controller). The results show that the proposed structure ensures smaller excursions of the DSG variables during the transition and, therefore, the possibility of a successful formation of the islanded system is extended to a wider range of operating conditions. Moreover, with the proposed control, the success of this transition does not depend on the islanding detection time, which is the main contribution of this paper. [ABSTRACT FROM AUTHOR]

High entrance capacitances of ultrahigh-voltage (UHV) transformers make it difficult to generate impulses with standard front time during the lightning impulse tests. In this paper, we discuss the feasibility of examining the longitudinal insulation design of a transformer via impulses with a front time longer than the standard value. We consider the following features: transient voltage distributions and insulation characteristics of longitudinal insulation. The transient voltages of two UHV windings are measured with a capacitive voltage sensor under impulses with different front times. The results demonstrate that the front time of the applied impulse has little effect on the gradient voltage distributions and the oscillatory component of the transient voltage, but it determines the front time of transient voltages. Therefore, the relationship between breakdown characteristics of the turn-to-turn insulation and the front time is studied. Experimental results revealed that whatever the insulation thickness, an increase in the front time of the applied impulse decreases the breakdown voltage, but this variation is within acceptable tolerance when the front time changes from 1.2 μs to 6 μs. Thus, as long as the front time is restricted within 6 μs, we can ensure the validity of lightning impulse tests on the examination of longitudinal insulation designs. [ABSTRACT FROM AUTHOR]

Aging of power system assets challenges utilities to maintain reliable service while minimizing their maintenance costs. Although malfunction of devices in the transmission or distribution networks can have catastrophic consequences, their failure is seldom unforeseen. Typically, the device aging occurs gradually over time until its health condition reaches a point of complete breakdown. The overall objective of this paper is to devise a non-invasive, online condition monitoring technique of circuit switchers for shunt capacitor banks. It aims to detect abnormal operation of these devices based on features derived from raw voltage and current waveforms, such that a predictive maintenance strategy may be employed before a complete failure occurs. The data analyzed in this paper are collected at a transmission capacitor bus, where the banks are equipped with pre-insertion impedance. Through a baseline range of values for selected parameters, the proposed technique is able to detect missing pre-insertion impedance during the capacitor energizing and failure of pre-insertion impedance switch out a few cycles after the energizing operation. [ABSTRACT FROM AUTHOR]

This paper addresses time-dependent rating calculations for groups of power cables with temperature-dependent losses. It is a continuation of a recent publication [1] – the presentation of a new transient calculation method for one cable – and offers an extension of the algorithm presented there to the problem of multiple cables subjected to variable loading. Whereas the standards offer simplified solutions for one load step and for equally loaded cables only, the new approximate algorithm is providing a complete analytical solution to this problem. In the following, this will be explained in detail and demonstrated by practical numerical examples. [ABSTRACT FROM AUTHOR]

Although the use of a series capacitor increases the transmission line (TL) capacity, it inverts the voltage/current and creates a sub-harmonic frequency that results in unintended operation of the protection devices, which further decreases the transient stability under fault conditions. Restoring the transient stability requires an accurate diagnosis of faults in TLs, which further requires exploration of in-depth features. This imposes challenges for the existing approaches as they demand a large amount of data. In this paper, a self-attentive weight-sharing capsule network (WSCN) is proposed to achieve robust and high classification performance while utilizing a limited amount of data. The capsule network, with the weight-sharing mechanism delivers robust performance in detecting and classifying the faults in the TL domain. Simultaneously, the self-attention layer highlights the more dominant features that make the network work upon the limited data effectively. A Western-System-Coordinating-Council WSCC 9-bus and 3-machine test model, which was modified with the series capacitor was studied to quantify the robustness of the self-attention WSCN. The results were compared with those of others networks to confirm the explicit classification ability of the proposed network. A study on real-world data obtained from power network was also conducted to validate the proposed self-attention WSCN. [ABSTRACT FROM AUTHOR]

Out-of-Step (OOS) protection against severe grid disturbances is one of the most serious concerns of the power industry. Many methods have been proposed to identify the OOS condition in which impedance-based relays are one of the most widely used ones. However, some problems have been reported associated with the performance and application of these techniques. Therefore, to alleviate some of these problems, this paper presents a new real-time approach based on the active power deviation to estimate the instability of a single synchronous generator in the multi-machine power system. The angular velocity and acceleration data obtained from the instantaneous active power are continuously calculated to determine whether or not the generator experiences an unstable condition. The instability can be distinguished by monitoring the polarity of extracted angular velocity and acceleration data during the power swing. The instantaneous active power is computed locally using voltages and currents measured at the terminals of each generator. Additionally, this method does not require such information as fault clearing time and network parameters. The reliability of the presented approach and its independence from the network configuration are successfully investigated by simulating various scenarios. Furthermore, simulation results demonstrate the straightforward and satisfactory application of this method. [ABSTRACT FROM AUTHOR]

Identifying the location of partial discharges (PDs) and providing clear information on insulation vulnerabilities in power systems can help utility companies to improve their maintenance efficiency. This paper presents a PD monitoring and locating system for medium-voltage switchgears using a set of transient earth voltage (TEV) detectors. A time-of-flight methodology was used to identify the switchgear affected by PD among a row of switchgears. A time sharing access mechanism, which was implemented by multiple high-frequency surface mounting relays integrated into each TEV detector, was applied to automatically identify the site of PD and minimize the number of expensive high-speed acquisition cards. The operation principle and implementation of the modular system were formulated, and the efficiency of the proposed system in identifying the switchgear with PD among three 10 kV metal-clad switchgears was confirmed experimentally. [ABSTRACT FROM AUTHOR]

This paper has the purpose to investigate the variation of electric field inside a HVDC cable insulation, and consequent potentiality of partial discharge inception in a defect upon load current variation. Simulations carried out by COMSOL Multiphysics show that load variations can incept partial discharges during cable operation at a constant voltage, depending on insulating material characteristics, defect size and location, and, in particular, the temperature and field dependence of electrical conductivity. Different values for temperature and field dependency of electrical conductivity, as well as of thermal conductivity and heat capacity are considered and their effect on the field distribution inside cavities near inner and outer semiconducting layer analyzed. For this purpose, a new analytical model to predict the behavior of PD amplitude and repetition rate from cable energization to steady state, including load variation, is introduced. The likelihood of PD inception during load transients, and not only voltage transients as previously demonstrated, poses further attention and constraints to HVDC cable design. [ABSTRACT FROM AUTHOR]

This paper shows a comparison between Vector fitting and rational Krylov fitting techniques for the determination of rational models concerning the fitting accuracy, the computational performances and the model order. Primarily, the mathematics behind the second technique are presented. It should be noted that rational Krylov fitting have never been used in transmission line modeling. A new procedure is proposed to use rational Krylov fitting instead of vector fitting in the universal line model (ULM). Furthermore, it is demonstrated that this procedure has several advantages over the traditional one. Two illustrative examples involving a transmission system are presented for validation of the new procedure. [ABSTRACT FROM AUTHOR]

Speed and selectivity of DC fault protection are critical for High-Voltage DC (HVDC) grids and present significant technical and economic challenges. Therefore, this paper proposes a non-unit protection solution that detects and discriminates DC faults based on frequency domain analysis of the transient period of DC faults. The representation of a generic HVDC grid section and the corresponding DC-side fault signatures in the frequency domain form the basis of a generalized approach for analytically designing a protection scheme based on Wavelet Transform (WT). The proposed solution is adaptive within its design stage and offers general applicability and immunity to system changes, while the protection settings are configured for optimized performance. The scheme is validated through offline simulations in PSCAD/EMTDC and the technical feasibility of the algorithm in the real world is demonstrated through the use of real-time digital simulation (using RTDS) and Hardware-in-the-Loop (HIL) testing. Both offline and real-time simulations demonstrate that the scheme is able to detect and discriminate between internal and external faults at a significantly high speed, while remaining sensitive to high impedance faults and robust to external disturbances and outside noise. [ABSTRACT FROM AUTHOR]

Due to global climate change, the effect of extreme weather on power systems has attracted extensive attention. In the prior-art grid resilience studies, the hurricanes or wildfires are mainly defended in terms of expected line damages, while they are prone to trigger short-circuit fault (SCF) evolved with dynamic influence in reality. In this paper, a fragile model is developed to evaluate the nodal SCF probability considering the insulation aging of equipment and extreme weather condition. Then, a response framework for extreme weather events is developed for a transmission system to defend the cascading impacts of expected SCFs. Specifically, switches are shifted to restrain the out-of-range short-circuit currents (SCCs) so that to ensure the SCFs can be removed by circuit breakers, generation rescheduling and load shedding are arranged to maintain the post-fault system transient stability. The above measures are optimized simultaneously by an integrated Mixed-Integer Nonlinear Programming (MINLP). Considering the error or uncertainty of weather event forecasts, a multi-state model is established to provide the most cost-effective grid resilience enhancement scheme, in which the expected urgent adaptions of the initial scheme subject to weather state transition is included in the overall cost. The proposed model and techniques are validated using the IEEE 39-bus New-England test system and realistic meteorological data. [ABSTRACT FROM AUTHOR]

This paper proposes a transient stability constrained controlled islanding (TSC-CI) approach against cascading events in power systems. Using the wide area measurements, the proposed islanding algorithm is implemented regarding the actual dynamic conditions of the resulted islands. Since the main concern, right after the execution of controlled network splitting, is the transient stability of synchronous machines, a transient energy function is utilized for proper network partitioning. The proposed transient stability criterion is expressed as a function of the transfer impedance between each pair of generators in the resulted islands. Transfer impedance between each pair of coherent generators determines the direction of network splitting for transient stability improvement. A multi-objective function is then introduced to provide the transient stability with least possible power imbalance, as a resilience measure, over the created islands considering operational and structural constraints. Based on the wide-area measurements, a proper weighting procedure is utilized to prioritize the critical islands, avoiding the unnecessary power imbalance in favor of transient stability improvement. The proposed TSC-CI model is formulated as a mixed integer linear programming (MILP) multi-objective model and is solved using CPLEX in GAMS. The obtained results are compared with the conventional islanding scheme using the IEEE 118-bus system. [ABSTRACT FROM AUTHOR]