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• Emphasizing load modeling's critical role in TRV calculation, particularly in substations with short lines. • Discussing load modeling's influence on CB selection, a factor ovelooked in current standards. • Investigating the distinct effects of active and passive loads on TRV calculation in real sub-transmission systems. • Providing a comprehensive system modeling for TRV analysis, incorporating finite element models to compute stray capacitances. In this paper, the selection of circuit breakers for a densely-meshed sub-transmission system with relatively short transmission lines is investigated. The results highlight the significance of modeling the load at the end of lines for accurately calculating the transient recovery voltage (TRV) following the short-circuit current interruption. According to IEEE standards, assuming load modeling at the end of the transmission lines can result in a maximum discrepancy of 20 % in the calculated TRV. Supposing that the load modeling leads to a moderating effect on the calculated TRV, the difference is considered as a safety margin, and load modeling is ignored. Nevertheless, the results presented here illustrate that this assumption strongly depends on characteristics of the studied network and the load type. In a network possessing the characteristics outlined in this paper, even a passive load as small as 15 % of the nominal capacity can significantly moderate the calculated TRV, with impacts reaching up to 38 %. Conversely, modeling an active load may result in a more severe TRV than if it were disregarded. The simulation results indicate that for networks with transmission lines longer than 70 km, neglecting to model passive loads on the transmission lines still yields sufficiently accurate calculations of TRV. [ABSTRACT FROM AUTHOR]

• Developing a comprehensive model of a transformer winding which incorporates skin effect, proximity effect, and eddy current losses in the core, • Studying the effects of the core at high frequencies using simulations and measurements, • Investigating the skin and proximity effects on the frequency response and impedance characteristics of transformers, • Performing empirical measurements to validate the reduced influence of the core at high frequencies. This paper investigates the high-frequency transformer losses attributed to eddy currents in both conductors and the core. A comprehensive model of a transformer winding is presented, meticulously incorporating skin and proximity effects. The winding resistance increase due to these effects is determined by applying analytical and finite element methods. The investigation highlights that eddy current losses in the magnetic core notably increase the resistance of each winding section in low frequencies. However, the impact of these losses diminishes as the winding length approaches the voltage wavelength at high frequencies. Consequently, the net magnetomotive force and flux in the core become negligible. As a result, the high-frequency impedance characteristics are remarkably similar for windings with and without a magnetic core. These findings are substantiated through rigorous simulations and empirical measurements. [ABSTRACT FROM AUTHOR]

Highlights • A new effective approach of online power system inertia calculation is presented. • An improved method of Estimation of the Rate of Frequency Change is proposed. • It increases the awareness of the varied and reduced system inertia in future power system. • The performance is validated by simulations, under laboratory conditions and real-life data. • The method uses wide area monitoring data and can be part of application of WAMPC scheme. Abstract Future developments in power systems, e.g. relatively larger generator sets, the virtual power plant and synthetic inertia concept and connection of generation assets over inverters, will cause the system inertia to vary significantly. During system operation, if the inertia of the system is significantly lower than anticipated at the planning stage, then the existing, deterministic protection and control may fail to ensure system stability. Therefore, the ability to accurately determine the inertia of individual system areas, and the system as a whole, online would be very useful. In this paper, an Inertia Calculation Application (ICA), which could be implemented as part of a Wide Area Monitoring Protection and Control scheme, is presented. The necessary wide area measurements must be processed during large disturbances to the active power balance of the system. The ICA has been validated by using computer simulations, under laboratory conditions and by using real-life data recorded by a transmission system operator. [ABSTRACT FROM AUTHOR]

Traditionally, extra high voltage (EHV) transmission systems consist exclusively of overhead lines (OHL) but in the recent years the utilization of high voltage AC (HVAC) power cables is increasing due to environmental, political and operational aspects. To study the impact of HVAC power cables on temporary overvoltages and slow front transients, a simulation model of the large-scale network is required. Guidelines concerning the modeling detail for the surrounding network need to be established to ensure sufficient accurate results. Existing guidelines for purely OHL transmission networks may not be adequate since the integration of HVAC power cables shift the harmonic impedance of the network to lower frequencies and that can affect the extent of the needed simulation model. Much effort can be saved from obtaining detailed data for an extensive but proper model, but a less detailed model might not describe the network behavior with sufficient accuracy. In this study, simplification options are considered, for which the time to construct and analyze a simulation model can be reduced. The simplification options are compared with the reference model (full detail), and judged based on three indicators for the level of accuracy: frequency and amplitude of the main resonance, and maximum over-voltage. As a result, guidelines on the extent of the simulated network and in which detail the model has to be built are proposed. The inverse approach, i.e. increasing level of detail until simulation results hardly change leads to similar results, but requires equivalent short-circuit networks for the rest of the grid. The method is tested for the Dutch 380 kV grid where an underground cable connection of 10.8 km length is introduced in 2013 and a second connection is planned for 2017. [ABSTRACT FROM AUTHOR]

This paper presents the modular-multilevel-converter (MMC) control interoperability (IOP) and interaction within the High Voltage Direct Current (HVDC)-based power system. IOP is a crucial issue in the large-scale HVDC grid with different suppliers. To accommodate future multi-vendor HVDC grids developments, this article comprehensively investigates the MMC control IOP issue. Firstly, the most commonly adopted proportional-integral (PI) control and other non-linear controllers, e.g., model predictive control (MPC), back-stepping control (BSC), and sliding mode control (SMC), are constructed for MMC. Then, the IOP simulations are carried out in a multi-terminal direct current (MTDC) system in real-time digital simulator (RTDS) environment. The most frequent transients of the practical projects, e.g., power flow changing, wind speed changing, and DC/AC grid faults, are simulated with eight different scenarios. Each scenario presents different control capabilities in maintaining system stability, more precisely, the scenarios with non-linear controllers show faster settling time and fewer DC voltage and power variations. Controller switchings are also achieved without bringing large system oscillations. This paper provides the optimal allocation strategy of controllers to cope with system transients. • Control interoperability study in power electronic converter dominated high voltage direct current system. • Modeling the classical and advanced controllers for modular-multilevel converter in a real-time digital simulator. • Power system oscillations investigation with various controller allocations. • Optimal controller settings against power system transients. [ABSTRACT FROM AUTHOR]

Power systems require a reliable supply and good power quality. The impact of power supply interruptions is well acknowledged and well quantified. However, a system may perform reliably without any interruptions but may have poor power quality. Although poor power quality has cost implications for all actors in the electrical power systems, only some users are aware of its impact. Power system operators are much attuned to the impact of low power quality on their equipment and have the appropriate monitoring systems in place. However, over recent years certain industries have come increasingly vulnerable to negative cost implications of poor power quality arising from changes in their load characteristics and load sensitivities, and therefore increasingly implement power quality monitoring and mitigation solutions. This paper reviews several historical studies which investigate the cost implications of poor power quality on industry. These surveys are largely focused on outages, whilst the impact of poor power quality such as harmonics, short interruptions, voltage dips and swells, and transients is less well studied and understood. This paper examines the difficulties in quantifying the costs of poor power quality, and uses the chi-squared method to determine the consequences for industry of power quality phenomenon using a case study of over 40 manufacturing and data centres in Ireland. [ABSTRACT FROM AUTHOR]

This paper proposes a novel approach for testing dynamics and control aspects of a large scale photovoltaic (PV) system in real time along with resolving design hindrances of controller parameters using Real Time Digital Simulator (RTDS). In general, the harmonic profile of a fast controller has wide distribution due to the large bandwidth of the controller. The major contribution of this paper is that the proposed control strategy gives an improved voltage harmonic profile and distribute it more around the switching frequency along with fast transient response; filter design, thus, becomes easier. The implementation of a control strategy with high bandwidth in small time steps of Real Time Digital Simulator (RTDS) is not straight forward. This paper shows a good methodology for the practitioners to implement such control scheme in RTDS. As a part of the industrial process, the controller parameters are optimized using particle swarm optimization (PSO) technique to improve the low voltage ride through (LVRT) performance under network disturbance. The response surface methodology (RSM) is well adapted to build analytical models for recovery time ( R t ), maximum percentage overshoot (MPOS), settling time ( T s ), and steady state error ( E ss ) of the voltage profile immediate after inverter under disturbance. A systematic approach of controller parameter optimization is detailed. The transient performance of the PSO based optimization method applied to the proposed sliding mode controlled PV inverter is compared with the results from genetic algorithm (GA) based optimization technique. The reported real time implementation challenges and controller optimization procedure are applicable to other control applications in the field of renewable and distributed generation systems. [ABSTRACT FROM AUTHOR]

Multi-terminal HVDC is an important new technology for bulk power transfer from large sources of remote renewables to large load centers. Recently MTDC grids have become more technically feasible due to improvements in VSC based converters, which can reverse currents and generally have greater control than LCC. Furthermore, emerging DC breakers technology unlocks the possibility of de-energizing only the faulted section of an MTDC without losing the whole grid. The protection relaying in such grids is not trivial: it must be very reliable, robust to the converter control system׳s effect on the post fault transients and act quickly, before the fault current develops beyond the presently modest interrupt capability of DC breakers. Within this challenging environment, this paper discusses in detail the important considerations for designing MTDC protection, including accurate modeling of the converter topology, associated control scheme and the DC link itself, supported by the results of novel transient simulation studies. This context is then used to appraise the suitability of the existing protection of HVDC and MTDC grids and suggests future work in the area. [ABSTRACT FROM AUTHOR]

This paper presents a novel wavelet based approach for fault location using voltage transient waveforms in power distribution systems. The proposed method includes two main stages. Firstly, the approximate location of the fault or fault section is determined using a new algorithm with discrete wavelet transform. The difference between arriving times of transient components in different measurement units is used for this purpose. The accurate location of the fault is determined in the second stage. Depending on the determined fault section, the difference between arriving times of transient components in different measurement units or the frequency content of the voltage transients are used. The time difference and frequency content are calculated using discrete and continuous wavelet transform (DWT and CWT) respectively. The proposed technique is implemented on an unbalanced 34 bus distribution system with two distributed generation units which is simulated in ATP–EMTP. The comparison of the results of the proposed method with previous works verifies its better accuracy and more robustness to fault conditions including fault inception angle and fault resistance. [ABSTRACT FROM AUTHOR]

Overvoltage transients due to lightning strikes is a common occurrence in power transmission lines which can ultimately lead to line outages. Simulation of transmission line networks under the influence of lighting plays a vital and essential role in predicting their transient behaviour. The transient behaviour of uniform transmission lines due to lightning has been studied extensively. However, in modern transmission line networks, nonuniformities and discontinuities such as nonparallel wire, line crossings or bends are becoming increasingly common. Therefore, in such cases, inaccuracies might be present in analysis performed using conventional uniform transmission line approaches. A recent publication by the authors analysed the transient behaviour of nonparallel and crossing transmission lines above perfect electric conductor (PEC) ground. However, the PEC assumption can only be used in a limited scope. In this paper a nonuniform transmission line model with external field coupling is presented to analyse the transient behaviour of a nonuniform wire structure due to the field coupling from a nearby lightning strike in the presence of lossy ground. Simulation results generated by the proposed model is compared with full-wave simulations and results published by other researchers. A case study to compare the results from the proposed model with a conventional uniform transmission line model under various ground-conductivity values and terminating conditions is also performed. The study indicates that the presence of nearby nonparallel lines can cause the transient behaviour of the structure to deviate from the behaviour predicted using uniform approaches. • The transient behaviour of a nonuniform wire structure a nearby lightning strike in the presence of lossy ground is studied. • Simulation results generated by the proposed model is compared with full-wave simulations and results published by other researchers. • Results from the proposed model have been compared with a conventional uniform transmission line model. • The study indicates that the presence of nearby nonparallel lines intensify the transient behaviour. [ABSTRACT FROM AUTHOR]

• Penstock water start time. • Water conduit transients. • Modelling of hydraulic conduits. • Solving travelling wave equations. • Power system dynamics for hydro generators. Almost all electrical power systems literature assumes that the shaft mechanical torque of a water turbine is linearly proportional to the wicket gate position and ignores the dynamics in the shaft torque due to the propagation of the water from the reservoir to the turbine. This propagation is usually modelled very simply using the water start-time equation and ignoring the travelling wave dynamics along possibly long conduits. This simplified modelling may introduce significant errors in the frequencies of oscillation which are in the range of 0.5–2 Hz and often fall within the range of slow oscillations in power system dynamics. This paper introduces the EMTP-H model for water dynamics based on the analogy between the hydraulic equations and the EMTP transmission line equations. The proposed model is validated with a field test and is applied to a simple power system frequency dynamics case. [ABSTRACT FROM AUTHOR]

• Decoupled single-phase Clarke-transformed networks can be used in transient studies. • Large ground-fault overvoltages occur in MV radial networks with ungrounded neutral. • An ATP ground-fault overvoltages parametric study with Clarke circuits is reported. • Results are compared with a full polyphase ATP model of the MV system under study. • Results agreement is excellent, with Clarke yielding much shorter computation times. The paper investigates the use of the Clarke transform in time-domain simulations, namely the evaluation of transient overvoltages in medium voltage (MV) distribution networks. A simplified Clarke equivalent circuit can be set up using readily available symmetrical components data used in steady-state studies, such as line series impedances and the aggregate MV network capacitance. Attention was focused on an existing 20 kV radial distribution system, a large mixed cable/overhead network operated with ungrounded neutral. The worst-case scenario of a single-line-to-ground (1LG) fault occurring at the end of a long overhead line was simulated by means of equivalent lumped-parameter Clarke circuits which were implemented and solved by means of ATP-EMTP. An extensive transient simulation parametric study was carried out using Clarke circuits with different degrees of simplification, and results were compared with those yielded by a complete 3-phase ATP model, evidencing a very good agreement and a drastic reduction of computation times. [ABSTRACT FROM AUTHOR]

• This work is first in the power quality literature to cover the role of collection and internal grid in the magnification of dip-staring transients. • It demonstrates that the amplitude and type of the transient at the WT terminals depend strongly on the properties of the collection grid. Using a simplified model may result in underestimation of the impact of the dip on the WTs. • It shows that it is essential to consider the dip-starting transients when assessing the impact of a voltage dip on WTs in a WPP. Moreover, that is also indispensable to include sufficient details of the collection grid of a WPP when studying FRT of such plants. This work shows that transients at voltage dips starting impact fault-ride-through of wind turbines. For fault-ride-through studies and manufacturer tests, it is therefore important to consider these transients and their magnification from the transmission grid through the collection grid to the wind turbines. Fault-ride-through studies in the literature do not consider the transient as a dip characteristic and employ overly-simplified models that do not consider the collection grid. This work studies in detail how the dip-starting transient changes during the propagation from the transmission grid to the wind-turbine terminals. It is also studied how this transient impacts the dynamic behaviour of the wind turbines in terms of the overvoltage on the DC-link of wind turbines based on doubly-fed induction generator (DFIG). The analyses are performed for several realistic configurations of a wind-power plant, all based on an existing installation. The results show that the magnitude of the transient is magnified when the resonant frequency of the collection grid is similar to the oscillation frequency of the transient. Moreover, the higher magnitude of the transient results in a significantly higher overvoltage on the DC-link. This work is the first in power quality literature to cover the collection and internal grid as a factor for the magnification of dip-staring transient. The main finding of this work is that the detailed models of the collection grid and the transients at the voltage dips starting must be not neglected when accessing the LVRT of wind turbines. It is strongly recommended to consider the details of the dip-starting transients and of the collection grid to assess the impact of dips on the wind turbines properly. [ABSTRACT FROM AUTHOR]

Abstract: The present work shows how a new genre of problem solution techniques called compressive sensing (CI) can be utilized for solving power quality problems. Specifically speaking, this work undertakes the power system transient recognition problem and solves it using sparse representation based classification (SRC) technique, a popular, recently emerging technique in CI. The method employs SRC based classification in combination with a feature extraction procedure, carried out using dual tree complex wavelet transform (DTCWT). The proposed method uses statistical characteristics to extract features from the DTCWT coefficients obtained from each signal and then these extracted features are used as input arrays for the SRC based classifier. The PQ disturbance events considered in this work include four different transient conditions, namely, transients due to capacitor switching, transformer inrush currents, transients due to motor switching and transients due to short circuit faults. The proposed algorithm could achieve perfect classification accuracy of the order of 100% and could comfortably outperform several similar contemporary methods known for identical power system transient classification problems. [Copyright &y& Elsevier]

Highlights: [•] A new power quality index is defined, tested and established. [•] New index is based on weighted sum of percentage energy deviation from pure signal. [•] 133 Signals as per IEEE 1159-2009 standard are used for validation of new index. [•] Real transients acquired by laboratory experimentations are used for validation. [•] New index is compared with THD and other discrete wavelet transform based index. [ABSTRACT FROM AUTHOR]

Abstract: The installation of long HVAC cables has recently become more common and it will likely increase during the next years. Consequently, the energization of long HVAC cables in parallel is also a more common condition. The energization of HVAC cables in parallel resembles the energization of capacitor banks in parallel and high inrush currents are associated to both energization phenomena. The existing IEEE standard C37.012-2005 for capacitive switching proposes a formula that can be used to estimate the peak magnitude and frequency of the inrush current. However, for sake of simplicity, the formula has several simplifications and it does not always provide accurate results. This paper proposes a new formula that can be used for the estimation of these two quantities for two HVAC cables in parallel. [Copyright &y& Elsevier]

Abstract: This paper reports the study carried out to asses the dynamic behaviour of a real wind farm during fault conditions in the distribution network. The objective is to obtain a transient model valuable for all kinds of operational conditions. The paper begins with a detailed modelling of the distribution network between the wind farm common busbar and the point of common coupling with the HV transmission system, for all types of short-circuits. The state-space network model, together with the electromagnetic transient model of each wind generator, configure a multi-machine wind farm model, with 572 state variables. Simulation results prove that multi-machine modelling is mandatory in order to tackle the wind farm contribution to symmetric and asymmetric short-circuits. [Copyright &y& Elsevier]

Abstract: In this paper, a novel algorithm, based on the Teager Energy Operator (TEO) of a sinusoidal waveform, proposed to detect and analyze the voltage disturbances. Most common power quality (PQ) disturbances at the distribution level such as voltage sags, notches, and capacitor switching are presented. These examples provide the basis for further characterization of other power quality events. Magnitudes of transient PQ events are located in the width of the signal. Furthermore, meaningful components of transients are analyzed. The whole method is implemented and tested over a sample representing recorded disturbances. Simulation and experimental results of different PQ disturbances show that the proposed method is fast and robust in detecting voltage disturbances and requires only a few samples to calculate the energy of a signal. [ABSTRACT FROM AUTHOR]

Abstract: An installed 900kW fixed speed stall controlled soft-started wind turbine connected to a weak distribution grid was modeled under start-up conditions. Generator and soft-start control and design parameters were not available, so a modeling process independent of this information was developed. Field measured transients were closely reproduced in simulation using a full-order generator model with generic parameters and a thyristor-based graduated interconnection to the grid. A Static VAR Compensator model was then added to the wind turbine model to explore a method of reducing start-up transients. It was found that for the specific transients modeled, a 300kVAR SVC supplied sufficient reactive power to limit line voltage variations to within 3% of steady state values. Model details and simulation output are presented in this paper. [Copyright &y& Elsevier]

Abstract: This paper presents a modelling methodology to analyse the impact of wind farms in surrounding networks. Based on the transient modelling of the asynchronous generator, the multi-machine model of a wind farm composed of N generators is developed. The model incorporates step-up power transformers, distribution lines and surrounding loads up to their connection to the power network. This model allows the simulation of symmetric and asymmetric short-circuits located in the distribution network and the analysis of transient stability of wind farms. It can be also used to study the islanding operation of wind farms. [Copyright &y& Elsevier]

Abstract: This paper presents a protection scheme that is capable of very fast isolation of faults in high voltage transmission systems. Proposed scheme comprises set of relays connected through a telecommunication network, located at different nodes of the system. Relays use wavelet coefficients of current signals to identify the fault directions relative to their location. Fault directions identified at different locations in the system can be combined to determine the faulted line (or busbar) and isolate it. A robust single ended traveling wave based fault distance estimation approach is proposed as a backup in case of communication failure. Investigations were carried out using time domain simulations in PSCAD/EMTDC for a high voltage transmission system. [Copyright &y& Elsevier]

Abstract: Simulators of the Electromagnetic Transients Program (EMTP) type are widely used for the study of high-frequency transients in power electric systems. For the study of electromechanical transients, where the main interest is to focus only on deviations from the ac waveform, the EMTP approach is not efficient. In this paper, a branch companion model that is suitable for both electromagnetic and electromechanical transients simulation is proposed. It processes analytic signals whose Fourier spectrum can be shifted in accordance with the objective of the study. The proposed method opens the way for a unified description of electromagnetic and electromechanical transients simulation. [Copyright &y& Elsevier]

Abstract: In this study, power quality (PQ) signals are analyzed by using Welch (non-parametric) and autoregressive (parametric) spectral estimation methods. The parameters of the autoregressive (AR) model were estimated by using the Yule–Walker method. PQ spectra were then used to compare the applied spectral estimation methods in terms of their frequency resolution and the effects in determination of spectral components. The variations in the shape of the obtained power spectra were examined in order to detect power system transients. Performance of the proposed methods was evaluated by means of power spectral densities (PSDs). Graphical results comparing the performance of the AR method with that of the Welch technique are given. The results demonstrate superior performance of the AR method over the Welch method. [Copyright &y& Elsevier]

Abstract: In this work, an accurate coupling capacitor voltage transformer (CCVT) model for electromagnetic transient studies is presented. The model takes into account linear and nonlinear elements. A support routine was developed to compute the linear 230kV CCVT parameters (resistances, inductances and capacitances) from frequency response data. The magnetic core and surge arrester nonlinear characteristics were estimated from laboratory measurements as well. The model is used in connection with the electromagnetic transients program (EMTP) to predict the CCVT performance when it is submitted to transient overvoltages, as are the cases of voltages due to the ferroresonance phenomenon and circuit breaker switching. The difference between simulated and measured results is fairly small. Simulations had shown that transient overvoltages produced inside the CCVT, when a short circuit is cleared at the CCVT secondary side, are effectively damped out by the ferroresonance suppression circuit and the protection circuit. [Copyright &y& Elsevier]

Abstract: This paper discusses the analysis of voltage disturbances in the time scale domain using Lifting Based Wavelet Transforms (LBWT) to quantify Power Quality (PQ) events. Characteristics of the investigated signals are generated on a time–frequency plane. Converter operation, load interruption/reenergizing, nonlinear loading and two types of capacitor switching, representing five common power quality events at the distribution level, are presented. These examples provide the basis for further characterization of other power quality events. Magnitudes of transient PQ events are located in the width of the signal. Furthermore, meaningful time and frequency components of transients are analyzed. The whole method is implemented and tested over a sample representing disturbances. Simulation results for five types of PQ events show that the proposed method is more efficient and faster in tracking signal dynamics than classical Wavelet Transforms (WT). [Copyright &y& Elsevier]

Abstract: This paper describes a method for the analysis of electromagnetic transients in multiphase transmission networks using the Numerical Laplace Transform. The proposed procedure is based on the superposition principle and is applied to switching and non-linear elements modeling. Switching operations are modeled as initial condition problems by means of injected current sources. In the case of non-linear elements, a piece-wise linear approximation is made, which reduces the problem to a sequence of switching operations. Several applications and comparisons with results obtained with the EMTDC and ATP programs are presented. [Copyright &y& Elsevier]

For a power industry undergoing deregulation, the growing number of power marketers and independent power generators increases the variability of power flow patterns in an interconnected power system, thereby making the prediction of critical operating conditions and limits more demanding. Consequently, transmission system operators need new tools to understand the effects of disturbances. Among these needs are tools to classify and identify disturbances as they occur on the power system. In this paper, power system disturbance data from the Northfield substation dynamic recording device in the US New England power system are analyzed and their features extracted. Then a classification method is used to identify the disturbance. Events with multiple disturbances are also addressed. A MATLAB-based software, called ‘Event Analyzer’, has been developed for the disturbance identification. [Copyright &y& Elsevier]

• Finite-element full-wave electromagnetic simulation of power transmission towers. • Incorporation of lossy, frequency-dependent soil and grounding electrodes in the model. • Determination of time-domain overvoltages induced across the string of insulators. • Development of a black-box model for incorporation in EMT-type simulators. • Determination of backflashover rate based on critical current. In order to calculate the overvoltages across the insulator strings of overhead lines, electromagnetic transient (EMT)-type simulators require a model of the tower and its grounding system. In this paper, an electromagnetic field simulation model based on the solution of the full-wave frequency-domain Maxwell's equations is established to obtain an EMT-compatible black-box representation of the transmission line tower for time-domain simulations. Details of the system, such as the grounding system consisting of counterpoise electrodes, shield wires, phase conductors, and tower cross arms and slant elements are considered in the simulation model. The frequency dependence of soil electrical parameters is included as well. The input data to the frequency-domain simulation model consists of the topology of the tower structure, grounding system, and the parameters of the surrounding medium. As a case study, the developed simulation model is applied to obtain the black-box representation (macromodel) of a 400 kV double circuit tower, where the lightning current injected at the tower top is the input, and the output is the induced voltage across the insulator strings. The critical current leading to the flashover of the insulator strings is calculated according to various existing flashover models and two different first stroke lightning current waveforms. Finally, a probabilistic analysis is performed to obtain the percentage of downward negative first strokes leading to a backflashover. [ABSTRACT FROM AUTHOR]

The recent changes in power systems give rise to new scenarios in which power transformers must continue to perform satisfactorily. Resonant transient phenomena caused by the interaction of the power grid and the transformer have recently led to serious failures. It is therefore essential to carry out network transient studies in order to evaluate potentially dangerous events of this type. This requires high frequency transformer models having an adequate representation of the damping. In this work, a white-box transformer model is presented together with a new and robust procedure for determining its parameters that enforces the stability of the model. In this way, substantial improvements are proposed with respect to models currently available in the literature in the methodological aspects needed for successful practical implementation. The methodology for determining basic impedance data, obtained using the Finite Element Method is first described and then a new methodology for the subsequent processing of these data to obtain the parameters of a stable and robust model is presented, using Vector Fitting in combination with Particle Swarm Optimization. The model has been successfully validated using a large power transformer as a case study within the scope of the JWG CIGRE A2/C4.52. [ABSTRACT FROM AUTHOR]

Electromagnetic Transient (EMT) simulation tools are typically developed using an imperative language. It is typically required to code the models using a specific numerical integration method and following existing solution algorithms. On the other hand, modern high-level, acausal and equation-based programming languages, such as Modelica, allow to formulate models that are easy to read, understand and develop by expressing what needs to be computed without stating how it should be computed. In this paper, the application of Modelica to EMT modeling is demonstrated and the benefits of the language for EMT-type simulations is established. This is done by using the declarative language concept for the development of two models, namely the Constant Parameter (CP) and the Wide-Band (WB) models. Two case studies are presented and used to validate the models and to compare accuracy and performance. [ABSTRACT FROM AUTHOR]

The reliability of distribution networks can be improved by islanding of part of the network when a short-circuit fault occurs. The voltage and frequency stability during transient can be evaluated with time-domain simulations using component-based DER and load models. However, simulations with this type of models have a relatively high computational burden which constrains periodical stability analysis of microgrids. This paper proposes an aggregation methodology which significantly decreases the computation time of fault-initiated islanding simulations with component-based models, while maintaining the nonlinear dynamics. The methodology is first validated by directly comparing the original and equivalent models during large voltage and frequency transients. Secondly, a fault-initiated islanding case study of a residential distribution network is performed. For both validations, a sensitivity analysis of model parameters and voltage variations is performed. The results indicate the methodology preserves the dynamics of DERs and loads, while significantly reducing the computational time. [ABSTRACT FROM AUTHOR]

• Simulation model for electrodes in frequency-dependent soil. • Effects of variable water content on ground electrode impedance. • Ground potential raise due to lightning considering soil water content. The increasing demand for electricity over the past few decades has been continuously urging for more reliable power systems. Power transmission line networks, due to their vast physical dimensions, are the most critical components of a complex power system concerning the direct lightning strike. Grounding systems are widely used to mitigate the transient overvoltages for the protection of electrical component and human safety. The goal of this paper is to investigate the effect of frequency dependence of soil electrical parameters on the performance of vertical and horizontal grounding electrodes subjected to direct lightning strike, considering a variable water content of soil. Four soil models that take into account the variation of soil water content are reviewed and compared. The numerical simulations are performed using a full-wave electromagnetic model based on the Finite Element Method (FEM) in the frequency range from 1 kHz to 5 MHz and applying different soil models with varying water content. Then, electromagnetic transient (EMT) compatible equivalent circuits of grounding electrodes are developed using Fast Relaxed Vector Fitting (FRVF). Typical first and subsequent return stroke current waveforms measured at two different locations are adopted. The results of this paper demonstrate the importance of considering the variation of soil water content in the transient response of grounding electrodes. Simulation results demonstrate that the frequency dependence of the soil parameters lead to a decrease of the potential rise of the grounding electrodes compared to the case where the soil parameters are assumed constant. The effect of soil water content on the grounding performance is also shown to be dependent on the soil type, value of soil water content, and length of electrode. [ABSTRACT FROM AUTHOR]

• A PEM Fuel Cell model covering the multi-physics transients is developed. • The representation of the model is based on electric circuit equivalents. • It is shown how the model is realized in a network simulator for electric transients. • The multi-physics PEMFC model is validated against experimental reference data. • A CHP application is carried out to verify the value of the modeling. Digital simulators of electric networks have been indispensable tools for engineers and researchers engaged in the analysis and design of electric power and energy systems. With the increasing interest to extend electric energy toward multi-energy systems covering multiple energy carriers, there arises a need to also extend the capabilities of today's simulators. Such an extension is proposed here in the form of a Proton Exchange Membrane Fuel Cell (PEMFC) model that covers the multi-physics transients involved in the flows of the electric current, hydrogen and other gases, and the enthalpy. The modeling makes use of given analogies to allow for a uniform representation of electric, pneumatic, and thermal transients based on electric circuit equivalents. It is shown how these models representing different transients are coupled as part of a PEM fuel cell system. Validation and application are carried out to verify the value of the modeling. The investigations are performed with an electric network simulator of type EMTP (Electromagnetic Transients Program) and so substantiate the possibility of using the latter for studying multi-energy systems. [ABSTRACT FROM AUTHOR]

• Sparse matrices are introduced in numerical routines applied to simulate the propagation of electromagnetic transient phenomena. • The main interest is to decrease the computational time and memory necessary for carrying out the simulations. • If damping resistances are applied together the sparse matrix techniques, the computational time and memory are decreased and results are not significatively influenced by the numerical oscillations. Modified numerical integration methods and modelling have been analysed for application to simulations of electromagnetic transient phenomena in transmission lines. Both objectives are considered: minimize numerical errors caused by Gibbs' oscillations and decrease the computational memory necessary for the mentioned simulations. For this, transmission lines have represented by cascades of π circuits. Based on this representation, modifications in numerical integration methods applied to the mentioned simulations can be introduced easily and the comparisons can be carried out quickly. In case of minimization of numerical errors, two approaches have checked. One of these options is related to the way how the numerical integration method is applied. The numerical integration has applied using 2-order matrices. These matrices are related to each π circuit applied to represent transmission lines. The other option is related to the damping resistance application. It has made by introducing damping resistances in the longitudinal structure of the π circuits. Sparse matrices have applied for decreasing the computational memory used during the simulations, because this matrix has a great quantity of null elements when the cascade of π circuits are represented by only one matrix with great order. Comparisons have carried out considering the non-modified numerical methods and the three modified methods suggested in this paper. [ABSTRACT FROM AUTHOR]