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Accurate knowledge of the admittance matrix of distribution grids is essential for grid operation and control as more renewable energy sources and electric vehicles are integrated into distribution grids. Due to limited observability, measurement noise, topology assumptions, and partial data availability, the exact calculation of the admittance matrix of distribution grids is challenging. In this paper, we present a method for estimating the admittance matrix of radial distribution grids. The proposed method uses low-cost measurement devices that record ten-minute-based measurements of voltage magnitudes, active power, and reactive power at a single end of the lines or transformers instead of second-based phasor measurements. The impedance of lines or transformers is then estimated using a statistical optimization subject to the distribution flow (DistFlow) model. The effectiveness of proposed method is demonstrated by simulation, laboratory testing, and field measurements taken from a distribution grid in Switzerland. • A novel method for estimating grid admittance has been introduced. • Low-cost measurements at ten-minute intervals have been utilized. • A statistical optimization technique using DistFlow model has been employed. • Validation through simulation and field experiments has been conducted. [ABSTRACT FROM AUTHOR]

Accurate transformer-customer relationship is critical for better operation and management of low-voltage distribution system. It is of high cost to establish and check the profiles of transformer-customer relationship by manual maintenance, which is often mistakenly archived too. This paper proposes a novel automatic identification method based on a deep Gaussian mixture model with voltage data. The improved model is built with feature extraction network, clustering network and splitting and merging network, which can adjust the number of transformers adaptively and dynamically. It can also identify outliers of the data and effectively solve the uncertainty and imbalance of samples. Finally, the proposed method is tested in a practical power supply district which has 2752 customers with 9 transformers. Five different test cases are set to verify its effectiveness. The method can dynamically adjust to the actual number of transformer under different initial clustering numbers. The experimental results demonstrate that our model achieves an accuracy of up to 98.8% in profiling unlabeled users, while achieving an error correction accuracy of 99% in the case of imbalanced users in the transformer area. To further validate the superiority of our model, we conducted robustness experiments under different user scales and data loss scenarios. The results consistently outperform current state-of-the-art algorithms. • An innovative deep Gaussian mixture model is proposed. • The model consists of feature extraction, clustering, splitting and merging network. • The unlabeled substations can be automatically profiled and verified by our model. • The high accuracy transformer-customer relationship identification is achieved. • The problem of sample imbalance can be solved by our model. [ABSTRACT FROM AUTHOR]

• Using the high-frequency principle to set fault direction identifiers (FDIs), the proposed fault location scheme mitigates the uncertain fault behavior exhibited by DFIG-based wind DGs. • The proposed fault location scheme relies solely on local current information and does not necessitate strict synchronization conditions. • The multi-agent system(MAS)-assisted relaying structure facilitates flexible and orderly fault location procedures. This paper proposes a multi-agent system (MAS)-assisted relaying scheme to identify the fault section in a distribution network (DN) that incorporates doubly-fed induction generators (DFIG)-based wind distributed generations (DGs). Firstly, to mitigate the adverse effect of the uncertain fault behavior of DFIGs on conventional directional elements, the fault direction identifiers (FDIs) are assigned at the I-type and II-type buses in the DN using the high-frequency component principle. Furthermore, a MAS-assisted relaying structure is devised to collect and integrate these identifiers and then accomplish the fault section location. On the one hand, since the setting of FDIs only requires local current information and the logical signals are solely exchanged within each predetermined associated region, the proposed fault section location scheme can be implemented in an immature DN with a limited number of voltage transformers or under weak synchronization conditions. On the other hand, the autonomy and cooperation of intelligent Agents also enable the proposed scheme to determine the fault section in a decentralized decision-making (DDM) mode, eliminating the dependence on strong central control and reducing redundant data transmission in a multi-bus distribution system. Simulation results under representative faulty scenarios validate the expected performance of the protection scheme. [ABSTRACT FROM AUTHOR]

• In this paper, the mechanism and the main suppression measures of ferroresonance were described. • A typical substation was taken as the research object, and the simulation models were established by using the electromagnetic transient analysis software ATP-EMTP. • Two measures to suppress ferromagnetic resonance are discussed, which can provide reference for the power sector to control ferromagnetic resonance. • When the neutral point of voltage transformer is grounded by resonance eliminator or the arc suppression coil, the ferroresonance of the system can be effectively suppressed. In order to monitor the three-phase voltage of the busbar to ground, the substation bus is usually connected to the electromagnetic voltage transformer by YN connection in the distribution network. Under certain conditions, the excitation characteristics are easy to saturate, and the inductance parameters match the system capacitance, excite ferromagnetic resonance, cause damage to fuses or voltage transformers, and seriously affect the safe and stable operation of the power system.By analyzing typical single-phase ground faults, this paper introduces the generation mechanism of ferromagnetic resonance and the main suppression measures at this stage. The electromagnetic transient analysis software ATP-EMTP is used to analyze the operation of a typical transformer, and the change of ferromagnetic resonance waveform after the neutral point of the voltage transformer is replaced by resonant eliminator and arc suppression coil grounding. The results show that when eliminating the single-phase ground fault, it is easy to cause the frequency division resonance of the system, and the use of these two measures can effectively suppress the ferromagnetic resonance of the system. [ABSTRACT FROM AUTHOR]

• Propose a fault analysis method of valve-side winding fault of converter transformer. • Fault analysis by classified time-period is closer to LCC-HVDC system actual situation. • Study the fault characteristics of the valve-side winding fault. • Point out the fault characteristics differences with ordinary transformer. This paper studies the fault analysis method and fault characteristics of valve-side winding fault of converter transformer in line-commutated-converter high-voltage direct-current (LCC-HVDC) system. The time periods are divided according to the valve conduction state within one power frequency cycle, and these time periods are classified based on the common features. For each type of time period, the flow path of the fault current is analyzed, the circuit equation is established, and the current on both sides of the transformer is solved. By combining the various time periods together, the expressions of the currents on both sides of converter transformer in one power frequency cycle after fault are obtained. The fault characteristics of the valve-side winding fault are studied based on the expressions, and the differences between them and ordinary transformer are pointed out. The research results show that when a single-phase grounding fault occurs in the valve-side winding of the low-voltage (LV) bridge converter transformer in LCC-HVDC system, both the grid-side current and the valve-side measured current contain a lot of harmonics, which will affect the operating characteristics of differential protection. Finally, a simulation model is built using PSCAD to verify the fault characteristics obtained from analysis. These works provide a theoretical basis for analyzing the action characteristics of converter transformer protection when the valve-side winding fault occurs and studying the new protection principle. © 2017 Elsevier Inc. All rights reserved. [ABSTRACT FROM AUTHOR]

• Lightning impulse circuit is accurately simulated. • Both full and chopped wave impulses are simulated in time domain. • The admittance model of transformer is fitted with measured one using vector fitting technique. • State space is employed to find lightning impulse waveforms. Performing insulation tests on power transformers is one of the most essential topics in high-voltage laboratories. According to international standards like IEC 60,076, full and chopped wave Lightning Impulse (LI) voltages must be applied to high voltage transformers. Since the wave shape parameters of the applied impulses must comply with the constraints that are specified in these standards, the setting of the impulse generator and test circuit must be modified correctly by test engineers. An accurate computerized simulation method is very desirable to help them accomplish this duty quickly and enhance the outcome results. Therefore, this paper proposes a method that is capable of simulating both Full Wave (FW) and Chopped Wave (CW) impulses in the time domain accurately. In this method, the frequency response of the transformer is measured and then the Vector Fitting (VF) technique is used to obtain its admittance model. Additionally, the transformer's impulse test circuit elements are precisely modelled. Afterwards, by solving the equations of the equivalent circuit in the state-space system, the time domain signals of all state variables are calculated. Finally, the output impulse voltages are computed. Several practical tests are accomplished on two power transformers to ensure the correctness of the proposed method. [ABSTRACT FROM AUTHOR]

Ferroresonance in single-phase, line-to-line connected transformers in an ungrounded distribution system with delta-connected capacitors is possible but has not been reported in recent literature. In this paper, a high voltage event that actually occurred in an ungrounded distribution network with multiple distribution transformers has been simulated for lessons learned. The hypothesis was that ferroresonance was the cause of the overvoltage event in the network after a single-phase event led to sustained voltages of 1.45 p.u. Simulations were performed and ferroresonance was found to be the possible cause for the overvoltages. One of the prevention solutions found to avoid the overvoltages was to balance the loads on the three phases. The increased deployment of CVR/VVO strategies leads to circuit configurations similar to that studied in this paper and could lead to an increased likelihood of ferroresonance, if it is not fully understood and addressed. The impact of additional single-phase solar inverters on the low voltage side of the transformers was also studied. The results obtained show that the effective loading of the transformers, which is the difference in the actual load connected and the output from the DER, proved to be the deciding factor for initiation of ferroresonance in such networks. • Overvoltage event observed on a utility's ungrounded distribution system. • Ferroresonance was the cause based on electromagnetic transient simulations. • Distributed Energy Resources (DER) added to study impact on ferroresonance. • With DER connected, overvoltage observed at higher values of transformer loading. • Phase balancing and capacitor bank switching avoided overvoltage event. [ABSTRACT FROM AUTHOR]

• Risk of voltage escalation in ungrounded MEV networks. • Electromagnetic transient calculations. • Voltage transformers installed with permanent insulation monitors prevent voltage escalation. In an ungrounded electrical system, successive reignitions and extinctions of a single-phase to ground fault is known to potentially generate a voltage escalation phenomenon. This is the so-called arcing ground fault whose theory has been thoroughly described in literature, even though very few practical occurrences have been experienced in existing electrical systems due to the specific and rare conditions that cause its appearance. This paper studies the risk of appearance of voltage escalation in the 10 kV auxiliary system of an industrial plant. It describes the physical principles at the origin of voltage escalation and presents a study conducted using an EMT-like program. The results show that, thanks to the presence of a permanent insulation monitor (PIM) connected to the system through voltage transformers, the successive reignitions and extinctions of single-phase faults cannot cause severe overvoltages or voltage escalation. The paper ends with some general conclusions. [ABSTRACT FROM AUTHOR]

• Calculation of the transmission line impedance to the fault location based on the conic section general equation and the solution of a homogeneous linear system. • Proposes a new time-domain distance protection algorithm based on the Lissajous figure of the measured voltage and current signals. • Three-phase implementation of the novel method by using a fractional time delay filter for the zero-sequence current compensation. • The proposed algorithm calculates the impedance accurately and presents faster stabilization and pickup times when compared to the conventional DFT based distance protection. • The proposed method has a good performance even during adverse conditions, such as the presence of CVT, CT saturation, power swing, and close-in faults. This paper presents the development, implementation, and testing of a time-domain distance protection algorithm based on the behavior of voltage and current signals over time and on the conic section general equation that describes this behavior. The authors used six existing electrical systems modeled in ATP/EMTP to perform fault simulations varying the fault type, phases involved, fault resistance, inception angle, and fault location. Besides, the authors carried out a sensitivity analysis regarding the sampling frequency influence, noisy data, dynamic behavior of capacitor voltage transformers, power swings, and close-in faults. Finally, the authors tested the proposed method against an actual fault event on a 500 kV Brazilian transmission line. The results presented in this paper indicate that the proposed solution is robust, fast, accurate, secure, and dependable. [ABSTRACT FROM AUTHOR]

• Capacitive voltage transformers are not suitable for measuring fast transients. • Capacitive electric field sensors are broadband solutions for transient measurement. • Installation of capacitive electric field sensors does not require de-energization. This paper presents a case study of measurements of switching transient overvoltages using a capacitive electric field sensor. The system was developed and installed in a substation after the failure of a surge arrester during the energization of a 420 kV transmission line. First, the closing operation was simulated in an electromagnetic transient simulation program to estimate the switching transient overvoltages for this event. Then, the concept and design of the measurement system are explained. The system calibration performed in the high-voltage laboratory is presented. Afterward, the installation of the measurement system in the substation is discussed, with a special focus on the calibration of the voltage ratio considering the physical configuration where it is installed. Finally, the measurement results are presented and compared with simulations. The results show that the transient overvoltages caused by the energization of the transmission line should not cause the failure of the surge arrester. The measurement system has been demonstrated to be capable of measuring switching transient voltages of a 420 kV transmission line and can be considered a flexible solution for the long-term monitoring of transient overvoltages in substations. [ABSTRACT FROM AUTHOR]

• Measuring CVT's frequency response in equivalent circuit mode improves accuracy. • Commercial FRA equipment are not suitable for low-frequency modeling of CVT. • Modeling nonlinear components is essential for an accurate CVT model. This paper presents the implementation of a nonlinear model of a Capacitive Voltage Transformer (CVT) to evaluate the transient response in conformity with the international standard IEC 61869-5. The work is divided into three main steps: electrical tests to measure the CVT parameters and frequency response; implementation of linear mathematical modeling and an optimization algorithm to estimate unmeasured parameters; and implementation of nonlinear modeling in a simulation program for electromagnetic transients. First, the requirements of the IEC standard about the transient response of CVT are presented. Then, some important aspects of CVT modeling are discussed. Afterward, a linear model of a CVT is used to estimate the parameter not measured in the laboratory. Then, a nonlinear model is implemented in ATP. The model is validated in the time domain by comparing the simulation results with some signals measured in the laboratory. Finally, simulations of the transient response tests are made following the IEC 61869-5. The results show that the model developed in this work is reliable and can be used for the simulation of ferroresonance and transient response tests following the IEC 61869-5 when no laboratory structure is available. [ABSTRACT FROM AUTHOR]

• An improved framework with a deep network for online measurement of CVT errors based on GRU and MTL is proposed. • The GRU-MTL architecture fully mines the coupling relationship between the six error data and enhances the predictive capability. • Three adaptive improvements are made to the GRU-MTL architecture to improve the training effect and prediction accuracy. • The improved GRU-MTL model outperforms other models and has a higher measurement accuracy of CVT error. The metering accuracy of capacitive voltage transformers (CVTs) affects the fairness of electricity settlement, and the online measurement of their metering errors has become a research hotspot. This error consists of amplitude and phase errors of three-phase voltages. In this paper, because the error data is characterized by time series, nonlinearity, and large volumes, the Gate recurrent unit is selected as a deep learning framework to predict it, the Batch normalization is proposed to prevent gradient disappearance, and the self-attentive mechanism is proposed to help learn long-time information. To measure all six errors simultaneously, multitask learning is employed to mine the shared information and coupling among multiple tasks, and the loss function is reconstructed to ensure simultaneous convergence of the six tasks. Experimental results show that, compared with the traditional single-task model, the accuracy indexes MSE, RMSE, and MAE of the proposed model are reduced by 95.1%, 81.1%, and 89.5% on average for all tasks, and the total training time is reduced by 57.8%; compared with other multitask models, the accuracy indexes are improved by orders of magnitude. This shows that the proposed model fully meets the requirements of both accuracy and speed for online measurement of CVT error. [ABSTRACT FROM AUTHOR]

The algorithms for the correction of transients in coupling capacitor voltage transformers (CCVTs) are generally designed from processing samples in the time domain. Therefore, they need to be embedded in the measurement, protection, and control devices, because in these instruments only the phasors are available for the development of dedicated applications. In this paper, as an extension of a method originally developed by these authors for the time domain correction of CCVT secondary voltage samples, a mathematical formulation is developed that demonstrates the applicability of the existing method for phasor compensation, regardless of the phasor estimation algorithm used. Three phasor estimation algorithms and two CCVTs with different topologies and voltage levels are used to evaluate the method's performance. ATP (Alternative Transients Program) was used to generate the oscillographic records, while the phasor estimation algorithms and their corrections were implemented in MatLab. From the results, it can be inferred that the evaluated method can be used to correct voltage phasor disturbances, providing more suitable signals for protection algorithms. • Correction of subsidence transient present in the CCVTs voltage phasor. • Advancements for reductions in fault detection time. • Improvements for high-performance phasor-based distance protection. [ABSTRACT FROM AUTHOR]

• LF signal injection for faulty feeder detection in unearthed MV network is possible. • LF signal is injected through primary windings of group of three IVTs. • It is possible to influence the IVT design to make it suitable for signal injection. • The capacitance to ground topology influences the signal distribution the most. Unearthed neutral is often used in industrial networks, which require continuous power supply, and in distribution networks where the network capacitive current is not too high; or it is uneconomical to move to compensated grounding. Unearthed networks can remain in operation during an earth-fault, but fast determination of the faulty line is the key for prevention of further fault escalation. Signal injection is one of the fault location methods often used in LV unearthed networks. The possibility of application of this method in MV networks is dependent on how to inject the signal into unearthed phases with voltages ranging from 10 to 35 kV. This paper presents the signal injection circuit, which consist of three inductive voltage transformers (IVTs). The specifics of IVT design for signal injection are discussed. The application of the LF signal injection during an earth-fault was simulated on the model of an unearthed distribution network in EMTP. An intermittent fault and ring-type network connection were considered as possible cases. [ABSTRACT FROM AUTHOR]

• A cost-efficient centralized topology integrated with distribution system is proposed to compensate the voltage drops. • Mathematical modelling is formulated to reveal the compensation mechanism, required compensation rating, and dc voltage. • A more generalized and practical power loss model based on scaling laws is formulated to analyze the loss reduction mechanism. • A comprehensive control scheme is developed to achieve the voltage quality improvement. Recently, an intelligent substation by the L-coupling static synchronous compensator integrated with a primary tapped transformer (L-STATCOM PTT) has been developed to perform multifunction operations, including stable power supply and desired power quality. While previous works mainly focus on current quality, the cost-effective compensation of voltage quality is still not involved. This paper firstly formulated the mathematical model of voltage compensation to reveal the compensation mechanism, and determine the required inverter rating and reasonable dc voltage for acceptable performance. Then its power rating reduction under different voltage drops has been mathematically revealed compared with conventional STATCOM. Furthermore, by considering the potential differences in IGBT switching time and collector-emitter on-state resistance, a more generalized and practical power loss model based on scaling laws is formulated. The proposed voltage compensation method can reduce compensator rating by 28.1% and power loss by 12.2% at the 10% voltage drop. Simulation and experiments are performed to prove the validity and effectiveness of the proposed topology over cost-effectiveness and load voltage support. [ABSTRACT FROM AUTHOR]

• Proposing a new non-pilot solution to speed up the second zone protection of distance relays. • Detecting the remote circuit breaker operation (RCBO) using the healthy phases data. • Developed method for both single- and three-pole circuit breaker (CB) mechanisms. • Investigating the proposed algorithm with various simulations besides some experimental tests. • Proposing an immune scheme to the CT saturation and CVT transients. Nowadays, power systems are operated closer to their stability margins and therefore, the need for faster protection algorithms is escalated. The second zone of distance protection is conventionally set to operate with some lengthy delay, which increases the fault clearing time for the end part of the protected line. Therefore, the acceleration of zone-2 for internal faults can remarkably increase the speed of line protection. This practice is traditionally performed by pilot protection systems, which rely on communication channels. This paper proposes a new non-pilot solution to this challenge using only local measurements. A phasor-based algorithm is proposed to detect the remote circuit breaker operation (RCBO) using the healthy phases data. The proposed method has been developed for both single- and three-pole circuit breaker (CB) mechanisms, which makes it practical for transmission and sub-transmission lines. Thanks to using the healthy phases data, the proposed method is immune to the current transformer saturation and capacitor voltage transformer transients. The performance of the proposed method is simulated and investigated in various critical situations besides some experimental tests. The obtained results approve the efficiency and robustness of the proposed method for different scenarios, e.g. noisy conditions, evolving faults, and parallel lines. [ABSTRACT FROM AUTHOR]

• A transformer-customer relationship identification framework according the data analysis of end-customers measurement data is proposed. • A novel joint optimization model for transformer-customer relationship identification is proposed. • This model is adapted to a variety of scenarios, overcoming the shortcomings of other methods and thus improving identification accuracy. • The effectiveness and universality of this model are verified using actual engineering data and simulation data. Transformer-customer relationship refers to the unique electrical connection relationship between end-customers and their corresponding transformers. Accurate identification of transformer-customer relationship is the foundation for operation and management in low-voltage distribution networks, which may be mistakenly archived due to manual maintenance. In this regards, this paper proposes a joint optimization model based on voltage silhouette coefficient and power loss coefficient to realize the identification of transformer-customer relationship. Specifically, the adjacency matrix is introduced with binary variables to represent whether or not the customer is connected to the corresponding transformer. The customer clustering and evaluation functions are proposed according their voltage correlations to build up the voltage silhouette coefficient objective. The power loss coefficient function is then proposed considering the power balance between transformer and total customers to obtain the minimized power loss coefficient objective. Then, the joint optimization model is proposed to update the adjacency matrix until the accurate transformer-customer relationship is found. Finally, the test results of the practical low-voltage distribution network verify the effectiveness and universality of the model, and prove that the model can improve the identification accuracy in the case of unsatisfactory data. [ABSTRACT FROM AUTHOR]

• Phase-resolved partial discharge patterns model and the Weibull distribution. • Detection of internal partial discharges. • Prototype Rogowski coils. • RC integrator module for partial discharge current evaluation. • Shunt detection and Rogowski detection. This paper deals with the influence of the characteristics of a Rogowski coil compared to a shunt sensor in the detection of partial discharges (PDs) in insulating oils. An experimental investigation is conducted on the activity of PDs and their inception voltage in mineral oil (MO) and palm kernel oil methyl ester (PKOME). This is done by using the analysis of the phase-resolved partial discharge patterns model and the Weibull distribution. The experimental set-up consists of a 220 V/50 kV high voltage transformer, a control box and a test cell containing four electrode configurations. The tests are conducted according to the IEC 61,294 for the determination of partial discharge inception voltage. The experimental results show that the characteristics of a Rogowski coil have a considerable effect on the waveform and the number of occurrences of detected discharges. The number of occurrences of discharges detected with the shunt sensor is reduced by almost 50% compared to those of a Rogowski sensor. The parameters obtained from the Weibull distribution show that PKOME has a higher resistance to discharge inception than MO. [ABSTRACT FROM AUTHOR]

Harmonics pose significant challenges to the operation of power systems, and active filters are a key technology to mitigate impacts. When there are multiple active filters in a network, they can be coordinated to achieve specific goals, e.g., provide maximal compensation at the point of common coupling. Mathematical optimization of power systems with representation of harmonics, i.e., harmonic optimal power flow, is a framework that develops solutions to such decision problems. This paper presents a formulation for harmonic optimal power flow in the rectangular current–voltage variable space, and solves it as a continuous nonlinear optimization problem. A key novelty of this work is the detailed treatment of transformers, and transformer harmonics due to saturation. Normally, transformer excitation is modeled in the time domain, so the inclusion of these effects in frequency-domain optimization models poses a nontrivial challenge, which is addressed here through a conversion to empirical multivariate nonlinear continuous functions (splines). The model is demonstrated numerically on a real-world test case. • Next to loads, transformers are a source of harmonics in power networks. • Active filters are a key technology to manage harmonics in industrial networks. • Coordinating active filters across voltage levels is a nontrivial decision problem. • We develop a nonlinear ac harmonic optimal power flow model and case study. [ABSTRACT FROM AUTHOR]

In this paper, an algorithm for calculating the asymmetric current distribution in parallel-connected high-voltage winding sections of a transformer during a short-circuit test caused by an asymmetric magnetic field is described. This calculation is essential in multi-winding transformers, such as traction transformers and rectifier transformers, which have a high-voltage winding consisting of sections connected in parallel, where the number of sections is equal to the number of low-voltage windings and only one low-voltage winding is shorted during the short-circuit test. An optimization method, combined with a semi-analytical transformer model based on conformal mapping, is used to calculate the magnetic field and find a combination of currents in parallel-connected sections assuming equal induced voltages, i.e. flux linkages, in each section. The results of the calculation show good agreement with measurements carried out on the traction transformer for an electric multiple unit. • To calculate the short-circuit voltage, the currents in parallel sections are needed. • The proposed method can be applied to a 2D and quasi 3D model of the transformer. • The calculations of the magnetic field are based on conformal mappings. • FEM is the only other method that can be used to calculate current distribution. • The proposed method is accurate, reliable and a good alternative to FEM. [ABSTRACT FROM AUTHOR]

• A frequency-domain algorithm for very fast transient over-voltage (VFTO) and its conducted disturbance via voltage transformer (VT) is proposed, which calculates the whole process in frequency domain with VFTO frequency spectrum and the VT transfer function for gas-insulated switchgear (GIS) substations. • The proposed algorithm has been applied in the simulation of VFTO and its disturbance in the 1000 kV Wuhu substation, of which the calculated and measured VFTO waveforms are in good agreement for verification. • The transfer functions of the two types 1000 kV GIS VT are obtained by the impulse test method, and the conducted disturbances are also calculated with their severity and suppression method analyzed. Simulation of very fast transient over-voltage (VFTO) and its conducted disturbance via voltage transformer (VT) is significant in guiding the insulation design and electromagnetic compatibility of gas-insulated switchgear (GIS) substations. However, the commonly used time-domain algorithms are difficult to deal with the complex frequency-dependent parameters in the VFTO simulation, and might introduce instability of conducted disturbance due to an active circuit synthesis of the VT in some cases. In this paper, a frequency-domain algorithm is proposed, which calculates the VFTO in the frequency domain and then directly uses the VFTO frequency spectrum and the VT transfer function to calculate the conducted disturbance without circuit synthesis. The proposed algorithm has been applied in the simulation of VFTO and its disturbance in the 1000 kV Wuhu substation. The calculated and measured VFTO waveforms are in good agreement, verifying the accuracy of the algorithm. The transfer functions of the 1000 kV GIS VT are obtained by the impulse test method, and the conducted disturbances are also calculated with their severity and suppression method analyzed. The proposed frequency-domain algorithm covers the whole calculation process from VFTO to its conducted disturbance and has good engineering application potential in the simulation, with better convenience and stability. [ABSTRACT FROM AUTHOR]

• A multi-timescale transient branch companion model for electrical elements with strong nonlinearity is proposed. • Simulation efficiency of nonlinear elements is improved based on the proposed shifted equivalent phasor (SEP) model. • Instantaneous and wide frequency-band envelope waveforms can be tracked within one simulation study. • There are no overshooting and oscillating phenomena even large time step is adopted based on the proposed flux SEP model. This paper proposes a multi-timescale transient branch companion model for electrical elements with strong nonlinearity, which is suitable for electromagnetic transient simulation for a wide range of time-scales. A shifted equivalent phasor (SEP) method for signals with wide frequency range is adopted combined with envelope signals and equivalent phasor-shift operator, and a saturable SEP based transformer model is established. A method of piecewise linearization based on Newton-Raphson is used to simulate excitation current under different simulation time step sizes in order to illustrate the overshoot phenomenon of piecewise linearization under large step sizes. Reducing the step size can effectively suppress the overshoot, but simulation efficiency will be sacrificed. This problem can be avoided in SEP model even if large time step is adopted and integrated simulation of both instantaneous and envelope (with larger timescale) waveforms can be efficiently tracked within one simulation run based on the determination of two parameters setting: shifting equivalent phasor and time step size. Finally, the transformer model and voltage source converter (VSC) are established and validated in MATLAB and PSCAD to prove its correctness and efficiency. [ABSTRACT FROM AUTHOR]

• Combined internal overvoltage occurs as a result of the disturbances and failures development in the distribution electrical grids. • In power supply schemes for remote low-power consumers on long-distance lines, quasi-stationary voltage increases are possible due to the capacitive effect of long lines. • Quasi-stationary overvoltages can pose a danger to all substation equipment insulation, including power transformers, voltage transformers, non-linear surge arresters, etc. • Long-term currents exceeding the rated currents of power transformers can lead to power transformers thermal damage or the appearance of insulation defects. • In the event of a violation of the longitudinal symmetry of phase voltages, priority switching is not switching off the load, but disconnecting the network element on which the combined violation occurred. One of the main conditions for the non-development of quasi-stationary overvoltages is the operational work of personnel and the well-established operation of the relay protection. For consumers more than 100km away, this is not always possible to provide. This paper presents simulation-based studies on the 110 kV substations located in the Euro-Arctic region of Russia. Longitudinal and transversal disturbances in the high voltage grid are studied. The influence of the harsh climatic conditions on the rate of occurrence of accidents is shown. The 110-kV section of the Kola power system was considered and the features of this section were analyzed. The simulation model of the investigated section was created in the ATP-EMTP software and a wire break of the overhead transmission line followed by a single phase-to-ground faults of the same phase was simulated. The influence of combined overvoltage on substations electrical equipment is considered. The simulation results show that overvoltages in power grids with one-way power supply can pose a significant threat to insulation of any substation equipment. [ABSTRACT FROM AUTHOR]

• An accurate and simple model for distribution transformers is introduced based on experimental measurements. • The study is carried out using two distribution transformers, 100 kVA and 500 kVA. • System identification of the impedance parameter is done. • The proposed model is experimentally evaluated and verified under unloaded and loaded conditions. • The proposed model achieves simple implementation and accurate results for unloaded and loaded transformers as well as under MVSG operation. High frequency transformer model represents a mandatory requirement to study transient performance of distribution systems under lightning strokes. In Finland, the distribution systems are frequently subjected to these strokes. In this paper, an accurate and simple model is introduced for the distribution transformers concerning two-port network implemented using transfer functions. Compromising between simplicity and accuracy facilitates the high frequency modeling of transformer for complicated networks. The study is done using two distribution transformers used in Finland, DYN5 (100 kVA) and DYN11 (500 kVA). The proposed model parameters of the two-port network are experimentally extracted for each transformer under the impulse tests. System identification of the impedance parameter is done using the denoised experimental signals for primary and secondary voltages and currents. The model is verified using experimental results under unloaded and loaded conditions. The evaluation is accomplished concerning time domain and frequency responses. Also, the proposed model is compared with published accurate model for loaded, unloaded, and medium voltage spark-gap (MVSG) operation of the first transformer, 100 kVA. The proposed model achieves simple implementation and accurate results for unloaded and loaded transformers as well as under MVSG operation. The simulated results are carried out using the MATLAB and ATPDraw programs. [ABSTRACT FROM AUTHOR]

• Transient voltages with fast rise time are critical to transformer insulation. • The impedance frequency response gives transformer resonance signature. • Highest transient voltage is in transformer's initial tap under resonance region. • Impact of distribution of transient voltages on transformer insulation. A transformer is one of the most important assets in an electric power generation system. Wind turbine step-up transformers have recently seen premature failures due to high-frequency repetitive transients. The high-frequency transients under resonance can severely damage the insulation of the transformer. Thus the paper initially analyses the distribution of transformer frequency response to identify critical resonance frequencies for the two transfer function frequency responses; namely, impedance frequency response and voltage ratio response. Transformer frequency response is compared with transient response to understand the influence of repetitive transient parameters; namely applied voltage amplitude, rise time, and duty cycle, on transient distribution along the transformer winding. Based on three variable experiments, the range of rise time was considered as less than 300 ns and 1000 ns, applied voltage considered was 100 V and 1000 V, and duty cycle considered was 10% and 50%. For the current transformer model, both the experimental and PSCAD simulation results indicated that a duty cycle of 50% and rise time < 300 ns were critically damaging to transformer insulation. The research helps in designing transformer insulation stressed under different transient parameters. [ABSTRACT FROM AUTHOR]

• CVT is divided into three subsystems including CVD, EMU and high voltage bushing. • 10-state, 9-state and 2-state Markov models for CVD, EMU and high voltage bushing, are presented, respectively. • A 10-state extensive Markov model is proposed for CVT by integrating the mentioned models. • The obtained results show the efficiency of the proposed Markov model. • Based on the sensitivity analysis, C2 and CR have a significant effect on the reliability of the CVT. Capacitor voltage transformer (CVT) is one of the most important instrument transformers widely used to prepare the voltage signal for control and protection equipment. The measuring accuracy of CVT plays an important role in the proper operation of the protection system. Therefore, maintaining the accuracy of CVT throughout its lifetime at the desired level is of great significance, and proper maintenance activities must be implemented regarding the equipment conditions. Evaluating the reliability of the CVT over its lifetime is necessary to determine the proper maintenance. Until now, no proper model has been suggested to evaluate the reliability of CVT. Therefore, this paper proposes a new Markov model to evaluate the reliability of CVT. To obtain the model, initially, a Markov model is presented for each subsystem of CVT including capacitor voltage divider (CVD), electromagnetic unit (EMU), and high voltage bushing. Then, by integrating these models, a 10-state extended Markov model is proposed for CVT. Finally, by combining similar states, a 3-state model including healthy, low-quality and failure states is obtained. The simulation results show that the second capacitive group in the CVD subsystem, and the compensating reactor in the EMU subsystem play a major role in the CVT performance. [ABSTRACT FROM AUTHOR]