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Inrush currents in electric power transformers are nonsinusoidal, high currents in magnitude that are generated due to saturation of flux in the transformer core during the energization. This paper proposes a decision technique for discrimination between inrush and internal faults currents in power transformers. The technique based on wave shapes recognition by criterion extracted of the instantaneous differential currents using the discrete wavelet transform based feature extraction technique. Adaptive Neuro-Fuzzy Inference System is used as a machine learning technique to classify inrush current and internal fault current of a power transformer. The criteria based on the calculation of energy contents in extracting coefficients D4 and D5 fed to ANFIS controller to give the decision. Simulation and lab tests have been implemented. The experimental results show the discrimination can be accurately made by quantifying the extracted features, also shows the technique is reliable, quite fast, and appropriate for different types of the power transformer. [ABSTRACT FROM AUTHOR]

A novel hybrid machine learning technique is proposed for the protection of three-phase power transformers in this study. Here, the developed model was tested across several types of current signal fault cases from different fault conditions and examined based on a laboratory-constructed transformer system, in which internal and external faults were created. The data gathered on signals were used to develop a novel hybrid model. A process for optimal feature identification was put forward, with machine learning classifiers being trained to classify faults. The methods used included orthogonal matching pursuit and discrete wavelet transform for extraction of statistical characteristics from unprocessed data. Following this, the bees algorithm (BA) was used to create an optimized subset, minimizing the amount of data needed and making the model more accurate. In order to distinguish normal operational conditions (inrush current) from faults, an optimized feature set was used as an input for three classification algorithms: the k-nearest neighbour, support vector machine, and artificial neural network. Training was conducted via k-fold cross-validation. Comparisons were made between the proposed approach and a comparable approach, which used the genetic algorithm (GA). This model was analysed based on specificity, accuracy, precision, recall, and F1 score. The findings from the experiment suggest that the model proposed here is suitable for fault identification in a range of conditions and faults.

Large induction motors can have a high inrush and run-up current during starting, often up to ten times the rated current. In weak supplies, this could be a problem, causing system stability issues and the voltage to dip below acceptable levels. In islanded systems, the capacity could be pulled below its maximum. There are several different starting methods possible, but they are often only suitable for smaller machines. One method not investigated is the use of parallel capacitor compensation during the starting because large induction motors are very inductive during the starting sequence, so that supplying reactive power may be more effective than supplying energy. This paper first investigates several different induction motors with increasing size and assesses their compensation requirements in terms of the reactive power requirement during starting. It is shown that they do generally become more inductive as they increase in size. It is illustrated, using simulations, that using parallel capacitors during starting can significantly reduce the starting current. The concept is tested using a small experimental motor. Series inductors are added to the motor to make it more inductive and look more like a larger machine. The experimental results confirm that the method works. It is also found that the parallel capacitors require series filters to prevent harmonic current, and these can be realized using inductors in series with the compensation capacitors. Point-on switching using solid-state relays was tested, and these can reduce the transient switch-on current over the first voltage cycle.

Two new methods, current change ratio (CCR) and percentage area difference (PAD) were proposed to solve a problem of how to distinguish between internal faults and inrush condition when transformer is switched on. This problem may delay operation or may mal-operate some protection schemes like deferential protection. The methods were concluded after observing and analyzing the behavior and shape of large number of both inrush and internal fault signals that had been obtained using a model transformer in a laboratory. The methods were practically tested on a three-phase transformer with rated power of 20 kVA at Cardiff University's laboratory and the data were processed using LabVIEW and MATLAB programs. The results showed that internal faults can be correctly distinguished from inrush condition within a short time (from 5 to 10 ms), particularly the minor internal faults such as the interturn fault which is submerged to inrush current and make it is too difficult to be detected. The advantages of these algorithms are simple in design and faster than the second harmonic method which is the most popular method used for solving this problem. [ABSTRACT FROM AUTHOR]

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The traditional pure switched‐capacitor equaliser brings the large inrush current and low energy density. This study proposes a series of resonant switched‐capacitor (ReSC) voltage equaliser, which realises energy transferred directly from source cells to target cells by the series ReSC converter. The ReSC converter eliminates the inrush current and improves the capacitors' energy density to increase the balancing speed. Meanwhile, all the switches are controlled by a pair of complementary PWM signals at a fixed operational frequency without voltage monitors. Both simulation and experiment are used to verify the system feasibility and theoretical analysis of the proposed circuit. In the same experimental condition, the proposed series resonant circuit reduces the inrush current. It improves the energy density of capacitors three times compared with the pure series switched‐capacitor equaliser. [ABSTRACT FROM AUTHOR]

This paper presents an improvement in an energizing strategy for a three-phase four-wire modular multilevel converter connected to an electric network as a static synchronous compensator. The new procedure comprises three stages. During the first stage, two thyristors control the current drained from the grid in order to charge the capacitors of the submodules of the positive and negative arms of two phases of the converter. After that, the energy stored in these capacitors is shared with the others submodule capacitors by firing the semiconductor switches of the converter in a strategic way. In addition of limiting the current drained from the grid during all the energizing procedure, the proposed strategy does not produce overvoltage at the converter dc terminals. The results of digital simulations are presented to validate the mathematical analysis and to demonstrate the performance of the proposed energizing strategy. [ABSTRACT FROM AUTHOR]

Energization of wind turbine transformers (WTTs) in wind power plants (WPPs) leads to high transient inrush current, which imposes high mechanical and thermal stresses on transformer windings and voltage sag problems at grid connection point (GCP). Therefore, to solve these problems, the inrush current should be suppressed during the energization time. The current research presents a modified transient current limiter (MTCL) for suppressing WTTs' inrush current. Two types of MTCL such as resistor type (R-type) and reactor type (L-type) are considered for this purpose. To investigate the efficiency of the MTCL, a WPP includes five wind turbines which are simulated by PSCAD/EMTDC software. The simulation results reveal that the MTCL is an effective means for suppressing the WTTs' inrush current and mitigating GCP voltage sag during energization. Besides, to show the effectiveness of the proposed MTCL, the performance of the MTCL is compared with the conventional TCL through simulation. [ABSTRACT FROM AUTHOR]

The article presents an extended computer model of a HTS (high temperature superconducting) 21 MVA 70/10.5 kV transformer with severalfold compound windings, based on a verified PSpice model of 10 kVA transformer with single windings. Two superconducting tapes: SCS12050 (with a copper stabiliser layer) and SF12050 (without a stabiliser) were used in the windings. The circuit model of the transformer developed in the PSpice environment considers Rhyner’s power law and the Jiles–Atherton description of the magnetic hysteresis loop. Two types of transient states were analysed: switching on a superconducting transformer to the network and an emergency short-circuit of the secondary winding. For the first five unidirectional current pulses generated when connecting an unloaded transformer to the network, the values of the pulse duration (γ angle), the temperature increase in the primary winding as well as the mean and effective values of the switch-on current were calculated. The waveforms of currents, voltages, changes in resistance and temperature during the operation short-circuit were compared for the windings with both types of HTS 2G (high temperature superconductor second generation) tapes. The conducted analysis allowed us to determine the influence of electrical and thermal parameters of the tapes in question on the effective limitation of current in transient states and the possibility of their use in the construction of superconducting transformer windings.

This paper describes the combination of discrete wavelet transforms (DWT) and artificial intelligence (AI), which are efficient techniques to identify the type of inrush current, analyze the origin and possible cause on the capacitor bank switching. The experiment setup used to verify the proposed techniques can be detected and classified the transient inrush current from normal capacitor rated current. The discrete wavelet transforms are used to detect and classify the inrush current. Then, output from wavelet is acted as input of fuzzy inference system for discriminating the type of switching transient inrush current. The proposed technique shows enhanced performance with a discrimination accuracy of 90.57%. Both simulation study and experimental results are quite satisfactory with providing the high accuracy and reliability which can be developed and implemented into a numerical overcurrent (50/51) and unbalanced current (60C) protection relay for an application of shunt capacitor bank protection in the future.

This paper proposes a power electronic module that uses a switched capacitor for retaining the integrity of the dc-link voltage of a variable speed drive (VSD) during a 0.2 s short-term power interruption (STPI). Ride-through was achieved through switched capacitor onto the dc bus. However, this technique presents a challenge of the high inrush currents during a ride through compensation. In this work both analytical and experimental investigations were conducted in order to reduce the in-rush currents and its impact on the performance of the VSD during the STPI. Inrush peak currents were reduced by approximately 90%. Experimental results showed torque pulsations of 12.8% and 14.3% at the start and end of dc-link voltage compensation, respectively. A method for sizing the switched capacitor and the inrush limiting resistors is proposed. This methodology is based on the use of readily available nameplate information of the VSD and the electric motor. The proposed module can be retrofitted to existing VSDs that are based on v/f control.

When the electromagnetic transients occur in a power transformer, an inrush current is generated in its winding. The inrush current not only affects the performance of the transformer windings, but also affects the lifetime of the transformer. Many factors affect the inrush current, the most influential ones among which are the closing phase angle and the residual flux. In this paper, a dry-type transformer simulation model is built to analyze the influence of the inrush current on the performance of transformer windings during no-load reclosing conditions. Firstly, the inrush current was generated in the transformer windings during the no-load reclosing operation under different residual fluxes. Secondly, the field-circuit coupling 3d finite element method is used to analyze the electromagnetic force at different locations of the transformer windings under the influence of different inrush currents. The results of winding structural parameter variations are obtained through electromagnetic-structural coupling simulation, and the electromagnetic forces are used as the input parameter for the structural analysis. Finally, the residual flux is generated by controlling the opening and closing angle of the transformer through the phase-controlled switch, and the winding electromechanical characteristics are tested under different residual fluxes. Finally, comparisons of the test and simulation results are drawn to verify the impact of the closing angle and residual flux on inrush current and the winding deformation during the no-load reclosing conditions.

This article investigates the effect of voltage sag propagation through the transformer due to various symmetrical and asymmetrical faults in a chronological manner. A unique and precise explanation has been presented for the propagation of voltage sag through different transformer connections. The effect of voltage sag has been studied in determining the healthiness of the transformer by analyzing the parameters of insulating oil obtained through dissolved gas analysis. For this purpose, different transformer winding connections have been considered, and the analysis has been performed considering the occurrence of all major faults. In order to justify and strengthen the proposal, a real-time case study has been deliberated for depicting the correlation of propagation of fault and its effect on transformer remaining in-service period through dissolved gas analysis. The study presented in this article will help utilities to understand the voltage sag propagation along with realizing its effect on transformer healthiness, thereby ensuring total system reliability and better economical service. [ABSTRACT FROM AUTHOR]

When a transformer is being energized, a large magnetic inrush current flows depending upon the closing phase angle of the circuit breaker and the residual magnetic flux in the iron core. One of the suppressing methods of the magnetic inrush current is to control the closing phase angle of the circuit breaker. Controlled switching has been used for no‐load transformer energization until now. However, the transformer may be switched under an on‐load state. In this case, a large magnetic inrush current will flow. To suppress the inrush current by using controlled switching, the information of the residual magnetic flux in the iron core is needed. The residual magnetic flux for no‐load transformers has been investigated. However, the residual magnetic flux for on‐load transformers was not investigated. In this paper, the residual magnetic flux for an on‐load transformer was investigated by experiments and calculations. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

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With the increase in power rating of electric vehicle chargers, concerns regarding their integration to the grid have likewise grown. This paper investigates a novel technique for start‐up of high‐power fast electric vehicle (EV) charger using a solid‐state inrush current limiter (SSICL). The circuit consists of three similar sets of bidirectional switch and limiting resistor that utilizes a control strategy based on a simple Kalman filter (KF) to suppress the converter start‐up current and transient over‐voltages. In fact, the built‐up profile of the inrush current is dictated in accordance with the estimated current by the slow dynamic KF. Compared to alternatives, such as start‐up resistors, the presented SSICL achieves faster start‐up with comparable peak current, tends to provide lower distortion, and provides a good balance between speed and efficiency. Simulation and experimental results confirm that the proposed method can considerably suppress inrush currents, while improving the total harmonic distortion. Comparing the behaviour of the SSICL to other relevant techniques underlines the advantages of the proposed solution. Furthermore, although the focus of this paper is start‐up of the fast EV chargers, the proposed circuit can act as an inrush‐current limiter in most AC to DC converter structures. [ABSTRACT FROM AUTHOR]

The three-stage modular solid-state transformer (SST) is a typical SST with a non-trivial start-up process. Specifically, special controls must be used to reduce the inrush current in the DC/DC stage during the start-up process of these SSTs. For the DC/DC stage based on the widely studied single-phase-shift dual active bridge (DAB), no inner-phase-shift control is required in its normal operations. In this case, using the existing inner-phase-shift control to reduce the inrush current increases the PWM channel requirements of microprocessors and the complexity of the control system. To address this problem, this paper proposes a long-dead-time control with easy implementation to reduce the inrush current without introducing additional PWM channel requirements. Fixed-dead-time and varied-dead-time controls are also proposed to reduce the inrush current when restarting three-stage modular SSTs with small cell capacitors without increasing the PWM channel requirements. The varied-dead-time control is faster than the fixed-dead-time one. The magnitude of the DAB current under these controls is calculated, and the overall time for fulfilling the proposed controls is estimated. The simulation and experiment on a 510 V/380 V 500 kW cascaded-full-bridge SST reveal that the proposed controls can effectively reduce the inrush currents during the start-up and restart processes of SSTs. [ABSTRACT FROM AUTHOR]

Copyright of DYNA - Ingeniería e Industria is the property of Publicaciones Dyna SL and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Switching-on a power transformer creates energizing inrush current (EIC) which in turn leads to sympathetic inrush current (SIC) in adjacent in-service transformers (ITs). Sympathetic inrush current changes the waveform and prolongs the duration of its origin EIC. This makes the analysis and estimation of EIC complicated. Occasionally, the stress of EIC can cause internal short circuit in energizing transformer. Detecting short-circuit fault current during EIC is a challenge for transformer differential protection. On the other hand, SIC may mislead the earth-fault protection of in-service-transformer. This paper, first, proposes a new dc equivalent-circuit based on the dc-component of inrush current, which is efficient in analyzing concurrent EIC and SIC. Next, based on this equivalent circuit, new predictive formulas for EIC and SIC are derived. By employing these formulas, the transformer internal fault during inrush current can be detected based on the comparison between the predicted and measured waveforms, which prevents differential protection from fail-to-trip. Also, the predicted SIC waveform can be compared with the measured waveform to prevent IT earth-fault protection from mal-trip, during inrush current. Finally, the proposed equivalent circuit and formulas are verified using the EMTP simulation results of a real grid, under different system conditions. [ABSTRACT FROM AUTHOR]

An unbalanced current protection relay (60C, 51NC) cannot operate fast enough to avoid catastrophic failure against high system fault currents within capacitor units because a traditional unbalanced current protection relay is not capable of classifying the transient fault current and switching inrush current. This article proposes a new algorithm for the detection and classification of these transient current signals. Simulations are performed using PSCAD/EMTDC programs. The outputs of the current signals are used as inputs for the discrete wavelet transform (DWT) process. The wavelet-based fault and inrush current detection and classification unit receive the captured transient current signals and use DWT to detect and analyze these signals. Actual inrush current signals from an experimental setup and shunt capacitor banks of 115 kV, 22 kV, and 6.6 kV are used to verify the proposed algorithm. The results show that the proposed algorithm provides good performance and accuracy in identifying faults and inrush currents. [ABSTRACT FROM AUTHOR]

Differential protection scheme is the main protection scheme of power transformers, which still holds the risk of sending false trips subject to inrush currents. This article aims to develop a differential protection scheme to discriminate power transformer magnetizing current from internal faults to decrease the risk of false trips. In this article, an accelerated convolutional neural network (CNN) based approach is designed for the discrimination between internal faults and inrush current. The main competitive advantage of the proposed algorithm is its capability in fusing the feature extraction and fault detection blocks into a single deep neural network block by enabling the network to discover important features automatically. The result of this point is that the algorithm is more efficient in terms of speed, hardware usage, and accuracy. The proposed method is applied to a simulated 230-kV network and an experimental prototype. Different cases with various external factors are simulated to calculate reliability indexes. The comparison between the accelerated CNN, conventional CNN, and nine widely used methods demonstrates the faster and more reliable performance of the proposed algorithm. [ABSTRACT FROM AUTHOR]

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The proposed method in the paper offers several key contributions. • Accurate detection of inrush current during the activation of crowbar and DC-link protection schemes in wind farms, specifically addressing the challenges of DFIG-based systems. • Effective differentiation between CT saturation and inrush currents, ensuring reliable detection in various operating conditions. • Utilization of harmonic phase signs (HPSs) for the 1st to 10th harmonics of the collector transformer (CTr) primary current, enabling real-time comparison with a predefined pattern for inrush current detection. • Distinguishing between inrush currents and internal faults of the CTr even during FRT of DFIGs, providing enhanced protection capabilities. As doubly-fed induction generator-based (DFIG-based) wind farms become increasingly widespread, the need for ongoing improvement in their protective studies becomes imperative. This paper focuses on the protection of the collector transformer (CTr), a key component of DFIG-based wind farms. One of the critical requirements for the differential protection of the CTr is the detection of inrush current. However, internal faults in the CTr can lead to harmonic interference and cause false detection of inrush current, resulting in the maloperation of the CTr differential protection. To address this issue, a new harmonic-based method for identifying inrush current in DFIG-based wind farms is introduced. The method is based on analyzing the phase signs of harmonics (HPSs) from the 1st to 10th harmonic. The proposed method ensures the accurate operation of the CTr differential protection and effectively distinguishes inrush currents from internal faults in the CTr. The effectiveness of the proposed method is validated through simulation and experimental results. The proposed method showcases several notable advantages, including precise detection of inrush current during the activation of crowbar and DC-link protection schemes, and effectively distinguishes between CT saturation and genuine inrush currents. [ABSTRACT FROM AUTHOR]

Under certain circumstances, after connecting a superconducting transformer to the power network, a high value current may flow through its windings. This current can exceed the critical value of the superconductor many times and cause the windings to lose their superconductive state. Loss of superconductive state of the windings may result in thermal interruption of their continuity as a result of conduction of a current of very high density. The mathematical relationships used to calculate the inrush current of conventional transformers do not work well for the calculation of superconducting transformers. This is due to the properties of superconducting materials used in the windings, first of all to the stepwise changes of the windings’ resistance when exiting the superconducting state and when returning to this state. The article presents the mathematical dependencies allowing to calculate the pulse waveforms of the inrush current of these transformers are derived. Basic electrical circuit sizes are used in the calculations, making the calculations quick and easy. Using the formulas, calculations of the inrush current of 8.5 kVA and 13.5 kVA superconducting transformers. The results were verified with the results of the inrush current measurements, achieving good compliance.

The main equipment responsible for connection and transmission of electric power from generating centers to consumers are power transformers. This type of equipment is subject to various types of faults that can affect its components, in some cases also compromising its operation and, consequently, the electric power supply. Thus, in this paper, electromagnetic, thermal, and structural analysis of power transformers was carried out with the objective of providing the operator with information on the ideal moment for performing predictive maintenance, avoiding unplanned shutdowns. For this, computational simulations were performed using the finite element method (FEM) and, from that, the different transformer operation ways, nominal currents, inrush current, and short-circuit current were analyzed. In this perspective, analyses of the effects that thermal expansion, axial forces, and radial forces exerted were carried out, contributing to possible defects in this type of equipment. As a study object, simulations were carried out on a 50 MVA single-phase transformer. It is important to emphasize that the simulations were validated with real data of measurements and with results presented in the current literature.

This work presents a study regarding the contribution of inrush current to the occurrence of mechanical fatigue in windings of a power transformer and, consequently, the impact of that phenomenon on the equipment's lifetime. In order to perform the study, electromagnetic transient simulations were used to determine the inrush current in a three-phase transformer model. Then, three-dimensional simulations, based on the finite element method, were performed to determine the electromechanical stresses in the equipment windings, under the effect of the inrush current. Next, the methods of critical plans and Palmgren–Miner were applied to perform the fatigue study. From the results, the impact of inrush current on the integrity of the transformer's windings was determined, as well as, to estimate the reduction of lifespan of the equipment due to inrush occurrence. The number of inrushes needed for the total reduction of lifespan of the equipment due to mechanical fatigue was also determined. The results can be used in the transformers design stage, with the purpose of assisting the evaluation of mechanical withstand of equipment, which can minimise the possibility of failure and increase its lifespan.