Inrush Phenomenon Research Articles

Practical Experiments with a Ready-Made Strategy for Energizing a Suitable Pre-Magnetized Three-Column Three-Phase Dy Transformer in Unloaded State for Inrush Current Computations

This article presents the results of an experimental verification of three-phase Dy transformer dynamics under no-load conditions. This study is motivated by previous ferroresonance analyses where the occurrence of inrush currents has been observed. The measurements covered all available electrical quantities in a transient state (12 measured and 3 additionally computed waveforms) during the device’s start-up under no-load conditions, as well as in a long-term steady state. A detailed analytical analysis is carried out for the obtained comprehensive set of measurement results. As a result of the conducted research, the mathematical model of the pre-magnetized three-phase Dy transformer is modified. Particular attention is paid to the issue of residual magnetism of the transformer core and its consideration in further research. The original strategy for energizing a three-column three-phase Dy transformer with a suitable pre-magnetization of its columns and original control switching system with a given/set value of the initial phase in the supply voltage is put to the test. The evolution of the induced inrush phenomenon up to the quasi-steady state under given (forced) conditions is documented (currents, voltages and the dynamics of changes taking place in the core (hysteresis loops)). This article represents a continuation of ongoing work on the study of transient states (dynamics of transformer inrush currents). At present, the Dy three-phase transformer is analyzed because of the requirements of industrial operators.

Varying the energisation condition to mitigate sympathetic inrush current

Transformers are generally easy to access and can contribute significantly to entire power system. When a transformer is turned on for the first time, it produces a magnetising inrush current which acts as a starting current. Energisation of transformer has a substantial impact on inrush current and transformer that are connected in parallel. Sympathetic inrush current is a phenomenon that appears when a transformer is switched-on in network whereas the other transformers that was earlier energised. Besides, when sympathetic inrush phenomena occur, the peak and period fluctuate significantly. In this paper, the transformers will be energised in three different ways and each condition will be explored in depth. The operation time of the transformer’s energisation whether it is energised simultaneously or at different times are tested and analysed in terms of their characteristics. It is performed using power system computer aided design (PSCAD) software, starting with a develop model of the energisation and then generate the outcomes. The results of the simulation demonstrate that energising the transformer in different ways can give different effect on the sympathetic inrush current, as well as the variables that affect it and methods for reducing it.

Application of Wavelet Packet and S Transforms for Differential Protection of Power Transformer

The differential protection of power transformers appears to be more difficult than any type of protection for any other part or element in a power system. Such difficulties arise from the existence of the magnetizing inrush phenomenon. Therefore, it is necessary to recognize between inrush current and the current arise from internal faults. In this paper, two approaches based on wavelet packet transform (WPT) and S-transform (ST) are applied to recognize different types of currents following in the transformer. In WPT approach, the selection of optimal mother wavelet and the optimal number of resolution is carried out using minimum description length (MDL) criteria before taking the decision for the extraction features from the WPT tree. In ST approach,the spectral energy index and the standard deviation (STD) are calculated from the S-matrix obtained by discrete S-transform. The two approaches are tested for generating a trip signal and disconnecting the transformer supply experimentally using 1KVA, 220/110V, 50Hz, ∆ / Y threephase transformer. The experimental results show that the trip signal is initiated faster in WPT approach while the transformer is disconnected from the supply after a delay of 10-15msec in thetwo approaches due to computer interface and the relay circuit used.

Dynamic simulation of the water inrush process in tunnel construction using a three-dimensional coupled discontinuous deformation analysis and smoothed particle hydrodynamics method

Water inrush, which is a major cause of geological disasters during tunnel excavation, can worsen construction conditions and result in casualties. It is thus important to study the water inrush process for mitigating and preventing water inrush disasters in tunnel construction. The tunnel water inrush problem involves solid–fluid interactions. The coupling of the parallel three-dimensional discontinuous deformation analysis (3-D DDA) and 3-D smoothed particle hydrodynamics (3-D SPH) is suitable for analyzing this problem. The critical condition of a block under water pressure was first investigated to check the validity of the 3-D coupled DDA-SPH method. Further, a tunnel water inrush phenomenon was reproduced, and the mechanical behaviors of tunnels were explained with respect to the thickness T of the rock wall, the initial water height H, and the presence and location of a water outlet. The simulation results show that the tunnel deformation increases as T decreases and as H increases. The tunnel deformation was found to be larger when there was a water outlet; however, the location of the outlet had a slight effect on the deformation.

A new high-frequency-based method for the very fast differential protection of power transformers

A novel algorithm based on Teager energy operator (TEO) and hidden Markov model (HMM) is presented for the transformer differential protection. After detecting any increase in the amplitude of the transformer differential currents, it is needed to know if the over-current is because of an internal fault or an inrush phenomenon. In this paper, a combination of TEO and HMM is used to discriminate between the magnetizing inrush currents and the internal fault currents. TEO is used to extract high frequency components of the signal and present them as a curve, named as Teager output curve (TOC). Then TOC is used as the input for HMM. First, the HMM training process is implemented by an adequate number of the train signals and then, the unknown signals are identified by the testing process. Fault type identification is also performed by the use of TOC. TOC has a separated region for each of the three phases. TEO of the zero-sequence signal has a peak for the grounded and no peak for the un-grounded faults. Train and test signals are generated by the simulated system in PSCAD/EMTDC. Results by means of the simulated and experimental data show the robustness of the proposed algorithm.

Discrimination of Transformer Inrush Currents and Internal Fault Currents Using Extended Kalman Filter Algorithm (EKF)

The discrimination of inrush currents and internal fault currents in transformers is an important feature of a transformer protection scheme. The harmonic current restrained feature is used in conventional differential relay protection of transformers. A literature survey shows that the discrimination between the inrush currents and internal fault currents is still an area that is open to research. In this paper, the classification of internal fault currents and magnetic inrush currents in the transformer is performed by using an extended Kalman filter (EKF) algorithm. When a transformer is energized under normal conditions, the EKF estimates the primary side winding current and, hence, the absolute residual signal (ARS) value is zero. The ARS value will not be equal to zero for internal fault and inrush phenomena conditions; hence, the EKF algorithm will be used for discriminating the internal faults and inrush faults by keeping the threshold level to the ARS value. The simulation results are compared with the theoretical analysis under various conditions. It is also observed that the detection time of internal faults decreases with the severity of the fault. The results of various test cases using the EKF algorithm are presented. This scheme provides fast protection of the transformer for severe faults.

Review of methodologies used for detection of magnetising inrush and fault conditions in power transformer

Reliable and dedicated protection of the transformer is the main requirement in the power system due to the supervision of a highly expensive equipment transformer. Non-linear characteristics of the transformer give mysterious behaviour for the period of inrush and Current transformer saturation conditions. As a stagnant and electro-magnetic device, a transformer is mainly affected by self-inrush. The saturation of the core itself is a very critical situation that becomes a hindrance for fault discrimination. This article involves the elaboration of the effect of inrush on transformer operation, past research practices for the discrimination of inrush with the faulty condition, and a definite method to recognise inrush and fault condition of the transformer. More than 50 reputed journal articles, various book chapters, and books with research thesis thoroughly studied to review the inrush and fault discrimination conditions. A comparative analysis is prepared to suggest the identification of transformer abnormalities. Moreover, a total harmonics distortion (THD)-based solution proposed for discrimination of the inrush condition from fault and other abnormal conditions using the short time Fourier transform technique. Hardware setup for 2 kVA transformer developed in the laboratory to study different inrush phenomena and altered fault conditions. Simulation and hardware results show various effects of inrush and fault with individual harmonic and THD-based analysis. The collection of real-time data is further used based on laboratory hardware setup to enhance real-time protection schemes for power transformers.

Spatiotemporal Evolution Characteristics of Fluid Flow through Large-Scale 3D Rock Mass Containing Filling Joints: An Experimental and Numerical Study

This study analyzes the spatiotemporal evolution characteristics of seepage through a large-scale rock mass containing a filling joint. Firstly, a conceptual model was established to characterize the geomechanical occurrence of a typical water-resistant slab adjacent to a water-bearing structure. Then, a special apparatus was developed to conduct a hydromechanical test of a 3D large-scale rock mass. For a certain boundary stress and inlet water pressure, the pore water pressure in the joint first experiences a dramatic increase before approaching a constant value, and the steady pore water pressure presents a linear decrease along the joint length. A water inrush phenomenon happens as a result of connected flowing channels induced by migration of fillings. Using the finite element of COMSOL multiphysics, the influences of filling joint permeability, matrix permeability, and joint thickness as well as the inlet water pressure on seepage evolution in the jointed rock mass were, respectively, investigated. The pore water pressure increases with all these factors, and the stable pressure values increase with the inlet water pressure but decrease along the joint length. The flow velocity undergoes an increase with both the joint permeability and inlet water pressure but presents constant values independent on the matrix permeability or joint thickness. The water pressure contour planes distributed along the flowing path generally transfer from a “long funnel” shape to a “short funnel” shape before reaching a series of parallel pressure planes perpendicular to the joint direction. By using the genetic algorithm, the coupling influences of these factors on the pore water pressure and flow velocity were investigated, and the decision parameters were optimized. The calculated values show a good agreement with the numerical results, indicating a good prediction of the seepage evolution through the filling joint.

Water Inrush Risk Assessment Based on AHP and Advance Forecast Approach: A Case Study in the Micangshan Tunnel

Water inrush is a serious geological disaster in tunnel. For the effective prevention and control of the occurrence of water inrush, a static‐dynamic water inrush risk assessment method is proposed by considering the Micangshan tunnel as an example. First, four possible types of water inrush phenomenon are identified based on the geological and hydrogeological conditions of the tunnel: water inrush in water‐bearing cracks, fault fracture zones, karst pipelines, and karst caves. Next, evaluation indexes that affect water inrush are determined. By combining the index weight value calculated by analytic hierarchy process (AHP) with the index quantitative value, the static water inrush disaster evaluation model is established, which provides a basis for tunnel design. Finally, with the combination of the static evaluation model and advanced forecast method, a dynamic risk prediction method of water inrush is established, which provides guidance for safe construction. The results confirm that the proposed method is a reliable theoretical basis for early assessment and prediction of tunnel water inrush disasters.

Assessment of the use of FEM for computation of Electromagnetic Forces, Losses and Design of Transformers

This paper reviews the published work on power transformers for assessment of the electrical, magnetic losses, and electromagnetic forces produced during normal and short circuit conditions. The different methods used are analytical method, integral equation method, boundary element method, and finite element method (FEM). The copper losses and eddy currents within the copper conductors/windings need to be assessed to estimate the heat generate and compute the efficiency. The knowledge of the eddy current losses in the core and stray losses in each component of the transformer can help in the design improvement. Earlier 2D FEM and now 3D FEM has been used to evaluate the electromagnetic flux under normal conditions, the effect of the in-rush phenomenon, assessment of mechanical strength of windings, insulation, computation of short circuit forces and need of the mechanical support components during fault conditions. This paper gives an overview of the work done on the computation of electromagnetic forces, losses, and design of transformers using FEM to assess the scope for further work.

Electromagnetic analysis of single-phase transformer banks under sympathetic inrush phenomenon

Electromagnetic analysis of single-phase transformer banks under sympathetic inrush phenomenon

Harmonic resonance overvoltage due to main transformer energization in large wind farms, Zafarana, Egypt

Abstract When a large power transformer is energized, a high magnitude inrush current may result in harmonic resonance overvoltage (HRO), if the system has resonance phenomenon. Such overvoltages may cause large stresses on the system. This paper investigates the energization of two winding 220/22 kV main power transformer in a weak system of a grid-connected wind farm. To achieve this aim, the produced HRO during the energization of the main power transformers and the affecting factors are investigated. In addition, the effect of the sympathetic inrush phenomena is studied. Finally, the sequential energization of the three-phase transformer and the pre-insertion resistor method are discussed as mitigation methods. The investigated scenarios and results will contribute in developing the power system restoration procedures for existent wind farm, its extension, and future-planned wind farms.

Application of the comprehensive identification model in analyzing the source of water inrush

Disasters caused by water inrush during production affect many coal mines in China. To rapidly and accurately prevent further water inrush, a comprehensive identification model combining hydrochemistry analysis, water source detection, and water channel exploration is proposed. The Schukalev classification method (SCM) is used to distinguish the concentration distribution of the main ion between the water inrush source and other aquifers, and hierarchical cluster analysis (HCA) is adopted to classify the water samples in the hydrochemistry analysis stage. Water channel exploration and water source detection are combined to test and verify the conclusion in the third stage. The comprehensive discrimination model is applied to the water inrush of the Buliangou coal mine. Hydrochemistry analysis shows that the percentages of the main ions (Na+, Ca2+, Mg2+, HCO3−, and Cl−) are greater than 30% (excluding SO42−), which is in agreement with the case of a sandstone aquifer. In the second stage, water source detection based on the transient electromagnetic method (TEM) and drilling technique indicates that not much water is present in the anomaly area around the water inrush location. The limited water recharge contradicts the phenomenon of water inrush. Water channel exploration is carried out, and an area with massive fissures that have water storage capacity is revealed. The conclusion is that the water conserved in the fissures flows out over a short time without the occurrence of stable recharge. The comprehensive discrimination model has complementary advantages and improves the water inrush prediction efficiency.

A Reconstruction of the WAMS-Detected Transformer Sympathetic Inrush Phenomenon

The transformer-energization inrush phenomenon is to be expected in cases when a single transformer is connected to an ac voltage source. The most commonly employed mitigation technique is to use mechanisms that enable per-phase controlled switching based on a residual magnetic-flux estimation in the transformer’s ferromagnetic iron core. However, the time scale of the phenomenon is relatively short and imposes no extremely critical issues to wide-scale power-system operation. On the other hand, the sympathetic inrush phenomenon that might occur among several parallel transformers exhibits much longer time constants (several seconds and even tens of seconds). The resulting system impact becomes longer lasting. Recently, national wide area monitoring system (WAMS) measurements indicated that a sympathetic interaction occurred between two parallel 400/110-kV transformers in the Slovenian power system. The cause was the magnetization of a second transformer in parallel to the already operational and fully loaded one in the substation in electrical vicinity to a large power plant. Apart from the WAMS, the event was also captured by the transformer’s over-current protection relay. The specifics of the event are described in this paper together with a dynamic-simulation-based reconstruction that is very important for the operator’s understanding of on-line, WAMS-provided, and system measurements.

Transformer Sympathetic Inrush Characteristics and Identification Based on Substation-Area Information

Sympathetic inrush is a magnetizing inrush phenomenon between two or more transformers when one of them is switched on under no load condition. This phenomenon has caused transformer differential protection maloperation in several cases due to the failure of the second harmonic restraint algorithm. Sympathetic inrush is very different in both the mechanism and characteristics from general inrush generated by a single transformer. Hence, a special identification method is required. The physical mechanism of sympathetic inrush between transformers is thoroughly studied to understand its characteristics, especially the alternate saturation feature. A substation-area information-based identification method is proposed to identify sympathetic inrush of the in-service transformer (IT). The alternate opposite saturation characteristic is used in the method. Both locking and unlocking criteria are designed for current differential protection to prevent the IT protection from maloperation. The proposed method is verified to be effective under different system conditions by simulations conducted in PSCAD and RTDS.

Inrush Blocking Scheme in Transformer Differential Protection

Transformer inrush currents during energization result in mal operation of transformer differential protection, due to the flow of magnetizing current only on the primary / source side of the transformer. A straightforward approach in recognition of inrush currents is proposed here by checking the core cause of this inrush phenomenon i.e. the magnetic saturation of the transformer core. In this method, the pseudocharacteristics were approximated in terms of orthogonal polynomials, and is used to distinguish between inrush currents and fault currents. In this method, the magnetic saturation characteristics of the transformer is modeled by defining pseudocharacteristics, and is obtained over a half cycle data window of the differential current. This data is assessed using certain criteria to detect inrush and fault currents. This method is not reliant on the transformer models and is precise in its output. The propositioned method was tested using delta-wye transformers under both fault and inrush scenarios.

Investigation into the Physical Mechanism and Influencing Factors of Sympathetic Inrush for Transformers in Series

Sympathetic inrush is a magnetizing inrush phenomenon generated in an in-service transformer, when a nearby transformer in series or in parallel is energized. This phenomenon lasts longer than the general inrush and can cause protection maloperation. The study focuses on sympathetic inrush in series transformers. Flux linkage expressions for the series transformers are derived using the Laplace transform. The residual flux, switching angle, and system impedance factors are included in the expressions. The physical mechanism of the sympathetic inrush for series transformers is explained based on the characteristics of the flux linkages. The interaction between the saturated transformers during sympathetic inrush is also investigated. Moreover, the influence of the switching angle, residual flux, system impedance, and load on the inrush currents is analyzed and validated.

Simulation of Inrush Dynamics for Unbalanced Distribution Systems Using Dynamic-Phasor Models

Microgrids are a possible solution to mitigate the extensive interruptions in service to end-use customers caused by abnormal events, such as major natural disasters. Microgrids can be formed by energizing portions of the distribution system to interconnect generators and critical loads. In this scenario, inrush dynamic currents can present a barrier for forming microgrids by preventing the necessary switching operations and/or damaging equipment. Modeling inrush dynamics, through electromagnetic transient tools, presents a challenge when the microgrid represents a large portion of a distribution feeder. This paper uses a dynamic-phasor approach to approximate the envelope of inrush dynamics; thus, allowing simulations of large systems. The two main contributions of this paper are: a) a new formulation to solve a set of dynamic-phasor differential equations representing an unbalanced distribution system; and b) modeling of transformer saturation in the dynamic-phasor frame. These two contributions allow the inrush phenomena to be simulated on full-size microgrid and/or distribution feeder models. The new formulation and model are implemented in GridLAB-D, and tested on IEEE test systems. The simulations showed good approximation as compared with a more accurate electromagnetic transient simulation in PSCAD software.

Sympathetic inrush current phenomenon with loaded transformers

The magnetization of a transformer is usually associated with the well-known phenomenon of inrush current. However, inrush current does not only affect the transformer being switched on. Instead, it has a significant impact also on all parallel connected transformers, which most certainly includes measurement transformers. This is known as a sympathetic inrush phenomenon. While the transformer being switched on might be a subject to a sudden high saturation level, the parallel transformers are gradually drawn to saturation as well, but of opposite polarity. The sympathetic inrush is expected in situations where the ohmic part represents a significant portion of the total system impedance. In contrast to other available literature on this subject, in this paper a modal approach to solving equivalent circuit by expressing differential equations in the state-equation form is selected. From the derivation of eigenvalues and eigenvectors, the phenomenon can be systematically investigated. A special attention will be given to circumstances, when the already operational transformer is fully loaded as within substations two or more transformers often operate in parallel, among which at least one of them is usually loaded. Finally, simulation results are compared to captured Wide Area Monitoring System (WAMS) measurements of the phenomenon and reasons for discrepancies are discussed.

Transformer Inrush Current Identification Based on EMD+TEO Methods

Transformers is one of the most important power system components, its role is to carry power conversion and transmission, transformer manufacturing technology continues to develop, but there is a surge of its problems, factors that have caused the transformer inrush load switching, transformers string parallel operation and fault lines, etc, as a transformer inrush phenomenon often can lead to malfunction of its protection, the correct identification is particularly important means of this paper, the combination of EMD and TEO transformer inrush and fault operation effective identification, theory and simulation confirms the validity and reliability of the algorithm.