Internal Overvoltages Research Articles

ДОСЛІДЖЕННЯ ПОДВІЙНИХ ЗАМКНЕНЬ НА ЗЕМЛЮ В МЕРЕЖАХ 10 КВ З ІЗОЛЬОВАНОЮ НЕЙТРАЛЛЮ

Reliability indicators of power supply systems as a whole are largely determined by the level of reliability of distribution electrical networks with a voltage of 6-35 kV. Emergency in networks with a voltage of 6-35 kV, operating in the mode with an isolated neutral, is often associated with internal overvoltages. Single-phase short-circuits occur in places of defects in the insulation of cable lines and substation equipment due to the aging of the insulation, non-compliance with the manufacturing technology of insulating structures at factories, non-compliance with the requirements of the instructions during the installation and operation of the equipment, as well as due to mechanical damage, which, sometimes at the time of their appearance, lead to only a partial decrease in electrical strength. Overloads lead to defects. Single-phase short-circuits in 10 kV networks that are not eliminated in time can lead to double earth faults. For protection in these networks, such protections as current cut-off, maximum current protection, current cut-off, zero-sequence current protection, current protections with blocking at minimum voltage are used. However, there are cases when there are cases of negative operation of such protections at a 10 kV substation during double earth faults. For example, a phase A short circuit occurred on the first line, which receives power from the buses of the investigated substation, and another short circuit (phase B) occurred on the second line. Instead of tripping the two damaged lines, the input circuit breaker tripped by mistake. This de-energized other (undamaged) lines that are fed from the same bus section as the damaged lines. Under such conditions, further studies of the currents in the lines during double circuits are required. For this, the use of the Matlab application program package is justified. This will significantly reduce the costs of physical modeling and verification of proposed proposals. The paper presents the results of studies of double earth faults in 10 kV networks with an isolated neutral in the "MATLAB" program package.

Calculation of voltage distribution along the transformer winding using the wide band transformer model

Electrical devices in transmission and distribution networks are submitted to fast front transients. These transients are often a cause of a failure in the power system. To simulate these phenomena, it is necessary to detailly model all the components of the power system. This paper concentrates on studding transformer internal failures due to voltage stress that can occur when fast front electromagnetic wave travels through the transformer. To prevent a failure, an internal insulation of the transformer has to be dimensioned to sustain these transient phenomena. Therefore, it is necessary to use advanced wideband transformer models for its dimensioning. In this paper, a wide band transformer model based on limited transformer geometry is used to simulate evaluate the stresses on internal transformer insulation in the case of fast front transients. The model is validated for calculation of internal overvoltages with the test case from CIGRE brochure.

Study on internal overvoltages of 220 kV submarine cable transmission system of the offshore oil–gas field group

Since the Electromagnetic Transient (EMT) process of the high-voltage submarine cable has a great influence on the safe and stable operation of the transmission system, it is necessary to analyze its EMT process under actual engineering. In this study, PSCAD (EMTDC) is used to simulate the internal overvoltages (power-frequency overvoltages and switching overvoltages) of the submarine cable transmission system. Under the working conditions considered in this study, the maximum power-frequency overvoltage in the system occurs in the case of single-phase fault earthing, which is 1.147 p.u. The maximum switching overvoltage is 1.321 p.u., and the 2% overvoltage is 1.276 p.u. The above results are all able to meet the regulations of the internal overvoltages. By simulating the overvoltages in the system, the operation states of the system can be obtained, which is of great significance to improve the transmission reliability of the submarine cable system.

Simulation of impulse current generator for testing surge arresters using frequency-dependent models

The object of research is the equivalent circuit of an impulse current generator designed for testing surge arresters. Calculation of the impulse current generator parameters when discharging a capacitor bank to a complex nonlinear load is a difficult task for an analytical solution. Until now, the application of surge arrester frequency-dependent models was limited to the problems of overvoltage computation. Surge arrester frequency-dependent models can predict the residual voltage with high accuracy. This is the reason to consider that surge arrester frequency-dependent models can be used for calculating the main parameters of impulse current generators designed for physical testing of surge arresters.
 The task of determining the equivalent circuit parameters required for getting a discharge current of a given waveform and amplitude in an impulse current generator scheme with a nonlinear load was solved using circuit simulation.
 This article presents the results of studying the processes in impulse current generator equivalent circuit. In the circuit a dynamic model of a surge arrester is used as the load model. For this, an equivalent circuit for the discharge path of the impulse current generator was drawn up. The parameters of the circuit elements (including the required number of capacitors and their charging voltage) are determined, which are necessary for getting a discharge current of a given standardized waveform and amplitude. The parameters of the discharge path are determined for surge arresters of three different voltage classes. It was found that the relative error when determining the residual voltage between the terminals of the surge arrester model does not exceed 3 %.
 The work contributes to the further development of circuit simulation of surge arresters and the expansion of the scope of surge arrester dynamic models. As a result of the research performed, the possibility of using surge arrester frequency-dependent models for determining the discharge current waveform in impulse current generators is shown. The research performed is relevant due to the fact that surge arresters have become a main tool for protecting the insulation of electrical network equipment against external and internal overvoltages

Overvoltage Impact on Internal Insulation Systems of Transformers in Electrical Networks with Vacuum Circuit Breakers

Vacuum circuit breakers are increasingly used as switching apparatus in electric power systems. The vacuum circuit breakers (VCBs) have very good operating properties. VCBs are characterized by specific physical phenomena that affect overvoltage exposure of the insulation systems of other devices. The most important phenomena are the ability to chop the current before the natural zero crossing, the ability to switch off high-frequency currents, and the rapid increase in dielectric strength recovery. One of the devices connected directly to vacuum circuit breakers is the distribution transformer. Overvoltages generated in electrical systems during switching off the transformers are a source of internal overvoltages in the windings. The analysis of the exposure of transformers operating in electrical networks equipped with vacuum circuit breakers is of great importance because of the impact on the insulation systems of switching overvoltages (SO). These types of overvoltages can be characterized by high maximum values and atypical waveforms, depending on the phenomena in the circuit breaker chambers, system configuration, parameters of electrical devices, and overvoltage protection. Overvoltages that stress the internal insulation systems are the result of the windings response to overvoltages at transformer terminals. This article presents an analysis of overvoltages that stress the transformer insulation systems, which occur while switching off transformers in systems with vacuum circuit breakers. The analysis was based on the results of laboratory measurements of switching overvoltages at transformer terminals and inside the winding, in a model medium-voltage electrical network with a vacuum circuit breaker.

Analysis of Internal Overvoltages in Transformer Windings during Transients in Electrical Networks

Due to the increasing requirements for the reliability of electrical power supply and associated apparatus, it is necessary to provide a detailed analysis of the overvoltage risk of power transformer insulation systems and equipment connected to their terminals. Exposure of transformer windings to overvoltages is the result of the propagation condition of electromagnetic waves in electrical networks and transformer windings. An analysis of transformer winding responses to transients in power systems is of particular importance, especially when protection against surges by typical overvoltage protection systems is applied. The analysis of internal overvoltages in transformers during a typical transient related to switching operations and selected failures is of great importance, particularly to assess the overvoltage exposure of insulation systems in operating conditions. The random nature of overvoltage phenomena in electrical networks implies the usage of computer simulations for the analysis of overvoltage exposures of electrical devices in operation. This article presents the analysis of the impact of transient phenomena in a model of a medium-voltage electrical network during switching operations and ground faults on overvoltages in the internal insulation systems of transformer windings. The basis of the analysis is simulations of overvoltages in the windings, made in the Electromagnetic Transients Program/Alternative Transients Program (EMTP/ATP) using a model with lumped parameters of transformer windings. The analysis covers the impact of the cable line length and the ground fault resistance value on internal overvoltage distributions.

GLIDARC reactor power supply with ignition improvement

PurposeThe research purpose of this paper is to confirm that internal overvoltages in the push–pull power inverter can be used to improve the discharge ignition in the gliding arc discharge (GAD) plasma reactor.Design/methodology/approachInvestigations are based on the acquisition of voltage changes that occur together with the development of the discharge column and the imaging the GAD with the use of a high-speed camera.FindingsThe power supply has the distinctive feature of not having the switching overvoltages completely extinguished, as it is in typical push–pull inverters. The overvoltages still exist but only dangerous peaks are cut off. The remaining ones, of a dumped resonance character (Figure 3), are transferred to the secondary coil of the transformer. Correctly shaped overvoltages are used for ignition improvement in the GAD reactor.Practical implicationsGAD plasma reactors have many applications for pollution control, disinfection and sterilization of surfaces and for plasma deposition, surface functionalization, as well as in agricultural and medical treatment. Investigations prove the push–pull inverter’s advantages in comparison with the transformer-type power supply. Properly configured push–pull inverters have good ignition properties and control options, allowing to generate, desirable for many applications, homogeneous non-thermal plasma.Originality/valueThe idea of using switching overvoltages in transistors of push–pull switching-mode power supplies is new and has not been previously used to improve discharges ignition in a non-thermal plasma reactor.

Time-Domain Implementation of Damping Factor White-Box Transformer Model for Inclusion in EMT Simulation Programs

White-box detailed transformer models are used by manufacturers for predicting internal overvoltages in transformer windings during the lightning impulse test. One such model is the d -factor model, which is based on a lumped-parameter description based on winding discretization with the inclusion of losses via an empirical, frequency-dependent damping factor. This paper shows a procedure for direct inclusion of the d -factor model in electromagnetic transients simulation programs for use in general studies of network overvoltages. Proper utilization of the model's diagonal structure is utilized in combination with real-valued arithmetic for maximum speed in transient simulations, with optional initialization from sinusoidal steady-state conditions. The model can be used both as a terminal equivalent and for calculation of internal voltages with use of an augmented input data set. The method is demonstrated for a single-phase three-winding transformer that is applied in a simulation study of transient resonant voltage buildup in the tertiary winding.

Internal Over-Voltage Identification Method of Distribution Network Based on AD-SVM Algorithm

An internal over-voltage identification method based on atomic decomposition(AD) and Support Vector Machine(SVM) is proposed. Firstly, the three-phase voltages of bus are decomposed by AD algorithm to get the optimal atoms. According to the atom frequency, the optimal atoms are constructed as characteristic atom matrix, which can reflect the time-frequency characteristics of the over-voltage signal. Features are obtained from the characteristic atom matrix by singular value decomposition. Finally, the features are input into the SVM to achieve the identification of seven types of internal over-voltages. The proposed method is verified on the simulation platform. The results show that the proposed method has high accuracy.

Comparison of transformer winding responses to standard lightning impulses and operational overvoltages

During operation transformers are typically subjected to overvoltages with different wave shapes. Even though overvoltage protection systems, such as metal oxide surge arresters, are often employed, the overvoltages generated in power systems can still have a significant impact on the insulation systems of the windings of a power transformer. There are numerous examples of transformer failures as a result of overvoltages occurring during operation, even in cases that standard impulse voltage tests show no indication of potential problems and the transformers are protected by surge arresters. Comparisons of the internal overvoltages generated in transformers during voltage tests as well as those generated during standard operation of electrical networks provides fundamental insights into these overvoltages; insights which in turn can be used to improve the construction of insulation systems and overvoltage protection approaches and ultimately increase the reliability of transformers. In this paper, internal overvoltages occurring in the windings of selected transformer during standard voltage tests are compared with those which occur during typical operation of a transformer in an electrical power system. Overvoltages measured in the windings of the transformer form the basis of this comparison. Measurements were conducted with the transformer being subjected to voltage impulse tests, as well as transient voltages with waveforms simulating overvoltages generated in electrical power systems. In order to investigate the response of the windings to voltage strokes, full and chopped lightning voltage impulses were applied to the transformer. The waveforms of overvoltages generated in the electrical power systems were modelled as a rectangular voltage stroke, sinusoidal voltage, voltage with shape of “sweep sinus” also often referred to as a chirp signal and voltage with shape of “sweep ramp”.

Скоростная характеристика фронта напряжения как информационного параметра в системах диагностики и защиты рентгеновского компьютерного томографа

Malfunctions that arise in the operation of diagnostics apparatus, can be divided into external, associated with poor-quality power supply system, and internal, resulting from the breakdown of blocks and elements of the devices themselves. The X-ray computer tomograph (RCT) is particularly sensitive to low-quality power supply. All the parts of the RCT possess high inductance L. Voltage drops result in internal overvoltages on the blocks of the RCT. To protect RCT, the use of high-speed characteristics of the voltage front change as the information parameter when detecting the emergency modes of the power supply network can treble the speed of the network mode detecting. Changes in the value of the speed characteristics at various values of the non-sinusoidal voltage, voltage deviations and frequency oscillations in the power supply network have been investigated. The range of the speed characteristic change for detecting voltage drops and voltage deviations have been developed.as well as the range for the normal operation of RCT and the range of the transition to an emergency mode

Calculation of internal overvoltages using a wide band transformer model based on limited information about transformer design

Specific insulation coordination studies might require an accurate modeling of power transformers. This is the case for instance when transfers of fast front overvoltages from the high voltage side to the low voltage side of a transformer are involved. Modeling accurately the transformer’s electromagnetic behavior for the frequency range of fast front overvoltages requires special care. There are numerous existing transformer models which are intended to model the wide band transformer’s behavior. However, many of them are too complex or require confidential information about the transformer design. This makes them not suitable for power engineering applications.In this paper a wide band transformer model based on limited information about the transformer design and compatible with an electromagnetic transients program is presented. Then, its application on calculating internal overvoltages distribution along transformer windings, taking into account different conditions on the outer transformer terminals, is shown for lightning impulse.

Very fast transient overvoltage waveshapes in a 500-kV gas insulated switchgear setup

Very fast transient overvoltages (VFTOs) may be generated in SF6 gas insulated switchgear (GIS) due to a series of arc ignitions and extinctions, during the operation of disconnecting switches and circuit breakers. They may also occur during internal flashover in the GIS. Some recent reports of failures in power transformers suggest high internal overvoltages associated with arc restrikes as a possible cause; such internal overvoltages might be generated by resonant frequencies affecting the transformer coils [1]. In order to understand this phenomenon better, Itaipu Binacional, a 14,000-MW Hydropower Plant, in cooperation with LACTEC, a High Voltage Research Laboratory, assembled a 31-meter-long GIS test setup including a disconnecting switch in order to study VFTOs occurring during disconnecting switch operation. It was found that the time to voltage peak, the inner frequency bandwidth, and the voltage peak values around 2.0 to 2.8 times the nominal voltage [2] lay within the ranges suggested in IEC 60071-1 [3]. However, the measured frequencies of the envelope of the transitory overvoltage shape were at least 10 times the upper limit suggested in [2] and [3]. Detailed results are presented below.

Przepięcia wewnętrzne w uzwojeniach transformatorów blokowych

The problem of winding insulation damage in unit transformers due to internal overvoltages have been described. The results of computer analyses on voltage distribution along transformer windings at different switching operations and faults in supply line are presented. For the calculations of voltage distribution mathematical model of typical transformer windings based on its real parameters have been used. Conclusions and recommendations are specified.(Internal overvoltages in unit transformer windings). Slowa kluczowe: transformatory blokowe, uzwojenia transformatorow, przepiecia wewnetrzne, symulacje komputerowe.

Analysis of fast transient voltage distributions in transformer windings under different insulation conditions

Evolution and operation of modern grids and networks necessitate reconsideration of several characteristics. For example transformers operating in power grid or in industrial environment are subjected nowadays also to transients or stimulus with different waveforms. Traditionally, in case of power transformers the stresses are related to lightning and switching phenomena. In industrial networks distribution transformers are exposed to many switching operations generating often overvoltages. Transformer windings subjected to such system disturbances are stressed due to the non-linear transient voltage distribution along the winding and potential resonance phenomena. Initial voltage distribution, reflects mainly the capacitive coupling between turns, windings, core and tank. The non-uniform distribution of voltage results in concentred stress in certain parts of the winding. Winding resonance might occur, if the frequency spectrum of incoming surge matches the winding natural frequencies. Therefore external transients occurring in power systems might trigger internal overvoltages with large maximum value in transformer windings. The frequency characteristic of the transformer winding may determine those regions in the frequency spectrum. The paper presents results of voltage distribution measurements performed on model windings with different constructions subjected to voltages with different rise time. The experimental setup allows for investigation of influence of the dielectric (air/oil) and temperature on the stress distribution along the winding. Also the impact of oil and temperature on frequency characteristics of internal voltages are shown. The presented results might be used both for design and optimization of transformers windings.

Internal Transient Over-Voltages in Large Fusion Coils

Magnets show a linear voltage distribution along the windings for slow changing currents. This simple behavior may be lost when fast transients occur, e.g. for fast discharge or earth fault events. Such transients may become critical with respect to the magnet lifetime if they are not considered in design and acceptance tests. The problem of internal transient over-voltages can become more essential for large magnets with high number of turns because the critical frequencies for transients are lowered due to the large number of capacitances and inductances of windings and winding packs. At KIT large fusion magnets such as POLO and the ITER Toroidal Field Model coil (TFMC) have been investigated in the large coil testing facility TOSKA. For these coils direct measurements of internal over-voltages were possible and could be compared to calculations of transient voltages. As a consequence a set of transient voltage calculations was executed at KIT for ITER coils. An overview of calculations and results for ITER TF and PF coils is given and consequences for high voltage testing are discussed.

Evaluation of the probability of arrester failure in a high-voltage transmission line using a Q learning artificial neural network model

One of the most popular methods of protecting high voltage transmission lines against lightning strikes and internal overvoltages is the use of arresters. The installation of arresters in high voltage transmission lines can prevent or even reduce the lines’ failure rate. Several studies based on simulation tools have been presented in order to estimate the critical currents that exceed the arresters’ rated energy stress and to specify the arresters’ installation interval. In this work artificial intelligence, and more specifically a Q-learning artificial neural network (ANN) model, is addressed for evaluating the arresters’ failure probability. The aims of the paper are to describe in detail the developed Q-learning ANN model and to compare the results obtained by its application in operating 150 kV Greek transmission lines with those produced using a simulation tool. The satisfactory and accurate results of the proposed ANN model can make it a valuable tool for designers of electrical power systems seeking more effective lightning protection, reducing operational costs and better continuity of service.

Application of the transfer function to recognition of resonance overvoltages in transformer windings

Power transformer insulation systems are subjected to many stresses during normal operation due to lightning and switching. If the spectrum of incoming surge voltage matches the winding one, a corresponding resonance will be excited. Therefore external transients occurring in power systems might trigger internal overvoltages of a large maximum value in transformer windings. Overvoltages having such a character have been the main cause of many power transformer failures. This paper presents a novel approach to the identification of sensitive zones in the transformer windings based on measurements of overvoltages inside the windings and the frequency dependences of admittance of the power transformer. The frequency characteristics of the transformer winding may determine those regions in the frequency spectrum. The presented methodology might be used both for design optimization and diagnostics of power transformers.

Online lightning and internal overvoltages monitoring system in 10 kV distribution networks

The design and realization of an online overvoltage monitoring system for distribution network are introduced. The system can capture both lightning and internal overvoltage waveforms automatically. A variable sampling technique is employed to solve the conflict between high-speed data acquisition and the data storage with large capacity by using complex programmable logic device (CPLD). The performances of low damping resistor–capacitor divider, signal conditioning circuit, data acquisition card, acquisition control, and waveform analysis software are analyzed in detail. The long-term operation shows that the system is effective in providing scientific support for fault analysis of power system. Copyright © 2010 John Wiley & Sons, Ltd.

Comparison of metal-oxide surge arresters circuit models and implementation on high-voltage transmission lines of the Hellenic network

The transmission lines of high-voltage networks are protected against lightning strikes and internal overvoltages with the use of overhead ground wires and surge arresters. The appropriate modelling of a power system's components is a significant issue, in order to obtain reliable theoretical simulation results for the lightning performance of the line that will be used in the design of a new line or the improvement of an existing one. Metal-oxide surge arresters present difficulties in their modelling owing to their dynamic characteristics, which depends on the rise time of the injected impulse current. Several equivalent circuit models have been proposed in the technical literature for the adequate simulation of this frequency-dependent behaviour. The current paper presents the most important surge arresters models, comparing the residual voltage that each model extracts for different lighting currents. For each model, the arresters failure probability is computed implementing them on Hellenic high-voltage transmission lines, performing a sensitivity analysis for the tower footing resistance and the arresters interval installation.