Voltage Distribution In Transformer Windings Research Articles

МАТЕМАТИЧНА МОДЕЛЬ ДЛЯ АНАЛІЗУ ПЕРЕХІДНИХ ПРОЦЕСІВ В ДВООБВИТКОВИХ ТРАНСФОРМАТОРАХ У МЕТОДІ ПРЯМИХ

A mathematical model of a two-winding transformer for research of fast transient processes in windings in the form of an electromagnetic circuit, taking into account the main magnetic flux, own and mutual interturns and interwinding dissipation fluxes, transverse turns and longitudinal interwinding capacitances of the winding, is developed by using the straights method. The created model simplifies engineering calculations of fast transient processes in windings by applying the methods of the theory of electromagnetic circuits instead of the methods of mathematical physics, which are difficult for practical application. Transient processes and voltage distribution in transformer windings, under the action of pulse overvoltage in the form of a standardized pulse, are researched. References 8, figures 4.

Capacitive voltage sensor array for detecting transient voltage distribution in transformer windings

Experimental investigation of transient characteristics is an important way to design transformer winding insulation structures. Of the previous measuring methods, there has been the problem of direct electrical connection with the winding, which may influence the transient characteristics. This paper presents a new non-invasive method for measuring the transient voltage distribution in transformer windings based on our previous study. The measuring method is based on the capacitive sensor with the stray capacitance between sensing electrode and transformer discs as the high-voltage arm and the film capacitance as low-voltage arm. An RC integrator and impedance adapter have been designed into the measurement circuit to enhance the sensor worked in the integrating mode and broaden its bandwidth respectively. The final measurement circuit can satisfy the need for measuring various kinds of waveforms, with 0.64 Hz lower cut-off frequency and 76 MHz higher cut-off frequency, respectively. The sensor array has been set up to measure the transient voltage distribution in a transformer winding. In this case, a mathematical conversion is put forward to decouple the influence of the adjacent discs on the corresponding disc to get the voltage distribution of different discs of the transformer winding.

Measurement and analysis of transient overvoltage distribution in transformer windings based on reduced-scale model

Non-uniform voltage distribution under transient voltage is a primary cause of breakdown of the major insulation of power transformers. Actual power transformer winding has no tap, and is unable to directly measure impulse voltage distribution. Research on the measurement of impulse voltage distribution is important and practical in engineering. According to the relationship between physical quantities, the present study established a simplified shrinkage ratio criterion. Then, the researchers designed a similar ratio 1/20, typed 10kVA 220V/2400V, tapped transformer reduced-scale model, and used the finite element simulation software to simulate the reduced-scale and the original model of the electromagnetic field. Finally, a test platform was built to measure nanosecond pulse voltage and lightning impulse voltage distribution in transformer winding. The researchers also studied the nanosecond pulse width and lightning wave tail time affecting the winding voltage distribution.

Computation of internal voltage distribution in transformer windings by utilizing a voltage distribution factor

In this paper, a method for the application of the black-box transformer models to the lumped-parameter transformer winding models is presented. The methodology is based on applying terminal transformer voltages as input parameters that could be provided by using the powerful black box vector fitting. Then, internal voltage distribution is determined by applying a lumped-parameter model approximation. In particular, the paper is focused on the direct computation of the internal voltage distribution, by avoiding a complicated procedure of solving the lumped-parameter winding model. The method is based on the transformation matrix utilization of the voltage distribution factors. This transformation matrix reflects the voltage distribution at specific internal points along the winding with respect to the input terminal voltages. At this stage, the inputs for the lumped-parameters model are provided by measured voltages at transformer terminals and the transformation matrix is determined through geometrical data of the transformer. The implementation of the proposed method with the black-box modeling approach in existing simulation software tools like EMTP is under development. The method is verified by comparing measured with computed waveforms.

Fractional Lumped Parameter Modeling of Transformer Windings under VFTO

Very fast transient overvoltage (VFTO) is very dangerous for the transformer which is directly connected to gas insulated substations (GIS). At the same time, due to the continuous increasing of operating frequency in the power system, under the high frequencies of the transmission line calculation and simulation process, it is necessary to consider the frequency-dependent properties of the parameter. In order to study the voltage distribution in transformer windings under VFTO, a new Lumped parameter model which takes into account the fractional characteristics is proposed, named fractional lumped parameter model (FLP). The model divides each turn of transformer windings into proper sections, whose parameters are all lumped parameters. Finally it shows the correctness and validity of the model through the comparation of the experiment datas and simulation results.

TRANSIENT VOLTAGE DISTRIBUTION IN TRANSFORMER WINDING (EXPERIMENTAL INVESTIGATION)

In this work, the non-liner voltage distribution in transformer winding is investigated that occurs due switching and lightning. Transformer winding is modeled in the alternative transients program (ATP) version of Electromagnetic transients program (EMTPRV). EMTP software is used to simulate the very fast transient overvoltage. An experimental setup that consist of , Recurrent surge generator, CRO and transformer winding model has been developed and voltages measured at different point along the transformer winding. Simulation results show good agreement with the experimental result.

Modelling, simulation and measurement of fast transients in transformer windings with consideration of frequency-dependent losses

For the specification of winding insulation of transformers, it is important to know the electrical stresses to which the winding can be exposed during fast transient oscillations. These oscillations occur during switching operations performed by circuit breakers, or when gas-insulated substations (GIS) are used. Therefore one of the priorities is to use a high-frequency transformer model capable to simulating fast transient oscillations in the windings. The model presented requires only information about the geometry of the winding and the core, as well the electrical and magnetic parameters for the used materials. In the transformer model, the frequency-dependent core and copper losses are included. Numerical computations are performed with and without the core losses being taken into account. Two types of measurement are taken to verify the validity of the model. First, the voltage transients are measured and computed by the application of a step impulse with a rise time of 50 ns. Then, the transformer is switched by a vacuum circuit breaker, and the multiple reignitions, which contain oscillations with a wide frequency range, are analysed. The results verify that the model is suitable to simulate the voltage distribution in transformer windings over a wide frequency range.

New approach to the analysis of impulse voltage distribution in transformer windings

A new calculation method for the analysis of transient voltage phenomena in HV power transformers is presented. Consideration is given to the electric, magnetic and current fields in the windings. An equivalent circuit is derived; this consists in the connection of two networks, one magnetic the other electric, whose components with their numerical values are deduced directly from the geometry of the transformer and from its electrical parameters. Application of the ensuing mathematical model to a real case of a 132 kV, 50,000 kVa power transformer is discussed.

Use of Thyrite in power transformers

ELECTROSTATIC SHIELDING has long been recognized as an effective means of controlling the impulse voltage distribution in transformer windings. Under certain conditions the same, or a better, result can be achieved by the use of Thyrite. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∗</sup> Developed as a material for lightning arresters, Thyrite is characterized by its nonlinear resistance; it passes a current proportional to a relatively high exponent of the applied voltage. Thyrite is a ceramic-bonded inorganic resistance material manufactured in the form of disks.

Lightning Studies of Transformers by the Cathode Ray Oscillograph

Study of transformers has been under way for some time to coordinate the strength of transformers and transmission line insulation under lightning conditions. This study has taken two forms: first, the transient dielectric strength of the line end of the winding and second, the distribution of transient voltages and therefore the stress caused by them throughout the winding. Theoretical studies and spark-gap tests of transient voltage distribution in transformer windings have been previously published by the Institute. Since then an extensive study of transient voltage phenomena has been made by the cathode ray oscillograph on power transformers connected to a short transmission line and subjected to artificial lightning waves sent along the line. The effects of the transmission line, concentrated inductance, and transformer entrance bushings on traveling waves were studied, as well as the effect of traveling waves of various service conditions in producing internal oscillations in ordinary transformers. The non-resonating transformer was stutdied under similar conditions. A striking agreement between the oscillographic records and theoretical conclusions previously published was found, sufficient to establish beyond any doubt the following conclusions: 1. Very high voltage oscillations occur throughout the entire winding of even a grounded neutral transformer. 2. Points of the winding near the grounded neutral may rise to 95 per cent of the crest voltage of a very short traveling wave (three microseconds long). 3. Entrance bushings have a negligible effect on the shape of the incoming traveling wave. 4.

Lightning studies of transformers by the cathode ray oscillograph

Study of transformers has been under way for some time to coordinate the strength of transformers and transmission-line insulation under lightning conditions. This study has taken two forms; first, the transient dielectric strength of the line end of the winding and second, the distribution of transient voltages and therefore the stress caused by them throughout the winding. Theoretical studies and spark-gap tests of transient voltage distribution in transformer windings have been previously published by the Institute. Since then an extensive study of transient voltage phenomena has been made by the cathode ray oscillograph on power transformers connected to a short transmission line and subjected to artificial lightning waves sent along the line. The effects of the transmission line, concentrated inductance, and transformer entrance bushings on traveling waves were studied, as well as the effect of traveling waves of various service conditions in producting internal oscillations in ordinary transformers: The non-resonating transformer was studied under similar conditions. A striking agreement between the oscillographic records and theoretical conclusions previously published was found, sufficient to establish beyond any doubt the following conclusions: 1. Very high-voltage oscillations occur throughout the entire winding of even a grounded neutral transformer. 2. Points of the winding near the grounded neutral may rise to 95 per cent of the crest voltage of a very short traveling wave (three microseconds long). 3. Entrance bushings have a negligible effect on the shape of the incoming traveling wave. 4. In case of sudden voltage changes, concentrated inductance in series with the transformer, unless by-passed by a suitable device, causes rise of voltage across the transformer terminals as well as internally in the windings. 5. Arc-over of line insulators by a traveling wave produces severe oscillations in a transformer the amplitudes of which are roughly proportional to the arc-over voltage of the line insulators. 6. Grading the insulation between high voltage and low voltage and ground in ordinary transformers with grounded neutral is a dangerous practise when the transformers are subject to lightning. 7. All the above conclusions apply to concentric winding core type as well as interleaved and shell type transformers. From theoretical studies the non-resonating type of transformer has been developed and its action checked by tests. This type of transformer eliminates voltage oscillation within the winding and therefore local concentration of transient voltage. An Appendix entitled Present Status of the Cathode Ray Oscillograph on the Measurements of Transients, by H. L. Rorden and J. C. Dowell, both of the General Electric Company, Pittsfield, Mass. is included in the complete paper.