Ordinary Transformer Research Articles

Study on valve-side winding fault characteristics of converter transformer in LCC-HVDC system

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

Characteristic analysis of zero-mode inrush current of high-impedance transformer

In recent years, zero-sequence overcurrent protection misoperated frequently when the high-impedance transformers are energized without load. This paper studies the characteristics of zero-mode inrush current of the high-impedance transformers, including amplitude and attenuation. Firstly, the corresponding relation of “winding arrangement – flux distribution – equivalent circuit parameters” of transformer was clarified. On the basis of analyzing single-phase and three-phase inrush current, the analytical expression and equivalent circuit of zero-mode inrush current were proposed. Then, according to the different winding arrangement, the different parameters of hollow inductance, self-leakage inductance and saturated mutual inductance of the high-impedance transformers and ordinary transformer were explained and calculated. Further, based on the different parameters and analytical expression, the zero-mode inrush current characteristics of high-impedance transformers were analyzed. The correctness of characteristic analysis of high-impedance zero-mode inrush current was verified via digital simulations and dynamic physical simulations tests.

Research on Magnetic-Valve Controllable Reactor Based on ANSYS

Magnetic-valve controllable reactor (MCR) has become many researcher’s topic of the day because of its versatile use in power systems. MCR utilizes the concept of magnetic saturation to control power flows in the power grid. It is as simple to operate and maintain, and reliable as an ordinary transformer. However, magnetic-valve controllable reactor works under a more variety of complex excitation condition because of the superposition action of AC and DC excitations. This paper carefully discusses the distribution of magnetic field of MCR core, provides an understanding of the range of inductance adjustments and further analyzes the working current waveform. Based on that, the finite element analysis software ANSYS Maxwell is used to design and examine a 3-D prototype model under different control voltage levels. The method of transient solution is applied for the reason being that it simultaneously has both AC and DC voltages. The AC voltage is kept constant while the DC voltage is varied from the minimum to the maximum rated value. The simulation results confirm that the magnetic-valve controllable reactor works in the saturation region of the magnetization curve under the combined excitation of AC and DC. The inductance adjustment range shows that the MCR inductance value can be smoothly and continuously varied. In addition, the working output current contains little odd-order harmonics that can be mitigated if filtering device is used or the magnetic valves are designed carefully. By observing the simulation results and analysis, one can gain a thorough understanding of MCR under actual working condition. It provides a reliable basis for the performance design of magnetic-valve controllable reactor.

The Prospect of Nano Vegetable Transformer Oil

This paper started with the performance of ordinary transformer oil, and then compared the Nano-modified vegetable oil with transformer oil. From the comparison of results, the author drew a conclusion that Nano vegetable oil must have broad prospects in the use of the transformer.

Analysis and Design Aspects of a Series Power Semiconductor Array with Digital Waveform Control Capability for Single Phase AC Voltage Regulators and Other Applications

A series connected power semiconductor array, with digital control capability could be used for developing single phase AC regulators or other applications such as AC electronic loads. This technique together with an ordinary gapless transformer could be used to develop a low cost AC voltage regulator (AVR) to provide better or comparable specifications with bulky ferro-resonant AVR types. One primary advantage of the technique is that digital control can be used to minimize harmonics. Commencing with a review of AC voltage regulator techniques for single phase power conditioning systems, an analysis and design aspects of this technique is presented with experimental results for AVRs. Guidelines on how to utilize the technique in a generalized basis is also summarized together with a summary of a technique for achieving harmonic control.

Pulsed power transmission line transformer based on modern cable technology

A high-voltage transmission-line pulse transformer has been constructed based on modern cable technology. The transformer has been successfully tested for output powers of 0.5 GW. The high-voltage cable is equipped with a resistive layer (semicon) on the inner conductor and on the inside of the outer conductor. Semicon cables are commonly used in high-voltage transmission of electrical power. The pulse transformer was built using a coaxial semicon cable, with the inner conductor used as secondary winding and the screen as primary winding. Such a transmission-line transformer works in the same way as an ordinary transformer. The input is transformed to the desired output using a step-up or a step-down configuration. An output voltage of 85 kV with 1-/spl mu/s duration was achieved into a 15 /spl Omega/ load. Because the windings are coaxial the magnetic leakage is kept low and, therefore, the coupling coefficient is high. This type of transformer is useful in applications where weight is an important factor. Another advantage is the simple design and that it can be manufactured at a low cost.

A ceramic transformer inverter for driving liquid crystal back light lamps

The Cold Cathode Fluorescent Lamps (CCFLs) used for back light sources of Liquid Crystal Displays (LCDs) require high voltage greater than 1000 V to be turned on. A conventional CCFL driver uses wound wire transformers to supply the high voltage. But the transformer keeps supplying the high voltage when the CCFL requires lower voltage. In this paper, the author proposes an inverter using a Ceramic Transformer (CTR) instead of the ordinary transformer. The lamp voltage is controlled using a V-F converter through the frequency characteristic of the CTR from turn on to usual operating voltage. The prototype CTR inverter using a conventional type CTR achieved better than 70—80% efficiency in 2 to 3 watts' systems

New Emission Bands of Forbidden Systems of Nitrogen in the Vacuum Ultraviolet Region

Using a 2-m vacuum spectrograph and an ordinary transformer discharge through mixtures of N2 with argon and xenon, we observed two new systems of N2 emission bands, one for each mixture, in the region between 1550 and 2250 A. According to the vibrational analysis, the bands, which were excited in the mixture of (N2+A), are due to the transition (a′1Σu−−X 1Σg+), and the bands, which were excited in the mixture of (N2+Xe), are due to the transition (3Σu−−X 1Σg+). The present results are in good agreement with those of Wilkinson and Mulliken whose observations were made for the same systems of bands in absorption.

Continuous Emission Spectra of Rare Gases in the Vacuum Ultraviolet Region

Using an ordinary transformer discharge, continuous emission spectra of krypton and argon similar to that of xenon reported recently were found in the regions from their resonance lines to about 1850 and 1650 A, respectively. The characteristics of all the rare gas continua were similar regarding their dependence on excitation conditions as well as their features. The origin of these continua is explained by a process similar to that for the xenon continuum. The helium and neon continua are briefly discussed in connection with the present work.

Field Measurements on Model Betatron and Synchrotron Magnets

Measurements have been made of the magnetic field distribution in the air gap and of flux leakage for a `groovecoil' betatron magnet and for a synchrotron magnet. The synchrotron magnet was a complete solid of revolution, and was intended to simulate a magnet built up from twenty C-shaped segments. The betatron magnet yoke resembled the ordinary shell-type transformer. The boundaries of the regions within which stable orbits could be established have been calculated. It was found that the betatron magnet could be converted to a synchrotron magnet simply by removing the central core, without reshaping the pole faces. The measurements were carried out on model magnets made of solid iron and excited by direct current. Search coils and a ballistic galvanometer were used to explore the fields.

A Laboratory Method of Producing High Potentials

Details are given of the experimental arrangement by which, using Tesla coils in oil, very high potentials have been produced and measured. Excited at the rate of 120 sparks per second Tesla coils have been operated at 3,000,000 volts in ordinary transformer oil at atmospheric pressure. In oil under a pressure of 500 pounds per square inch, voltages as high as 5,200,000 have been produced with intermittent excitation. These voltages (peak values) are measured by a simple capacity-potentiometer, in which an insulated electrode "picks up" a known fraction of the total voltage, this fractional voltage being measured by means of a sphere gap. Measurements are given of the voltage-distribution along Tesla coils. Calculations and measurements of the efficiency and power-output of such coils show that at 120 sparks per second, a coil operating at 5,000,000 volts provides sufficient power, if used to accelerate helium nuclei in a suitable vacuum-tube, to yield the equivalent of about 2,600 grams of radium.

Effect of Transient Voltages on Power Transformer Design

When subject to transient voltage excitation, local concentration of voltage takes place in an ordinary transformer in which the capacitance charging current of the coils to ground is supplied through the winding, or the ratio of inductance and capacitance of the various parts throughout the winding is not constant. Calculations and tests of voltage distribution in the winding, caused by the impact of (a) damped high-frequency oscillations, and (b) unidirectional traveling waves, are given. In order to make the analysis clearer, the transformer winding is considered as a network of inductances and capacitances, and this term ``network'' is used throughout the paper. Certain simplified and typical networks are considered. Transformers having one terminal grounded, such as are used in three-phase star connection, particularly in high voltage systems, are frequently built with the insulation graded to other windings and ground, in the order of the normal frequency voltage stress. The danger of such a practise is shown in power transformers which are subject to transient overvoltage, since voltage oscillation in the winding may raise the voltage to ground at intermediate points above the terminal voltage, unless the design of the winding eliminates oscillation. The theoretical and experimental data given show that the distribution and magnitude of voltage stresses existing during recognized standard insulation tests are essentially different from stresses created by transient voltages. This permits the construction of transformers that would satisfactorily pass standard insulation tests but at the same time would not be suitable for average service.

Design Methods for Soft Magnetic Materials in Radio

The present paper is concerned with a study of design methods for soft magnetic materials such as those used in radio broadcast receivers and similar apparatus. The design of such devices as interstaged transformers calls for more than choosing points on curves in a standard handbook and computing mechanical or electrical details by ordinary power transformer theory. Radio transformer design is usually complicated by the effect of passing d-c space current through the winding or by the use of an air-gap to prevent saturation of the core by uni-directional current. The procedure to follow when one of these disturbing effects is present is not very difficult, but it becomes somewhat involved if an efficignt design is wanted when both are present. In this paper an attempt is made to present a workable method of taking account of these disturbing effects. The method intvolves the determination of the apparent and maximum obtainable permeabilities of the magnetic materials so that a proper conclusion nay be arrived at by an application of simple engineering methods.

Abridgment of effect of transient voltages on power transformer design

When an ordinary transformer is subject to transient voltage excitation, local concentration of voltage takes place in which the capacitance charging current of the coils to ground is supplied through the winding. This is because the ratio of inductance and capacitance of the various parts throughout the winding is not constant. Calculations and tests of voltage distribution in the winding, caused by the impact of (a) damped high-frequency oscillations, and (b) unidirectional traveling waves, are given. In order to make the analysis clearer, the transformer winding is considered as a network of inductances and capacitances, and this term “network” is used throughout the paper. Certain simplified and typical networks are considered. Transformers having one terminal grounded, such as are used in three-phase star connection, particularly in high-voltage systems, are frequently built with the insulation graded to other windings and ground, in the order of the normal frequency voltage stress. The danger of such a practise is shown in power transformers which are subject to transient overvoltage, since voltage oscillation in the winding may raise the voltage to ground at intermediate points above the terminal voltage, unless the design of the winding eliminates oscillation. The theoretical and experimental data given show that the distribution and magnitude of voltage stresses existing during recognized standard insulation tests are essentially different from stresses created by transient voltages. This permits the construction of transformers that would satisfactorily pass standard insulation tests but at the same time would not be suitable for average service. A new type of a transformer called “non-resonating,” for use on grounded neutral systems, is described. In transformers of this type, voltages of all frequencies distribute uniformly along the windings, as the possibility of internal voltage resonance is eliminated by a proper balance of distributed capacitance and inductance of the winding. This is accomplished principally by means of conducting surfaces (shields) placed outside of the winding and connected to its line terminal. The action of the shields is similar to that of the shielding ring on an insulator string. It neutralizes the effect of the capacitance current from the inside surface of the winding to ground, by supplying to every point of the winding a “charging” current equal to the “discharging” current of that point to ground. In some cases, the application of the shield reduces the local stresses to one-eightieth. Up to the present time, the total capacity of this new type of transformer exceeds half a million kva.

Application of Heaviside's Resistance Operators to the Theory of the Air-core Transformer and Coupled Circuits in General

In circuits possessing constant inductance, resistance and capacity the differential equations for the currents and the voltages are linear with constant co-efficients, and may therefore be solved by aid of the known simple properties of symbolic operators. The symbolic operator method proves to be very compendious in problems concerning the determination of the primary and secondary currents and voltages of transformers whenever the applied E.M.F. can be expressed as an exponential function of the time, or when it consists of a sudden application of a constant or an exponential function, and also when it is impulsive. In the Paper the method is first applied to a pair of indirectly coupled circuits - i.e., circuits that are insulated from each other. The cases of pure magnetic coupling, pure electric coupling and of mixed magnetic and electric coupling are treated in turn and shown to be operationally all of the same form. Then general operational equations are obtained for direct coupling and for mixed direct and indirect coupling - this latter, of course, includes all types of autotransformers, both high and low frequency. Here again the operational equations are of the same form as in the case of magnetic coupling. The equations readily show that if the primary applied voltage be a pure sine or cosine alternating E.M.F. the well-known circle diagram for determining the instantaneous magnitudes and phases of the primary and secondary currents in an ordinary transformer is applicable to the most general type of transformer, and is capable of certain extensions. It is then shown that a similar circle diagram can be used for obtaining the primary and secondary currents in any general transformer under the application of the quasi-steady conditions of a slowly-damped alternating voltage. A further new theorem arises here, which affirms that a similar graphical method of computation can be used for deducing the voltages at the terminals of the primary and secondary condensers in a high-frequency transformer under steady or quasi-steady alternating current. In the second section of the Paper it is shown how to calculate accurately the free periods and the damping factors of a coupled system with two degrees of freedom, and approximate formulae are also deduced. The third section passes to the investigation of the currents and voltages produced in the circuits by sudden application or removal of a constant, a harmonic, or a damped harmonic E.M.F., and of those produced by the application of electromotive impulses. For this purpose formulae expressing the effect of the various types of symbolic operator occuring in the Paper, on impulsive E.M.F.s and on sudden applications, are deduced and tabulated. Using these tables of integrals it is very easy to write down quickly the solutions appropriate to any given transformer under the action of any of the types of E.M.F. specified. These tables include all the cases likely to arise in wireless telegraphy and in ordinary transformers. As an example of the application of the methods the problem of a primary spark-circuit and its antenna is worked out under resonance conditions, and for various couplings and time graphs are made of the primary and secondary currents and voltages. A further example is the solution for the incidence of a natural electric wave (a stray) on a receiving antenna and its resonant secondary.