Air-core Transformer Research Articles

A 25-MW Soft-Switching HVDC Tap for<tex>$pm$</tex>500-kV Transmission Lines

This paper presents an optimization of a previously proposed high-voltage dc current transmission system (HVDC) tap system where the power rating of the soft-switching dc-dc converter has been increased to supply medium loads (up to 25 MW) from /spl plusmn/500-kV HVDC transmission lines. This dc-dc converter is connected in series with a pole of the HVDC transmission line. It drains energy to a dc capacitor through an air-core transformer and a single-phase diode rectifier connected at the secondary. A pulsewidth-modulation (PWM) voltage-source converter is directly connected to this dc capacitor to supply ac loads. The HVDC tap comprises all those components and can supply high-quality power to isolated or interconnected load areas. The controller of the HVDC tap is independent from the control systems of the main HVDC converter stations. It does not need any communication channel to the control of the main HVDC converter stations. A complete bipolar /spl plusmn/500-kV HVDC system, together with a refined model of dc transmission line were implemented to simulate the performance of the proposed HVDC tap. It has revealed to be very robust and to be an attractive solution for feeding small loads from long dc transmission lines.

Proposal of DC shield reactor type superconducting fault current limiter

Saturated DC reactor type superconducting fault current limiter (SFCL) had been proposed two years ago. It was classified to rectifier type SFCL. The changing inductance value with the operating mode has superior characteristics to reduce voltage sag during step increase of the load current. But it has the disadvantage of its weight. In this paper, rectifier type SFCL with shielded reactor has been proposed. The reactor which has superconducting ring or tube inside its winding is substituted to the DC link of the rectifier. The configuration looks like an air core transformer with secondary short winding. When the current through the bulk shield-ring reaches to a certain level, the flux penetrates to the shield body and finite impedance appears in the primary winding. In other words, when the surface flux density exceeds its critical flux density, the flux penetrates into the bulk superconductor, and increases equivalent inductance. The equivalent transient resistance of the shield was represented as a function of exponential of the time. Using this equivalent transient resistance, the transient impedance was expressed. The transient wave analysis using EMTDC (electro-magnetic transients in DC systems) has been described. Simulated waveforms are shown considering the source inductance, the leakage inductance, the coupling coefficient and the forward voltage drop of the semiconductor. And voltage sag was also investigated with 50% step load increase. Preliminary design was also performed. The coil size and number of turns are designed to obtain adequate inductance for the current limitation, and the central magnetic field of the coils are calculated. There is optimal aspect ratio to minimize the magnetic field with restriction in outer diameter of the coil.

Development of a superconducting transformer using self-resonant techniques for DC∕DC power conversion

The design and implementation of a novel superconducting air-core power transformer for power conversion applications is described. The transformer is constructed using high temperature superconducting tape. The high temperature superconducting tape is made using Ag-clad (Bi,Pb)2Sr2Ca2Cu3O10+x using the oxide power-in-tube technique. Self-resonance techniques are used so that a lower switching frequency operation can be used. The loss calculation to demonstrate the improved efficiency that can be obtained with the proposed transformer is also presented. Experimental results are given to demonstrate the effectiveness of the proposed system for power conversion applications.

Interfacing transformer for a pulsed load current exceeding 1 MA

In much pulsed power experimentation a capacitor bank is discharged into an inductive load, but although sufficient energy may be available in the capacitors their voltage rating may considerably exceed that necessary for the load and the current delivered during the experiment may accordingly be too low. This paper describes a novel design of air-cored transformer that has been used as an interfacing or matching device in such a situation, where peak load currents between 1 and 2 MA were required.Design considerations led to the use of an air-cored autotransformer connection wound with copper sheet conductors. Although thick wide conductors and clamping are needed to prevent deformation due to high magnetic pressure, the transformer is nevertheless relatively simple, easy to make, lightweight and inexpensive.This paper describes the design and the winding arrangement of the transformer that was constructed, and presents typical experimental results.

Controlling confinement with induced toroidal current in the flexible Heliac TJ-II

A method to control plasma particle and energy confinement in the TJ-II Heliac device is reported. A small toroidal current is induced in the plasma with the aid of a 0.2 Wb air core transformer. Plasma particle and energy confinement improve (degrade) with negative (positive) plasma current. For typical TJ-II discharges, plasma density and temperature profiles broaden considerably when plasma current is sufficiently negative, accounting for a 40% increase in stored energy. The experimental results agree qualitatively with the paradigm of instability growth rate modifications with magnetic shear.

Calculation of inductances of high frequency air-core transformers with superconductor windings for DC–DC converters

A simplified Neumann's formula to calculate the inductance of a transformer with spiral shape is proposed. The advantage of using the proposed method is to avoid the cumbersome and tedious integration in Neumann's formula; the solution usually includes elliptic functions that are complicated and long power series. The stray capacitance of the transformer is also calculated. The experimental measurement agrees well with the calculation.

Fabrication and test of force-balanced-coil type air-core superconducting transformer using parallel conductor

A small experimental superconducting air-core transformer was fabricated and tested. It is an autotransformer with a fundamental coil configuration of a kind of helical one. It has a configuration of force-balanced-coil (FBC). The test results show that the transformer works effectively as a power transformer. The obtained output power is about 1 kVA. Because of the absence of iron-core and also because of its force-free configuration, the operation noise of the transformer was very small.

Development of a flyback polymer thick film planar transformer using Mn-Zn ferrite for HF switch mode DC-DC converter

In this work, air core and ferrite core polymer transformers were developed using polymer thick film screen-printing technology. These transformers were printed on 5 × 5cm alumina substrate. The magnetic core material was home-made high permeability Mn–Zn ferrite. An efficiency of 93% and a coupling factor of 0.9 at 500kHz were achieved. This transformer design has five primary turns connected in series and one secondary turn consisting of five turns connected in parallel, the primary and the secondary windings being fully interleaved. All the layers, including the magnetic (Mn–Zn), isolative and conductive layers of the transformers, are made from polymer paste materials. Due to their higher heat radiation, higher power density and lower profile over conventional transformers, these transformers can be used in DC–DC converters for switch mode power supply (SMPS) applications.

A high-power high-voltage multi-pulse twin-output driver for radiation generators

This paper presents the second phase of a research programme aimed at producing twin series of high-voltage pulses from the single discharge of a capacitor into multiple arrays of exploding metallic wires. Details of the design and construction of a generator that is able to produce either three 200 kV pulses, two 250 kV pulses or a single 500 kV pulse, on resistive loads between 50 and 100 Ω at each output, are given. In addition to describing the design of the exploding wire arrays, the paper also details other ancillary equipment that is needed for the generator. This includes the two compact air-cored transformers that provide the twin series of output pulses at the required voltage. The experimental performance of the generator is shown to be close to that predicted by numerical modelling.Results are presented with the resistive loads replaced by x-ray heads, and ways by which the system can be further developed to drive microwave sources are discussed.

Magnetic field and electromagnetic force analysis of 3-phase air-core superconducting power transformer

The superconducting windings of the air-core superconducting power transformer have possibilities of being exposed to a higher magnetic field than those of the iron-core transformer because of no special paths for magnetic flux. Thus, in this paper, the magnetic field and electromagnetic force acting on the windings of an experimental 3-phase air-core superconducting transformer are analyzed. From the results, it has become clear that the superconducting windings of the air-core transformer are exposed to a 3-dimensional magnetic field containing a rotational component.

HVDC tapping using soft switching techniques

Soft switching techniques have been applied to develop a novel dc/dc power converter for feeding small loads (under 5 MW) with low cost and high reliability, from HVDC transmission lines. This dc/dc converter is connected in series with a pole of the HVDC transmission line. It drains energy to a dc capacitor through an air-core transformer. From this dc capacitor, a voltage-source converter may be directly connected to supply ac loads. This HVDC tap can supply high quality ac power to isolated areas, with or without ac power generation. The controller of the HVDC tap is independent from the master control of the HVDC link. It does not need any communication channel to the control of the main HVDC converter stations. Simulation results have revealed that the proposed HVDC tap can be a very good option for feeding small loads along the HVDC transmission line. It generates high quality ac voltage and is extremely robust.

Experience with the ECRH system of ASDEX-Upgrade

The 140 GHz ASDEX-Upgrade ECRH system uses four gyrotrons to generate a total power of 2 MW in a Gaussian beam for 2 s. External magnetic perturbations, originating from the poloidal stray magnetic field of the tokamak air core transformer, and also from the cryomagnet of the adjacent gyrotron, influence the startup of the oscillation and the electron beam deposition in the collector. The transmissions are partly quasioptical and partly corrugated HE-11 waveguide lines. Their losses have been determined calorimetrically.

Multimegawatt solid state rf driver for generating rotating magnetic fields

A radio frequency (rf) system using solid state switching has been designed and constructed that is capable of driving a pair of loop antenna with a circulating power of 60 MW for millisecond pulses. The primary application of this system is to produce an oscillating magnetic field in the frequency range of 0.2–0.7 MHz, to generate and sustain a large plasma current. The maximum power transfer from the rf source was measured to be greater than 10 MW. The driver consisted of 12 insulated gate bipolar transistor modules driven in parallel, and was capable of switching 20 kA at 1700 V. A sinusoidal current waveform with large circulating currents was obtained on the antenna by incorporating the antenna in a high Q (60), parallel LCR resonant circuit. The low impedance switch driver was matched to the antenna load through a 20:1 step-up air core transformer with a coupling efficiency of 99%.

Equivalent Circuit Model of an Inductive RF Discharge with a Helical External Coil

An inductive RF discharge with a helical external coil has been analyzed by an equivalent circuit model including both the inductive and capacitive coupling branches. A true inductive discharge is considered as a co-axial one-turn secondary winding of an air-core transformer, while a capacitive current is assumed to flow from a high voltage coil plate to a grounded electrode via a quartz glass and sheaths. The dependence of the plasma density on antenna coil voltage is calculated as a function of the excitation frequency and gas pressure. It is shown that the mechanism of the E-H mode jump, accompanied by the hysteresis phenomenon, can be attributed to the nonlinear dependence of the inductively absorbed power on the plasma density.

High frequency planar transformer with helical winding structure

A high frequency planar transformer with helical windings is proposed. This transformer combines the advantages of the planar magnetic core transformer and air core transformer. The experimental results show that the input impedance and the voltage ratio of this new transformer are much higher than its air core counterpart. The numerical result shows that the flux is evenly distributed and totally enclosed inside the planar ferrite. The electromagnetic interference (EMI), generated from the windings of transformer, is significantly reduced by using magnetic ferrite cores.

Generation of multiple pulses with extremely short pulse repetition interval

A new type of multipulse generator is proposed for the generation of high power pulses with an extremely short interpulse repetition interval. In this system, an air-core step-up transformer is used with a magnetic switch and a pulse forming line, which is charged in the double-resonance mode. Numerical simulations of this system have shown that 600 kV, 56 ns pulses, matched to a 20 /spl Omega/ impedance can be produced with a minimum pulse separation of 2.7 /spl mu/s. A small system was constructed to demonstrate the production of double pulses experimentally. The system contains two first stage capacitors of 150 nF each, a 1:15 air-core step-up transformer, a magnetic switch using cobalt based material, a second stage capacitor of 1.6 nF, and an 80 /spl Omega/ load. Double-pulses 100 ns wide, peak voltages of 85 kV (first pulse) and 90 kV (second pulse) have been successfully generated with an interval of 3.75 /spl mu/s between pulses, when the first stage capacitors were charged to 14 kV (for the first pulse) and 17 kV (for the second pulse).

円筒誘導結合型プラズマの放電モード遷移の研究

The mode transitions of cylindrical inductively coupled plasmas have been studied on the basis of the equivalent circuit model included both inductive (H-mode) and capacitive (E-mode) coupling branches. The pure inductive discharge is considered as a co-axial one-turn secondary winding of an air-core transformer, while a capacitive current is assumed to flow from a high voltage coil plate to a grounded electrode via a quartz glass and sheaths. The dependence of the plasma density on RF voltage amplitude is calculated as a function of the ratio of angular excitation frequency to electron-neutral collision frequency, ω/νc. It is found that the E-H mode jump with the hysteresis phenomenon occurs at ω/νc>1, on the other hand the plasma density continuously increases from E- to H-mode at ω/νc<1.

Ferrite Core Effects in a 13.56 MHz Inductively Coupled Plasma

We investigate the effects of adding a ferrite core to the rf coil in a cylindrical inductively coupled plasma, thereby increasing the mutual inductance between the rf coil and the plasma at 13.56 MHz. This could reduce resistive power loss, Pcoil(loss)=Icoil2Rcoil, in the copper coil because of lower required rf drive current to ignite and sustain a plasma. The air core transformer circuit model of Piejak et al.. is modified to account for the effects of the ferrite core on the rf circuit. Specifically, circuit elements accounting for core power losses and the new primary inductance created by the addition of the core are included. The plasma electron density created by inductive drive, with and without the core was the same at the same total rf power. Any energy savings in the copper coils are offset by the large core losses at 13.56 MHz due to hysteresis and eddy currents.

空心トランスを用いた高繰り返しパルス電力装置の開発

空心トランスを用いた高繰り返しパルス電力装置の開発

The electromagnetic basis of the transformer model for an inductively coupled RF plasma source

An air-cored transformer model is commonly used to analyse macroscopic measurements of the electrical properties of inductively coupled, H-mode RF discharges. In this paper, we explicitly expose, via an electromagnetic theory of the H-mode plasma produced within a planar coil ICP, the manner in which the values of the circuit elements of the transformer representation are related to the values of the electron number density and effective collision frequency of the generated plasma. A comparison of experimental measurements and theoretical predictions lends credence to the view that the transformer model of H-mode discharges has a very useful role to play in the design and understanding of inductively coupled RF plasma sources.