High Frequency Power Transformer Research Articles

Investigation of Partial Discharge Transformation Characteristics in Polyimide (PI) Insulations under High-Frequency Electric Stress.

This research delves into the primary issue of polyimide (PI) insulation failures in high-frequency power transformers (HFPTs) by scrutinizing partial discharge development under high-frequency electrical stress. This study employs an experimental approach coupled with a plasma simulation model for a ball-sphere electrode structure. The simulation model integrates the particle transport equation, Poisson equation, and complex chemical reactions to ascertain microscopic parameters, including plasma distribution, electric field, electron density, electron temperature, surface, and space charge distribution. The effect of the voltage polarity and electrical energy on the PD process is also discussed. The contact point plays a pivotal role in triggering partial discharges and culminating in the breakdown of PI insulation. Asymmetry phenomena were found between positive and negative half-cycles by analyzing the PD data stage by stage. A significant number of PDs increased at every stage and the PD amplitude was higher during the negative cycle at the initial stage, but in later stages, the PD amplitude was found to be higher in the positive half-cycle, and scanning electron microscopy (SEM) revealed that the maximum damage occurred near the contact point junction. The simulation results show that the plasma initially accumulates the electron density near the contact point junction. Under the action of the electric field, plasma starts traveling at the PI surface outward from the contact point. Before the PD activity, all parameters have higher values in the plasma head. The microscopic parameters reveal maximum values near the contact point junction, during PD activities where significant damage takes place. These parameter distributions exhibit a decreasing trend over time as when the PD activity ends. The model's predictions are consistent with the experimental data. The paper lays the foundation for future research in polymer insulation design under high-frequency electrical stress.

Application Prospects of Hybrid Magnetic Circuits in High Frequency Power Transformers

In medium to high frequency power transformers, apart from minimizing the power loss in the core and copper, the design of its thermal circuit plays an important role in containing the temperature rise in different parts. It is extremely complex because there exist multiple loss centers with different heat removal characteristics. Moreover, the power loss is concentrated around a small core volume and the rest of it is available for convection and radiation. This article proposes to use a hybrid core configuration to improve the heat removal feature of the transformer. This article establishes through practical validation on suitability i.e., the magnetic compatibility of cores in the hybrid magnetic circuit. It further explores the application prospect of the proposed concept where the thermal performances have been compared using different core types in two different application domains. In the first case, the core could always remain energized to its rated value, and in the second design, both the windings could always draw their respective rated current.

Forced Air-Cooling Thermal Design Methodology for High-Density, High-Frequency, and High-Power Planar Transformers in 1U Applications

Due to increased loss per unit volume in power transformers when operating at higher frequency, the increase in cooling system size can outweigh high-frequency transformer (HFT) size reduction benefits. Conventional air-cooling approach simply adopts oversized cooling fans with high airflow to keep the transformer below a temperature limit, resulting in a large footprint and high power consumption. For many applications using air cooling, more challenges arise when multiple transformers are placed in a dimension-constrained space in which free airflow from the cooling system is easily disrupted. This work seeks to address this tradeoff by presenting a high-density, low-profile, forced air-cooling system for HFTs. The combination of an epoxy resin cast transformer winding and converging air duct is proposed to minimize the heat transfer barrier from both the internal and external structures of the high-frequency power transformer. The air-cooling system design process is demonstrated with a 15-kW load test featuring two 500-kHz planar Litz-wire transformers. The proposed transformer cooling system achieves a power density of 635 W/in3, a peak height of 43 mm, and a cooling power consumption of 6.8 W. Compared to using a 120 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times120$ </tex-math></inline-formula> mm cooling fan, peak winding temperature is reduced to 8%, the cooling system volume is reduced to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times $ </tex-math></inline-formula> , and a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times $ </tex-math></inline-formula> reduction of cooling power consumption is achieved.

CALCULATION OF FAST FRONT OVERVOLTAGES IN VICINITY OF A POWER TRANSFORMER

During their lifespan all the primary equipment installed in power system is subjected to transients. Therefore, it is necessary to adequately dimension and protect the equipment. However, in the networks with high share of renewables, power electronics, cables or near GIS, the failure can occur even though good practice of insulation coordination are fulfilled.&#x0D; In this paper high frequency power transformer model is presented. Such model is based on measurements of admittance matrix and is adequate for simulation of fast front transient in EMTP like software. Additionally, simulations of lightning and switching overvoltages are given for the observed power transformer unit.

Analysis, Design, and Experimental Results for a High-Frequency ZVZCS Galvanically Isolated PSFB DC-DC Converter over a Wide Operating Range Using GaN-HEMT

This paper investigates the potential of the emerging gallium nitride (GaN) high-electron mobility transistors (HEMT) power devices to meet certain power conversion challenges. The advantages of utilizing GaN HEMT transistors in a high-frequency, high-power isolated DC-DC topology are explored experimentally. Using the GaN HEMT’s parasitic elements, e.g., output capacitance, and the leakage inductance of the transformer, a soft switching zero-voltage zero-current switching (ZVZCS) phase shift converter is proposed. Accordingly, the freewheeling current is terminated, and soft switching is realized for most of the primary and secondary active devices. Furthermore, without using any additional circuitry, the overshoot voltage across the bridges of active rectifier diodes is clamped at their voltage level. In addition, a high-frequency power transformer is optimized to minimize the overall transformer losses (e.g., winding and core losses). Combined the conductor types, e.g., litz wire and copper foil, shows good electrical and thermal performance by reducing the AC and DC resistance. Finally, a 5 kW, 100–250 kHz prototype is built and tested. The experimental results show a conversion efficiency of up to 98.18% for the whole converter.

Practical Study of Mixed-Core High Frequency Power Transformer

The design of medium- to high-frequency power electronics transformer aims not only to minimize the power loss in the windings and the core, but its heat removal features should also allow optimal use of both core and copper. The heat removal feature (e.g., thermal conduction) of a transformer is complex because there exist multiple loss centers. The bulk of total power loss is concentrated around a small segment of the core assembly where windings are overlaid. The primary winding is most constrained thermally. For superior use of core and copper, the temperature rise in different segments of the transformer should be well below their respective safe operating limits. In practice, cores of same soft-magnetic materials are traditionally used. To achieve superior temperature profile and for better long-term performance, this article proposes to use the mixed-core configuration. The new core(s) would replace the parent ones from the segment where windings are laid. The characteristic features of new cores would share increased burden of heat removal from the transformer. To obtain the qualitative insight of magnetic and thermal performance, the proposed mixed-core transformer would be thoroughly validated practically in two different high-power applications. In the first case, the core is always energized to its rated value, and in the second one, windings are always energized at respective rated current capacity.

Clarification of the Method for Determining Distribution of Coil and Core in Small-capacity High-frequency Wireless Power Transformer of Coil Volume and Loss

Clarification of the Method for Determining Distribution of Coil and Core in Small-capacity High-frequency Wireless Power Transformer of Coil Volume and Loss

Study on dielectric properties, electric modulus, and impedance spectroscopy of Ni–Ca ferrite nanoparticles

The dielectric properties of the Ni1-xCaxFe2O4 nanoparticles (x = 0.02, 0.04, and 0.1) synthesized by the citrate precursor method and sintered at two different temperatures, 600 °C and 1000 °C, were investigated by using a complex impedance technique in the range of 100 Hz to 5 MHz. Also, the temperature effect of the dielectric properties of the two sintered Ni0.9Ca0.1Fe2O4 samples at different frequencies was studied in the range of 313 K–773 K showing their semiconducting nature. The frequency dispersion of ε′, ε" and tanδ for all the samples is attributed to the interfacial polarization mechanism, which follows the Maxwell-Wagner model. The variation of the ac conduction in the samples depends primarily on the electron hopping between Fe3+↔Fe2+ ions and hole hopping between Ni2+↔Ni3+ ions in the octahedral sites. The Cole-Cole (Zʺ vs. Zʹ) plots characterize by two overlapping semicircles represented by an electrical circuit consisting of two parallel R–C in series, manifesting the grain and grain boundary effect on the electrical properties of the samples. The (Mʺ vs. Mʹ) plots insure the electric stiffness behavior of this type of ferrites. Moreover, in the present ferrite samples, the lower values of the σʹac, ε′, εʺ, and tanδ at high frequencies, make them suitable for use as an inductor-core for high-frequency power transformers for telecommunication purposes.

Boundary Condition Independent Thermal Network Modeling of High-Frequency Power Transformers

With increasing global power demands and the trend of miniaturization of power converters, electronic components are being pushed toward their thermal limits. This has accentuated the need for accurate and low-cost thermal models of electronic components. Lumped Parameter Thermal Network Modeling (LPTNM) is commonly used to evaluate the thermal performance of electronic components, but unlike semiconductors, transformer LPTNM is not standardized in the academic community. Due to this lack of standardization, most proposed transformer thermal models have not been evaluated for Boundary Condition Independence (BCI). This study presents a BCI thermal modeling approach for power transformers based on LPTNM. A thermal network model is developed by discretizing the transformer geometry into smaller volumes and evaluating the thermal resistances by approximating the three-dimensional heat transfer problem as a piecewise one-dimensional model. The network model/geometry discretization is iteratively improved for a set of different boundary conditions, resulting in a BCI model. The modeling approach is experimentally and numerically validated on a PQ40/30 transformer operating in a 3.3 kW switch mode power supply. The BCI transformer thermal model closely matches both numerical and experimental results with a fraction of the computational cost of the numerical model. The proposed BCI modeling approach provides a low-cost alternative to detailed thermal models and can predict transformer thermal performance for varied applications.

Magneto-thermal coupling simulation and experimental verification for a three-winding high-frequency transformer

As a core component of the power electronic transformer (PET) in DC network, the multi-level high-frequency power transformer has received great attention due to the insulation material fatigue problems resulting from the hot-spot temperature rises. To solve this problem, a three-winding high-frequency transformer for 10 kVA PET application is designed and made in the laboratory, and the loss and temperature rise distribution is calculated by means of the finite element (FE) electromagnetic-thermal coupling simulation. The influence of temperature on the hysteresis and loss properties of core material has been carefully considered and measured. The influence of skin effect and end effect on the winding loss is taken into account through the establishing three-dimensional FE model. Besides, the convective heat transfer coefficient is solved based on the principle of heat transfer instead of the empirical coefficient method. By compared with the experimental results, the calculated results are validated to be effective in predicting the loss and hot-spot temperature rises of the transformer.

High-Frequency Surface Insulation Strength with Nanoarchitectonics of Disiloxane Modified Polyimide Films.

High-frequency power transformers are conducive to the reliable grid connection of distributed energy sources. Polyimide is often used for the coating insulation of high-frequency power transformers. However, creeping discharge will cause insulation failure, therefore, it is necessary to use disiloxane for the purpose of modifying the molecular structure of polyimide. This paper not only introduces 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane (GAPD) with a molar content of 1%, 2%, and 5% to polyimide, but also tests both the physical and chemical properties of the modified film and the high frequency creeping dielectric strength. The results show that after adding GAPD, the overall functional groups of the material do not change, at the same time the transfer complexation of intermolecular charge and the absorption of ultraviolet light increase. There is no phase separation of the material and the structure is more regular and ordered, moreover the crystallinity increases. The overall dielectric constant and the dielectric loss tangent value show different trends, which means that the former value increases, while the latter value decreases. In addition, the resistivity of the surface and the volume increase, which is the same as the glass transition temperature. The mechanical properties are excellent, and the strength of bulk breakdown is mounting. The insulation strength of the high frequency creeping surface has been improved, which will increase with larger contents of GAPD. Among them, the relative change of the creeping flashover voltage is not obvious, and the creeping discharge life of G5 is 4.77 times that of G0. Further analysis shows that the silicon-oxygen chain links of the modified film forms a uniformly dispersed Si-O-Si network in the matrix through chemical bonds and charge transfer complexation. Once the outer matrix is destroyed, it will produce dispersed flocculent inorganic particles which have the role of protecting the inner material and improving the performance of the material. Combined with the ultraviolet light energy absorption, the increase of deep traps, the reduction of dielectric loss, and the improvement of thermodynamic performance, can better improve the high-frequency creeping insulation strength of polyimide film and its potential application value.

Study on High Frequency Surface Discharge Characteristics of SiO2 Modified Polyimide Film.

Polyimide (PI) can be used as a cladding insulation for high frequency power transformers, and along-side discharge can lead to insulation failure, so material modification techniques are used. In this paper, different doped nano-SiO2 are introduced into polyimide for nanocomposite modification. The results of testing the life time of high-frequency electrical stress along-side discharge show that the 10% SiO2 doping has the longest life time. The results show that: for composites prone to corona, their flashover causes more damage, and both positive half-cycle and polarity reversal discharges are more violent; compared to pure PI, the positive half-cycle and overall discharge amplitude and number of modified films are smaller, but the negative half-cycle is larger; at creeping development stages, the number of discharges is smaller, and the discharge amplitude of both films fluctuates in the mid-term, with the modified films having fewer discharges and the PI films discharging more violently in the later stages. The increase in the intensity of the discharge was greater in the later stages, and the amplitude and number of discharges were much higher than those of the modified film, which led to a rapid breakdown of the pure polyimide film. Further research found that resistivity plays an important role in the structural properties of the material in the middle and late stages, light energy absorption in the modified film plays an important role, the distribution of traps also affects the discharge process, and in the late stages of the discharge, the heating of the material itself has a greater impact on the breakdown, so the pure polyimide film as a whole discharges more severely and has the shortest life.

Surface discharge characteristics and influencing factors of polyimide insulations under high‐frequency sinusoidal voltages

The high-frequency power transformer (HFPT) has become a vital component of the power system. For HFPT insulations, surface discharge is a serious problem. Polyimide (PI) is a widely used insulating material for HFPT's. A needle-plate electrode structure-based experimental setup was developed to produce and measure the surface discharge. This paper first time used the fluid-kinetic model by adopting the finite element simulation method to address the micro-level study of surface discharge. This paper aimed at the discharge phenomenon of the PI under the direct action of high-frequency sinusoidal voltage and has not considered the overvoltage phenomenon caused by the waveform parameters that is the actual high-frequency action on the PI material. The development of surface discharge at various stages was analysed. Particle densities and electric field distributions versus discharge time were determined. Frequency, temperature, and air pressure have a significant effect on surface discharge behaviour. As a result, the applied voltage reaches its peak value at high-frequency, so the flashover time of surface discharge is reduced under the high frequency. The electron density of the streamer development increases as the temperature rises, and the time to flashover the channel becomes shorter. The higher the electron density and the longer the discharge channel develops while the gas pressure is lower. The simulation results have high accuracy and good agreement with experimental data.

High Frequency Solid State Transformer Design Considerations for Power Systems Applications

The conventional power frequency (50 or 60 Hz) transformers are economical, highly reliable and quite efficient but they suffer with certain drawbacks like sensitive to harmonics, voltage drop under load, no protection from system disruptions and overloads, poor performance under dc offset load unbalances and no scope to improve power factor. These transformers with copper wound wires on iron cores are unable to respond to control signals as power generations become distributed and intermittent. So, the need of electronic based regulated power supply with software based remote intelligence has become essential. Also, to easily connect the new energy sources to the grid and to improve the power quality by harmonic filtering, voltage sag correction and highly dynamic control of the power flow, a new type of transformer based on power electronics, known as SST has been introduced. The SST realizes voltage transformation, galvanic isolation, power quality improvements such as instantaneous voltage regulation, voltage sag compensation and power factor correction. It is a collection of high-powered semiconductor components, high frequency power transformer and control circuitry which is used to provide a high level of flexible control to power distribution networks. The SST is a high frequency switched Power Electronic Devices (PEDs) based transformer with high controllability that enables flexible connectivity between existing medium voltage power distribution network, low voltage AC residential system and envisioned DC residential system. In this paper a systematic constructional detail of a SST with a power rating of 2 kVA, operating frequency of 20 kHz and voltage rating of 600/60 V as a scaled-down prototype used for power converter topologies is presented. The design is simple and it avoids the difficulty of choosing massive amounts of empirical parameters.

ZVZCS SRI Guides Optimal Use of Copper and Core for Air-Cooled Nanocrystalline Transformer for Induction Heating

In induction heating applications, active and passive power components play important roles for proper functioning of the power controllers. For suitability study, the characterization of active power components is well defined and documented. Characterization of capacitors is also well defined. They are commercially available as well. High-frequency power magnetic components, such as transformers and inductors, are not readily available. For better power density and reliable operation of controllers, their qualitative evaluation is necessary. The parametric set to define the suitability of finished high-frequency magnetic components is large and wide, have multiple and complex heat sources. For input materials, multiple choices are available. Two main ingredients in high-frequency magnetic components are the magnetic core and litz-wire conductor. This article, using inherent features of ZVZCS SRI topology, explores practical methods to validate the optimal design of the high-frequency power transformer for induction heating applications. The method is able to evaluate the usable current density of the litz wire conductors as well as the upper limit of usable flux density of nanocrystalline cores.

The high frequency surface discharge behaviours of modified polyimide films: the effects of phenyl thioether in the backbone

High frequency surface discharge is one of the main causes of insulation failure in the high frequency power transformer. In order to extend the endurance lifetime and investigate the behaviours of surface discharge, a series of polyimide films containing phenyl thioether groups were prepared and tested in a self-built experimental platform with a high frequency voltage source. First, the surface discharge inception voltages on the films were tested under high frequency sinusoidal voltage. Then, with the application of continuous high frequency voltage, surface discharge tests were conducted to observe the entire evolution process of surface discharge including its initiation, self-sustaining, streamer and the ultimate flashover. Combining with the frequency-domain dielectric spectra and x-ray diffraction tests, effects of phenyl thioether content on the evolution process in each stage were analyzed. Experimental results and analyses indicate that the discharge inception voltage is opposite to the dielectric loss factor. The depth and distribution of traps caused by crystal and amorphous regions play the different role in various stages of surface discharge. The endurance lifetime of polyimide film containing 40% phenyl thioether was the longest and the effects of which were further discussed based on the discharge behaviour analysis. The paper demonstrates that molecular-structure modification is an effective method to improve the high frequency electrical performance of polyimide.

Design and Construction of an Atmospheric Pressure HF Plasma Source

Atmospheric pressure plasma sources used in many industrial, environmental, light sources, biological, and medical applications. Aiming these prospects, an effort has been remunerated for the design and construction of a cost-effective atmospheric pressure high frequency (30 KHz) plasma source. The major sections include in this source are, (i) DC power suppliers, (ii) A set of Class A push-pull power amplifiers, (iii) Ferrite core high-frequency power transformer, (iv) Impedance matching network, and (v) Electrode design for plasma production. Two power suppliers have been considered; one is a 12V DC for biasing the base of the 2N3055 power, and the other is a 90V DC for biasing the Class A push-pull power amplifier. Class A push-pull power amplifier is chosen because of its high efficiency of about 50% concerning other types of amplifiers. Ferrite core high-frequency power transformer is designed with a turn ratio of 1:3 in order to step up the voltage to a certain level for producing atmospheric pressure plasma. An impedance matching network is designed to transfer maximum power from the source to the plasma load.

Increasing the arc burning stability during electric arc surfacing using ribbon electrode

The issues of stability of the arc during surfacing with a ribbon electrode are considered. It is shown that the electric arc movement along the end of the electrode is not progressive, but chaotic. In this case, the surfacing process is accompanied by periodic short circuits of the ribbon electrode on the workpiece. It was established experimentally that in order to improve the quality of the deposited metal, it is necessary to provide an operating mode with minimal fluctuations of the arc current. To solve this problem, it is necessary to realize the possibility of rapid destruction of the jumper, which is formed when the electrode is shorted on the workpiece, and to ensure reliable re-excitation of the electric arc. Regarding this, it is is proposed a method and a system for improving arc stability during arc surfacing with a ribbon electrode, consisting of an inverter with a microcontroller control, matching high-frequency power transformer, the secondary winding of which is connected through a capacitor in parallel to the welding arc. When a short circuit is detected between electrode and the workpiece (by catching the reduction of the voltage on the arc gap), the generator is turned on at the resonant frequency of the circuit formed by the capacitor and the inductance of the secondary circuit. A high-frequency component with an amplitude of several kA is superimposed on the main operating current, which leads to rapid heating and destruction of the jumper. The destruction is detected by the control system by «seeing» the increase of voltage on the arc gap. After that, the generator is turned off until the next short circuit is detected

Zero-Voltage and Zero-Current Switching PWM DC–DC Converter Using Controlled Secondary Rectifier With One Active Switch and Nondissipative Turn-Off Snubber

Soft switching pulse-width modulation (PWM) dc–dc full-bridge converter with controlled rectifier on the secondary side of the high-frequency power transformer is presented in this paper. An active secondary switch, including new nondissipative passive energy recovery turn-off snubber on the secondary side and modified PWM control algorithm, provide conditions for zero-voltage and zero-current turn-on and zero-current turn-off of IGBT transistors on the primary side of the high-frequency dc–dc converter. The simple passive energy recovery snubber ensures also zero-voltage turn-off of the transistor on the secondary side of the power transformer. The principle of operation is explained and analyzed and consequently verified on a 2 kW, 50 kHz laboratory model of the converter.

Partial discharge behavior and ground insulation life expectancy under different voltage frequencies

Power electronic transformers, namely solid state transformers, have aroused a widespread attention due to the integration issue of large scale renewable energy in modern power system. High frequency power transformer (HFPT) is one of its key components operating at several to tens of kHz. Under such high frequency voltage, accelerated insulation aging and premature insulation failure occur frequently. Partial discharge (PD) is considered as the main cause of electrical performance deterioration in HV systems or LV ones where overvoltage or surge may occur. Therefore, it is necessary to study the PD behavior and insulation life under different voltage frequencies, in order to have a better understanding of premature failure mechanism. In this paper frequency dependence of space charge and PD properties is firstly analyzed theoretically and mathematically, followed by the results of electrical aging tests conducted on polyimide (PI) films that were subjected to surface PDs under 5-30 kHz sinusoidal voltage. Discharge characteristics and lifetime of PI film under different frequencies have been discussed, along with the correlation between lifetime and PD. Both theoretical analyses and experimental results show that average PD amplitude and total PD number per cycle increase first and then decrease afterwards with voltage frequency, which means that the frequency induced inflection phenomenon exists in the tested frequency range. Synergistic action of high frequency induced thermal effect and reduced space charge dissipation time under high frequency is the reason for this novel phenomenon. Besides, strong linear correlation has been found between lifetime and PD characteristic-total PD amplitude per second, which reveals the accelerated aging mechanism under high frequency electrical stress.