Showing papers on "Isolation transformer published in 2014"

Optimal ZVS Modulation of Single-Phase Single-Stage Bidirectional DAB AC–DC Converters

Abstract: A comprehensive procedure for the derivation of optimal, full-operating-range zero voltage switching (ZVS) modulation schemes for single-phase, single-stage, bidirectional and isolated dual active bridge (DAB) ac-dc converters is presented. The converter topology consists of a DAB dc-dc converter, receiving a rectified ac line voltage via a synchronous rectifier. The DAB comprises primary and secondary side full bridges, linked by a high-frequency isolation transformer and a series inductor. ZVS modulation schemes previously proposed in the literature are either based on current-based or energy-based ZVS analyses. The procedure outlined in this paper for the calculation of optimal DAB modulation schemes (i.e., combined phase-shift, duty-cycle, and switching frequency modulation) relies on a novel, more accurate, current-dependent charge-based ZVS analysis, taking into account the amount of charge that is required to charge the nonlinear parasitic output capacitances of the switches during commutation. Thereby, the concept of “commutation inductance(s)” is shown to be an essential element in achieving full-operating-range ZVS. The proposed methods are applied to a 3.7 kW, bidirectional, and unity power factor electric vehicle battery charger which interfaces a 400 V dc-bus with the 230 Vac, 50-Hz utility grid. Experimental results obtained from a high-power-density, high-efficiency converter prototype are given to validate the theoretical analysis and practical feasibility of the proposed strategy.

LLC Resonant Converter With Matrix Transformer

Abstract: In this paper, a high-efficiency high power density LLC resonant converter with a matrix transformer is proposed. A matrix transformer can help reduce leakage inductance and the ac resistance of windings so that the flux cancellation method can then be utilized to reduce core size and loss. Synchronous rectifier (SR) devices and output capacitors are integrated into the secondary windings to eliminate termination-related winding losses, via loss and reduce leakage inductance. A 1 MHz 390 V/12 V 1 kW LLC resonant converter prototype is built to verify the proposed structure. The efficiency can reach as high as 95.4%, and the power density of the power stage is around 830 W/in3.

Transformer-Less Grid Feeding Current Source Inverter for Solar Photovoltaic System

Abstract: High efficiency and operating life of grid feeding solar photovoltaic (PV) inverters are demanded. Due to reduced dc-link capacitor requirement, current source inverter (CSI) offers higher reliability than the voltage source based solar inverter. However, conventional three-phase pulse width modulated (PWM) current source based solar inverter injects high earth leakage current into the grid. In order to suppress this current, an isolation transformer can be used. Use of this transformer increases the cost and size, and decreases overall efficiency. In order to address the aforementioned limitations, a modified CSI is proposed in this paper. The proposed inverter suppresses the earth leakage current without using an isolation transformer, thereby increasing the efficiency and reducing cost as compared to conventional current source based solar inverters. A mathematical model of the system is derived based on which controller for the operation of the inverter is designed. The effectiveness of the scheme is verified through detailed simulation study. To confirm the viability of the scheme, experimental studies are carried out on a scaled-down laboratory prototype.

Flux Balancing of Isolation Transformers and Application of “The Magnetic Ear” for Closed-Loop Volt–Second Compensation

Abstract: Semiconductor switches possess nonideal behavior which, in case of isolated dc-dc converters, can generate dc-voltage components which are then applied to the isolation transformer. This dc-voltage component is translated into a dc flux density component in the transformer core, increasing the risk of driving the core into saturation. In this paper, a novel noninvasive flux density measurement principle, called “The Magnetic Ear,” based on sharing of magnetic path between the main and an auxiliary core is proposed. The active compensation of the transformer's dc magnetization level using this transducer is experimentally verified. Additionally, a classification of the previously reported magnetic flux measurement and balancing concepts is performed.

Power Electronic Traction Transformer: Efficiency Improvements Under Light-Load Conditions

Abstract: Power electronic transformer (PET), a converter technology that utilizes power semiconductors in combination with medium-frequency transformers, is considered a promising solution for certain applications requiring flexible galvanic isolation. Among are those where space occupied by bulky low-frequency transformers is of concern and/or where advanced power quality control features are needed. In this paper, the PET for a single-phase traction on-board application is discussed with emphasis on the efficiency improvements and reductions of energy consumption during the operation on the vehicle. Several control algorithms devised to improve efficiency under light-load conditions are tested on a low-voltage prototype of the PET, and experimental results are presented demonstrating the effectiveness of the proposed algorithms.

Impact of High PV Penetration on Distribution Transformer Insulation Life

Abstract: The reliable operation of distribution systems is critically dependent on detailed understanding of load impacts on distribution transformer insulation systems. This paper estimates the impact of rooftop photovoltaic (PV) generation on a typical 200-kVA, 22/0.415-kV distribution transformer life under different operating conditions. This transformer supplies a suburban area with a high penetration of roof top photovoltaic systems. The transformer loads and the phase distribution of the PV systems are significantly unbalanced. Oil and hot-spot temperature and remnant life of distribution transformer under different PV and balance scenarios are calculated. It is shown that PV can significantly extend the transformer life.

Iron Losses in a Medium-Frequency Transformer Operated in a High-Power DC–DC Converter

Abstract: The three-phase dual-active bridge is a dc-dc converter, which is highly suitable for high-power applications Among others, this is due to the medium-frequency transformer in the ac link, which provides galvanic isolation The transformer is operated with a square-shaped voltage waveform The flux density in the transformer core is piecewise linear However, for the sake of simplicity, the magnetic flux is often assumed sinusoidal Thereby, the actual iron losses generated in the core material are misinterpreted This paper discusses the difference between the exact piecewise linear and the sinusoidal course in terms of iron losses Silicon steel with a thickness of 018 mm is measured at a frequency of 1000 Hz, comparing the sinusoidal excitation with the actual one The measurements are validated using the improved generalized Steinmetz equation Finally, the transformer core losses are evaluated when the dc-dc converter is operated under load The results are confirmed through measurement

Characterization of the Voltage and Electric Field Stresses in Multi-Cell Solid-State Transformers

Abstract: Solid-State Transformers (SSTs) are a promising technology since they enable a reduction in weight and volume of transformers while integrating new functionalities and services in the grid. However, a new kind of electric stress for the insulation occurs in this type of power converter, given that low-frequency medium-voltage stresses are mixed with high- frequency stresses generated by the converter's switching actions. This paper analyzes, in time and frequency domains, the voltage stresses appearing in cascaded converters, employing converter cells based on two-level or three-level bridge legs. The highest electric fields occur in the medium-frequency transformers of the converter cells' DC-DC converters, which provide galvanic isolation within the SST. Numerical simulations of the electric field distributions in these transformers are presented for the different frequency components and the impact of the converter topology on the insulation stress is highlighted. Furthermore, it is shown that the dielectric losses of the transformer can be ne- glected despite the presence of high-frequency harmonics. Finally, a transformer insulation concept based on semiconducting tape is proposed. Index Terms—Solid-State Transformers, Multi-cell converter, Medium-frequency transformer, Electric field, Insulation.

Transformer winding deformation diagnostic system using online high frequency signal injection by capacitive coupling

Abstract: Transformer winding deformation is common among all sorts of transformer failures. Cumulative deformation can eventually burgeon into catastrophic faults and result in entire network outage. It is possible to detect the early signs of faults with continuous online monitoring transformer. However, frequency response analysis (FRA) is considered to be a useful and accurate tool for sufficient detection. This paper aims at proposing a novel diagnostic system for online monitoring power transformer winding deformation based on FRA. In order to realize online monitoring transformer winding, the system uses the capacitive coupling method to inject controllable nanosecond pulses, which function as the excitation signal of winding, and to obtain the response signal. This proposed method may extend frequency range for analysis and perhaps could early detect minor winding movement and looseness. Transformer experiments show great prospect in the application of the system.

Electrical breakdown in dielectric liquids-a short overview

Abstract: Power transformers are utilized to convert high voltages normally used in electrical power transmission to lower voltages more suitable for consumers. A transformer consists essentially of a magnetic iron core with primary and secondary copper windings. The alternating current flowing in the primary winding induces a magnetic flux in the core, which in turn creates a current in the secondary windings. If there is a difference in the number of turns in the primary and secondary windings, the secondary voltage will be scaled up or down proportionally to the ratio of the turns. In this way, a high voltage can be transformed to a low voltage. However, the desire to convert increasingly greater electrical loads using smaller power transformers results in both higher electrical and thermal stresses. The materials utilized to insulate the different electrically conductive components from each other must be designed to withstand those stresses. The insulating media often consist of pressboard and an insulating liquid. The liquid performs a double duty as it not only insulates the conductive parts but also functions as a liquid coolant. Here we are primarily interested in the oil as an insulating medium.

Analysis and Design of Energy Regenerative Snubber for Transformer Isolated Converters

Abstract: An energy regenerative passive snubber for transformer isolated converters is proposed. The snubber is implemented on the transformer's primary and secondary windings. The proposed snubber significantly reduces the voltage spike across the switch caused by the transformer's primary inductance upon switch turn-off and facilitates the fast ramping up of the transformer secondary current. In addition, the proposed snubber provides lossless zero voltage turn off and zero current turn on conditions for the power switch. Experimental example of a flyback converter has shown measured efficiency exceeding 90%. This paper describes the principle of operation and presents approximate theoretical analysis and design guidelines of the proposed snubber. Simulation and experimental results are also reported. The proposed energy regenerating snubber is best suited for flyback and SEPIC converters and can also be adapted to other transformer isolated topologies.

Trifoliate Three-Phase Contactless Power Transformer in Case of Winding-Alignment

Abstract: This paper proposes a trifoliate three-phase contactless power transformer. This transformer can achieve balanced electrical behavior between three phases at winding alignment. For this transformer, models are presented with accompanying detailed explanations. The models reproduce the electrical behavior more precisely than conventional models. A contactless power transfer system using a prototype of the proposed transformer and the resonant capacitances estimated by the models works effectively and attains good performance levels. In addition, in winding misalignments with slight displacements, the system is able to maintain these high-performance levels.

Wind Turbine Generator–Battery Energy Storage Utility Interface Converter Topology With Medium-Frequency Transformer Link

Abstract: A medium-voltage (MV) wind turbine generator (WTG)-battery energy storage (BESS) grid interface converter topology with medium-frequency (MF) transformer isolation is introduced in this paper. The system forms a three-port network in which several series stacked ac-ac converters transform the low-frequency (50/60 Hz) utility MV into MF (0.4 to 2 kHz) ac voltage by modulating it with MF square wave. This voltage is then fed to the MF transformer primary windings. The secondary and tertiary windings interface with the WTG side and the BESS side, respectively, after power conversion. The power generated by WTG is transferred to the MF transformer secondary windings through a three-phase pulse width modulation (PWM) rectifier and a three-phase PWM inverter, whereas the power transfer between the BESS and the tertiary winding occurs through a three-phase PWM inverter. It is shown that the utility grid sinusoidal currents, the battery current, and the WTG output currents can be controlled to be of good quality using PI and DQ control strategies. Thus, the proposed MF transformer-based three-port topology results in smaller converter weight/volume. Moreover, the control can handle voltage sags/swells and provide low voltage ride-through capability. Simulation waveforms along with experimental results are shown as proof of concept.

Gapped transformer design methodology and implementation for LLC resonant converters

Abstract: In the LLC resonant converter, the air gap is generally positioned in the core of the transformer for proper magnetizing inductance. Traditional transformer design methods assume infinite permeability of the core and no energy stored in the core. The improved design methodology for the gapped transformer is proposed with the optimum relative permeability and gap selection to meet the temperature rise and the magnetizing inductance requirements. The magnetizing current influences the magnetic flux in the core leading to the core saturation and core loss, while the resonant current contributes to the winding loss. The transformer design for a 200 W, 90 kHz LLC resonant converter is presented and experimental results validate the proposed methodology.

Medium voltage power distribution architecture with medium frequency isolation transformer for data centers

Abstract: In this paper, a medium-voltage (MV) data center power distribution system (DC-PDS) architecture using medium-frequency (MF) link transformer isolation is introduced. The proposed approach significantly improves power density while maintaining high efficiency compared to conventional line-frequency based solutions. The approach also contributes to a reduction in PVC or copper used in conventional DC-PDS. First the MV transformed from the utility, is interfaced with a MV switch gear system and a diesel power generator (DPG) system. Then this MV is converted to the low voltages (LVs) required by the loads via MF transformer. The MF transformer primary side windings are connected to stacked AC-AC converters. The LV secondary windings are interfaced with several load systems and battery energy storage system (BESS) using different topologies like boost power factor corrector (PFC), PWM inverter, etc. The presence of MV switch gear and MF transformer in the architecture results in higher efficiency and power density.

A method to measure the DC bias in high frequency isolation transformer of the dual active bridge DC to DC converter and its removal using current injection and PWM switching

Abstract: The Dual Active Bridge (DAB) bidirectional dc to dc converter has gained a significant popularity in high voltage, high power due to its several topological advantages. Due to non-ideal switching characteristics and delays in gating, DC bias may be generated in the high frequency transformer leading to increased loses and eventually saturation and failure of the transformer and the switches. This paper proposes a simple way to measure the dc bias in the transformer flux using the primary and secondary winding current. A simple duty cycle modulation of the primary bridge is proposed to eliminate the DC bias in lower power topologies. This method has been verified through experimental results. For high power applications a current injection method through a separate tertiary winding is proposed, and verified such that the magnetizing current is supplied separately by the tertiary winding eliminating the possibility of transformer saturation.

Diagnostics of a Transformer's Active Part With Complementary FRA and VM Measurements

Abstract: This paper presents the concept of complementary measurements with the use of two methods that allow for a more detailed assessment of the mechanical condition of a transformer's active part. These methods are frequency-response analysis (FRA) and vibroacoustic method (VM). The VM was interpreted separately in steady and transient conditions, thus allowing, together with the FRA measurements, the determination of possible source of vibrations. This paper presents a theoretical introduction, experimental measurements performed on a real transformer with introduced controlled deformations and, finally, an example of a practical application.

Winding design of a high power medium frequency transformer

Abstract: The availability of improved magnetic materials and higher voltage rating semiconductor devices have made the design of high power medium frequency transformers feasible. The paper presents a simple design procedure to determine the core dimensions based on the area product method. A winding design procedure to use foil conductors in the high voltage side to achieve ac-to-dc resistance ratio close to 1 is also shown, providing a possible replacement to the litz wires having low window utilization factor. The paper demonstrates a design example of 13.8kV/690V, 10kHz, 50kVA transformer to validate the design methodology and the winding losses are validated using 2-D FEM.

Medium-Voltage Current-Source Converter Drives for Marine Propulsion System Using a Dual-Winding Synchronous Machine

Abstract: Medium-voltage (MV) drives are increasingly used in high-power marine applications for running thrusters and main propulsion motors. In this paper, an MV drive solution employing active front-end current-source converters is proposed for a synchronous-motor-based propulsion system. The proposed solution includes two independent drives, each to control one of the two sets of three-phase windings of the synchronous motor. A dedicated communication link between the drives allows continuous load sharing and robust system operation in a master-follower drive configuration. Field-oriented control with the use of an absolute encoder is implemented for providing high starting torque and smooth speed control over a wide speed range including a 30% overspeed region. The major advantages of the proposed solution include simple structure, increased system power rating, redundant operation, low-harmonic input/output waveforms, improved reliability, elimination of a bulky input isolation transformer, and parallel drive control without the need of a complex coordinated inverter gating system. In addition, the system offers input power factor compensation and dynamic braking to allow operation on a generator-based supply system. Field test results obtained on a 14-MW ship propulsion system are provided to demonstrate the system performance.

PWM Strategies for Common-Mode Voltage Reduction in Current Source Drives

Abstract: The common-mode voltage (CMV) in the motor drive system could damage the motor insulation and induce destructive bearing current. Various reduced CMV space vector modulation (RCMV SVM) methods have been proposed in both voltage source converter (VSC) and current source converter systems. Most of them reduce the CMV by avoiding the use of zero-state vectors. Recently, a zero-state vector selection strategy for CMV reduction in CSCs has been proposed, which allows the use of zero-state vectors and achieves reduced CMV without affecting the modulation index range and the harmonics performance of converters. This paper further investigates three possible zero-state vector selection strategies in RCMV SVM for current source drives where the PWM CSR and CSI are connected back to back. Among the three methods, Method 1 and Method 2 are suitable for CMV peak value minimization, while Method 3 is proposed for the average CMV reduction. These methods can then be considered for different types of drives (with or without isolation transformer) to reduce the insulation stress or to limit the resonance in the common mode circuit. Details of the three zero-state vectors selection methods are presented. Both simulation and experimental results are provided to verify their effectiveness.

A Wideband Current Transformer Bridge

Abstract: The development and testing results of a new current transformer (CT) test set at Physikalisch-Technische Bundesanstalt are described The operation range for the ratio based bridge is intended for frequencies from 167 Hz up to 20 kHz Equal setups of magnetically shielded current comparators with primary windings for rated currents from 100 mA up to 5 A convert the secondary currents of the standard CT and of the transformer under test (TUT) into proportional voltages The errors of the TUT are calculated from the subsequent measurement of the complex voltage ratio This current bridge and the voltage-ratio-based method allow comparing CTs with nonequal ratios At 50 Hz, the measuring system offers an extended measurement uncertainty of less than 2 ppm or μrad, respectively The bridge is verified for use with uncertainties of below 200 ppm or 400 μrad at a frequency of 20 kHz

Single-phase ac–dc–ac multilevel five-leg converter

Abstract: In this study, a multilevel five-leg single-phase ac–dc–ac converter and its generalisation are presented. The converter is suitable to be used as an uninterrupted power supply or a unified power quality conditioner without an isolation transformer. The operating principles, the pulse-width modulation strategy and the control system for the proposed converter are presented. The proposed single-phase multilevel ac–dc–ac converter is composed mostly of low-power switches capable of generating input and output voltages with low harmonic content. The proposed configuration is compared with the conventional solution in terms of harmonic distortion, semiconductor losses and ratings of switches. Experimental results demonstrate the feasibility of the proposed configuration. The experimental verification of the proposed topology and control strategy was obtained by using IGBTs with dedicated drives and a digital signal processor TMS320F28335 with plug-in boards and sensors.

Common-Mode Noise Cancellation by an Antiphase Winding in Multilayer Isolated Planar Transformer

Abstract: Common-mode (CM) noise is always present in switch-mode power supplies All International Electrical Commission class I appliances must have their chassis directly connected to the earth ground, which provides a low impedance path for CM noise to transfer from primary to secondary via the isolated transformer A proposed planar transformer construction is analyzed and a comparison of CM noise cancellation performance with wire-wound transformers is presented Planar transformers are found to have better noise cancellation throughout the conducted electromagnetic interference range, thanks to a better winding plane matching to the primary CM noise source winding plane A planar transformer with two primary winding planes is investigated and a construction with antiphase winding is proposed for best CM noise attenuation

Design, measurement and equivalent circuit synthesis of high power HF transformer for three-phase composite dual active bridge topology

Abstract: High voltage high frequency (HF) transformer provides the isolation between high and low dc link voltages in dual active bridge (DAB) converters. Such DAB converters are finding wide applications as an intermediate DC-DC converter in transformerless intelligent power substation (TIPS), which is proposed as an alternative for conventional distribution-transformer connecting 13.8 kV and 480 V grids. The design of HF transformer used in DAB stage of such application is very challenging considering the required isolation, size and cost. In this paper, the specification generation, design, characterization, test and measurement results on a 10kHz HF transformer are presented, highlighting the above challenges.

Reduction of Stray Losses in Flange–Bolt Regions of Large Power Transformer Tanks

Abstract: In large power transformers, the presence of stray currents in the structural elements near the high current bushings can be considerable, and this leads to hot spots. This work presents a practical analysis of overheating in the bolts that join the tank and the cover, which are near the high current bushings of the transformer. Overheating results are analyzed and discussed for the case of a 420-MVA transformer. The hot spots in the flange-bolt regions are discovered by thermal maps that are obtained during power transformer operation as a part of a preventive maintenance program. In this paper, we use copper links to ensure the connection of both the cover and tank body, significantly reducing the overheating of the flange-bolt region. The copper link solution has been validated by measurements. We have used calibrated measurement instruments in all the experiments. Moreover, a 3-D finite-element analysis of the geometry of interest has been used to verify the copper link solution.

High frequency link multi-winding power electronic transformer using modular multilevel converter for renewable energy integration

Abstract: Grid integration of multiple renewable energy sources using a power electronic transformer (PET) topology is presented in this paper. A modular multilevel converter (MMC) is proposed as the power converter on the high voltage grid side to generate high frequency sinusoidal voltages. A single multi-winding high frequency transformer (HFT) interfaces different wind generators/photovoltaic arrays using many H-bridge converters and PWM inverters. The leakage inductance of the transformer acts as the smoothening filter for the current through the HFT. With sinusoidal voltages and smooth currents through the transformer, there is significant reduction in transformer losses and also natural commutation of leakage energy is obtained. The operating principle, modulation and control of the proposed PET is validated by simulations in MATLAB/Simulink.

Communication within a power inverter using transformer voltage frequency

Abstract: A power inverter may include a transformer serving as an isolation barrier. The power inverter may include a first controller on one side of the isolation barrier. The first controller may encode the frequency of an input voltage of the transformer with one or more operating conditions of a direct current power supply electrically coupled to the inverter. As second controller on the other side of the isolation barrier may determine a frequency associated with an output voltage of the transformer. The second controller may decode the frequency associated with the output voltage of the transformer to determine the encoded operating conditions.