Showing papers on "Isolation transformer published in 2011"

Quasi-Z-Source-Based Isolated DC/DC Converters for Distributed Power Generation

Abstract: This paper presents new step-up dc/dc converter topologies intended for distributed power generation systems. The topologies contain a voltage-fed quasi-Z-source inverter with continuous input current on the primary side, a single-phase isolation transformer, and a voltage doubler rectifier (VDR). To increase the power density of the converter, a three-phase auxiliary ac link (a three-phase inverter and a three-phase isolation transformer) and a three-phase VDR are proposed to be implemented. This paper describes the operation principles of the proposed topologies and analyzes the theoretical and experimental results.

High-Frequency Transformer Isolated Bidirectional DC–DC Converter Modules With High Efficiency Over Wide Load Range for 20 kVA Solid-State Transformer

Abstract: This paper presents the design of new high-frequency transformer isolated bidirectional dc-dc converter modules connected in input-series-output-parallel (ISOP) for 20-kVA-solid-state transformer. The ISOP modular structure enables the use of low-voltage MOSFETs, featuring low on-state resistance and resulted conduction losses, to address medium-voltage input. A phase-shift dual-half-bridge (DHB) converter is employed to achieve high-frequency galvanic isolation, bidirectional power flow, and zero voltage switching (ZVS) of all switching devices, which leads to low switching losses even with high-frequency operation. Furthermore, an adaptive inductor is proposed as the main energy transfer element of a phase-shift DHB converter so that the circulating energy can be optimized to maintain ZVS at light load and minimize the conduction losses at heavy load as well. As a result, high efficiency over wide load range and high power density can be achieved. In addition, current stress of switching devices can be reduced. A planar transformer adopting printed-circuit-board windings arranged in an interleaved structure is designed to obtain low core and winding loss, solid isolation, and identical parameters in multiple modules. Moreover, the modular structure along with a distributed control provides plug-and-play capability and possible high-level fault tolerance. The experimental results on 1 kW DHB converter modules switching at 50 kHz are presented to validate the theoretical analysis.

Small-size light-weight transformer with new core structure for contactless electric vehicle power transfer system

Abstract: A contactless power transfer system for electric vehicles is required to have high efficiency, a large air gap, good tolerance to misalignment in the lateral direction, and be compact and lightweight. A new 1.5 kW transformer has been developed to satisfy these criteria using novel H-shaped cores, which is more efficient, more robust to misalignment, and lighter than previously employed rectangular cores, and its characteristics are described. An efficiency of 95% was achieved across a 70 mm air gap. The results of test at wide air gap of 100mm, temperature rise test and 3kW operation test are also presented.

A Three-Phase Step-Up DC–DC Converter With a Three-Phase High-Frequency Transformer for DC Renewable Power Source Applications

Abstract: This paper presents a new three-phase step-up dc-dc converter with a three-phase high-frequency (HF) isolation transformer in an average current-mode controlled closed loop. This converter was developed for industrial applications where the dc input voltage is lower than the output voltage, for instance, in installations fed by battery units, photovoltaic arrays, or fuel cell systems. The converter's main characteristics are reduced input ripple current, step-up voltage, HF transformer, reduced output-voltage ripple due to three-pulse output current, and the presence of only three active switches connected to the same reference, this being a main advantage of this converter. By means of a specific switch modulation, the converter allows two operational regions, each one depending upon the number of switches in overlapping conditions-if there are two switches, it is called R2 region, and if there are three switches, it is called R3 region. An average current-mode control strategy is applied to input-current and output-voltage regulation. Theoretical expressions and experimental results are presented for a 6.8-kW prototype, operating in the R2 region, and for a 3.4-kW prototype, operating in the R3 region, both in continuous conduction mode.

Apparatus for transferring energy using onboard power electronics with high-frequency transformer isolation and method of manufacturing same

Abstract: An apparatus for transferring energy using onboard power electronics with high-frequency transformer isolation includes a power electronic drive circuit comprises a dc bus and a first energy storage device coupled to the dc bus A first bi-directional dc-to-ac voltage inverter is coupled to the first energy storage device and to the dc bus, and a first electromechanical device coupled to the first bi-directional dc-to-ac voltage inverter A charging system coupled to the dc bus via a charge bus comprises a receptacle configured to mate with a connector coupled to a voltage source external to the power electronic drive circuit and an isolation transformer configured to electrically isolate the charge bus from the receptacle A controller configured to cause the charging system to supply a charging voltage to the dc bus based on a voltage received from the voltage source external to the power electronic drive circuit

Magnetic induction communication system for an implantable medical device

Abstract: A communication system for an active implantable medical device. The communication system includes an isolation transformer a coil coupled to the isolation transformer, and first and second communication components each coupled to the isolation transformer such that the first and second communication components are electrically isolated from the coil, and such that the isolation transformer enables the first and second communication components to communicate, via magnetic induction (MI) using the coil, with at least one external component.

Effect of Repeated Impulses on Transformer Insulation

Abstract: Power transformer plays a vital role in electrical power system. Many of the transformer failures reported are due to insulation failure in windings. The sources of failures are mainly due to impulse overvoltages of different waveshapes and also due to repeated application of impulses. In this paper an attempt has been made to analyze the degradation of transformer insulation for repeated applications of different impulse voltages. As oil impregnated paper (OIP) constitute major insulation of transformer winding, detailed experimental analyses are carried out for OIP insulations using standard test cell for impulse waves of varying front times from 0.064 μs to 1.4 μs and tail times from 6.0 μs to 67 μs. Exponential mathematical models for the voltage-number (V-N) characteristics are derived and used for predicting the number of impulses that OIP can withstand for any wavefront and wavetail. The accuracy of the models are checked with the experimental values.

High step-up continuous input current LCCT-Z-source inverters for fuel cells

Abstract: Z-source inverters (ZSIs) are suited for applications which require a large range of voltage gain, such as in fuel cells. The present paper extends the concept of ZSIs. New type high step-up LCCT-Z-source inverter dedicated for fuel cells is proposed in the paper. Compared to recently proposed modifications of basic Z-source inverter the new topology has the advantages of available continuous input current, improved boost ratio and simple topology. Thanks to application of transformer with turns ratio higher than one the proposed LCCT-Z-source can operate with lower shoot-through ratio and higher modulation index compared to ZSI. The existence of two built-in DC-current-blocking capacitors connected in series with transformer windings prevents the transformer core from saturation. Simulation and experimental results are provided to verify the reliability of the proposed topology.

Continuous uninterruptable AC grounding system for power system protection

Abstract: A continuous grounding system for use in an alternating current system including a transformer is disclosed. The system includes a switch assembly connected between a transformer neutral of a transformer and a ground, the switch assembly having an open position and a closed position, the open position disrupting the path through the switch assembly between the electrical connection and the transformer neutral, and the closed position establishing a path connecting the electrical connection to the transformer neutral through the switch assembly, wherein in normal operation of the alternating current electrical device the switch assembly remains in a closed position. The system also includes a DC blocking component positioned in parallel with the switch assembly and connected between the transformer neutral and the ground. The system further includes a control circuit configured to control the switch assembly, the control circuit including a sensor configured to actuate the switch assembly to an open position upon detection of a predetermined harmonic signal threshold at one of the transformer phases or a predetermined threshold of DC current between the transformer neutral and ground.

Multivariate Analysis of Transformer Resonant Overvoltages in Power Stations

Abstract: In this paper, cable-transformer resonant overvoltages are studied on the low-voltage (LV) side of a 410-MVA generator step-up (main) transformer, due to ground fault and energization. The transformer is placed in a network configuration often found in Norwegian hydropower stations. The overvoltages are calculated when systematically varying the length of the high-voltage (HV) side cable and that of the cable between the main transformer LV side and the station transformer. This multivariate analysis is performed efficiently, using frequency-domain calculations and the inverse Numerical Laplace Transform. Time-domain simulations are used for analyzing the critical cases in detail. Both types of calculations make use of a blackbox, wideband model of the transformer, and novel procedures are introduced for initializing computations from correct 50-Hz initial conditions. Among the findings are that extreme overvoltages can occur on the station transformer if current-limiting reactors are placed in series with the cable between the main transformer LV side and the station transformer.

High voltage power transformer insulation design

Abstract: The large power transformer life span depends heavily on insulation — its condition, materials, composition, geometry, etc. The high voltage transformer insulation requires a very focused analysis in the design stage. The large power transformer is subjected to different overvoltages during factory testing and in operation, therefore all resulting electric stresses have to be modeled in the electric field program and compared to the industry accepted withstand curves. The overvoltages include transient voltages from impulse testing, both lightning and switching, as well as power frequency voltages generated during the induced and applied voltage tests. The distributions of electric stresses under these different electric excitation conditions are different. The transient voltage programs with MLCR components (mutual- and self-inductance, capacitance, resistance) are used to establish exact voltage distribution inside the transformer. The electric field analysis for complex units is performed for all energized components, including turn-to-turn, section-to-section, winding-to-winding, winding to core, phase-to-phase, between leads, from winding and leads to constructional parts, bushing to tank, etc. The electric stresses are checked under two main conditions: (i) strike in oil, (ii) creeping discharge along solid insulation. The paper will discuss the analysis for selected transformers and explain the design process needed to maintain the dielectric stresses under critical values.

Switching control strategy to extend the ZVS operating range of a Dual Active Bridge AC/DC converter

Abstract: A switching control strategy to extend the zero-voltage-switching (ZVS) operating range of a Dual Active Bridge (DAB) AC/DC converter to the entire input-voltage interval and the full power range is proposed. The converter topology consists of a DAB DC/DC converter, receiving a rectified AC line voltage via a synchronous rectifier. The DAB comprises a primary side half bridge and secondary side full bridge, linked by a high-frequency isolation transformer and inductor. Using conventional control strategies, the soft-switching boundary conditions are exceeded at the higher voltage conversion ratios of the AC input interval. A novel pulse-width-modulation strategy to fully eliminate these boundaries and its analysis are presented in this paper, allowing increased performance (in terms of efficiency and stresses). Additionally, by using a half bridge / full bridge configuration, the number of active components is reduced. A prototype converter was constructed and experimental results are given to validate the theoretical analyses and practical feasibility of the proposed strategy.

A Class of High-Input Low-Output Voltage Single-Step Converters with Low Voltage Stress on the Primary-Side Switches and High Output Current Capacity

Abstract: Power conversion from an input voltage of several kilovolts to a low load voltage is of great significance in various applications, but poses serious challenges. In this paper, a new converter, which is able to realize such a large step-down conversion in a single step, is proposed by introducing a novel concept of dc-dc multiphase conversion and n-phase interleaving rectification. The proposed structure is formed by n switch pairs in the primary side, an n-phase isolation transformer with the primary windings connected to dc blocking capacitors, and an n-phase current multiplier in the output side. The switching patterns applied to the switch pairs have a phase difference of 360° \mathord/ \vphantom 360°n n, and the output inductor currents are interleaved correspondingly, making necessary a smaller output filter. For a Vi input voltage and Io load current, the converter features Vi/n voltage stress on the primary-side switches, and Io/n current stress on the secondary-side inductors and diodes. Thus, the magnetic size of the inductors is considerable reduced. The primary-side switches are commutated with zero-voltage-switching (ZVS). Therefore, rather than using insulated-gate bipolar transistors (IGBTs) or MOSFETs with higher voltage ratings, the most available, notable performing 500/600 V MOSFETs can be used in the proposed converter with several kilovolts supply voltage, allowing for a higher operation frequency and lower conduction losses. Compared with an input-series-output-parallel (ISOP) connection of full-bridge (FB) isolated converters, for the same voltage stress on the switches, the proposed converter requires half of the number of transistors and inherently balances the input voltage among the switch pairs. The switching mechanism of a typical switch pair in the kth interval Ts/n of a switching cycle is analyzed. A dc analysis was carried out to determine the dc conversion ratio and the ZVS conditions in an analytical form. It allows for a tradeoff design of the converter, such that to minimize the duty-cycle loss and maximize the ZVS load range. A 1500/48-V, 2-kW prototype with four switch pairs was designed, implemented, and evaluated. The experimental results prove the soft switching of the switches, the low voltage stress across the primary-side switches, and the low current flowing through the rectifier's diodes and inductors. The efficiency measured at nominal power rating was 90.75%.

Analysis, Design, Modeling, and Implementation of an Active Clamp HF Link Converter

Abstract: This paper deals with the system oriented analysis, design, modeling, and implementation of active clamp HF link three phase converter. The main advantage of the topology is reduced size, weight, and cost of the isolation transformer. However, violation of basic power conversion rules due to presence of the leakage inductance in the HF transformer causes over voltage stresses across the cycloconverter devices. It makes use of the snubber circuit necessary in such topologies. The conventional RCD snubbers are dissipative in nature and hence inefficient. The efficiency of the system is greatly improved by using regenerative snubber or active clamp circuit. It consists of an active switching device with an anti-parallel diode and one capacitor to absorb the energy stored in the leakage inductance of the isolation transformer and to regenerate the same without affecting circuit performance. The turn on instant and duration of the active device are selected such that it requires simple commutation requirements. The time domain expressions for circuit dynamics, design criteria of the snubber capacitor with two conflicting constrains (over voltage stress across the devices and the resonating current duration), the simulation results based on generalized circuit model and the experimental results based on laboratory prototype are presented.

100-KVA micro-grid energy storage bidirectional converter

Abstract: The invention provides a 100-KVA micro-grid energy storage bidirectional converter, comprising a controller, a three-phase bridge-type inverter, an adapter and an isolation transformer The high-voltage end of the isolation transformer is connected with an alternating-current bus; the alternating bus is connected with a power grid via an alternating-current breaker; the lower-voltage end of the isolation transformer is respectively connected with the alternating-current output end of the three-phase bridge-type inverter and the alternating-current input end of the adapter; the direct-current input end of the three-phase bridge-type inverter is provided with a direct-current breaker; the direct-current output end of the adapter is provided with the direct-current breaker; the sampling signal input end of the controller is respectively connected with the direct-current input end/alternating-current output end of the three-phase bridge-type inverter and the direct-current output end/ alternating-current input end of the adapter; the controller controls the outputs of the three-phase bridge-type inverter and the adapter; the controller is also used for controlling an alternating-current contactor, the direct-current breaker and the alternating-current breaker which are connected with the three-phase bridge-type inverter and the adapter; and the controller is provided with a communication interface and a man-machine device interface The technical scheme is suitable for energy exchanging between different energy storage batteries and the power grid in manners of charging and discharging

Power quality improvement based on novel power electronic transformer

Abstract: This paper presents a novel topology of power electronic transformer. In the design process, the AC/DC, DC/AC, AC/AC converters and high frequency transformer have been used. One matrix converter operates as AC/AC converter in power electronic transformer. The proposed power electronic transformer performs typical functions and has advantages such as power factor correction, voltage sag and swell elimination, voltage flicker reduction and protection capability in fault situations. Power quality improvement with proposed power electronic transformer has been verified by the simulation results.

Power supply apparatus and method for a backlight system

Abstract: A power supply system includes a transformer having a primary winding on its primary side for coupling to a power source and one or more secondary windings on its secondary side A first control circuit is disposed on the primary side of the transformer for controlling a current flow in the primary winding A second control circuit disposed on the secondary side of the transformer, and the second control circuit is configured to provide a regulated output voltage In the power supply system, the first control circuit is configured to generate a control signal for controlling the current flow in the primary winding without using a feedback control signal from the secondary side of the transformer

Six-Phase Machine Drive System With Reversible Parallel AC–DC–AC Converters

Abstract: This paper proposes a reversible six-phase machine drive system. It is composed of two parallel three-phase to three-phase dc-link converters without isolation transformers. The proposed topology permits one to reduce the harmonic distortion in the input converter and decreases the power processed by converter switches. Furthermore, the proposed configuration can guarantee the feasibility of the system when high-power applications are considered. The model and the control strategy have been developed. The simulation and experimental results are presented.

Fault Current Limitation in Power Network by the Superconducting Transformers Made of 2G HTS

Abstract: The fault current limiting feature of the SC transformer with the HTS 2G windings provide protection and significantly reduce wear and tear for circuit breakers and other substation power equipment. A comparison of the short circuit currents of the 64 MVA transformer wound with copper and YBCO 2G HTS tapes is made in this paper. The results of the analysis show that the YBCO 2G tapes enable to built the superconducting transformer with the ability to limit the short-circuit currents. The superconducting windings require special considerations for the induced electromagnetic forces in order to limit them below the allowable tensile stresses. Strongly anisotropic Y-123 tape used in the winding of the transformer will be exposed to different orientations and amplitudes of the local magnetic field. If the transformer is going to act also as a fault current limiter, the uniformity of the transition of the transformer winding is crucial to avoid sectional responses of the transformer. Therefore it is essential to conduct systematic current-magnetic field measurements to define `constant current domain' for the conductor at a given operating temperature.

High voltage transformer bushing problems

Abstract: Bushing is an important part of power transformer. It is used to insulate the incoming or outgoing conductor into or out of a grounded barrier, in power transformer case is the transformer main tank. The bushings connect the windings of the transformer to the supply line and insulate the feed through conductor from the transformer main tank. From the physical and electrical point of view, bushing appears to be the weakest part of the transformer. The starting of the bushing breakdown process could not be detected easily and could lead to the sudden failure of the bushing, even the failure of the whole transformer. Currently, any time-based-maintenance conducted in PLN P3B Jawa Bali based on manufacturer's manual had been proved not sufficient to recognize any degradation in the bushing. Sometimes it is very hard to find the real root cause of the failure, and getting only the suspected cause of failures. This paper presents some field experiences concerning transformer's bushing failures, along with some facts and findings related to the failure.

Dc-ac inverter with high frequency isolation transformer

Abstract: The novel DC-AC inverter topology with high frequency isolation transformer consists of an input DC-DC converter with high frequency isolation transformer and an output full-bridge unfolding converter with four transistors provides the output AC voltage from a DC source. The input DC-DC converter has two primary side controllable switches and a single rectifier on the secondary side, two resonant capacitors, a resonant inductor, an output inductor and a high-frequency isolation transformer, which does not store DC energy. The duty ratio D of the primary side switches is modulated by the rectified AC voltage to result in an output rectified AC voltage, which is unfolded into an AC sinusoidal output voltage by the output full-bridge unfolding converter.

Medium frequency transformer for rail application using new materials

Abstract: A converter system based on medium frequency dc/dc-converters can replace the conventional traction transformer saving both mass and energy. A medium-frequency-transformer design-procedure is introduced that allows a clear representation of transformers total mass and efficiency under consideration of thermal aspects. measurements with a prototyped mf-transformer verify the calculated results.

The influence of square voltage waveforms on transformer insulation break down voltage

Abstract: The use of Voltage Source Converters (VSCs) in HVDC transmission and, more generally, in flexible AC transmission systems (FACTS), raises concerns over the endurance of turn/turn insulation of grid/converter interface transformers. In this paper, tests performed on oil-impregnated paper specimens are discussed. The results indicate clearly that shorter rise times and higher repetition frequencies have a strong impact on both partial discharge inception voltage and breakdown voltage. These results suggest that deeper investigation might be important to understand the long term behavior of these insulation systems.

ZVS Converter With Parallel Connection in Primary Side and Series Connection in Secondary Side

Abstract: This paper presents an active snubber dc/dc converter to realize the zero voltage switching of power switches. Thus, the switching losses on power switches can be reduced. The primary windings of two transformers are connected in parallel to reduce the transformer current stresses. To reduce the transformer secondary winding turns at high-output-voltage applications and distribute output power to two transformers, two transformer secondary windings are connected in series. Thus, the transformer primary currents are automatically balanced. A full-wave diode rectifier is used at the secondary side to realize full-wave rectification at the output side and reduce voltage stress on rectifier diodes. Finally, the performance of the proposed converter is evaluated on a 600-W (400 V/1.5 A) laboratory prototype.

Laboratory validation of the relationship between Geomagnetically Induced Current (GIC) and transformer absorbed reactive power

Abstract: This paper presents the theory and simulation work to establish a relationship between Geomagnetically Induced Current (GIC) flowing through a transformer's windings and the reactive power absorbed by that transformer's core. This relationship is determined in laboratory for various levels of GICs injected under different loading conditions of a transformer. This obtained relationship validates the results obtained through electromagnetic transients simulation using PSCAD/EMTDC and also correlates well with actual field data. This study is one of the first laboratory simulation studies of the effect of GIC, reported in literature.

Design and Implementation of a Rectangular-Type Contactless Transformer

Abstract: In this paper, a rectangular contactless transformer is proposed to be used in a contactless power supply system. The proposed rectangular contactless transformer maintains a high coupling coefficient even when it has a relatively large air gap. The characteristics of the proposed transformer are compared to a transformer using a general EE core with a variable air gap. The proposed system is verified through simulation and experimental results. The experimental results demonstrate that the efficiency of the contactless power transmission system using a contactless transformer based on a rectangular core is over 20% greater than the system based on the EE core.

Impact of power system harmonics on insulation failure of distribution transformer and its remedial measures

Abstract: The ageing of insulation denotes physical and chemical changes, which takes place as a result of electrical stresses in the power system network. In the modern days, a large use of nonlinear devices has resulted into increased distortion in load voltages. The current drawn by all such devices contains harmonics. The losses as well as insulation are more in the equipment at harmonic frequencies than the fundamental frequency. The thermal circuit capability at the fundamental frequency is inadequate to dissipate increased losses due to harmonic current. Hence excessive current drawn during steady state and dynamic power quality problems resulting into the higher temperature rise. The increased use of the non-linear devices in today's network is responsible for faster degradation of insulating material. The transformer transfers electrical energy from one point circuit to another. The additional heating experienced by a transformer depends on the harmonic content of the load current & the design principals of the transformer. In this paper, an attempt has been made to discuss the procedure for loss calculations and analysis of impact of the temperature rise on conventional oil immersed and derated transformer under the non-linear load conditions. Some remedial measures are also suggested to reduce the impact of harmonics on insulation failure.

DC-to-DC power conversion

Abstract: Systems and methods for managing interaction between inverter-based DC and other power systems are disclosed. In one embodiment, a 3-phase isolation transformer is fluxed to create a 3-phase rotating field from the output of a source inverter. An inductive filter turns that output into three sine waves. A secondary inverter regenerates the system, sometimes after the isolation transformer is fluxed, and by advancing or retarding the secondary inverter's phase, current (and, thus, the DC voltage and power direction) is controlled. In another embodiment, an inverter is supplied by a DC source. The inverter is controlled to match its output voltage, current, and phase to a live AC grid, then the two are connected. The inverter frequency is then driven to advance the phase of the inverter in relation to the grid. Alternatively, the inverter voltage is then driven at a level greater than that of the grid.

Clamping force sensor assembly for monitoring transformer degradation

Abstract: A clamping force sensor assembly for a transformer includes at least one sensor disposed within a casing. The design and configuration of the sensor assembly is such that it accurately measures clamping force values placed on the windings, without adversely impacting the operation of the transformer and with the ability to continue operating under electrical and thermal changes within the transformer. The sensor assembly can include loading members that distribute the pressure evenly over the casing that contains the sensor. The output of the sensor can indicate to an operator the extent of pressure changes experienced by the transformer windings. The output can be coupled with a process by which the damage to or the possibility of failure of the transformer is readily evaluated.