Showing papers on "Isolation transformer published in 2012"

Power electronic traction transformer technology

Abstract: Combining modern high-power semiconductor devices with constantly improving magnetic materials opens up the possibility to replace bulky low frequency transformers with a new medium voltage medium frequency conversion structures. While there are still challenges to be addressed related to these so called power electronic transformers, a steadily increasing development effort is evident and considered in various contexts. Traction applications seem to be the first ones where proliferation of these new galvanic isolated power electronic converters is expected. In this particular application field, substantial weight and volume reduction could be achieved while providing additional functionality at the same time. In this paper a survey of recent R&D efforts in this field is presented.

Analysis of Transformer Inrush Current and Comparison of Harmonic Restraint Methods in Transformer Protection

Abstract: It is well known that differential protection is most suitable for transformer protection. However, inrush current due to transformer energization can appear as fault to the protective relay. To improve the security while maintaining the required levels of sensitivity, many restraint methods have been proposed to inhibit operation of the differential element. This paper first will analyze the magnetizing inrush current during transformer energization with a simplified excitation curve. It derives mathematical equations to compute the inrush current based on the residual flux and saturation flux for the worst case energization event. This paper reviews several popular restraint methods used today. A conceptual logic diagram is provided for each method. Advantages and disadvantages of different methods are analyzed for various systems. Finally, using data recorded from real life events and data generated by digital simulations, a performance comparison of different methods is provided.

Vibration and Audible Noise Characteristics of AC Transformer Caused by HVDC System Under Monopole Operation

Abstract: When the HVDC system works in monopole ground return mode, dc current flows through the neutral of ac power transformer with the neutral grounded, which will enhance its vibration and audible noise. In this paper, dc bias experiments on ac power transformers carried out on 500-kV power transformers both in the transformer factory and the substation are described; the experimental results show how the vibration and audible noise of the transformer change under the dc bias current or windings voltage. The frequency characteristics of the vibration and audible noise of the dc-biased transformer are discussed under the conditions with and without loads. The direct connections between the vibration peak value and the dc bias current, and between the sound pressure and the dc bias current are revealed. Long-term online monitoring shows that the vibration of the transformer changes seriously even if there is not a dc system operating in monopole earth return mode, so when we analyze the influence of dc bias on the vibration and audible noise of the transformer, other influential factors should be expelled.

High frequency bus converter with low loss integrated matrix transformer

Abstract: The trend in isolated DC/DC converters is increasing output power demands and higher operating frequencies. Improved topologies and semiconductors can allow for lower loss at higher frequencies. A major barrier to further improvement is the transformer design. With high current levels and high frequency effects the transformers can become the major loss component in the circuit. High values of transformer leakage inductance can also greatly degrade the performance of the converter. Matrix transformers offer the ability to reduce winding loss and leakage inductance. This paper will study the impact of increased switching frequencies on transformer size and explore the use of matrix transformers in high current high frequency isolated applications. This paper will also propose an improved integrated matrix transformer design that can decrease core loss and further improve the performance of matrix transformers.

Switching control strategy for full ZVS soft-switching operation of a Dual Active Bridge AC/DC converter

Abstract: A switching control strategy to enable Zero-Voltage-Switching (ZVS) over the entire input-voltage interval and the full power range of a single-stage Dual Active Bridge (DAB) AC/DC converter 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 primary and secondary side full bridges, 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. Recently we presented a novel pulse-width-modulation strategy to fully eliminate these boundaries, using a half bridge — full bridge DAB configuration. In this papers the analysis is extended towards a full bridge — full bridge DAB setup, providing more flexibility to minimize the component RMS currents and allowing increased performance (in terms of efficiency and volume). Experimental results are given to validate the theoretical analysis and practical feasibility of the proposed strategy.

Hybrid distribution transformer: Concept development and field demonstration

Abstract: Today's distribution system is expected to supply power to loads for which it was not designed. Moreover, high penetration of distributed generation units is redefining the requirements for the design, control and operation of the electric distribution system. A Hybrid Distribution Transformer is a potential cost-effective alternative solution to various distribution grid control devices. The Hybrid Distribution Transformer is realized by augmenting a regular transformer with a fractionally rated power electronic converter, which provides the transformer with additional control capabilities. The Hybrid Distribution Transformer concept can provide dynamic ac voltage regulation, reactive power compensation and, in future designs, form an interface with energy storage devices. Other potential functionalities that can be realized from the Hybrid Distribution Transformer include voltage phase angle control, harmonic compensation and voltage sag compensation. This paper presents the concept of a Hybrid Distribution Transformer and the status of our efforts towards a 500 kVA, 12.47 kV/480 V field demonstrator.

Single-Phase Transformer Inrush Current Reduction Using Prefluxing

Abstract: Power transformers can experience large inrush currents upon energization, the severity of which depends on the source strength, the leakage impedance and residual flux of the transformer, and the angle of the applied voltage at energization. A novel inrush current reduction strategy has been implemented which involves setting a single-phase transformer's residual flux to a known polarity after the transformer has been de-energized, a process called “prefluxing,” and controlling the instant of transformer energization based on the flux polarity, seeking not to eliminate inrush current but to substantially reduce it. Unlike a popular suggested solution, this strategy does not require prior knowledge of the transformer's flux. The device used for prefluxing is simple in construction and operates at substantially lower voltage levels when compared to the transformer's rated voltage. The presented strategy has been successfully implemented on an 18-kVA laboratory transformer with inrush current levels reduced below the rated current of the transformer even when accounting for typical breaker deviations. This paper describes the operation of the reduction strategy, including theory, device sizing, and implementation, and presents the successful laboratory results, all of which provide the basis for implementing inrush current reduction in three-phase transformers using a three-pole circuit breaker.

Power electronic transformer (PET) converter: Design of a 1.2MW demonstrator for traction applications

Abstract: A power electronic transformer converter made of cascaded multiple cells with a medium frequency link can replace the bulky traditional transformer in various applications with several advantages, including reduced size and weight, and energy savings. This paper describes the design of a 1.2MW power electronic transformer (PET) demonstrator for traction applications. Several design challenges, i.e. the selection of the IGBT modules, the design of the multiple-functional medium frequency transformer, and the mechanical arrangement to achieve the high voltage insulation between the grid voltage and ground potential, have been discussed. The experimental results, obtained from the developed PET demonstrator, are presented.

A matrix converter-based topology for high power electric vehicle battery charging and V2G application

Abstract: In this paper, a new three-phase converter topology based on a 3x1 matrix converter (MC) is proposed for Plug-in Hybrid or Battery (PHEV/BEV) electric transit buses. In the proposed approach, the MC directly converts the low frequency (50/60 Hz, three-phase) input to a high frequency (6 kHz, one-phase) AC output without a dc-link. The output of the matrix converter (MC) is then processed by a PWM rectifier via a high frequency (HF) isolation transformer to interface with the EV battery system. The MC-PWM rectifier system is made to operate like a dual active bridge (DAB), facilitating bi-directional power flow suitable for charging and Vehicle-to-Grid (V2G) application. The digital control of the system ensures that the input currents are of high quality under both charging and discharging operations. Due to the absence of dc-link electrolytic capacitors, power density of the proposed rectifier is expected to be higher. Analysis, design example and extended simulation results are presented for a three-phase 208 V LL , 50kW charger.

Isolated dynamic current converters

Abstract: Isolated Dynamic-Current (“Dyna-C”) converters are converters that convert incoming 3-phase AC or DC power to a mix of DC and AC power via an isolation link. In various embodiments, the isolation link is a high-frequency isolation transformer. Isolated Dyna-C converters may provide a high-frequency galvanic isolation and are able to convert three-phase AC power to three-phase AC power, or three-phase AC power to DC and vice versa. The topology is minimal and the costs are low. Isolated Dyna-C converters provide fast current responses and keep the losses low by using a simplified two-stage conversion and providing a magnetizing current that is dynamically controllable and tailored to the load. An isolated Dyna-C converter may synthesize currents at its input or output ports with an arbitrary phase that is relative to the grid or load voltages, thereby enabling a full independent control over the active and reactive power at its ports.

A Start-Up Scheme for a Three-Stage Solid-State Transformer With Minimized Transformer Current Response

Abstract: This letter is focused on developing a start-up scheme for a three-stage solid-state transformer that includes a high-frequency transformer. The proposed scheme is aiming at minimizing the high-frequency transformer current during the start-up transient. As a result, the input-inrush current is also eliminated. The scheme is implemented with no extra cost by synchronizing the start-up of rectifier stage and dc-dc converter stage. The scheme is compared with other start-up methods. The theoretical analysis and experimental results are provided to verify the proposed start-up scheme.

Design, Modeling, and Analysis of a Compact Planar Transformer

Abstract: This paper presents a new design of a planar transformer. Over the surface of a flat core, meander-type design was engraved, so that symmetrically adjusted primary and secondary coils, of the same meander-type, can fit into the engraved design. Primary and secondary coils were covered with another flat core consequently forming a compact planar transformer. Windings of primary and secondary coils are printed on both sides of PCB. Conductive stripes of a winding from upper and bottom layer are connected by vias. The transformer was analyzed when the primary and secondary coils were without a core and with a core. High frequency parameters of the transformer were obtained by finite element modeling software and Impedance Analyzer HP4194A in the frequency range from 50 kHz to 1 MHz. The transformer is intended to be used in DC-DC converters (for switching frequency up to several hundred kHz).

Extended Park's vector approach-based differential protection of three-phase power transformers

Abstract: This study presents a new scheme for power transformers protection, which is based on the analysis of the harmonic content of the differential current Park's vector modulus. The proposed method is able to detect turn-to-turn winding insulation failures, and to distinguish them from magnetising inrush current transients. Experimental and simulation results are presented and discussed.

A novel matrix converter based resonant dual active bridge for V2G applications

Abstract: Dual-active bridges (DABs) can be used to deliver isolated and bidirectional power to electric vehicles (EVs) or to the grid in vehicle-to-grid (V2G) applications. However, such a system essentially requires a two-stage power conversion process, which significantly increases the power losses. Furthermore, the poor power factor associated with DAB converters further reduces the efficiency of such systems. This paper proposes a novel matrix converter based resonant DAB converter that requires only a single-stage power conversion process to facilitate isolated bi-directional power transfer between EVs and the grid. The proposed converter comprises a matrix converter based front end linked with an EV side full-bridge converter through a high frequency isolation transformer and a tuned LCL network. A mathematical model, which predicts the behavior of the proposed system, is presented to show that both the magnitude and direction of the power flow can be controlled through either relative phase angle or magnitude modulation of voltages produced by converters. Viability of the proposed concept is verified through simulations. The proposed matrix converter based DAB, with a single power conversion stage, is low in cost, and suites charging and discharging in single or multiple EVs or V2G applications.

FRA vs. short circuit impedance measurement in detection of mechanical defects within large power transformer

Abstract: Power transformers are supposed to be and remain in service in various environmental circumstances under different electrical and mechanical stresses. Base on failure history in power transformers obtained from four corners of the globe one of the major problems in transformers is mechanical defect. A number of monitoring and diagnostic methods have been introduced to recognize transformer active part displacement and winding deformation. Frequency response analyses and short circuit impedance measurement have been employed as two common diagnosis methods in large power transformer winding deformation recognition. On the other hand, researchers are expressing an increased concern about power transformer condition monitoring in the smart grid context. Hence, all of off-line methods need to move towards on-line applications. One of the challenges is finding reasonably accurate method which can provide sufficient information about transformer winding condition. In this study, mechanical defects of windings and their causes are investigated in detail. Frequency response analyses and short circuit impedance measurement as two popular methods in transformer winding deformation diagnosis will be employed to get insight into transformer active part condition. A large power transformer has been taken as a case in order to put the capability and sensitivity of abovementioned methods into test. Onsite test results on this giant transformer winding show that frequency response analyses method is capable to provide far more information as to the healthy or defected condition and physical movements of the transformer's windings and core compared to the other method.

Effects of current and voltage harmonics on distribution transformer losses

Abstract: The drive to improve energy efficiency and reduce electrical loading has resulted in energy efficient lighting such as compact fluorescent lamps (CFLs) replacing conventional incandescent lamps However, the presence of such non-linear loads has brought about the injection of voltage and current harmonics into electrical networks As transformers are the interface between the supply and the non-linear loads, the investigation of their effects on transformer losses is of great importance These harmonics can cause excessive loss and abnormal temperature rise in the transformers, thus reducing their operational life span This paper investigates the impact of current and voltage harmonics of the loads on a single-phase 25kVA distribution transformer Harmonic spectra of a range of non-linear loads including CFL, LED tube, PC and fluorescent lamp are obtained A single-phase inverter is used for harmonic generation to simulate power supply harmonics injected into the transformer Open circuit and short circuit tests are conducted on the transformer under the effect of harmonics, and the impacts on core loss are analyzed

Power density of piezoelectric transformers improved using a contact heat transfer structure

Abstract: Based on contact heat transfer, a novel method to increase power density of piezoelectric transformers is proposed. A heat transfer structure is realized by directly attaching a dissipater to the piezoelectric transformer plate. By maintaining the vibration mode of the transformer and limiting additional energy losses from the contact interface, an appropriate design can improve power density of the transformer on a large scale, resulting from effective suppression of its working temperature rise. A prototype device was fabricated from a rectangular piezoelectric transformer, a copper heat transfer sheet, a thermal grease insulation pad, and an aluminum heat radiator. The experimental results show the transformer maintains a maximum power density of 135 W/cm3 and an efficiency of 90.8% with a temperature rise of less than 10°C after more than 36 h, without notable changes in performance.

A New Algorithm to Avoid Maloperation of Transformer Differential Protection in Substations With an Inner Bridge Connection

Abstract: In substations with an inner bridge connection, the two main transformers are generally connected in parallel. The differential protection of the transformer, under normal operating conditions, may maloperate when the other transformer is switching on through the bridge breaker after scheduled maintenance. This paper analyzes the factors that may lead to the maloperation of transformer differential protection and concludes that local transient saturation of the current transformer (CT) caused by the decaying dc component in the magnetizing inrush is the main cause of the maloperation of the transformer differential protection. By detecting the time difference between the instant of sudden changes occurring in the transformer wye-side current and the instant of the differential current increasing, a new algorithm based on the time differential method is proposed in this paper, which can prevent the maloperation of transformer differential protection in substations with an inner bridge connection. On the basis of detailed analysis and simulation work, the new algorithm is verified to be applicable under the conditions of sympathetic inrush and CT saturation.

A review of minimisation of output DC current component methods in single-phase grid-connected inverters PV applications

Abstract: Nowadays, it is a general trend to increase the Distribution Generator (DG) based on renewable energy such as photovoltaic (PV) system. This generator can affect the grid by causing a number of problems due to injection of DC current into the grid. If this DC current is not properly prevented, result problems in distribution network such as increased corrosion in underground equipment and transformer saturation. This paper reviews the minimization methods which employ to minimize the output DC current component in single-phase grid-connected inverters photovoltaic applications. These methods are categorized into four classifications. The basic approach is to make use of an isolating transformer to prevent the DC current. Another technique is to use a half-bridge inverter which has the capability of blocking the DC current flow to the AC side. It is also possible to use a DC blocking capacitor in the inverter output. A more recent method is to use current sensing and control techniques to monitor and calibrate the DC link current sensors.

Computation of 3-D Magnetic Leakage Field and Stray Losses in Large Power Transformer

Abstract: A time-stepping finite element method combining analytical and numerical solutions for laminated-material homogenization is proposed to calculate 3-D magnetic leakage field and its structural parts stray losses in a 380 000 kVA/500 kV single-phase power transformer, and the high-order harmonics of field components can be considered. To verify the method and computation code, a supplementary model of Team Problem P21C-M1 is made. Measured and computed results to the model indicate the validity of the method proposed.

Analysis of fast transient voltage distributions in transformer windings under different insulation conditions

Abstract: Evolution and operation of modern grids and networks necessitate reconsideration of several characteristics. For example transformers operating in power grid or in industrial environment are subjected nowadays also to transients or stimulus with different waveforms. Traditionally, in case of power transformers the stresses are related to lightning and switching phenomena. In industrial networks distribution transformers are exposed to many switching operations generating often overvoltages. Transformer windings subjected to such system disturbances are stressed due to the non-linear transient voltage distribution along the winding and potential resonance phenomena. Initial voltage distribution, reflects mainly the capacitive coupling between turns, windings, core and tank. The non-uniform distribution of voltage results in concentred stress in certain parts of the winding. Winding resonance might occur, if the frequency spectrum of incoming surge matches the winding natural frequencies. Therefore external transients occurring in power systems might trigger internal overvoltages with large maximum value in transformer windings. The frequency characteristic of the transformer winding may determine those regions in the frequency spectrum. The paper presents results of voltage distribution measurements performed on model windings with different constructions subjected to voltages with different rise time. The experimental setup allows for investigation of influence of the dielectric (air/oil) and temperature on the stress distribution along the winding. Also the impact of oil and temperature on frequency characteristics of internal voltages are shown. The presented results might be used both for design and optimization of transformers windings.

A novel predictive phase shift controller for bidirectional isolated dc to dc converter for high power applications

Abstract: In this paper a novel predictive algorithm has been proposed for bi-directional dc-dc converters with high frequency transformer isolation. The converter is a dual active bridge converter. The proposed algorithm is a faster alternative to the classical PI based phase shift controller. This mode of control can remove dc bias in the isolation transformer within several switching cycles hence preventing transformer saturation. Aspects of the proposed algorithm have been implemented on a hardware test bed and verified.

Design of Power Electronic Transformer Based on Modular Multilevel Converter

Abstract: Power electronic transformer acts as a key component of the future automation in smart grid, which is the replacement of conventional distribution transformers by a power transformer. In this paper , the optimum design of a power electronic transformer(PET) based on topology of Modular multilevel converter (M2C). M2C is an emerging and highly attractive topology intended for high- or medium-voltage power electronic devices. In the design process, the modulation scheme of M2C is analyzed. A control method for balancing the M2C DC buses has been introduced. The PET includes a dual-active-bridge DC/DC converter. The DC-DC converter realizes magnetic isolation of PET and reduces the transformer size. Meanwhile, both primary-side and secondary-side switches can achieve the zero-voltage-switching(ZVS) .The PET performs power transformer's functions, at the same time, which has advantages such as power factor correction, elimination of voltage sag and swell, and reduction of voltage flicker in load side.

Transformer Electrical Insulation [Editorial]

Abstract: This Special Issue of Transactions on Dielectrics and Electrical Insulation is dedicated to transformer insulation. The transformer is a very essential apparatus in an electric power system and its reliability is of utmost importance as a transformer failure results in a very costly and difficult to predict interruption of energy delivery. In turn, transformer's performance depends heavily on its insulation system; therefore the insulation is perhaps the most critical transformer part.

3D calculation and modeling of eddy current losses in a large power transformer

Abstract: Elimination of hot spots and reduction of eddy current losses in structural parts is one of the important constituents of transformer design. In this work, the eddy current losses in the clamping frame, transformer tank and electromagnetic shielding are calculated using a 3D finite element method. The clamping frame, transformer tank and electromagnetic shielding are modeled by surface impedance method. The paper analyses the effects of electromagnetic shielding and magnetic shunts on the eddy current loss reduction in the transformer tank.

Power supply system

Abstract: A power supply system drives a plurality of loads, and includes a power stage circuit, a pulse width modulation controller, a first isolation transformer, at least two second transformers, and at least two rectifier circuits The power supply system converts external power signals into first alternating current (AC) power signals, and converts the first AC power signals into second AC power signals The second transformers either boost or buck the second AC power signals, and each has a primary winding connected to a secondary winding of the first isolation transformer in parallel and a center tap grounded Each of the rectifier circuits is connected to two ends of a secondary winding of a corresponding one of the second transformers, and rectifies the boosted or bucked second AC power signals to output direct current (DC) power signals to a corresponding one of the plurality of loads

Multi-functional grid-connected inverter and grid-connected inverter control method

Abstract: The invention belongs to the field of electrical engineering, distributed power generation and power quality control, and aims to provide a multi-functional grid-connected inverter and a grid-connected inverter control method. The grid-connected inverter comprises a direct-current bus bar with a capacitor and three sets of single-phase full-bridge inverter circuits sharing the direct-current bus bar, the direct-current output ends of a distributed power supply or a grid-connected energy-storing generating set are connected to both ends of the capacitor of the direct-current bus bar, the output of each set of single-phase full-bridge inverter circuit is connected to a filter, and the output ends of each filter are connected with a distribution network of the 380V voltage class through a single-phase step-up isolation transformer. According to the invention, the grid-connected inverter equipment is effectively utilized to carry out effective control on the power quality of points of interconnection, so that the grid-connected inverter is multi-purpose and multi-functional, the cost performance of the grid-connected inverter is enhanced, the investment, operation and maintenance cost of a system can be reduced, and the reliability of the system can be enhanced. The principle of the provided control scheme is clear, and the control scheme is highly operable and reliable in operation.

Bushing characteristic impacts on on-line Frequency Response Analysis of transformer winding

Abstract: Diagnosis of electrical equipment, particularly n and power transformers has been quite important for quite a long time. Frequency Response Analysis (FRA) has been introduced years ago and has been employed as an off-line diagnosis test since last decade to investigate transformer mechanical integrity. As smart high voltage monitoring concept is under developing, on-line transformer winding deformation diagnosis has been investigated and discussed significantly over the last years. Hence, on-line diagnosis method implementations have become important accordingly. This study has concentrated on issues arising while on-line FRA is going to be employed on transformers. Existing off-line and on-line transformer winding deformation diagnosis methods are discussed and an on-line frequency response analysis measurement setup is proposed. A profound insight to the challenges associated with on-line FRA measurement has been provided. Influence of on-line setup on frequency response trace sensitivity, accuracy and interpretation has been highlighted. In the end, practical study has been performed on a 66 kV interleaved winding connected various types of capacitive bushings to explore the boundary condition for on-line FRA setup design and to address fundamental challenges.

A New Concept of High-Voltage DC–DC Conversion Using Asymmetric Voltage Distribution on the Switch Pairs and Hybrid ZVS–ZCS Scheme

Abstract: A new concept of high-voltage dc-dc power conversion is presented in this paper. Its distinctive feature lies in distributing the high input voltage asymmetrically between two primary-side series-connected switch pairs. This allows using switches of optimal ratings in their respective class: the low-voltage switch pair implemented with MOSFETs only, and the high-voltage switch pair implemented with insulated gate bipolar transistors (IGBTs) only. As the switches in a switch pair are of the same type and voltage rating, they are maximally utilized. With an active snubber on the secondary side of the isolation transformer, a hybrid zero-voltage-switching-zero-current-switching (ZCS) scheme, which is different from the zero-voltage and zero-current-switching technique, is realized and makes all IGBTs be zero-current-switched and all MOSFETs be zero-voltage-switched from very light load to full load condition with minimal circulating energy. The ZCS snubber energy is completely released to the load, leading also to a duty-cycle gain. The operating principles, dc analysis, and design guidelines will be given. A 2-kW, 1500/48-V experimental prototype has been built and evaluated. The measured efficiency of the converter is found to be 92.4% at the rated condition.

Integrated magnetics with isolated drive circuit

Abstract: A switch-mode power converter includes a power isolation transformer and a drive transformer having their various windings collectively wound on a magnetic core having a center leg and outer legs. A primary winding and one or more secondary windings of the power transformer are wound on the center leg, and first and second windings of the drive transformer are wound on an outer leg. A primary control circuit controls one or more primary switches to supply the input voltage to the primary winding. A secondary control circuit controls secondary switches connected between the secondary windings and a load. Another control circuit controls operation the primary and secondary control circuits based at least in part on a feedback signal. The drive transformer windings are further configured to provide isolation between the primary control circuit and the synchronous rectifier control circuit.