Showing papers on "Isolation transformer published in 2018"

High-Frequency Transformer Design for Modular Power Conversion From Medium-Voltage AC to 400 VDC

Abstract: This paper presents a high-frequency modular medium-voltage AC (4160 VAC or 13.8 kVAC) to low-voltage DC (400 VDC) system that is scalable in order to be used for different scale microgrids. A 15 kW, 500 kHz DC/DC converter is demonstrated as the most important stage of the system overall, which can be scalable to a 225 kW 4160 VAC to 400 VDC system. Motivation of a CLLC resonant converter and its design parameters determination is carefully illustrated. The high-frequency transformer is the key element for the DC/DC converter. Then, the paper focuses on high-frequency transformer design to realize high-voltage insulation, high efficiency, and high density at the same time. Based on a split winding UU core transformer structure, transformer insulation materials and dimension parameters are determined referring to the IEEE insulation standard. The transformer magnetic loss model is reviewed based on which loss design tradeoff is carefully analyzed. Different working frequency impact on transformer design over three different core materials is also presented. Finally, a 500 kHz transformer prototype has been developed and demonstrated with the IEEE standard required insulation tests. A whole CLLC resonant converter is also present with experiments results. The converter holds outstanding 98% efficiency and 2.9 kW/L power density.

Soft-Switching Solid-State Transformer (S4T)

Abstract: This paper presents a new topology for a fully bidirectional soft-switching solid-state transformer (S4T) The minimal topology, featuring 12 main devices and a high-frequency transformer, does not use an intermediate dc voltage link, and provides sinusoidal input and output voltages The S4T can be configured to interface with two- or multiterminal dc, single- or multiphase ac systems An auxiliary resonant circuit creates zero-voltage-switching conditions for main devices from no-load to full-load, and helps manage interactions with circuit parasitic elements The modularized structure allows series and/or parallel stacking of converter cells for high-voltage and high-power applications

A Wavelet-Based Transformer Differential Protection With Differential Current Transformer Saturation and Cross-Country Fault Detection

Abstract: The current transformer (CT) saturation phenomenon has been one of the main problems for the power transformer differential protection, leading to incorrect current measurements and relay misoperation. This paper proposes a fast and efficient transformer differential protection scheme with additional differential CT saturation and cross-country fault detection modules after the external fault detection, all of them based on the differential wavelet coefficient energy with border distortions in order to stabilize the relay during external faults and distinguish accurately CT saturation from cross-country internal faults. The proposed method was assessed by using representative simulations of internal faults, transformer energizations, and external faults with CT saturation followed by cross-country internal faults, and good results were achieved.

A Dual Active Bridge Converter With an Extended High-Efficiency Range by DC Blocking Capacitor Voltage Control

Abstract: A Dual active bridge (DAB) converter can achieve a wide high-efficiency range when its input and output voltages are equal, assuming a 1:1 turns ratio for its isolation transformer. If its input or output voltage is doubled, efficiency of the DAB will drop significantly, because of the introduction of the hard switching and high circulating power. Thus, a new modulation scheme has been proposed, whose main idea is to introduce a voltage offset across the dc blocking capacitor connected in series with the transformer. Operational principle of the proposed modulation has been introduced, before analyzing its soft-switching area and circulating power mathematically. The final modulation scheme is not difficult to implement, but can help the DAB achieve soft switching, low circulating power, and thereby high efficiency, even with its input or output voltage doubled. These features have been verified by experimental results obtained with a 1.2-kW prototype.

Load Control Using Sensitivity Identification by Means of Smart Transformer

Abstract: The higher variability introduced by distributed generation leads to fast changes in the aggregate load composition, and thus in the power response during voltage variations. The smart transformer, a power electronics-based distribution transformer with advanced control functionalities, can exploit the load dependence on voltage for providing services to the distribution and transmission grids. In this paper, two possible applications are proposed: 1) the smart transformer overload control by means of voltage control action and 2) the soft load reduction method, that reduces load consumption avoiding the load disconnection. These services depend on the correct identification of load dependence on voltage, which the smart transformer evaluates in real time based on load measurements. The effect of the distributed generation on net load sensitivity has been derived and demonstrated with the control hardware in loop evaluation by means of a real time digital simulator.

A New Leakage Flux-Based Technique for Turn-to-Turn Fault Protection and Faulty Region Identification in Transformers

Abstract: Power transformer, as one of the most important apparatus in power system, must be protected against the turn-to-turn faults. Although the various methods based on the terminal currents and/or voltages have been presented for transformer protection, the flux-based methods can be used to achieve the more accurate, sensitive, and secure results. Since the symmetrical form of the magnetic flux distribution will be disturbed due to any fault occurrence in the transformer windings, it can be considered as an appropriate criterion to achieve a suitable protection algorithm, which can detect the fault occurrence and identify the faulty phase/region. In this paper, a new leakage flux sensor is introduced and then a flux-based technique is proposed to detect and identify the faulty phase in transformers to apply in online conditions. By using the proposed technique, the faulty region can be identified in offline conditions, as well. The experimental results (which are done on a distribution transformer) show that this technique is secure in the face of overfluxing and transformer energizing conditions and can detect turn-to-turn faults with high sensitivity. Moreover, tap changer operation, imbalanced loads, and/or unsymmetrical voltage sources cannot influence the proposed technique security.

Design of a 10-kV·A Soft-Switching Solid-State Transformer (S4T)

Abstract: The soft-switching solid-state transformer (S4T) employs only 12 main active devices and an auxiliary resonant circuit to implement a bidirectional solid-state transformer, with an attractive feature of achieving a full range of zero-voltage-switching conditions for all the main devices. This paper covers detailed design of the power stage, auxiliary resonant circuit, and control of the S4T. The high-frequency transformer is an essential element for the S4T, and it has a unique feature of dc-biased flux. Design of such a high-frequency transformer is also discussed in detail in this paper. Soft startup, shutdown, and fast dynamic response under load transients are also attractive behaviors because of the low inertia of the S4T. Experimental results from a 208-V/10-kV·A S4T unit are presented.

A Transformer-Less Unified Power Quality Conditioner with Fast Dynamic Control

Abstract: A single-phase transformer-less unified power quality conditioner (TL-UPQC) is presented. Apart from having no isolation transformer, the proposed structure utilizes four switching devices only, forming two half-bridge voltage-source inverters—one connected in parallel with the load and another one connected in series with the ac mains. The two inverters share the same dc link. The parallel inverter, which is controlled by a hysteresis current controller, is used to shape the current drawn from the ac mains and regulate the dc-link voltage. The series inverter, which is controlled by a boundary controller with second-order switching surface, is used to regulate the steady-state load voltage and provide voltage sag/swell ride-through. A dc-link capacitor voltage balancing control that coordinates the operations of the hysteresis and boundary controllers is designed. Modeling, design, and analysis of the whole system will be given. A 1 kVA, 110 V, 60 Hz prototype has been built and evaluated on a setup with a nonlinear load. The steady-state and transient responses under a voltage sag will be given. Experimental results are favorably compared with the theoretical predictions and the performance of other UPQCs.

Multiphase Quasi-Z-Source DC–DC Converters for Residential Distributed Generation Systems

Abstract: A multiphase quasi-Z-source (qZS) dc–dc converter was proposed for distributed energy generation applications It contains single-switch qZS isolated dc–dc cells with a voltage doubler rectifier These cells are connected in parallel at the input side and in series at the output side to increase the dc voltage gain A dc voltage blocking capacitor in series with the isolation transformer results in resonance that could be utilized for soft-switching Two design approaches were proposed: considering phase shedding dependent on the input voltage and without it The former targets wide input voltage range applications, while the latter is better suited for high input current applications An experimental prototype rated for 300 W was tested with two types of isolation transformers designed according to the two presented approaches It could be used as a PV module integrated converter with wide input voltage regulation range The experimental results prove efficiency improvement from the phase shedding Resonance frequency variations caused by the utilization of the multilayer ceramic capacitors and their possible influence on switching losses are discussed

A Novel DC Current Injection Suppression Method for Three-Phase Grid-Connected Inverter Without the Isolation Transformer

Abstract: Due to current sensor errors, tolerance of power switching devices, and asymmetry of PWM gating driving pulses, grid-connected inverters without isolation transformer usually have certain amount of dc components injected to the ac grid. Many efforts, such as using a blocking capacitor, a current dc component feedback control, and a voltage dc component feedback control have been introduced to try to suppress the dc injection to the grid. This paper proposes a novel control strategy of suppressing dc current injection to the grid for a three-phase inverter by accurately sensing the dc component of line voltages of three-phase inverter and adding a dc component control loop. A dc suppression control scheme is presented, and the suppression performance of the proposed dc rejection method is evaluated and compared with the traditional method. Finally, the control scheme is verified on a 20-kW three-phase grid-connected inverter.

Bidirectional soft-switching dc–dc converter for battery energy storage systems

Abstract: The study introduces a bidirectional dc-dc converter with current- and voltage-fed (VF) ports that features soft switching in both buck and boost operating modes. The converter can be used for integration of low-voltage DC sources, such as batteries into a dc bus of considerably higher voltage or a dc link of a grid side inverter. Zero current switching, assisted with the leakage inductance of the isolation transformer, can be achieved at the current-fed side along with zero voltage switching of the VF side, assisted by snubber or intrinsic capacitances of the transistors. Soft switching can be maintained over a wide range of voltage and power levels, regardless of the energy transfer direction. Converter operation is described and theoretical findings were verified with experimental results obtained by means of a 300 W prototype operating at a switching frequency of 100 kHz and designed for integration of a 24 V battery into 400 V dc bus. The converter proposed is compared in terms of efficiency to other competing soft-switching full-bridge topologies implemented with the same components.

Analytical Modeling and Implementation of a Coaxially Wound Transformer With Integrated Filter Inductance for Isolated Soft-Switching DC–DC Converters

Abstract: In this paper, we propose an approach for integrating a series filter inductance in a coaxially wound transformer (CWT) as a potential alternative to conventional solenoidally wound transformers (SWT) for high-power isolated soft-switching dc–dc converters. The critical elements that determine the size and the performance of soft-switching dc–dc converters are the isolation transformer and the filter elements. While conventional SWTs using a stray magnetic field as a filter inductance often suffer from a considerably increased loss, heat congestion, and electromagnetic interference, a CWT is rarely affected by high frequency and fringing effects. The desired properties are still valid while the required filter inductance is fully integrated with an isolation transformer using the proposed method. In addition, the evenly distributed magnetic fields in a concentric geometry enable accurate electrical and mechanical analysis and a specific application-oriented design. In this paper, the operating principles, design procedure, and loss modeling of a CWT with an integrated filter inductance are introduced. Its functionality and suitability for high-frequency applications were proven by theoretical analysis and experiments. A prototype transformer with an integrated inductance 4 mH was facilitated and evaluated in a 15 kV-SiC mosfet -based 6 k–400 Vdc stage for a solid-state transformer of up to 6.5 kW at a switching frequency of 20 kHz.

Solid-State Transformer for Grid Interface of High-Power Multipulse Rectifiers

Abstract: This paper proposes a medium-frequency (MF) solid-state transformer (SST) based grid interface for high-power multipulse rectifiers. In the proposed approach, the low-frequency ac (grid) voltage is transformed to MF voltage via front-end converters. The three single-phase MF outputs from these converters are fed to the primary side of a three-phase, five-limb multiwinding isolation transformer (or alternately three one-phase transformers). The secondary side windings from the transformer are configured in a certain “zig-zag” arrangement, which are connected to the inputs of three-phase diode rectifier modules as in a typical multipulse system. The proposed topology can better than double the power density by reducing volume (or weight) of the required front-end isolation transformer while maintaining the same input current quality as a conventional low-frequency transformer based approach. Usage of an open-loop square wave pulse width modulation for the front-end converters makes this approach simple to implement. This paper explains the proposed concept taking an adjustable speed drive application's perspective using modeling, simulation, and experimental results from a scaled down laboratory prototype.

Single-Phase AC–DC–AC Multilevel Converter for Grid Overvoltage Based on an H-Bridge Connected in Series to the Five-Leg Converter

Abstract: In this work, an ac–dc–ac multilevel power converter for grid overvoltage is investigated. The developed configuration is composed of one single-phase ac–dc–ac three-leg module and two H-bridges. One of them is connected to the shared part of the system to generate multilevel voltages at the input and output of the structure, and the other one is connected to the grid side to compensate the grid overvoltage. The converter can be employed in applications with same input and output frequency, such as uninterrupted power supply and unified power quality conditioner without isolation transformer. System model, a space vector pulsewidth modulation strategy to balance the individual dc-link voltages, and an overall control strategy to adjust the grid power factor, the amplitude, and the frequency of the load voltage are presented. The proposed converter is compared in terms of harmonic distortion and semiconductor losses with conventional structures. Simulation and experimental results demonstrate the operation of the proposed topology.

Three-Phase Lines to Single-Phase Coil Planar Contactless Power Transformer

Abstract: This paper proposes a three-phase lines to single-phase coil planar contactless power transformer. The transformer has the primary consisting of three-phase line cables arrayed in parallel on a plane and the secondary of single-phase coil. The proposed transformer is studied for application to automatic guided vehicles (AGVs). Uniform power transfer that is independent of the position of the AGVs is an important issue for this transformer, because of the sparse arrangement of the primary lines. To ensure that the transformer achieves uniform power transfer, the structure and dimensions of the primary are studied using numerical analysis. Additionally, an equivalent model of the proposed transformer, which is applicable to the imbalanced conditions, is presented. The equivalent model proves analytically that balanced three-phase voltage and balanced three-phase current conditions, which can be created using appropriate circuit configurations, enable uniform power transfer. The results of experiments in which the prototype of the transformer is applied to the target AGV confirm that under balanced-current conditions, which can be generated using a three-phase LCL circuit, the proposed transformer can transfer sufficient uniform power to drive the AGV.

Comparative study between three-leg and four-leg current-source inverter for solar PV application

Abstract: Generation of electricity from the non-conventional energy sources requires efficient equipment to transfer energy to the grid. Highly efficient and good operating life of photovoltaic inverters and transformers are required. Normally voltage source inverters are used for solar PV application. But current source inverters are more reliable than the voltage source inverters. This paper discusses the implementation of a three-leg current source based inverter using three-phase space vector pulse width modulation. This will inject high earth leakage current to the grid and also results in high common mode voltage. To minimize the leakage current an isolation transformer can be used. Due to the addition of this equipment, size and cost increases, and the total efficiency decreases. To overcome this limitation Four-leg current source inverter is proposed. In this proposed method, we can suppress the common mode voltage and the earth leakage current. In this paper provides comparative results of 3-leg and 4-leg current source inverters.

A 25kW SiC Universal Power Converter Building Block for G2V, V2G, and V2L Applications

Abstract: This paper presents a universal power converter building block (PCBB) with isolation transformer which can perform various functionalities such as charging (G2V), vehicle to grid (V2G) regenerative capability, and vehicle to load (V2L) feature. This converter can output different types of voltage, such as AC or DC, high voltage or low voltage, singlephase or three-phase. With different combinations of modularized PCBB, various voltage and power rating can also be achieved. The control algorithms for each operation mode are explained, including G2V, V2G, and V2L. Simulation results are accompanied to validate its capability. To achieve high power density, SiC MOSFET power module with high frequency switching is adopted along with the isolation transformer of nanocrystalline core. Overall, this paper extends a promising circuit topology of multiple practical and essential features for vehicle electrification.

Pseudo-Partial-Power Converter without High Frequency Transformer for Electric Vehicle Fast Charging Stations

Abstract: This paper presents a new partial power converter (PPC) for the DC-DC stage of electric vehicle (EV) fast charging stations. The proposed converter handles only a fraction of the total power delivered from the grid to the battery, increasing the overall system efficiency and power density, while potentially reducing the cost of the charger. The proposed topology is based on a switched capacitor between the AC terminals of an H-bridge converter, and does not require high frequency isolation transformers to provide a controllable voltage source between the DC-link and the battery. The proposed concept can be implemented using interleaved power cells, which can improve the power quality, reduce inductor size, and enable scalability for higher power rating chargers. The operating principle, partial operation analysis, control scheme, and simulation results are presented to validate the proposed concept.

Analysis and Design of a Wireless Charger for Underwater Vehicles Fed from a Constant Current Distribution Cable

Abstract: Existing conductive power supplies for undersea electronic systems are typically fed from constant low-current distribution with a seawater return In such systems, power converters are connected in series along the distribution cable to supply power to sensor loads and underwater vehicles Isolated voltage fed resonant converters are often used for the advantages of soft-switching and low component stress When supplying underwater vehicles, there is further motivation to develop a wireless solution to improve reliability by removing the seawater exposed contacts This can be accomplished by replacing the isolation transformer of existing resonant converters with series-series compensated coupled coils to create an inductive power transfer (IPT) solution The introduction of the IPT system changes many factors in the design that need to be considered This paper presents detailed analysis and design of wireless charger for underwater vehicles fed from a constant current distribution cable The design is validated with experimental results for a 330 W prototype supplied from a 1 A source operated underwater with a switching frequency of 250 kHz

Electron series resonance in a magnetized 13.56 MHz symmetric capacitive coupled discharge

Abstract: A 13.56 MHz capacitive coupled radio-frequency (RF) argon discharge under transverse magnetic field has been investigated. The discharge is operated in a push-pull mode using a 1:1 isolation transformer with its centre tap grounded to a RF generator. The power delivered to the plasma has been calculated from phase-calibrated RF current/voltage waveforms measured on the secondary side of the isolation transformer. An equivalent electrical circuit of the discharge has been described to determine the net plasma impedance. It is found that in the presence of magnetic field, the discharge impedance exhibits a series resonance as the RF power level is increased gradually. However, in the un-magnetized case, the discharge remains entirely capacitive. A qualitative discussion has been given to explain the role of external magnetic field in achieving the series resonance.

A Reconstruction of the WAMS-Detected Transformer Sympathetic Inrush Phenomenon

Abstract: The transformer-energization inrush phenomenon is to be expected in cases when a single transformer is connected to an ac voltage source. The most commonly employed mitigation technique is to use mechanisms that enable per-phase controlled switching based on a residual magnetic-flux estimation in the transformer’s ferromagnetic iron core. However, the time scale of the phenomenon is relatively short and imposes no extremely critical issues to wide-scale power-system operation. On the other hand, the sympathetic inrush phenomenon that might occur among several parallel transformers exhibits much longer time constants (several seconds and even tens of seconds). The resulting system impact becomes longer lasting. Recently, national wide area monitoring system (WAMS) measurements indicated that a sympathetic interaction occurred between two parallel 400/110-kV transformers in the Slovenian power system. The cause was the magnetization of a second transformer in parallel to the already operational and fully loaded one in the substation in electrical vicinity to a large power plant. Apart from the WAMS, the event was also captured by the transformer’s over-current protection relay. The specifics of the event are described in this paper together with a dynamic-simulation-based reconstruction that is very important for the operator’s understanding of on-line, WAMS-provided, and system measurements.

Power System Compensation Using a Power-Electronics Integrated Transformer

Abstract: This paper presents a new transformer, that is, the custom power active transformer (CPAT)—which integrates shunt and series equivalent circuits within the transformer's magnetic structure. Thus, it provides power system services using a single transformer. The CPAT equipped with a power converter can be utilized in distribution systems to control grid current and load-voltage waveforms while operating as a step-up or step-down transformer between the grid and load. Moreover, it can provide other services that any typical shunt-series compensation arrangement provides. The design and analysis of a single-phase CPAT are presented, showing the effect of coupling between windings and transformer parameters affecting CPAT operation. In this paper, control of the CPAT in a unified power-quality controller application is investigated to attenuate grid-current and load-voltage harmonics as well as compensate for reactive power requirements and mitigate grid inrush current. Through simulation and experimental implementation, the merits and performance of the CPAT were validated.

Custom Power Active Transformer for Flexible Operation of Power Systems

Abstract: This paper presents a new transformer, i.e., the custom power active transformer (CPAT) which integrates both series and shunt power conditioning through power electronics into a single transformer. This is achieved through a distinct design of the magnetic circuit and auxiliary windings of the transformer. In this paper, a single-phase CPAT is proposed as well as a preface into its extension to multiphase systems. Through its magnetic equivalent circuit model, several design considerations and control limitations are revealed in the paper. Analysis of the resulting CPAT structure shows some prospects in material saving as well as size and cost reduction when compared to the traditional multitransformer-based configuration. In this paper, the proposed single-phase CPAT is utilized in a distribution system application, where the control architecture is designed to attenuate voltage and current distortions at both the load and the grid side, respectively. Performance and effectiveness of the proposed CPAT are evaluated through simulation and experiments.

A 7.8 kV nanosecond pulse generator with a 500 Hz repetition rate

Abstract: Pseudospark switches are widely used in pulsed power applications. In this paper, we present the design and performance of a 500 Hz repetition rate high-voltage pulse generator to drive TDI-series pseudospark switches. A high-voltage pulse is produced by discharging an 8 μF capacitor through a primary windings of a setup isolation transformer using a single metal-oxide-semiconductor field-effect transistor (MOSFET) as a control switch. In addition, a self-break spark gap is used to steepen the pulse front. The pulse generator can deliver a high-voltage pulse with a peak trigger voltage of 7.8 kV, a peak trigger current of 63 A, a full width at half maximum (FWHM) of ~30 ns, and a rise time of 5 ns to the trigger pin of the pseudospark switch. During burst mode operation, the generator achieved up to a 500 Hz repetition rate. Meanwhile, we also provide an AC heater power circuit for heating a H2 reservoir. This pulse generator can be used in circuits with TDI-series pseudospark switches with either a grounded cathode or with a cathode electrically floating operation. The details of the circuits and their implementation are described in the paper.

Common-Mode Overvoltage Mitigation in a Medium-Voltage Pump Motor Transformerless Drive in a Mining Plant

Abstract: The mining process requires different types of pumps to transfer fluids from one point to another. In the majority of applications, those pumps are driven by variable speed motor drives in order to improve the energy efficiency of the overall system. Large pumps, ranging from hundreds of kW to a few MW, usually demand medium-voltage converters. Transformerless converters reduce the footprint required for the installation, but give rise to additional concerns related to the common-mode voltages applied to the system and motor. The absence of an isolation transformer along with the use of pulse-width modulated rectifiers are the root causes of this issue. This concern increases when long cables are needed to connect the converter to the motor. This paper presents an application where three-level neutral point clamped transformerless medium-voltage converters are applied to drive motor pumps in a mining plant. The passive filtering techniques used to address the common-mode voltage issues are analyzed through simulations. The proposed solutions are field tested with successful results.

Phase Shift Modulation and DC-Link’s Voltage Balancing Control for a DAB DC-DC Converter

Abstract: To improve the electrical energy consumption in modern transportation systems, a 1.5 kV DC bus in the future railway application has been considered by researchers and manufacturers. One of the preferred systems for this application is a high-frequency power electronics transformer to replace the existing 60 Hz power transformer. This solid-state transformer (SST) is made of multiple modules, and each module is composed of the AC/DC converter connected to a dual active bridge (DAB) DC-DC converter. A performing topology for the DAB is a three-level neutral point clamped (NPC) inverter on the left side and a two-level full bridge converter on the right side, both connected through a high-frequency isolation transformer. Due to its multilevel voltage on the primary side, its soft switching properties and the low number of components capability, the NPC based DAB converter is chosen to meet this smart transformer’s requirements. This study proposes a novel control algorithm of the phase modulation and the voltage balancing perspective for future intelligent transformers.

A Gate Driver Design for Medium Voltage Silicon Carbide Power Devices with High dv / dt

Abstract: The use of silicon carbide (SiC) devices in medium voltage (MV) applications has become a possibility due to the development of reliable MV SiC power devices. However, when SiC devices are used in these MV applications, they are exposed to a high voltage peak stress (of up to 15 kV across the primary and secondary side of the gate driver) and a very high $dv/dt$ (of up to 100 kV/μs across the isolation transformer). The gate driver design is very critical for proper functioning of the MV devices under the presence of such high dv/dt. This paper presents a design of an improved gate driver power isolation method, with a low coupling capacitance between the primary and the secondary side. The footprint of the isolation transformer is minimized to meet the clearance and insulation requirements. Comparisons have been drawn with an existing gate driver topology, on the basis of size of the gate driver, and common mode performances for different $dv/dt$ . Experimental results are provided to validate both the gate driver designs. The testing and analysis is carried out on a 10 kV SiC MOSFET developed and packaged by Wolfspeed. In addition, a brief discussion on the insulation standards for these kinds of applications is provided. The gate driver concept is aimed at providing a benchmark for building an efficient and reliable method to drive MV SiC devices.