Input current shaped ac-to-dc converters

Input current shaping techniques for ac-to-dc converters were investigated. Input frequencies much higher than normal, up to 20 kHz were emphasized. Several methods of shaping the input current waveform in ac-to-dc converters were reviewed. The simplest method is the LC filter following the rectifier. The next simplest method is the resistor emulation approach in which the inductor size is determined by the converter switching frequency and not by the line input frequency. Other methods require complicated switch drive algorithms to construct the input current waveshape. For a high-frequency line input, on the order of 20 kHz, the simple LC cannot be discarded so peremptorily, since the inductor size can be compared with that for the resistor emulation method. In fact, since a dc regulator will normally be required after the filter anyway, the total component count is almost the same as for the resistor emulation method, in which the filter is effectively incorporated into the regulator

The Proposal for Implementation of Controlled Power Rectifier (3000/4000KW) in MTA New York City Transit (MTA-NYCT)

The MTA New York City Transit (MTA-NYCT) will require a robust and reconfigurable power system capable of supplying high power in order to be able to provide services based on for cased future forecast growth of the city population. A critical component in such a system is the Phase Controlled Rectifier. As such, the issues associated with the inclusion of a power electronics rectifier need to be addressed. These issues include input Alternating Current (AC) interface requirements, the output Direct Current (DC) load profile, and overall stability in the output voltage for Train car loads. Understanding these issues, providing possible solutions and determining the means of assuring smooth compatibility with MTA New York City Transit (MTA-NYCT) Traction Power systems is the focus of this thesis. By using a Simulink® model of an actual MTA-NYCT Traction Power System, actual train car load, 12 -Pulse count, high power rectifiers were exercised. The Simulink® results are compared between the Traction Power Systems of Uncontrolled Rectifier and Controlled Rectifier analysis results. In subway normal operation hour, with uncontrolled rectifier systems, subway cars load current level are 2800 Amps to 3600 Amps, and Voltage level 450 VDC to 600 VDC in running condition. In this Simulation, with controlled rectifier system, subway cars load current level are 3200 Amps to 4000 Amps, and Voltage level 550 VDC to 625 VDC established. These experiments led to the conclusion that increasing the continuous current and the overall stability in the output voltage, reducing the harmonics, there are tradeoffs in terms of complexity and size of the passive components, and optimization based on source and load specifications is also required.

Pulse Tripling Circuit and Twelve Pulse Rectifier Combination for Sinusoidal Input Current

In this paper, a novel pulse tripling circuit (PTC) is suggested, to upgrade a polygon autotransformer 12-pulse rectifier (12-PR) to a 36-pulse rectifier (36-PR) with a low power rating. The kVA rating of the proposed PTC is lower compared to the conventional one (about 1.57% of load power). Simulation and experimental test results show that the total harmonic distortion (THD) of the input current of the suggested 36-PR is less than 3%, which meets the IEEE 519 requirements. Also, it is shown that in comparison with other multi-pulse rectifiers (MPR), it is cost-effective, its power factor is near unity and its rating is about 24% of the load rating. Therefore, the proposed 36-PR can be considered as a practical solution for industrial applications

A Three-Phase Single-Stage AC-DC ZVZCS PWM Full-Bridge Converter

It is standard practice to use two separate power converters to convert an ac input voltage to a desired and isolated dc output voltage. A front-end ac-dc converter is used to convert the input ac voltage into an intermediate dc voltage which is then fed into a dc-dc converter with transformer isolation. The front-end converter also performs input power factor correction (PFC) to shape the input currents to be sinusoidal and in phase with the input voltages to maximize the use of the available source power. Conventional two-stage power conversion, however, requires two power con­ verters and there has been considerable interest to try to integrate the PFC and dc-dc conversion functions in a single power converter to reduce cost and complexity. Although many of these single-stage converters have been proposed for low power, single-phase applications, there have been relatively few higher power three-phase converters that have been proposed. This is due to the challenges faced when trying to perform PFC and dc-dc conversion for a wider load range. A new three-phase, single-stage ac-dc full-bridge converter is proposed in this thesis. The outstanding features of the new converter are that it is relatively simple and it can perform PFC using standard phase-shift pulse width modulation (PWM). In the thesis, derivation of the converter is discussed and its general operation is re­ viewed. The modes of operation of the converter are explained in detail and analyzed and the results of the analysis are used to develop guidelines for its design. The feasibility of the proposed converter is confirmed with experimental results that were obtained from a prototype and are presented in this thesis

A Three-Phase Single-Stage AC-DC ZVZCS PWM Full-Bridge Converter

It is standard practice to use two separate power converters to convert an ac input voltage to a desired and isolated dc output voltage. A front-end ac-dc converter is used to convert the input ac voltage into an intermediate dc voltage which is then fed into a dc-dc converter with transformer isolation. The front-end converter also performs input power factor correction (PFC) to shape the input currents to be sinusoidal and in phase with the input voltages to maximize the use of the available source power. Conventional two-stage power conversion, however, requires two power con­ verters and there has been considerable interest to try to integrate the PFC and dc-dc conversion functions in a single power converter to reduce cost and complexity. Although many of these single-stage converters have been proposed for low power, single-phase applications, there have been relatively few higher power three-phase converters that have been proposed. This is due to the challenges faced when trying to perform PFC and dc-dc conversion for a wider load range. A new three-phase, single-stage ac-dc full-bridge converter is proposed in this thesis. The outstanding features of the new converter are that it is relatively simple and it can perform PFC using standard phase-shift pulse width modulation (PWM). In the thesis, derivation of the converter is discussed and its general operation is re­ viewed. The modes of operation of the converter are explained in detail and analyzed and the results of the analysis are used to develop guidelines for its design. The feasibility of the proposed converter is confirmed with experimental results that were obtained from a prototype and are presented in this thesis

Multilevel Converters: An Enabling Technology for High-Power Applications

| Multilevel converters are considered today as the state-of-the-art power-conversion systems for high-power and power-quality demanding applications. This paper presents a tutorial on this technology, covering the operating principle and the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in-depth and self-contained information, while recent advances and state-of-the-art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not-familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.Ministerio de Ciencia y Tecnología DPI2001-3089Ministerio de Eduación y Ciencia d TEC2006-0386

Topological issues in single phase power factor correction

The equipment connected to an electricity distribution network usually needs some kind of power conditioning, typically rectification, which produces a non-sinusoidal line current due to the nonlinear input characteristic. With the steadily increasing use of such equipment, line current harmonics have become a significant problem. Their adverse effects on the power system are well recognized. They include increased magnitudes of neutral currents in three- phase systems, overheating in transformers and induction motors, as well as the degradation of system voltage waveforms. Several international standards now exist, which limit the harmonic content due to line currents of equipment connected to electricity distribution networks. As a result, there is the need for a reduction in line current harmonics, or Power Factor Correction - PFC. There are two types of PFC’s. 1) Passive PFC, 2) Active PFC. The active PFC is further classified into low-frequency and high-frequency active PFC depending on the switching frequency. Different techniques in passive PFC and active PFC are presented here. Among these PFC’s we will get better power factor by using high-frequency active PFC circuit. Any DC-DC converters can be used for this purpose, if a suitable control method is used to shape its input current or if it has inherent PFC properties. The DC-DC converters can operate in Continuous Inductor Current Mode – CICM, where the inductor current never reaches zero during one switching cycle or Discontinuous Inductor Current Mode - DICM, where the inductor current is zero during intervals of the switching cycle. In DICM, the input inductor is no longer a state variable since its state in a given switching cycle is independent on the value in the previous switching cycle. The peak of the inductor current is sampling the line voltage automatically. This property of DICM input circuit can be called “self power factor correction” because no control loop is required from its input side. In CICM, different control techniques are used to control the inductor current. Some of them are (1) peak current control (2) average current control (3) Hysteresis control (4) borderline control. These control techniques specifically developed for PFC boost converters are analyzed. For each control strategy advantages and drawbacks are highlighted and information on available commercial IC's is given. This high frequency switching PFC stage also has drawbacks, such as: it introduces additional losses, thus reducing the overall efficiency; it increases the EMI, due to the highfrequency content of the input current. Some of the EMI requirements are discussed. But the level of high-frequency EMI is much higher with a considerable amount of conduction and switching losses. This highfrequency EMI will be eliminated by introducing an EMI filter in between AC supply and the diode bridge rectifier. The efficiency will be improved by reducing the losses using soft switching techniques such as ‘Zero Voltage Switching’- (ZVS), ‘Zero Voltage Transition’ (ZVT), and ‘Zero Current Switching’- (ZCS). We study circuit techniques to improve the efficiency of the PFC stage by lowering the conduction losses and/or the switching losses. Operation of a ZVT converter will be discussed, in which the switching losses of the auxiliary switch are minimized by using an additional circuit applied to the auxiliary switch. Besides the main switch ZVS turned- on and turned-off, and the auxiliary switch ZCS turned-on and turned-off near ZVS. Since the active switch is turned- on and turned-off softly, the switching losses are reduced and the higher efficiency of the system is achieved

Addressing control and capacitor voltage regulation challenges in multilevel power electronic converters

Multilevel power electronic converters are the current industry solutions for applications that demand medium voltage, reasonable efficiency, and high power quality. The proper operation of these types of power converters requires special control, modulation methods, and capacitor voltage regulation techniques. Both developing capacitor voltage regulation methods and addressing their associated issues with such fall within the primary focus of this dissertation. In this dissertation an investigation was conducted on the capacitor voltage regulation constraints in cascaded H-bridge multilevel converters with a staircase output voltage waveform. In the proposed method, the harmonic elimination technique is used to determine the switching angles. A constraint was then derived to identify modulation those indices that lead to voltage regulation of the capacitor. This constraint can be used in optimization problems for harmonic minimization to guarantee capacitor voltage regulation in these types of converters. Furthermore, a capacitor voltage regulation method was developed using redundant state selection for a flying capacitor active rectifier. This method reduces the number of switching instances by using both online and offline state selection procedure. Additionally, a start-up procedure is proposed that pre-charges the all of capacitors in the rectifier to both avoid overstressing the switches and obtain a smoother start-up. Finally, a flexible capacitor voltage regulation method is proposed that provides the ability to control the voltage of the capacitors in both cascaded H-bridge and hybrid multilevel converters. In this method, the capacitor voltage in each individual H-bridge cell is independently regulated by controlling the active power of each cell

Multi-Frequency Modulation and Control for DC/AC and AC/DC Resonant Converters

Harmonic content is inherent in switched-mode power supplies. Since the undesired harmonics interfere with the operation of other sensitive electronics, the reduction of harmonic content is essential for power electronics design. Conventional approaches to attenuate the harmonic content include passive/active filter and wave-shaping in modulation. However, those approaches are not suitable for resonant converters due to bulky passive volumes and excessive switching losses. This dissertation focuses on eliminating the undesired harmonics from generation by intelligently manipulating the spectrum of switching waveforms, considering practical needs for functionality.To generate multiple ac outputs while eliminating the low-order harmonics from a single inverter, a multi-frequency programmed pulse width modulation is investigated. The proposed modulation schemes enable multi-frequency generation and independent output regulation. In this method, the fundamental and certain harmonics are independently controlled for each of the outputs, allowing individual power regulations. Also, undesired harmonics in between output frequencies are easily eliminated from generation, which prevents potential hazards caused by the harmonic content and bulky filters. Finally, the proposed modulation schemes are applicable to a variety of DC/AC topologies.Two applications of dc/ac resonant inverters, i.e. an electrosurgical generator and a dual-mode WPT transmitter, are demonstrated using the proposed MFPWM schemes. From the experimental results of two hardware prototypes, the MFPWM alleviates the challenges of designing a complicated passive filter for the low-order harmonics. In addition, the MFPWM facilitates combines functionalities using less hardware compared to the state-of-the-art. The prototypes demonstrate a comparable efficiency while achieving multiple ac outputs using a single inverter.To overcome the low-efficiency, low power-density problems in conventional wireless fast charging, a multi-level switched-capacitor ac/dc rectifier is investigated. This new WPT receiver takes advantage of a high power-density switched-capacitor circuit, the low harmonic content of the multilevel MFPWMs, and output regulation ability to improve the system efficiency. A detailed topology evaluation regarding the regulation scheme, system efficiency, current THD and volume estimation is demonstrated, and experimental results from a 20 W prototype prove that the multi-level switched-capacitor rectifier is an excellent candidate for high-efficiency, high power density design of wireless fast charging receiver