Design and Realization of a Novel Buck-Boost Phase-Modular Three-Phase AC/DC Converter System with Low Component Number

Scalability and modularity are key features for future power converters, such that these systems can be easily employed in many applications with different electrical specifications. In this thesis, the potential of a new bidirectional phase-modular three-phase AC/DC converter with buck-boost capability is evaluated by means of studying two potential application cases and developing a hardware prototype for one of them. The DC-DC inverting buck-boost converter is a well-known and established topology. By connecting three such systems in parallel, a phase-modular bidirectional buck-boost DC-AC converter employing a minimum number of active components results, where for given AC voltage amplitudes, an arbitrary DC voltage can be generated and vice versa. Such a three-phase converter was not yet described in literature and this project aims at investigating the fundamental topology properties, as well as its performance limits. A hardware demonstrator is designed for one potential application in order to verify the basic operation and the expected high performance in terms of efficiency and power density.Comment: 80 pages, Master Thesis at ETH Zuric

A two-stage power converter for welding applications with increased efficiency and reduced filtering

The power supply technology used in welding applications changed dramatically from manually tap-controlled 50Hz bulky transformers which had large leakage inductance to provide stable arc burn to switch-mode fast controlled highfrequency power electronics. Nowadays, the typical converter configuration consist of a diode rectifier supplying via a large electrolytic capacitor a smooth DC-link voltage to a high switching frequency H-bridge inverter that steps down the voltage and provides isolation via a high frequency transformer whilst operating with adjustable dutycycle to maintain the output current constant. This topology allows for important size reduction since the size of magnetics decreases rapidly with the increase of the frequency. This paper proposes a more complex two-stage configuration with a buck DC/DC converter operating at a reduced switching frequency to feed adjustable voltage to an H-bridge inverter, which is operating always with the required voltage at 50% dutycycle, enabling in addition the minimization of the output filter size and of the switching losses

A two-stage power converter for welding applications with increased efficiency and reduced filtering

The power supply technology used in welding applications changed dramatically from manually tap-controlled 50Hz bulky transformers which had large leakage inductance to provide stable arc burn to switch-mode fast controlled highfrequency power electronics. Nowadays, the typical converter configuration consist of a diode rectifier supplying via a large electrolytic capacitor a smooth DC-link voltage to a high switching frequency H-bridge inverter that steps down the voltage and provides isolation via a high frequency transformer whilst operating with adjustable dutycycle to maintain the output current constant. This topology allows for important size reduction since the size of magnetics decreases rapidly with the increase of the frequency. This paper proposes a more complex two-stage configuration with a buck DC/DC converter operating at a reduced switching frequency to feed adjustable voltage to an H-bridge inverter, which is operating always with the required voltage at 50% dutycycle, enabling in addition the minimization of the output filter size and of the switching losses

A Modular Active Front-End Rectifier with Electronic Phase-Shifting for Harmonic Mitigation in Motor Drive Applications

In this paper, an electronic phase-shifting strategy has been optimized for a multiparallel configuration of line-commutated rectifiers with a common dc-bus voltage used in motor drive application. This feature makes the performance of the system independent of the load profile and maximizes its harmonic reduction ability. In order to further reduce the generated low-order harmonics, a dc-link current modulation scheme and its phase-shift values of multidrive systems have been optimized. Analysis, simulations, and experiments have been carried out to verify the proposed method

DC/DC Conversion and Distributed Grid based Solution of HVDC Tapping

The increasing demand for efficient and reliable energy transmission solutions has underscored the significance of High Voltage Direct Current (HVDC) technology in modern power systems. HVDC systems have proven instrumental in addressing challenges such as offshore electricity transmission, interconnection of archipelagic nations, and long-distance power transport. However, the need for innovative approaches to address specific challenges, such as connecting multiple islands' energy systems, has led to the exploration of HVDC tapping as a potential solution. HVDC tapping, a concept analogous to different voltage levels in interconnected AC grids via power transformers, presents a promising approach for connecting multiple islands' energy systems. This concept involves the utilization of a DC/DC conversion method to reduce the voltage of the main HVDC link and transmit a small portion of its nominal power. The limitations in power magnitudes and the remote locations of tapping stations introduce operational and maintenance challenges, emphasizing the need for cost-effective, reliable, and user-friendly tapping converters.To address these concerns and fill the research gap in HVDC tapping, this paper proposes an HVDC tapping solution containing a DC/DC converter and a related DC distribution system. The proposed solution incorporates a Modular Multilevel Converter (MMC) and a diode-bridge rectifier connected by a medium frequency transformer, offering cost advantages and ease of control. Furthermore, a DC distribution grid is utilized for connecting several islands at the low voltage side, with a novel control system for the DC/DC converter to ensure the stability of the low-voltage DC grid. The significance of this study lies in addressing the challenges associated with connecting decentralized and geographically intricate power systems, with a specific focus on the potential of HVDC technology in the context of the HVDC GREEN project. The proposed HVDC tapping solution can enhance power quality, and voltage stability of islands’ power system. The study also presents novel fault-ride-through (FRT) control strategies to enhance the reliability of the DC distribution system, enabling the DC/DC converter to function as a current source during fault conditions, thereby preventing power outages.<br/

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

Power factor-corrected transformerless three-phase PWM converter for UPS applications

This thesis describes the research of a new transformerless three phase PWM converter for uninterruptible power supplies (UPS) applications. The removal of the bulky three phase transformer in larger power UPS can provide a significant saving in weight and cost of the overall system. The converter consists of a new four-wire rectifier coupled with a four-wire inverter via a dc bus. The supply and load neutral may be connected together without any neutral current flowing into the utility regardless of the load on the inverter. This allows the load to be at the same potential as the utility. The rectifier, inverter and complete UPS and control system are described in detail and simulation results are used extensively to back up the theory. An experimental prototype of the four-wire rectifier provides further confirmation of the principles. A further proposal to digitize the system is given. This would reduce the size of the required control circuit and simplify the hardware requirements