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

Electrical performance characteristics of high power converters for space power applications

The first goal of this project was to investigate various converters that would be suitable for processing electric power derived from a nuclear reactor. The implementation is indicated of a 20 kHz system that includes a source converter, a ballast converter, and a fixed frequency converter for generating the 20 kHz output. This system can be converted to dc simply by removing the fixed frequency converter. This present study emphasized the design and testing of the source and ballast converters. A push-pull current-fed (PPCF) design was selected for the source converter, and a 2.7 kW version of this was implemented using three 900 watt modules in parallel. The characteristic equation for two converters in parallel was derived, but this analysis did not yield any experimental methods for measuring relative stability. The three source modules were first tested individually and then in parallel as a 2.7 kW system. All tests proved to be satisfactory; the system was stable; efficiency and regulation were acceptable; and the system was fault tolerant. The design of a ballast-load converter, which was operated as a shunt regulator, was investigated. The proposed power circuit is suitable for use with BJTs because proportional base drive is easily implemented. A control circuit which minimizes switching frequency ripple and automatically bypasses a faulty shunt section was developed. A nonlinear state-space-averaged model of the shunt regulator was developed and shown to produce an accurate incremental (small-signal) dynamic model, even though the usual state-space-averaging assumptions were not met. The nonlinear model was also shown to be useful for large-signal dynamic simulation using PSpice

Distributed control of a fault tolerant modular multilevel inverter for direct-drive wind turbine grid interfacing

Modular generator and converter topologies are being pursued for large offshore wind turbines to achieve fault tolerance and high reliability. A centralized controller presents a single critical point of failure which has prevented a truly modular and fault tolerant system from being obtained. This study analyses the inverter circuit control requirements during normal operation and grid fault ride-through, and proposes a distributed controller design to allow inverter modules to operate independently of each other. All the modules independently estimate the grid voltage magnitude and position, and the modules are synchronised together over a CAN bus. The CAN bus is also used to interleave the PWM switching of the modules and synchronise the ADC sampling. The controller structure and algorithms are tested by laboratory experiments with respect to normal operation, initial synchronization to the grid, module fault tolerance and grid fault ride-through

Study of the generator/motor operation of induction machines in a high frequency link space power system

Static power conversion systems have traditionally utilized dc current or voltage source links for converting power from one ac or dc form to another since it readily achieves the temporary energy storage required to decouple the input from the output. Such links, however, result in bulky dc capacitors and/or inductors and lead to relatively high losses in the converters due to stresses on the semiconductor switches. The feasibility of utilizing a high frequency sinusoidal voltage link to accomplish the energy storage and decoupling function is examined. In particular, a type of resonant six pulse bridge interface converter is proposed which utilizes zero voltage switching principles to minimize switching losses and uses an easy to implement technique for pulse density modulation to control the amplitude, frequency, and the waveshape of the synthesized low frequency voltage or current. Adaptation of the proposed topology for power conversion to single-phase ac and dc voltage or current outputs is shown to be straight forward. The feasibility of the proposed power circuit and control technique for both active and passive loads are verified by means of simulation and experiment

A comparative study of electric power distribution systems for spacecraft

The electric power distribution systems for spacecraft are compared concentrating on two interrelated issues: the choice between dc and high frequency ac, and the converter/inverter topology to be used at the power source. The relative merits of dc and ac distribution are discussed. Specific converter and inverter topologies are identified and analyzed in detail for the purpose of detailed comparison. Finally, specific topologies are recommended for use in dc and ac systems

An Integral Battery Charger with Power Factor Correction for Electric Scooter

This paper presents an integral battery charger for an electric scooter with high voltage batteries and interior-permanent-magnet motor traction drive. The battery charger is derived from the power hardware of the scooter, with the ac motor drive that operates as three-phase boost rectifier with power factor correction capability. The control of the charger is also integrated into the scooter control firmware that is implemented on a fixed-point DSP controller. Current-controlled or voltage-controlled charge modes are actuated according to the requirements of the battery management system, that is embedded into the battery pack. With respect to previous integrated chargers, the ac current is absorbed at unitary power factor with no harmonic distortion. Moreover, no additional filtering is needed since the pulsewidth modulation ripple is minimized by means of phase interleaving. The feasibility of the integral charger with different ac motors (induction motor, surface-mounted phase modulation motor) is also discussed, by means of a general model purposely developed for three-phase ac machines. The effectiveness of the proposed battery charger is experimentally demonstrated on a prototype electric scooter, equipped with two Li-ion battery packs rated 260 V, 20 A

Boost converter fed high performance BLDC drive for solar PV array powered air cooling system

This paper proposes the utilization of a DC-DC boost converter as a mediator between a Solar Photovoltaic (SPV) array and the Voltage Source Inverters (VSI) in an SPV array powered air cooling system to attain maximum efficiency. The boost converter, over the various common DC-DC converters, offers many advantages in SPV based applications. Further, two Brushless DC (BLDC) motors are employed in the proposed air cooling system: one to run the centrifugal water pump and the other to run a fan-blower. Employing a BLDC motor is found to be the best option because of its top efficiency, supreme reliability and better performance over a wide range of speeds. The air cooling system is developed and simulated using the MATLAB/Simulink environment considering the steady state variation in the solar irradiance. Further, the efficiency of BLDC drive system is compared with a conventional Permanent Magnet DC (PMDC) motor drive system and from the simulated results it is found that the proposed system performs better

Low Power AC-DC and DC-DC Multilevel Converters

AC-DC power electronic converters are widely used for electrical power conversion in many industrial applications such as for telecom equipment, information technology equipment, electric vehicles, space power systems and power systems based on renewable energy resources. Conventional AC-DC converters generally have two conversion stages – an AC-DC front-end stage that operates with some sort of power factor correction to ensure good power quality at the input, and a DC-DC conversion stage that takes the DC output of the front-end converter and converts it to the desired output DC voltage. Due to the cost of having two separate and independent converters, there has been considerable research on so-called single-stage converters – converters that can simultaneously perform AC-DC and DC-DC conversion with only a single converter stage. In spite of the research that has been done on AC-DC single-stage, there is still a need for further research to improve their performance. The main focus of this thesis is on development of new and improved AC-DC single-stage converters that are based on multilevel circuit structures (topologies) and principles instead of conventional two-level ones. The development of a new DC-DC multilevel converter is a secondary focus of this thesis. In this thesis, a literature survey of state of the art AC-DC and DC-DC converters is performed and the drawbacks of previous proposed converters are reviewed. A variety of new power electronic converters including new single-phase and three-phase converters and a new DC-DC converter are then proposed. The steady-state characteristics of each new converter is determined by mathematical analysis, and, once determined, these characteristics are used to develop a procedure for the design of key converter components. The feasibility of all new converters is confirmed by experimental results obtained from proof-of-concept prototype converters. Finally, the contents of the thesis are summarized and conclusions about the effectiveness of using multilevel converter principles to improve the performance of AC-DC and DC-DC converters are made

Dynamic Interactions of a Double-stage Photovoltaic Power Converter: Modelling and Control

Photovoltaic (PV) systems are a promising renewable source to achieve green energy targets and be part of the electricity generation. Lots of efforts have been devoted to increase the penetration level of PV systems and its share in the generated electricity. Power quality is one of the challenges that impact the penetration level of PV systems. It is important to ensure high power quality from PV systems to allow more installations to the grid. So, PV power quality issues have to be addressed properly. It was reported that the poor power quality of the PV systems might be caused by many reasons such as the large amount of PV power fluctuation, the low level of current from the PV system, and large populations of PV inverters. In addition to the aforementioned reasons, recently it was suggested that perturb and observe (P&O) controller is another source of harmonics which result in a deprived PV power quality. This newly reported problem is based on experimental observations without full understanding of the generation mechanism of these harmonics in the PV system, the relation between the P&O controller design and the generated harmonics, and the effect of these harmonics on the rest of the system. Thus, in-depth analysis of the harmonics in PV systems due to P&O controller and a solution to eliminate these harmonics are demanded. Therefore, in this research an investigation is carried out to explore P&O related harmonics in a double-stage grid-connected PV system. First, regarding the P&O related harmonics full explanation of how harmonics are generated due to the perturbing nature of the P&O controller is provided, a modelling approach is suggested to identify the frequency and the amplitude of the variations in the DC bus due to the P&O controller, the effect of different factors (e.g. weather conditions, system parameters, system operating point, and P&O architecture) on the induced harmonics are investigated. Secondly, regarding the effect of the P&O related harmonics on the rest of the system an intense simulation analysis is provided to explore the possible effect of the P&O related harmonics on increasing the interaction between the system power stages. This can help to set system design recommendations and guidelines such as sizing the dc-link capacitance and designing the system controllers. Finally, a novel mitigation solution is proposed to supress the P&O related harmonics. That can help to reduce the dynamic interaction between system power stages and improve the power quality of the PV system