Design and Evaluation of High Efficiency Power Converters Using Wide-Bandgap Devices for PV Systems

The shortage of fossil resources and the need for power generation options that produce little or no environmental pollution drives and motivates the research on renewable energy resources. Power electronics play an important role in maximizing the utilization of energy generation from renewable energy resources. One major renewable energy source is photovoltaics (PV), which comprises half of all recently installed renewable power generation in the world. For a grid-connected system, two power stages are needed to utilize the power generated from the PV source. In the first stage, a DCDC converter is used to extract the maximum power from the PV panel and to boost the low output voltage generated to satisfy the inverter side requirements. In the second stage, a DC-AC inverter is used to convert and deliver power loads for grid-tied applications. In general, PV panels have low efficiency so high-performance power converters are required to ensure highly efficient PV systems. The development of wide-bandgap (WBG) power switching devices, especially in the range of 650 V and 1200 V blocking class voltage, opens up the possibility of achieving a reliable and highly efficient grid-tied PV system. This work will study the benefits of utilizing WBG semiconductor switching devices in low power residential scale PV systems in terms of efficiency, power density, and thermal analysis. The first part of this dissertation will examine the design of a high gain DC-DC converter. Also, a performance comparison will be conducted between the SiC and Si MOSFET switching devices at 650 V blocking voltage regarding switching waveform behavior, switching and conduction losses, and high switching frequency operation. A major challenge in designing a transformerless inverter is the circulating of common mode leakage current in the absence of galvanic isolation. The value of the leakage current must be less than 300mA, per the DIN VDE 0126-1-1 standard. The second part of this work investigates a proposed high-efficiency transformerless inverter with low leakage current. Subsequently, the benefits of using SiC MOSFET are evaluated and compared to Si IGBT at 1200 V blocking voltage in terms of efficiency improvement, filter size reduction, and increasing power rating. Moreover, a comprehensive thermal model design is presented using COMSOL software to compare the heat sink requirements of both of the selected switching devices, SiC MOSFET and Si IGBT. The benchmarking of switching devices shows that SiC MOSFET has superior switching and conduction characteristics that lead to small power losses. Also, increasing switching frequency has a small effect on switching losses with SiC MOSFET due to its excellent switching characteristics. Therefore, system performance is found to be enhanced with SiC MOSFET compared to that of Si MOSFET and Si IGBET under wide output loads and switching frequency situations. Due to the high penetration of PV inverters, it is necessary to provide advanced functions, such as reactive power generation to enable connectivity to the utility grid. Therefore, this research proposes a modified modulation method to support the generation of reactive power. Additionally, a modified topology is proposed to eliminate leakage current

Bipolar and unipolar schemes for confined band variable switching frequency PWM based inverter

The single phase inverter performance through the unipolar and bipolar strategies has been previously analyzed based on the constant switching frequency pulse width modulation (CSFPWM). However, the confined band variable switching frequency PWM (CB-VSFPWM) is currently proposed as a new variable switching frequency PWM technique through unipolar strategy to facilitate the design of high order filter, to reduce the switching losses, and to reduce the current total harmonics distortion (THD) as well. To evaluate the performance of a single phase inverter based on the CBVSFPWM through bipolar strategy, this paper presents a comparative study of the CB-VSFPWM based inverter performance using the unipolar PWM and the bipolar PWM strategies. The study adopts MATLAB/Simulink to simulate the inverter and to analyze the simulation results in terms of harmonics spectrum, total harmonic distortion (THD), and fundamental components. The analysis of the study results gives an indication about the appropriate type of CB-VSFPWM strategy (unipolar PWM or bipolar PWM) to guarantee the desired performance of the connected inverter in terms of the electrical grid standards like THD, and harmonics spectrum of the inverter current

New Modulation Technique to Mitigate Common Mode Voltage Effects in Star-Connected Five-Phase AC Drives

Star-connected multiphase AC drives are being considered for electromovility applications such as electromechanical actuators (EMA), where high power density and fault tolerance is demanded. As for three-phase systems, common-mode voltage (CMV) is an issue for multiphase drives. CMV leads to shaft voltages between rotor and stator windings, generating bearing currents which accelerate bearing degradation and produce high electromagnetic interferences (EMI). CMV effects can be mitigated by using appropriate modulation techniques. Thus, this work proposes a new Hybrid PWM algorithm that effectively reduces CMV in five-phase AC electric drives, improving their reliability. All the mathematical background required to understand the proposal, i.e., vector transformations, vector sequences and calculation of analytical expressions for duty cycle determination are detailed. Additionally, practical details that simplify the implementation of the proposal in an FPGA are also included. This technique, HAZSL5M5-PWM, extends the linear range of the AZSL5M5-PWM modulation, providing a full linear range. Simulation results obtained in an accurate multiphase EMA model are provided, showing the validity of the proposed modulation approach.This work has been supported in part by the Government of the Basque Country within the fund for research groups of the Basque University system IT978-16 and in part by the Government of the Basque Country within the research program ELKARTEK as the project ENSOL (KK-2018/00040)

Beijing converters: bridge converters with a capacitor added to reduce leakage currents, DC-bus voltage ripples, and total capacitance required

Abstract: Isolation transformers and bulky electrolytic capacitors are often used in power electronic converters to reduce leakage currents and voltage ripples but this leads to low power density and reduced reliability. In this paper, an auxiliary capacitor is added to the widely used conventional full-bridge converter to provide a path for, and hence significantly reduce, the leakage current. The operation of the full-bridge converter is split into the operation of a half-bridge converter and a dc-dc converter so that the ripple energy can be diverted from the dc-bus capacitor to the auxiliary capacitor. Hence, the dc-bus capacitor can be significantly reduced while maintaining very low voltage ripples on the dc bus because it is only required to filter out switching ripples. The auxiliary capacitor is designed to allow high voltage ripples because its voltage is not supplied to any load. Accordingly, the auxiliary capacitor can also be very small as well. As a result, the total required capacitance becomes very small. The reduction ratio of the total capacitance is significant, which makes it cost-effective to use film capacitors instead of electrolytic capacitors. The proposed converters can be also operated as an inverter without any restriction on power factor because the adopted four switches are all bidirectional in terms of power flow. Experimental results for both rectification and inversion modes are presented to demonstrate the performance of the proposed converter in reducing the ripples, the leakage currents, and the total capacitance needed, with comparison to the conventional bridge converter without the auxiliary capacitor

H8 Inverter for Common-Mode Voltage Reduction in Electric Drives

This paper presents a modified two-level three-phase inverter for the reduction of the leakage current. With respect to a traditional two-level inverter, the proposed solution reduces the common-mode voltage (CMV), both in amplitude and frequency. Between the dc source and the traditional three-phase bridge, two active dc-decoupling devices and a voltage-clamping network have been added. A dedicated control strategy was developed adopting a modified space vector pulse-width modulation, oriented to the reduction of the CMV. Simulations showing the good performance of the solution are presented. A preliminary prototype was developed and experimental results are presented

Transformerless grid-connected inverters for photovoltaic systems

The increasing demand for electrical power, along with the decreasing stock of traditional energy sources, has caused a growing interest towards microgeneration from renewable power sources. In particular, photovoltaic energy (PV) has witnessed an increasing attention and the scientific community has concentrated its efforts in order to develop innovative solutions for the integration of PV systems into the existing distribution grid. In this thesis PV converters for grid-connected systems without galvanic insulation were studied. A particular solution of a multilevel-based converter was employed in a PV application and its behavior was thoroughly analyzed. Extensive simulations and experimental results confirmed the feasibility and the good performance of the proposed solution

Active common-mode filter for PV transformerless grid-connected inverters

During the last years, it has been possible to witness a steady and progressive increase of energy production from renewable resources. In particular, the greatest increment has been registered for photovoltaic applications, due to the possibility to install low power implants easily integrated in the urban ambient, the so-called domestic photovoltaic. A photovoltaic system can be islanded, when the energy is extracted from the panels for supplying local loads, as in the case of remote agricultural areas, or grid-connected, where the energy recovered from the panels is directly injected into the mains. Until now, where there was the possibility, grid-connected system has been considered the easiest and most efficient solution for photovoltaic plants. Following these considerations, in the last years there has been a remarkable proliferation, in both academic and industrial field, of new solutions for grid-connected inverters that were designed to maximize efficiency and reliability. Initially, grid-connected inverters were realized employing a line frequency transformer, which, establishing a galvanic insulation between the photovoltaic source and the grid, facilitated the design issues. Nevertheless, because of its bulky dimension, costs, and additional power losses, the use of transformers was progressively abandoned. Nowadays transformerless inverters are the most efficient grid-connected converters commercialized and some companies arrive to claim values of 98% of efficiency for their products. Nevertheless, the absence of a galvanic insulation between the photovoltaic source and the grid gives rise to issues, such us ground leakage currents and possible DC current injection into the grid that must be addressed. The ground leakage current phenomenon was proven to be due to the presence of a parasitic capacitance between the PV cells and the metal structure of the panels, usually grounded for safety reason. A survey of the actual solutions to avoid the arising of ground leakage current in transformerless single-phase systems was elaborated in this work, and a novel classification for transformerless inverters was proposed as well. The principal causes of ground leakage current were investigated, and the contribution to the phenomenon of the common-mode voltage generated at the output of the grid-connected inverters during their operation was analyzed. In fact the common-mode voltage at the output of the converters generates currents that flow in the parasitic capacitance throughout the connection to the ground of the neutral wire of the grid at the MV/LV transformer. For this reason the ground leakage current is also known as common-mode current. A novel approach to cancel the common-mode voltage variations at the output of a transformerless grid-connected converter was proposed. This solution relies on an active common-mode filter connected at the output of the power converter. It is constituted by a common-mode transformer properly supplied by a low-power full-bridge. The proposed solution is applicable to both stand-alone and grid-connected converters. In particular in this work the active filter was applied to a full-bridge power converter topology driven by the efficient 3-level (unipolar) PWM. The feasibility of the proposed solution and the capacity to operate with power factor different from one was proven through extensive simulations in Simulink/Plecs environment, and confirmed with experimental results. On this purpose, a converter prototype was designed and built. It embeds all the components for enabling the connection to the mains in accordance to the Italian legislation CEI 0-21

Emerging Power Electronics Technologies for Sustainable Energy Conversion

This Special Issue summarizes, in a single reference, timely emerging topics related to power electronics for sustainable energy conversion. Furthermore, at the same time, it provides the reader with valuable information related to open research opportunity niches