Advanced Control Techniques for Efficiency and Power Density Improvement of a Three-Phase Microinverter

Inverters are widely used in photovoltaic (PV) based power generation systems. Most of these systems have been based on medium to high power string inverters. Microinverters are gaining popularity over their string inverter counterparts in PV based power generation systems due to maximized energy harvesting, high system reliability, modularity, and simple installation. They can be deployed on commercial buildings, residential rooftops, electric poles, etc and have a huge potential market. Emerging trend in power electronics is to increase power density and efficiency while reducing cost. A powerful tool to achieve these objectives is the development of an advanced control system for power electronics. In low power applications such as solar microinverters, increasing the switching frequency can reduce the size of passive components resulting in higher power density. However, switching losses and electromagnetic interference (EMI) may increase as a consequence of higher switching frequency. Soft switching techniques have been proposed to overcome these issues. This dissertation presents several innovative control techniques which are used to increase efficiency and power density while reducing cost. Dynamic dead time optimization and dual zone modulation techniques have been proposed in this dissertation to significantly improve the microinverter efficiency. In dynamic dead time optimization technique, pulse width modulation (PWM) dead times are dynamically adjusted as a function of load current to minimize MOSFET body diode conduction time which reduces power dissipation. This control method also improves total harmonic distortion (THD) of the inverter output current. To further improve the microinverter efficiency, a dual-zone modulation has been proposed which introduces one more soft-switching transition and lower inductor peak current compared to the other boundary conduction mode (BCM) modulation methods. In addition, an advanced DC link voltage control has been proposed to increase the microinverter power density. This concept minimizes the storage capacitance by allowing greater voltage ripple on the DC link. Therefore, the microinverter reliability can be significantly increased by replacing electrolytic capacitors with film capacitors. These control techniques can be readily implemented on any inverter, motor controller, or switching power amplifier. Since there is no circuit modification involved in implementation of these control techniques and can be easily added to existing controller firmware, it will be very attractive to any potential licensees

New multifunctional isolated microinverter with integrated energy storage system for PV applications

This paper proposes a novel multifunctional isolated microinverter which is able to extract the maximum available power from a solar photovoltaic module and inject it into the power grid, while simultaneously charging a battery energy storage system (BESS). The proposed microinverter integrates a novel DC–DC power converter and a conventional DC–AC power converter. The DC–DC power converter is able to send electrical energy to the secondary side of a high-frequency transformer and to the BESS, using only two power switches. Throughout this paper, the converter topology, the operation modes, the control algorithms, and the development of a laboratory prototype of the proposed microinverter are described in detail. Moreover, simulation and experimental results are presented to demonstrate the feasibility of the proposed solution.This work has been supported by FCT—Fundação para a Ciência e Tecnologia within the R&DUnits Project Scope: UIDB/00319/2020. This work has been supported by the FCT Project QUALITY4POWERPTDC/EEI-EEE/28813/2017. Luis A. M. Barros is supported by the doctoral scholarship PD/BD/143006/2018 granted by the Portuguese FCT foundation. Mohamed Tanta was supported by FCT PhD grant with a reference PD/BD/127815/2016 granted by the Portuguese FCT agency. Tiago J. C. Sousa is supported by the doctoralscholarship SFRH/BD/134353/2017 granted by the Portuguese FCT agency

Development of Multiport Single Stage Bidirectional Converter for Photovoltaic and Energy Storage Integration

The energy market is on the verge of a paradigm shift as the emergence of renewable energy sources over traditional fossil fuel based energy supply has started to become cost competitive and viable. Unfortunately, most of the attractive renewable sources come with inherent challenges such as: intermittency and unreliability. This is problematic for today\u27s stable, day ahead market based power system. Fortunately, it is well established that energy storage devices can compensate for renewable sources shortcomings. This makes the integration of energy storage with the renewable energy sources, one of the biggest challenges of modern distributed generation solution. This work discusses, the current state of the art of power conversion systems that integrate photovoltaic and battery energy storage systems. It is established that the control of bidirectional power flow to the energy storage device can be improved by optimizing its modulation and control. Traditional multistage conversion systems offers the required power delivery options, but suffers from a rigid power management system, reduced efficiency and increased cost. To solve this problem, a novel three port converter was developed which allows bidirectional power flow between the battery and the load, and unidirectional power flow from the photovoltaic port. The individual two-port portions of the three port converter were optimized in terms of modulation scheme. This leads to optimization of the proposed converter, for all possible power flow modes. In the second stage of the project, the three port converter was improved both in terms of cost and efficiency by proposing an improved topology. The improved three port converter has reduced functionality but is a perfect fit for the targeted microinverter application. The overall control system was designed to achieve improved reference tracking for power management and output AC voltage control. The bidirectional converter and both the proposed three port converters were analyzed theoretically. Finally, experimental prototypes were built to verify their performance

A 1-MHz Series Resonant DC-DC Converter with a Dual-Mode Rectifier for PV Microinverters

The photovoltaic (PV) output voltage varies over a wide range depending on operating conditions. Thus, the PV-connected converters should be capable of handling a wide input voltage range while maintaining high efficiencies. This paper proposes a new series resonant dc-dc converter for PV microinverter applications. Compared with the conventional series resonant converter, a dual-mode rectifier is configured on the secondary side, which enables a twofold voltage gain range for the proposed converter with a fixed-frequency phase-shift modulation scheme. The zero-voltage switching turn-on and zero-current switching turn-off can be achieved for active switches and diodes, thereby, minimizing the switching losses. Moreover, a variable dc-link voltage control scheme is introduced to the proposed converter, leading to a further efficiency improvement and input-voltage-range extension. The operation principle and essential characteristics (e.g., voltage gain, soft-switching, and root-mean-square current) of the proposed converter are detailed in this paper, and the power loss modeling and design optimization of components are also presented. A 1-MHz 250-W converter prototype with an input voltage range of 17-43 V is built and tested to verify the feasibility of the proposed converter

Fuel cell power conditioning multiphase converter for 1400 VDC megawatts stacks

Thesis (PhD (Electrical Engineering))--Cape Peninsula University of Technology, 2019Energy systems based on fossil fuel have demonstrated their abilities to permit economic development. However, with the fast exhaustion of this energy source, the expansion of the world energy demand and concerns over global warming, new energy systems dependent on renewable and other sustainable energy are gaining more interests. It is a fact that future development in the energy sector is founded on the utilisation of renewable and sustainable energy sources. These energy sources can enable the world to meet the double targets of diminishing greenhouse gas emissions and ensuring reliable and cost-effective energy supply. Fuel cells are one of the advanced clean energy technologies to substitute power generation systems based on fossil fuel. They are viewed as reliable and efficient technologies to operate either tied or non-tied to the grid to power applications ranging from domestic, commercial to industrial. Multiple fuel cell stacks can be associated in series and parallel to obtain a fuel cell system with high power up to megawatts. The connection of megawatts fuel cell systems to a utility grid requires that the power condition unit serving as the interface between the fuel cell plant and the grid operates accordingly. Different power conditioning unit topologies can be adopted, this study considers a multilevel inverter. Multilevel inverters are getting more popularity and attractiveness as compared to conventional inverters in high voltage and high-power applications. These inverters are suitable for harmonic mitigation in high-power applications whereby switching devices are unable to function at high switching frequencies. For a given application, the choice of appropriate multilevel topology and its control scheme are not defined and depend on various engineering compromises, however, the most developed multilevel inverter topologies include the Diode Clamped, the Flying Capacitor and the Cascade Full Bridge inverters. On the other hand, a multilevel inverter can be either a three or a five, or a nine level, however, this research focuses on the three-level diode clamped inverters. The aim of this thesis is to model and control a three-level diode clamped inverter for the grid connection of a megawatt fuel cell stack. Besides the grid, the system consists of a 1.54 MW operating at 1400 V DC proton exchange membrane fuel cell stack, a 1.26 MW three-level diode clamped inverter with a nominal voltage of 600 V and an LCL filter which is designed to reduce harmonics and meet the standards such as IEEE 519 and IEC 61000-3-6. The inverter control scheme comprises voltage and current regulators to provide a good power factor and satisfy synchronisation requirements with the grid. The frequency and phase are synchronised with those of the grid through a phase locked loop. The modelling and simulation are performed using Matlab/Simulink. The results show good performance of the developed system with a low total harmonic distortion of about 0.35% for the voltage and 0.19% for the current

Design and Implementation of PV-Firming and Optimization Algorithms For Three-Port Microinverters

With the demand increase for electricity, the ever-increasing awareness of environmental issues, coupled with rolling blackouts, the role of renewable energy generation is increasing along with the thirst for electricity and awareness of environmental issues. This dissertation proposes the design and implementation of PV-firming and optimization algorithms for three-port microinverters. Novel strategies are proposed in Chapters 3 and 4 for harvesting stable solar power in spite of intermittent solar irradiance. PV firming is implemented using a panel-level three-port grid-tied PV microinverter system instead of the traditional high-power energy storage and management system at the utility scale. The microinverter system consists of a flyback converter and an H-bridge inverter/rectifier, with a battery connected to the DC-link. The key to these strategies lies in using static and dynamic algorithms to generate a smooth PV reference power. The outcomes are applied to various control methods to charge/discharge the battery so that a stable power generation profile is obtained. In addition, frequency-based optimization for the inverter stage is presented. One of the design parameters of grid-tied single-phase H-bridge sinusoidal pulse-width modulation (SPWM) microinverters is switching frequency. The selection of the switching frequency is a tradeoff between improving the power quality by reducing the total harmonic distortion (THD), and improving the efficiency by reducing the switching loss. In Chapter 5, two algorithms are proposed for optimizing both the power quality and the efficiency of the microinverter. They do this by using a frequency tracking technique that requires no hardware modification. The first algorithm tracks the optimal switching frequency for maximum efficiency at a given THD value. The second maximizes the power quality of the H-bridge micro-inverter by tracking the switching frequency that corresponds to the minimum THD. Real-time PV intermittency and usable capacity data were evaluated and then further analyzed in MATLAB/SIMULINK to validate the PV firming control. The proposed PV firming and optimization algorithms were experimentally verified, and the results evaluated. Finally, Chapter 6 provides a summary of key conclusions and future work to optimize the presented topology and algorithms