Control of a Satellite Based Photovoltaic Array for Optimum Power Draw

This thesis analyzes the general performance and design requirements of photovoltaic(PV) systems, and specifically how they relate to the design of a system intended to supply power to a rotating satellite. The PV array geometry was discussed, different DC-DC converter topologies were analyzed, and optimum array geometry and converter topologies were determined. The potential reference quantities for use in control of the system are examined. Due to its comparably greater linearity with respect to changes in apparent load and its relative insensitivity to insolation changes, voltage was determined to be the best reference quantity for use in stable tracking of the maximum power operating point of photovoltaic modules. The preceding work is used to design and model a photovoltaic system for a rotating satellite ensuring the supply of the maximum available power as well as stable operation. Simulations of the system are performed at rotational velocities up to 300 rev/min and its behavior is analyzed to demonstrate the validity of the preceding work. It was concluded that: ● parallel connected photovoltaic panels provide greater efficiency than series connected panels. ● Buck, Boost, and Cuk Converter architectures are best suited to PV applications ● PV Voltage is the best reference quantity for use in stable control of PV systems

A Class-E-Based AC-DC converter for PFC applications

Connection of nonlinear utility load har increased through resent years and is expected to continue increasing. Nonlinear utility load injects harmonic content into the grid and reduces voltage quality for nearby consumers. To limit harmonic content from nonlinear load, the International Electrotechnical Commission requires power supplies to be designed according to IEC 61000-3-2. Fulfilling this standard for nonlinear load is done by power factor correction (PFC). Conventionally, pulse width modulation (PWM) converter has been used for PFC converters as they provide high efficiency with a simple control technic. However, as PWM converters switch by hard-switching, that limits the switching frequency through switching loss and generates EMI, resonant converters has become more attractive. Resonant converters operate at soft-switching where the voltage across and/ or current through is zero in the switching moment. This reduces switching loss and EMI, and allow for high switching frequency. High switching frequency is desired as it enables high power density. Through this thesis, two resonant converters using high switching frequency has been proposed. These converters are based on a Class-E converter as it has low noise and high efficiency when switching at high frequency. The thesis includes a mathematical model for both converts, simulation and experimental testing result. Result from testing differs from calculated and simulated values, and troubleshooting for one of the converters has been conducted. Through troubleshooting and a second test with changed parameters, the performance of the converter increased compared to the first test. Due to lack of time, the debugging process was not completed and will be a part of future work

Power electronic converter design handbook

Nowadays, power electronic converters play an essential role in the majority of consumer electronic devices and are widely used in industrial applications. Since most of these applications are supplied through the AC grid, the use of rectifiers and DC-DC conveters are mandatory to adapt the grid voltage to the application requirements. In this book, most used AC-DC rectifier topologies and DC-DC converter topologies are thoroughly discussed. Basics of each converter, equations for the power losses evaluation and passive elements design are described. Moreover, the medium frequency transformer required by several of the studied DC-DC converters is analysed in depth. Therefore, this book pretends to be a handbook with a wide scope, which could be used for academic purposes or even by engineers

Modelling and control of integrated PV-converter modules under partial shading conditions

It has been well-recognized that non-uniform solar irradiation of photovoltaic (PV) panels causes electrical mismatching of cells and may result in reduced output power and cell thermal breakdown. Bypass diodes are commonly used, but challenges exist into obtaining the maximum power point tracking in these partially shaded PV panels for each weather condition. This is due to that there are multiple peak power points present in their Power-Voltage characteristic curves which makes difficult to locate the global maximum power point. The work presented in this thesis studies in detail the converter topologies and control methods which can be used in the PV power generation systems to overcome effectively the shortcomings caused by partial shading. The proposed topology is an integrated bi-directional Cuk converter and PV-panel module. The particular example investigated includes two PV panels connected across two terminals of the Cuk converter. The features of this system in power harness are studied under partial shading conditions, its superior performance in power generation is demonstrated through simulation and practical tests. The generated power is 30% higher than that from a two PV panel system using only bypass diodes. To develop the control schemes for the above system a detailed study was performed leading to the derivation of the transfer function model describing the dynamic responses of voltages across the two PV panels corresponding to the variations of converter switch duty ratio. Experimental verification of this confirms that the model is sufficiently accurate for the application of controller design and tests. A novel maximum power point tracking scheme is developed. This consists of a switching selection scheme and a model based on an optimal control algorithm. The former determines which switch-diode pair in the bidirectional Cuk converter to be active according to measured light levels on each PV panel and the ability to predict the optimal voltage values across the individual PV panels under any practical irradiance and temperature levels. The performance of the controller is tested in simulation as well as in practice under various modes of partial shading, all giving desired results in achieving the maximum power generation. The final contribution lies in the design and construction of an experimental prototype consisting of an inner bidirectional Cuk converter across two PV panels and a terminal boost converter, controlled by DSP-based microcontroller. This setup enables further development and verification of the control schemes for this integrated converter and PV-panel system. Keywords: Photovoltaic Systems, Partial Shading, Cuk Converter, DC-DC Power Converters, Solar Power Generation, Maximum Power Point Tracking, Bypass Diode

Supercapacitor technologies for renewable energy application

Supercapacitors are increasingly gaining popularity since the advent of “world campaign” for clean energy generation. They are characteristically high power-dense energy storage devices with very low ESR. They are known to have more cycle life and less prone to explosion than the conventional lithium-ion batteries. With Research activities on the rise, supercapacitors could be potential alternatives for the less durable battery energy storage systems. This paper explores the behaviour of different supercapacitor technologies as energy storage devices. Three different technologies are analysed, they include; The Electrochemical double-layer capacitor, the hybrid supercapacitor and the battery-type supercapacitor. More emphasis was laid on the battery type supercapacitor as the latest supercapacitor technology with a capacitance of 40,000F. Empirical tests were conducted on the devices with the charge and discharge characteristic curves obtained to further analyse their behaviours. To investigate the supercapacitor’s storage capability, a boost converter was designed. The converter enables the 2.7V supercapacitor power a 12V LED. A 150W push-pull converter was also designed to enable a bank of supercapacitors to power a 120V LED floodlight. This was done to demonstrate the commercial applicability of the battery type supercapacitor for street lighting system. Detailed design procedures of the two converters are described, including the Printed circuit board and the push-pull transformer

Isolated Single-stage Power Electronic Building Blocks Using Medium Voltage Series-stacked Wide-bandgap Switches

The demand for efficient power conversion systems that can process the energy at high power and voltage levels is increasing every day. These systems are to be used in microgrid applications. Wide-bandgap semiconductor devices (i.e. Silicon Carbide (SiC) and Gallium Nitride (GaN) devices) are very promising candidates due to their lower conduction and switching losses compared to the state-of-the-art Silicon (Si) devices. The main challenge for these devices is that their breakdown voltages are relatively lower compared to their Si counterpart. In addition, the high frequency operation of the wide-bandgap devices are impeded in many cases by the magnetic core losses of the magnetic coupling components (i.e. coupled inductors and/or high frequency transformers) utilized in the power converter circuit. Six new dc-dc converter topologies are propose. The converters have reduced voltage stresses on the switches. Three of them are unidirectional step-up converters with universal input voltage which make them excellent candidates for photovoltaic and fuel cell applications. The other three converters are bidirectional dc-dc converters with wide voltage conversion ratios. These converters are very good candidates for the applications that require bidirectional power flow capability. In addition, the wide voltage conversion ratios of these converters can be utilized for applications such as energy storage systems with wide voltage swings

Design of module level converters in photovoltaic power systems

The application of distributed maximum power point tracking (DMPPT) technology in solar photovoltaic (PV) systems is a hot topic in industry and academia. In the PV industry, grid integrated power systems are mainstream. The main objective for PV system design is to increase energy conversion efficiency and decrease the levelized cost of electricity of PV generators. This thesis firstly presents an extensive review of state-of-the-art PV technologies. With focus on grid integrated PV systems research, various aspects covered include PV materials, conventional full power processing DMPPT architectures, main MPPT techniques, and traditional partial power processing DMPPT architectures. The main restrictions to applying traditional DMPPT architectures in large power systems are discussed. A parallel connected partial power processing DMPPT architecture is proposed aiming to overcome existing restrictions. With flexible ‘plug-and-play’ functionality, the proposed architecture can be readily expanded to supply a downstream inverter stage or dc network. By adopting smaller module integrated converters, the proposed approach provides a possible efficiency improvement and cost reduction. The requirements for possible converter candidates and control strategies are analysed. One representative circuit scheme is presented as an example to verify the feasibility of the design. An electromagnetic transient model is built for different power scale PV systems to verify the DMPPT feasibility of the evaluated architecture in a large-scale PV power system. Voltage boosting ability is widely needed for converters in DMPPT applications. Impedance source converters (ISCs) are the main converter types with step-up ability. However, these converters have a general problem of low order distortion when applied in dc-ac applications. To solve this problem, a generic plug-in repetitive control strategy for a four-switch three-phase ISC type inverter configuration is developed. Simulation and experimental results confirm that this control strategy is suitable for many ISC converters.The application of distributed maximum power point tracking (DMPPT) technology in solar photovoltaic (PV) systems is a hot topic in industry and academia. In the PV industry, grid integrated power systems are mainstream. The main objective for PV system design is to increase energy conversion efficiency and decrease the levelized cost of electricity of PV generators. This thesis firstly presents an extensive review of state-of-the-art PV technologies. With focus on grid integrated PV systems research, various aspects covered include PV materials, conventional full power processing DMPPT architectures, main MPPT techniques, and traditional partial power processing DMPPT architectures. The main restrictions to applying traditional DMPPT architectures in large power systems are discussed. A parallel connected partial power processing DMPPT architecture is proposed aiming to overcome existing restrictions. With flexible ‘plug-and-play’ functionality, the proposed architecture can be readily expanded to supply a downstream inverter stage or dc network. By adopting smaller module integrated converters, the proposed approach provides a possible efficiency improvement and cost reduction. The requirements for possible converter candidates and control strategies are analysed. One representative circuit scheme is presented as an example to verify the feasibility of the design. An electromagnetic transient model is built for different power scale PV systems to verify the DMPPT feasibility of the evaluated architecture in a large-scale PV power system. Voltage boosting ability is widely needed for converters in DMPPT applications. Impedance source converters (ISCs) are the main converter types with step-up ability. However, these converters have a general problem of low order distortion when applied in dc-ac applications. To solve this problem, a generic plug-in repetitive control strategy for a four-switch three-phase ISC type inverter configuration is developed. Simulation and experimental results confirm that this control strategy is suitable for many ISC converters

DC-DC CONVERTER FOR POWER COLLECTION IN WIND FARMS

Offshore wind farms have grown rapidly in number in recent years. Several large-scale offshore wind farms are planned to be built at further than 100 km from the United Kingdom coast. While high-voltage high-power installations have addressed the technical issues associated with reactive power flow in AC transmission, reactive power can be avoided by using High-Voltage Direct Current transmission (HVDC). Reactive power causes problems when transmission distances are long, therefore, HVDC transmission is now being considered for wind farm grid connection. However, as wind farms constitute weak systems Line Commutated Converter (LCC) based HVDC is not viable and newer Modular Multilevel Converter (MMC) based Voltage Source Converters (VSC) are needed for the AC-DC conversion. One of the key components in such systems is the DC-DC converter, which is required to act as the interface between the generation, transmission, and distribution voltage levels, and reduces the power conversion stages, avoiding transformers typically used in AC grid integration systems. In addition, there is no high-power Medium-Voltage MV DC-DC converter available for offshore wind farm energy systems at present. The specification requirements of high-power MV DC-DC converters can be set once the output characteristics of the wind turbine generators have been reviewed. An offshore wind farm with MVDC-grid collection does not exist today, but it is a promising alternative, although specification analysis of high power MV DC-DC converters is necessary. The work reported in this thesis aims to introduce two types of high power MV DC-DC converter topologies, for offshore wind farm energy systems, termed single-stage, and multi-stage converters. Ways of reducing losses by soft switching and reduction in the number of components are considered. Both topologies are based on the Marx principle where capacitors are charged in parallel and discharged in series to achieve the step-up voltage transformation. During doldrums, light and calm wind, and for maintenance work, it is necessary to supply the offshore wind farm with auxiliary power. This thesis proposes a novel Bidirectional Modular DC-DC converter (BMDC) and evaluates its performance. The simulation results show that the proposed BMDC allows up to 5% of the wind farm’s power rating to be drown from the onshore substation. This means that the proposed DC-DC converter is capable to provide bidirectional power flow. For offshore wind farm application, BMDC can be inserted between the offshore wind farm and onshore substation. The studies, in this thesis, are based on an input DC collection at 6 kV with the DC to DC converter stepping up the voltage to 30 kV. The proposed system is integrated and simulated with the DC offshore wind farm and a Voltage Source Converter (VSC) in the onshore station. The steady-state simulation results, to transmit the power between two different voltage levels, and the dynamic performance of the proposed converter were investigated. The advantages of the proposed converter include its simple design and that it does not require an AC transformer; hence can easily be implemented in an offshore wind farm since it requires less weight and size on the platform in the sea, which ultimately results in minimal cost. Furthermore, the proposed converter can ride through a fault which complies with the UK Grid code. However, in this case, it is necessary to provide protection systems such as a large chopper resistor for energy absorption or de-loading the wind turbine. Finally, the proposed integrated BMDC converter showed its suitability for offshore wind farms as well as improving their reliability