The increase in the consumption of energy year after year emphasizes the importance of power production by photovoltaic (PV) systems. Despite an increase in the use of PV systems, accurate solar power [kWh] daily harvest predicting data are not readily available. Accurate predicted solar power data is necessary because the data is helpful to designers who need to optimally size a PV panel before installation. Moreover, accurately predicted max dc power can indicate whether the PV panel is operating efficiently and economically or not. This thesis develops an approach to predict max solar power based on a Linear Regression model. The approach, which ia a simple regression was implemented using measured data on a response variable, a max solar power (Pmax), and predictor variables such as Global Horizontal (GH), Plane of Array (PA), Short Circuit Current (Isc), Open Circuit Voltage (Voc), and Panel Temperature (Temp). The statistical results of the linear regression model produced reasonable values which agreed with those of the measured data from the solar panel. / text

The U.S. Navy has developed in their ships, and is continually improving, electric propulsion, ship service power, and electric loads. The latest topology under design is the integrated power system (IPS). The IPS entails the all electric ship concept with electric propulsion, direct current (DC) distribution, and modular technology. In the all electric ship concept, ship propulsion and ship service loads are powered by alternating current (AC) generation. For the IPS, power electronics conversion is to be utilized to convert alternating current (AC) generation to direct current (DC) distribution. As state-of-the-art power electronics, the Navy plans to use power electronic building blocks (PEBB) technology in its IPS. A U.S. naval shipboard power system is required to be a highly reconfigurable system to enhance its survivability and reliability. Reconfiguration is a change in the shipboard power system state for various reasons such as new topology, changing missions and emergencies. It was decided to study the behavior of a PEBB-based integrated shipboard power system during reconfiguration. Since no real time operation data was available, the problem was studied through the simulation of reconfiguration scenarios on a scaled-down computer model of an IPS in MATLAB. Reconfiguration scenarios were determined and staged, and an AC/DC power system stability assessment methodology was applied by decoupling the IPS test system around an intrazonal bus. The coupled system of the test IPS, consisted of two dynamic 4160 VAC generators, two rectifiers, two DC-DC converters between the rectifiers' output looped bus and the downstream intrazonal 775V busses, inverters, buck converters, AC loads and DC loads. There was modeling of excitation perturbations which introduced errors in the assessment of the stability requiring an approximation analysis. The study found that the DC bus of interest was stable for all nine reconfiguration scenarios staged, but it found that other busses were not stable for two of the scenarios. The study further found that lower stability margins occurred at lower frequencies of about 1Hz for stable scenarios. It concluded that there were tangible benefits to advancing the shipboard power system architecture to the IPS topology because of the good stability results.

In recent years, there has been lots of emphasis put on the development of renewable energy. While considerable improvement on renewable energy has been made, there are some inherent limitations for these renewable energies. For example, for solar and wind power, there is an intermittent nature. For the fuel cell, the dynamics of electro-chemical reaction is quite slow compared to the electric load. This will not be acceptable for modern electric application, which requires constant voltage of constant frequency. This work proposed and evaluated a new power circuit that can deal with the problem of the intermittent nature and slow response of the renewable energy. The proposed circuit integrates different renewable energy sources as well as energy storage. By integrating renewable energy sources with statistical tendency to compensate each other, the effect of the intermittent nature can be greatly reduced. This integration will increase the reliability and utilization of the overall system. Moreover, the integration of energy storage solves the problem of the slow response of renewable energy. It can provide the extra energy required by load or absorb the excessive energy provided by the energy sources, greatly improving the dynamics of overall system. To better understand the proposed circuit, "Dual Active Bridge" and "Triple Active Bridge" were reviewed first. The operation principles and the modeling were presented. Analysis and design of the overall system were discussed. Controller design and stability issues were investigated. Furthermore, the function of the central controller was explained. In the end, different simulations were made and discussed. Results from the simulations showed that the proposed multi-port DC-DC power converter had satisfactory performance under different scenarios encountered in practical renewable energy application. The proposed circuit is an effective solution to the problem due to the intermittent nature and slow response of the renewable energy.

The U.S. Navy has developed in their ships, and is continually improving, electric propulsion, ship service power, and electric loads. The latest topology under design is the integrated power system (IPS). The IPS entails the all electric ship concept with electric propulsion, direct current (DC) distribution, and modular technology. In the all electric ship concept, ship propulsion and ship service loads are powered by alternating current (AC) generation. For the IPS, power electronics conversion is to be utilized to convert alternating current (AC) generation to direct current (DC) distribution. As state-of-the-art power electronics, the Navy plans to use power electronic building blocks (PEBB) technology in its IPS. A U.S. naval shipboard power system is required to be a highly reconfigurable system to enhance its survivability and reliability. Reconfiguration is a change in the shipboard power system state for various reasons such as new topology, changing missions and emergencies. It was decided to study the behavior of a PEBB-based integrated shipboard power system during reconfiguration. Since no real time operation data was available, the problem was studied through the simulation of reconfiguration scenarios on a scaled-down computer model of an IPS in MATLAB. Reconfiguration scenarios were determined and staged, and an AC/DC power system stability assessment methodology was applied by decoupling the IPS test system around an intrazonal bus. The coupled system of the test IPS, consisted of two dynamic 4160 VAC generators, two rectifiers, two DC-DC converters between the rectifiers' output looped bus and the downstream intrazonal 775V busses, inverters, buck converters, AC loads and DC loads. There was modeling of excitation perturbations which introduced errors in the assessment of the stability requiring an approximation analysis. The study found that the DC bus of interest was stable for all nine reconfiguration scenarios staged, but it found that other busses were not stable for two of the scenarios. The study further found that lower stability margins occurred at lower frequencies of about 1Hz for stable scenarios. It concluded that there were tangible benefits to advancing the shipboard power system architecture to the IPS topology because of the good stability results.

Transient stability studies are primarily concerned with the generator response of ac power systems and use only steady state type equations to model HVdc converter terminals. These equations are adequate for small disturbances at the converter terminals but cannot accurately represent a converters behaviour during, and through its recovery of, a significant transient disturbance. A detailed three phase electromagnetic analysis is necessary to describe the converters correct behaviour. This thesis describes an accurate and effective hybrid method combining these two types of studies, for analyzing dynamically fast devices such as HVdc converters within ac power systems. Firstly, conventional techniques are reviewed for both a transient stability analysis of power systems and for an electromagnetic transient analysis of HVdc converters. This review deals in particular with the two programs that constitute the hybrid developed in this thesis. Various techniques are then examined to efficiently and accurately pass the dynamic effects of an HVdc link to an ac system stability study, and the dynamic effects of an ac system to a detailed HVdc link study. An optimal solution is derived to maximise the inherent advantages of a hybrid. Finally, the hybrid is applied to a test system and its effectiveness in performing its task is shown.

A novel single phase rectification technique with a new architecture and control scheme is proposed. The new rectifier consists of switched capacitor branch in parallel with the diode bridge rectifier. The switched capacitor branch includes a capacitor and a bidirectional switch arranged in series so the switch can control the charging and discharging of the capacitor. The control strategy is carefully designed to ensure the output voltage of the rectifier is above a chosen threshold level and to maintain high input power factor with reduced line current harmonics. Circuit configuration, design parameters, principles of operation and the mathematical analysis are presented. The new architecture provides a reduction in the size of the DC side capacitor. This reduction can be as low as less than 10% of the size of the typical smoothing capacitor in the conventional single phase rectifier. The proposed concept is verified by the experimental results over a range of case studies. A novel buck-boost DC-DC converter architecture is also proposed. This converter utilises the close inversely-coupled inductors topology in both its conversion stages (buck and boost). The new converter aims to reduce the switching noise that usually accompanies the buck and boost circuits. This can be done by maintaining a continuous flow of current in both converter stages which results in a large reduction in the back e.m.f induced in the main inductor and thus reduces the switching noise. The new converter architecture also provides a unique design of the passive clamped circuit. This circuit is used to recycle the leakage energies of the coupled inductors which results in an efficiency improvement of the converter and to limit the voltage stress on the power switches. Circuit con figuration, principles of operation and the transfer function are presented. The proposed concept is verified by the experimental and the simulated results of a range of case studies. The highest achieved efficiency observed in the experiments was 97:7%.

In his thesis, CONDUCTED EMC MODELLING IN MODERN DC-DC POWER CONVERTERS, the conducted electromagnetic effects of high-density high-switching frequency modern power converters are studied. The noise source and noise path were modelled and the results calibrated to accredited noise levels. A VHF frequency model was developed and verified using different analysis packages and compared for accuracy, affordability and ease of use. Noise modes were separated and verified for accuracy. The mechanisms of the noise modes were studied and noise mitigation techniques presented. / Thesis (PhD)--University of Pretoria, 2017. / Electrical, Electronic and Computer Engineering / PhD / Unrestricted

This dissertation presents various systematic design techniques for photovoltaic (PV) source simulators to serve as a convenient tool for the dynamic performance evaluation of solar power conditioning systems and their maximum power point tracking algorithms. A well-designed PV source simulator should accurately emulate the static and the dynamic characteristic of actual PV generator. Four major design features should be adopted in any PV source simulator: (i) high power-stage efficiency, (ii) fast transient response-time, (iii) output impedance matching with actual PV generator, and (iv) precise reference generation technique. Throughout this research, two different PV source simulator systems are designed, modeled, and experimentally verified. The design of the first system focuses mainly on creating new reference generation techniques where the PV equivalent circuit is used to precisely generate the current-voltage reference curves. A novel technique is proposed and implemented with analog components to simplify the reference signal generator and to avoid computation time delays in digital controllers. A two-stage LC output filter is implemented with the switching power-stage to push the resonant frequency higher and thus allowing a higher control-loop bandwidth design while keeping the same switching ripple attenuation as in the conventional one-stage LC output filter. With typical control techniques, the output impedance of the proposed simulator did not  match the closed-loop output impedance of actual PV generator due to the double resonant peaks of the two-stage LC output filter. Design procedures for both control and power-stage circuits are explained. Experimental results verify the steady-state and transient performance of the proposed PV source simulator at around 2.7 kW output. The design concept of the first simulator system is enhanced with a new type of PV source simulator that incorporates the advantages of both analog and digital based simulators. This simulator is characterized with high power-stage efficiency and fast transient response-time. The proposed system includes a novel three-phase ac-dc dual boost rectifier cascaded with a three-phase dc-dc interleaved buck converter. The selected power-stage topology is highly reliable and efficient. Moreover, the multi-phase dc-dc converter helps improve system transient response-time though producing low output ripple, which makes it adequate for PV source simulators. The simulator circuitry emulates precisely the static and the dynamic characteristic of actual PV generator under different environmental conditions including different irradiance and temperature levels. Additionally, the system allows for the creation of the partial shading effect on PV characteristic. This dissertation investigates the dynamic performance of commercial and non-commercial solar power conditioning systems using the proposed simulator in steady-state and transient conditions. Closed-loop output impedance of the proposed simulator is verified at different operating conditions. The impedance profile --magnitude and phase- matches the output impedance of actual PV generator closely. Mathematical modeling and experimental validation of the proposed system is thoroughly presented based on a 2.0 kW hardware prototype. The proposed simulator efficiency including the active-front-end rectifier and the converter stages at the maximum power point is 96.4%. / Ph. D.

Offshore wind power is attracting increasing levels of research and investment. The use of HVDC transmission and the development of DC grids are topics with similar high levels of interest that go hand in hand with the development of large scale, far from shore wind farms. Despite increased capital cost of some components, DC power transmission can have significant advantages over AC transmission, in particular in the offshore environment. These advantages are well established for large scale, long distance point to point transmission. This thesis assesses the suitability of a multi-terminal DC power collection network, with short cables and relatively small amounts of power, addresses a number of the technical challenges in realising such a network and shows methods for overall system cost reduction. Technical and modelling challenges result from the interaction between power electronic DC-DC converters and the cables in a DC transmission network. In particular, the propagation of the ripple current in bipole DC transmission cables constructed with a metallic sheath and armour is examined in detail. The finite element method is used to predict the response of the cable to the ripple current produced by the converters. These results are used along with wave propagation theory to demonstrate that cable design plays a crucial role in the behaviour of the DC system. The frequency dependent cable models are then integrated with time domain DC-DC converter models. The work in the thesis is, broadly, in two parts. First, it is demonstrated that care and accuracy are required in modelling the cables in the DC transmission system and appropriate models are implemented and validated. Second, these models are combined with DC-DC converter models and used to demonstrate the practicality of the DC grid, make design recommendations and assess its suitability when compared with alternative approaches (e.g. AC collection and/or transmission).

Harvesting energy from ultra-low-power vibration energy sources typically employs a rectifier circuit as the first power conditioning stage. The Schottky diode has a 0.15 V - 0.2 V threshold voltage and can not extract energy efficiently at low voltage. Other technologies such as MOSFET bridge or active diode are designed to minimize the voltage drop to reduce the conduction loss. However, these designs require either additional power supplies to operate comparators or have a larger threshold turn-on voltage than Schottky. Therefore, most rectifiers have an unresponsive or significant low-efficiency zone when the input power is low. This dissertation will elaborate on a backward diode based self-powered micro-circuit diode that will operate in the extremely weak or low alternating source applications, where the existing approaches offer poor outcomes. This proposed micro-circuit diode was compared to a Schottky diode in several experiment setup. The micro-circuit based half-wave rectifier circuit harvested 3.1 mV DC at a 239.5 Ohm load when the input magnitude is 50 mV while the Schottky diode was unable to convert this ultra-low AC power. This dissertation also provides the analysis of two alternating sources, the oscillatory electromagnetic generator and the piezoelectric energy harvester, to conduct experiments in a more realistic context. The micro-circuit diode shows excellent advantages in electromagnetic generator experiment, the micro-circuit based half-wave rectifier circuit harvested 5.16 mV DC at a 0.5 kOhm load when the input magnitude is 40 mV. However, due to the large leakage current in negative resistance region, this micro-circuit is unable to show advantages in piezoelectric energy harvester applications.