This Master of Applied Science thesis presents a single phase grid connected DC/AC inverter with reactive power (VAR) control for residential photovoltaic (PV) applications. The inverter, utilizing the voltage sourced inverter (VSI) configuration, allows the local residential PV generation to actively supply reactive power to the utility grid. A low complexity grid synchronization method was introduced to generate the parallel and orthogonal components of the grid voltage in a highly computationally efficient manner in order to create a synchronized current reference to the current control loop. In addition, the inverter is able to use a small long life film type capacitor on the DC-link by utilizing a notch filter on the voltage control loop. Simulations were performed on PSCAD/EMTDC platform and a prototype was also developed in the lab to prove the effectiveness of the controllers and the grid synchronization method.

[Abstract]: The disadvantages of using solar energy are its capital cost (which is about A$6/W), in comparison to that of conventional sources of energy (which is about A$1.80/W), and its conversion efficiency, which in commercially available Photovoltaic (PV) systems is less than 20%. Consequently, for utility connected PV generation to become a viable alternative energy source, its efficiency needs to be improved, its cost reduced, and the quality of power supplied by the inverters must meet stringent standards.This dissertation describes the research work carried out to optimise the conversion efficiency and to minimise the cost of a single-phase, hysteretic current controlunipolar switched inverter system, for use as an interface between solar panels and the grid network. The 1 kW (peak power) PV system being considered does not useenergy storage batteries and the inverter output is connected to the grid supply without the use of a power transformer. Improvements in the efficiency of such aninverter system often come at the expense of the quality of its output power and an increase in cost. However, in the proposed inverter system the harmonics of theoutput current has been improved without compromising its overall efficiency or its cost. An improvement in power quality has been achieved using a novel AC splitinductorfilter network that reduces electromagnetic interference, prevents unwanted operation of the inverter switches, attenuates switching frequency harmonics,minimises low frequency harmonics and provides an average value of the inverter output current necessary for the removal of DC offset currents.An improvement in inverter efficiency and a reduction in cost has been achieved by omitting the 50 Hz power transformer (transformerless) and by optimising theinverter current control strategies. In Australia, some power supply authorities permit transformerless PV inverters of less than 10 kW rating to be connected to their supply system. However, avoiding the use of transformers can lead to magnitudes of DC offset current outside the limits specified by Australian Standard 4777.2, 2005 being injected into the grid supply. In this project a new cost effective DC offset current controller that removes DC offset current from the output of the inverter has been realised. This result translates into two primary benefits; firstly, a saving of about20% in the cost of the power transformer and in the cost of providing additional solar panels to overcome transformer power losses, and secondly the DC offset controller can also be utilised in inverter applications where power transformers are used, to prevent distortion of the magnetising current.The novel design procedure proposed in this thesis for a current controller takes into account intentional and unintentional switching circuit delays, and yields higherefficiencies without sacrificing power quality or increasing the cost of the inverter system. The inclusion of the effect of circuit delays in the design procedure issignificant as it is shown that delay not only has an adverse effect on the performance of the current controller but also on the efficiency and the power quality of the inverter system.Of paramount importance for the successful completion of this project was the relationship between switching circuit delays and the level of low frequencyharmonics generated by unipolar switched inverters. Theoretical analysis is developed to show why circuit delays, inverter DC input voltage and the inductanceof the current loop, are responsible for low frequency harmonics in unipolar switched and not in bipolar switched inverters. It has also been established that unipolarswitched inverters can be designed to operate within the limits specified by the Australian Standard 4777.2, 2005 and that the low frequency harmonics can be maintained at acceptable levels.For a current controller using unipolar switching, the choice of only one of four equivalent switching combinations of the inverter switches leads to suppression of switching noise, and prevents unwanted switching without the need for additional filters. Results are presented to demonstrate the unique advantage of unipolarswitching over bipolar switching.

Thesis (M.S.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 57-58). Also available on the World Wide Web.

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%.

It is common practice to design water chiller units and heat exchangers in such a way that they do not operate within the transition region. This is mainly due to the perceived chaotic behaviour as well as the paucity of information in this region. Due to design constraints or change of operating conditions, however, exchangers are often forced to operate in this region. This is even worse for enhanced tubes as much less information within this region is available. It is also well known that the entrance has an influence on where transition occurs, adding to the woes of available information. The purpose of this study is thus to obtain heat transfer and friction factor data in the transition region of fully developed and developing flows inside smooth and enhanced tubes, using water as the working fluid, and to develop correlations from these results. The use of different inlets, tube diameters and enhanced tubes was also investigated with regards to the commencement of transition. Heat transfer and pressure drop data were obtained from six different types of tubes with diameters of 15.88 mm (5/8′′) and 19.02 mm (3/4′′). Low fin enhanced tubes with a fin height to diameter ratio of 0.4 and helix angles of 18◦ and 27◦ were investigated. Heat transfer was obtained by means of an in-tube heat exchanger with the cooling of water being used as the test fluid. Reynolds numbers ranged between 1 000 and 20 000 while Prandtl numbers were in the order of 4 to 6. Uncertainties in heat transfer coefficient and friction factors were on average below 2.5% and 10% respectively. Adiabatic friction factor results showed that the use of different inlets influenced the commencement of transition. The smoother the inlet profile the more transition was delayed, confirming previous work done. The effect of increasing tube diameters had a slight delay in transition. Enhanced tubes caused transition to occur at lower Reynolds numbers which was accounted for by the fin height and not the helix angle. Heat transfer results showed that transition occurred at approximately the same Reynolds number for all the different inlets and enhanced tubes. This was attributed to the secondary flow forces influencing the growing hydrodynamic boundary layer. These secondary flow forces also influenced the laminar heat transfer and diabatic friction factors with both these parameters being higher. Turbulent enhanced tube heat transfer results were higher than those of the smooth tube, with the tube with the greatest helix angle showing the greatest increase. Correlations were developed for all the tubes and their inlets and predicted all the data on average to within 3%. / Thesis (PhD)--University of Pretoria, 2010. / Mechanical and Aeronautical Engineering / unrestricted

This thesis describes the design of semiconductor source for laboratory measurements of switchgears. The proposed source is able to use 16-250Hz frequency, fulfilling the standard value of voltage and current distortion. There are explained the theoretical knowledge of various types of inverters without transformer in the thesis. Furthermore, a circumferential proposal of source is made, which includes calculating the desired properties of individual components, their choice from offers from various manufacturers, and subsequently is constructed the total design solution. It is described the control and management of designed source and it is evaluated for the function finally.

No description available.

This thesis proposes a complete modeling and control design methodology for a multifunctional single-phase bidirectional PWM converter in renewable energy systems. There is a generic current loop for different modes of operation to ease the transition between different modes, including stand-alone inverter mode, grid-tied inverter mode, grid-tied rectifier mode and grid-tied charger/discharger mode. Under stand-alone mode operation, ac voltage regulation is of importance because of the sensitive loads. In this thesis, different multi-loop-based control schemes are investigated and compared, especially between the load current feedback control, PR control and capacitor current loop control. It shows that PR controller reduces the steady-state error, while load current feedback controller improves the transient response. However, the load current feedback controller and capacitor current loop controller presents unstable outputs under some filter load condition. Single-phase d-q frame control is also studied. In order to ease the implementation effort, an unbalanced d-q frame control is proposed to achieve zero steady-state error voltage regulation without generating β-axis component. Based on the same principle, a d-q frame-based single-phase PLL is also proposed to achieve the fast dynamic response with the zero steady-state error phase tracking. The entire control system is verified on a modified 7 kW single-phase PWM converter prototype with a simple DSP-based digital implementation. The load step response test is presented under different modes of operation. The controllers for stand-alone mode are also done under no load, 1 kW resistive load, 1kVar capacitive load, and non-linear load conditions verifying that the single-phase d-q achieves 70% steady-state error improvement if taking the normal PID controller as the baseline design. In the end, the proposed PLL is compared with the standard PLL by experiments showing that the steady-state error can be reduced by 80%. / Master of Science

Today, small distributed power generation (DG) systems are becoming more common as the need for electric power increases. Small DG systems are usually built close to the end-user and they take advantage of using different energy sources such as wind and solar. A few examples are hybrid cars, solar houses, data centers, or hospitals in remote areas where providing clean, efficient and reliable electric power is critical to the loads. In such systems, the power is distributed from the source side to the load side via power electronic converters in the system. At low and medium power applications, the task is often left to single phase inverters where they are the only interface between sources connected to DC bus and loads connected to an AC bus. Much has been done for the control of single phase inverters in the past years; however, due to the requirements of stand alone systems and the time-varying nature of the converter, its controller design is still quite difficult, and especially so if its critical functionality within the system is taken into consideration. Part of the challenge is also due to the fact that the load is not known at all time, further complicating the controller design. This thesis proposes a different method of control for single phase inverters used in low and medium power DG systems. The new control method takes advantage of the well-known DQ transformation and analysis mostly employed for three phase converters' analysis and control design. Providing a time-invariant model of single phase inverters is the main feature of DQ transformation. In addition to that, control design of the inverter in DQ frame becomes similar to those of DC-DC and three phase converters making it easier to achieve superior performance under different operation conditions while achieving a robust controller. The transformation requires at least two independent phases for each state variable in the system; thus a second phase must be created. This thesis proposes the creation of an imaginary circuit based on the real circuit of the inverter to provide the second required phase for transformation. The state variables of the imaginary circuit are obtained by differentiating the state variables of the main inviter's circuit. The differentiation can be implemented in DSP so there is no need for additional hardware in the system, making it more attractive and cost effective method. The DQ controller not only provides superior transient response, it also provides zero steady-state error as well as low output voltage THD under nonlinear load operation. The entire controller can be implemented in a digital control board which is becoming more common in power electronics converters within the past decade. Analysis and design of a DQ controller for a 2.5kW single phase full-bridge inverter is presented in this study with the final results implemented in a FPGA/DSP based digital controller board. / Master of Science

This thesis focuses on the single-phase voltage-source inverter for use in photovoltaic (PV) electricity generating systems in both stand-alone and grid-tied applications. In many cases, developments in single-phase PV systems have followed developments in three-phase systems. Time-variant systems are more difficult to control than time-invariant systems. Nevertheless, by using suitable transformation techniques, time-variant systems can often be modelled as time-invariant systems. After the transformation, the control signals that are usually time-variant (often varying sinusoidally in time) become time-invariant at the fundamental frequency, and are hence much easier to deal with. With this approach, synchronous rotating frame control techniques have been previously proposed for high performance three-phase inverter applications. The transformation theory cannot be applied directly in single-phase systems without modification, and the d-q components would not be time-invariant in situations where harmonics, resonances or unbalance is present. Single-phase inverter controller designs based on the use of a synchronous rotating reference frame have been proposed, but such designs do not always perform as well as expected. This thesis aims to improve single-phase voltage-source inverters. The main objective is to address, in terms of cost, efficiency, power management and power quality, the problems found with single-phase designs based on a synchronous rotating frame single-phase inverter controller. Consequently, this thesis focuses on a novel controller approach in order to obtain a more reliable and flexible single-phase inverter. As the first step, this thesis investigates the single-phase inverter switching gate-drive algorithms and develops a form of space-vector pulse-width-modulation (SVPWM) in order to reduce total harmonic distortion. The results of the new SVPWM algorithm demonstrate its superior performance when compared with sinusoidal pulse-width-modulation (SPWM) which is often used with single-phase inverters. The second step, which is further reviewed and presented in this thesis, is the modelling of the single-phase inverter control based on the synchronous rotating frame. A mathematical analysis is conducted to determine the mechanism of the coupling that exists between the voltage phase and amplitude terms, and a new transformation strategy is proposed based on using the voltage phase as a reference at the Park transformation stages, and the current phase as a reference for the current at the transformation stages. The line-frequency components of the feedback signals are transformed to time-invariant components, thus eliminating the ripple and reducing the computational burden associated with the controller stage. Consequently, the inverter feedback controller stage is designed so that the coupling terms are decoupled within the controller itself. The effectiveness of the techniques proposed in this thesis are demonstrated by simulation using the MATLAB/SIMULINK environment. The proposed technique was also investigated through a practical implementation of the control system using a Digital Signal Processor (DSP) and a single-phase inverter. This practical system was tested up to 1 kW only (limited by the available inverter hardware). Nevertheless, the correlation between the simulation and the practical results is high and this gives confidence that the developed mechanism will allow the 2.5kW goal to be achieved. Practical test cases illustrate the effectiveness of the models. In addition, the comparisons between experimental and simulation results permit the system's behaviour and performance to be accurately evaluated. With the development of the new controller, small-scale single-phase renewable energy systems will become more useful in the field of power quality management through their ability to separately control the phase and amplitude of the output voltage. Consequently, incorporation of this type of generator within the national electrical distribution network, as distributed generators (DG) at low-voltage level, can assist with power quality management at the consumer side of the grid. In addition, such a generator can also operate in stand-alone mode if the grid becomes unavailable. The third step in this thesis investigates small-scale single-phase renewable energy systems operating as decentralized distributed generators within a local network. This operation is achieved by controlling the inverter side using the quantities measured at the common coupling point between the grid and the inverter, without requiring other extensive communications. Thus, the small-scale single-phase renewable energy distributed generator systems will contain only a local controller at each installation.