This research addresses some of the difficulties faced when operating voltage controlled inverters with synchronous generators in microgrids. First, an overview of microgrid value propositions is provided, and the problems faced when attempting to use microgrids to provide improved power quality are discussed. Design considerations for different types of microgrids are provided to enable microgrids to deliver the desired functionality without adding unnecessary cost. The main body of this research investigates the poor transient load sharing encountered between voltage controlled inverters and synchronous generators in islanded operation. Poor transient load sharing results in high peak inverter rating requirements and high cost. The tradeoff between power quality and power sharing is highlighted, and methods to improve transient load sharing are proposed. The use of current limiting to protect inverters during faults and overloads is also investigated. Stability problems are identified when using simple inverter current limiting methods when operating in parallel with synchronous generators. Virtual impedance current limiting is proposed to improve transient stability during current limiting. The methods proposed in this thesis for mitigating inverter overloads and faults will allow for more reliable and cost effective application of inverter based distributed energy resources with synchronous generators in microgrids.

Several steps are followed in order to evaluate the cycle as the title suggests. The diffusion absorption refrigerator (DAR) cycle performance is evaluated when using helium or hydrogen as auxiliary gas. A slight increase in COP is found when using helium, but it is not sufficient to justify the cost. A secondary simulation of an alternate dual-pressure cycle using a pump is done as feasibility comparison with the same parameters as the diffusion cycle. It was found that the second cycle is not acceptable due to high evaporator temperatures needed to ensure liquid enters the pump instead of partially evaporated solution. This would greatly increase the work input required for what essentially becomes a compressor. Optimisation of the DAR is evaluated by simulating the use of a rectification column and the effects of different design points on overall performance. Meteorological data for Potchefstroom, South Africa is used to perform a yearly analysis on the simulated cycle and to specify a suitable design point. The use of a radiative cooling system as heat sink for the system is then investigated and incorporated into the system model. Finally, the performance characteristics of the simulated DAR cycle are discussed, verified and compared with available data from similar research. It is shown that a 40% solution aqua-ammonia-hydrogen cycle driven by 526 kW of solar thermal energy at 130°C and a system pressure of 1.5 MPa can easily achieve a COP over 0.4 with an air-cooled absorber at 40°C and a water-cooled condenser at 35°C. A 231 kW refrigeration capacity at an average evaporator temperature of –20°C is achieved, satisfying the requirements for a domestic refrigeration system. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

Several steps are followed in order to evaluate the cycle as the title suggests. The diffusion absorption refrigerator (DAR) cycle performance is evaluated when using helium or hydrogen as auxiliary gas. A slight increase in COP is found when using helium, but it is not sufficient to justify the cost. A secondary simulation of an alternate dual-pressure cycle using a pump is done as feasibility comparison with the same parameters as the diffusion cycle. It was found that the second cycle is not acceptable due to high evaporator temperatures needed to ensure liquid enters the pump instead of partially evaporated solution. This would greatly increase the work input required for what essentially becomes a compressor. Optimisation of the DAR is evaluated by simulating the use of a rectification column and the effects of different design points on overall performance. Meteorological data for Potchefstroom, South Africa is used to perform a yearly analysis on the simulated cycle and to specify a suitable design point. The use of a radiative cooling system as heat sink for the system is then investigated and incorporated into the system model. Finally, the performance characteristics of the simulated DAR cycle are discussed, verified and compared with available data from similar research. It is shown that a 40% solution aqua-ammonia-hydrogen cycle driven by 526 kW of solar thermal energy at 130°C and a system pressure of 1.5 MPa can easily achieve a COP over 0.4 with an air-cooled absorber at 40°C and a water-cooled condenser at 35°C. A 231 kW refrigeration capacity at an average evaporator temperature of –20°C is achieved, satisfying the requirements for a domestic refrigeration system. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

Radio Frequency IDentification (RFID) is a contactless, automatic identification wireless technology primarily used for identifying and tracking of objects, goods and humans. RFID is not only limited to identification and tracking applications. This proliferating wireless technology has been deployed in numerous securities sensitive applications e.g. access control, e-passports, contactless payments, driver license, transport ticking and health cards. RFID inherits all the security and privacy problems that are related to wireless technology and in addition to those that are specific to RFID systems. The security and privacy protection schemes proposed in literature for wireless devices are mostly secured through symmetric/asymmetric keys encryption/decryption and hash functions. The security of all these cryptographic algorithms depends on computationally complex problems that are hard to compute using available resources. However, these algorithms require cryptographic operations on RFID tags which contradict the low cost demand of RFID tags. Due to limited number of logic gates in tags, i.e., 5K-10K, these methods are not practical. Much research effort has done in attempt to solve consumer's privacy and security problem. Solutions that prevent clandestine inventory are mostly application layer techniques. To solve this problem, a new RFID physical layer scheme has been proposed namely Direct Sequence Backscatter Encryption (DSB Enc). The proposed scheme uses level generator to produce different levels before transmitting the signal to the tag. The tag response to the signal sent by the reader using backscatter communications on the same signal which looks random to the eavesdropper. Therefore eavesdropper cannot extract the information from reader to tag and tag to reader communication using passive eavesdropping. As reader knows the different generated levels added to the carrier signal, it can remove the levels and retrieve the tag's messages. We proposed a lightweight, low-cost and practically secure physical layer security to the RFID system, for a supply chain processing application, without increasing the computational power and tag's cost. The proposed scheme was validated by simulations on GNU Radio and experimentation using SDR and a WISP tag. Our implementation and experimental results validate that DSB Enc is secure against passive eavesdropping, replay and relay attacks. It provides better results in the presence of AWGN channel.

A wind turbine needs to be controlled to ensure its safe and optimal operation, especially during high wind speeds. The most common control objectives are to limit the power and rotational speed of the wind turbine by using pitch control. Aero Energy is a company based in Potchefstroom, South Africa, that has been developing and manufacturing wind turbine blades since 2000. Their most popular product is the AE1kW blades. The blades have a tendency to over-speed in high wind speeds and the cut-in wind speed must be improved. The objective of this study was to develop an active pitch control system for wind turbines. A prototype active pitch control system had to be developed for the AE1kW blades. The objectives of the control system are to protect the wind turbine from over-speeding and to improve start-up performance. An accurate model was firstly developed to predict a wind turbine’s performance with active pitch control. The active pitch control was implemented by means of a two-stage centrifugal governor. The governor uses negative or stalling pitch control. The first linear stage uses a soft spring to provide improved start-up performance. The second non-linear stage uses a hard spring to provide over-speed protection. The governor was manufactured and then tested with the AE1kW blades. The governor achieved both the control objectives of over-speed protection and improved start-up performance. The models were validated by the results. It was established that the two-stage centrifugal governor concept can be implemented on any wind turbine, provided the blades and tower are strong enough to handle the thrust forces associated with negative pitch control. It was recommended that an active pitch control system be developed that uses positive pitching for the over-speed protection, which will eliminate the large thrust forces. Keywords: pitch control, wind turbine, centrifugal governor, over-speed protection, cut-in wind speed, blade element-momentum theory, rotor, generator, stall, feathering. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2009.

A wind turbine needs to be controlled to ensure its safe and optimal operation, especially during high wind speeds. The most common control objectives are to limit the power and rotational speed of the wind turbine by using pitch control. Aero Energy is a company based in Potchefstroom, South Africa, that has been developing and manufacturing wind turbine blades since 2000. Their most popular product is the AE1kW blades. The blades have a tendency to over-speed in high wind speeds and the cut-in wind speed must be improved. The objective of this study was to develop an active pitch control system for wind turbines. A prototype active pitch control system had to be developed for the AE1kW blades. The objectives of the control system are to protect the wind turbine from over-speeding and to improve start-up performance. An accurate model was firstly developed to predict a wind turbine’s performance with active pitch control. The active pitch control was implemented by means of a two-stage centrifugal governor. The governor uses negative or stalling pitch control. The first linear stage uses a soft spring to provide improved start-up performance. The second non-linear stage uses a hard spring to provide over-speed protection. The governor was manufactured and then tested with the AE1kW blades. The governor achieved both the control objectives of over-speed protection and improved start-up performance. The models were validated by the results. It was established that the two-stage centrifugal governor concept can be implemented on any wind turbine, provided the blades and tower are strong enough to handle the thrust forces associated with negative pitch control. It was recommended that an active pitch control system be developed that uses positive pitching for the over-speed protection, which will eliminate the large thrust forces. Keywords: pitch control, wind turbine, centrifugal governor, over-speed protection, cut-in wind speed, blade element-momentum theory, rotor, generator, stall, feathering. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2009.

The Rigi research group has built an SVG (Scalable Vector Graphics) Graph Visualization Engine to visualize, interactively explore and annotate software structures. The biggest difficulty we experienced during the customization of this engine for different domains is that we need to create idiosyncratic generators every time. Each generator is created by different developers using different approaches. If the subject information model does not contain layout information, developers of the generator are also responsible for writing algorithms to calculate layout. In this thesis, we present a customization framework for our SVG Graph Visualization Engine to provide flexible customization using third party libraries to construct specific SVG Graph Generators. The customization framework consists of documentation for the existing graph engine and a componentized abstract generator that can be extended for a variety of information domains. We also created two reference implementations for the abstract generator and included them in the template solution project to exemplify the usage of the customization framework. We also validated the template solution with an end user to build a generator for a new domain. Our customization framework greatly eases the process of building SVG Editor Generators for domain-specific visualization engine.

Random variate genration is an important tool in statistical computing. Many programms for simulation or statistical computing (e.g. R) provide a collection of random variate generators for many standard distributions. However, as statistical modeling has become more sophisticated there is demand for larger classes of distributions. Adding generators for newly required distribution seems not to be the solution to this problem. Instead so called automatic (or black-box) methods have been developed in the last decade for sampling from fairly large classes of distributions with a single piece of code. For such algorithms a data about the distributions must be given; typically the density function (or probability mass function), and (maybe) the (approximate) location of the mode. In this contribution we show how such algorithms work and suggest an interface for R as an example of a statistical library. (author's abstract) / Series: Preprint Series / Department of Applied Statistics and Data Processing

This research work focuses on the reactive power compensation of the permanent magnet synchronous generator (PSG) in wind power plants. PSG feeds a fixed voltage dc grid through a rectifier bridge. In variable speed operation, the PSG will be able to build torque only in small speed range. This is due to the fixed magnet of the PSG. External reactive power compensation provides an attractive solution to overcome this problem. Different reactive power compensation configurations were examined. Statics synchronous series compensation and a shunt passive filter were chosen as a compensation method. Simulation and implementation of small wind power plant were performed. The wind power plant consists of the synchronous generator, inverter, rectifier, coupling transformers and shunt passive filter. The experimental results agree to the proposed theory and simulation results. / Der Schwerpunkt meiner Promotion ist die Blindleistungskompensation bei einem permanenterregten Synchrongenerator. Der Synchrongenerator speist das Gleichsspannungsnetz über ein Gleichrichter. In der Drehzahlvariablen Betriebsverhalten können Nachteile auftreten. Die Folge ist, dass bei konstanter Gleichspannung und fester Erregung durch die Permanenterregung nur ein sehr kleiner Drehzahlbereich mit vernünftiger Drehmomentausbeute bedienbar ist. Ein möglicher Ausweg wäre eine variable Kompensationsspannung. Verschiedene Kompensationsverfahren wurden untersucht. Ein Series Active Filter und ein Shunt Passive-Filter wurden als Blindleistungskompensation gewählt. Im Rahmen meiner Dissertation beschäftige ich mich mit dem Aufbau und der Simulation einer Windkraftanlage. Diese besteht aus einem permanenterregten Synchrongenerator, einem Wechselrichrter, einem Gleichrichter, drei Transformatoren und einem passiven Filter. Das Versuchsergebnis zeigt, dass die Theorie mit der Simulation übereinstimmt.

Surface gravity waves in the world’s oceans contain a renewable source of free power on the order of terawatts that has to this date not been commercially utilized. The division of Electricity at Uppsala University is developing a technology to harvest this energy. The technology is a point absorber type wave energy converter based on a direct-driven linear generator placed on the sea bed connected via a line to a buoy on the surface. The work in this thesis is focused mainly on the energy transport of ocean waves and on increasing the transfer of energy from the waves to the generator and load. Potential linear wave theory is used to describe the ocean waves and to derive the hydrodynamic forces that are exerted on the buoy. Expressions for the energy transport in polychromatic waves travelling over waters of finite depth are derived and extracted from measured time series of wave elevation collected at the Lysekil test site. The results are compared to existing solutions that uses the simpler deep water approximation. A Two-Body system wave energy converter model tuned to resonance in Swedish west coast sea states is developed based on the Lysekil project concept. The first indicative results are derived by using a linear resistive load. The concept is further extended by a coupled hydrodynamic and electromagnetic model with two more realistic non-linear load conditions. Results show that the use of the deep water approximation gives a too low energy transport in the time averaged as well as in the total instantaneous energy transport. Around the resonance frequency, a Two-Body System gives a power capture ratio of up to 80 percent. For more energetic sea states the power capture ratio decreases rapidly, indicating a smoother power output. The currents in the generator when using the Two-Body system is shown to be more evenly distributed compared to the conventional system, indicating a better utilization of the electrical equipment. Although the resonant nature of the system makes it sensitive to the shape of the wave spectrum, results indicate a threefold increase in annual power production compared to the conventional system.