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Table of Contents
Quantitative Feedback Theory Design of Line Current Commutated HVDC Control Systems
An Exploratory Case Study on the Requirements Business Processes of a Typical South African High Technology Systems Engineering Company
Cross layer Extended Parameter Call Admission Control for Future Networks
State Space Model Extraction of a Natural Circulation U-Tube: A Network Approach
Quantitative Feedback Theory Design of Line Current Commutated HVDC Control Systems by L. Chetty and N.M. Ijumba
Abstract: Line Current Commutated (LCC) HVDC systems consists of uncertain plants. These uncertainties are result of changes/disturbances in the ac networks or in the LCC HVDC system itself. Further uncertainties can be introduced due to simplified system modelling techniques. Quantitative Feedback Theory is a frequency-domain technique that utilises the Nichols chart to achieve a robust design over a specified region of uncertainty. The Quantitative Feedback Theory design philosophy was applied to design the LCC HVDC control system parameters. The stable start-up and step responses of the LCC HVDC system, for varying ac system conditions, conclusively validate the Quantitative Feedback Theory design method of the LCC HVDC control system parameters.
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An Exploratory Case Study on the Requirements Business Processes of a Typical South African High Technology Systems Engineering Company by S. Schulze and L. Pretorius
Abstract: This article describes the findings of an exploratory case study into the requirements management and engineering processes of a South African Systems Engineering Company and how these processes affect the business of the company. For the purpose of this article the name of the company is kept confidential. This case study identifies processes that resulted in both successful and unsuccessful project performance, reasons for the problems, successful processes and ideas for practice improvement. The findings illustrate which processes are used, why requirement processes are important, and provide useful insights on causes of project failure and situations to be avoided. The findings also show where the main focus for company improvement should lie. This case study culminates in recommendations on ways to improve the current requirements processes of the company and proposes a theory on the use of requirements management and requirements engineering processes in the company.
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Cross layer Extended Parameter Call Admission Control for Future Networks by T. Walingo and F. Takawira
Abstract: The Next Generation Network (NGN) is to deliver anything, anytime anywhere with full quality of service (QoS) guarantees. The network designers need to overcome the NGN’s challenges namely; heterogeneous wireless access environments, multiple traffic types, flexible bandwidth allocation and cross layer design issues among others. To guarantee quality of service for these NGN’s, a call admission control scheme addressing the main challenges of NGN’s is presented. This is a cross layer call admission control (CAC) scheme featuring multiple traffic types. The featured model effectively combines call level and packet level call admission control issues. It is based on Code Division Multiple Access (CDMA) air interface which together with Orthogonal Frequency Division Multiple (OFDM) access are the most popular air interface technologies. Traditionally, signal to interference ratio (SIR) has solely been used as the call admission control parameter for CDMA networks. However, the results indicate that due to the nature of the multimedia traffic more parameters need to be incorporated in the call admission control scheme. The presented CAC scheme uses extended user specified QoS parameters of signal to interference ratio (SIR) and delay to accept or reject a call and guarantee a certain call blocking probability QoS metric. The results from the developed model clearly indicate that a CAC algorithm incorporating more parameters outperforms one with less admission control parameters.
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State Space Model Extraction of a Natural Circulation U-Tube: A Network Approachs by K.R. Uren and G. van Schoor
Abstract: The Reactor Cavity Cooling System (RCCS) in a High Temperature Gas-cooled Reactor (HTGR) provides protection to the concrete structures surrounding the reactor pressure vessel. The RCCS comprises stand pipes circulating water. These pipes may fundamentally be considered as U-tubes. Since the RCCS is critical in case of a Loss of Flow Accident (LOFA), it is very important to characterise the dynamics of such a system both for systems and control engineering purposes. Detailed Computational Fluid Dynamic (CFD) models do exist, but these models are too complex for the purposes mentioned. The majority of thermohydraulic simulation codes utilise a network approach towards representing thermohydraulic systems. This concept is used to extract state space models from graphic representations of such systems. The purpose of this paper is to illustrate the application of a State Space Model Extraction (SSME) method applied to a fundamental thermohydraulic system, namely a U-tube. The solution of the extracted state space model is compared with a validated systems CFD code called Flownex® and shows good correlation.
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