AFRICON 2004 POWER SPECIAL ISSUE

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1. Switched reluctance motor measurements and simulation models
2. Design and performance evaluation of a medium power PM-assisted reluctance synchronous traction machine using bonded PM-sheets
3. Stability analysis of multicell converters using Gerschgorin circles
4. The impact of closed-loop power flow control strategies on power system stability characteristics in a single-generator system
5. Torque performance of optimally designed three- and five-phase reluctance synchronous machines with two rotor structures
6. Linear model of a closed three-shaft Brayton cycle
7. Model for the apportionment of the total voltage drop in combined medium and low voltage distribution feeders
8. Voltage management by the apportionment of total voltage drop in the planning and operation of combined medium and low voltage distribution systems
9. Rainfall rate modeling for LOS radio systems in South Africa 
10. A public key management scheme and threshold-multisignature scheme for mobile ad hoc networks
11. Design and implementation of a fractional-N frequency synthesizer for cellular systems

Switched reluctance motor measurements and simulation models by S Wang, B Burton and RG Harley
Abstract: This Paper describes an experimental process used to produce a model in Matlab and Simulink for a prototype 8/6 Switched Reluctance Motor, and some methods used to validate the model. The inductance, flux and torque characteristics of switched reluctance motors are nonlinear, and must usually be numerically determined from real experimental data. Torque can be measured directly or calculated from measured phase current and rotor position, and numerically determined inductance. This paper explains how two different lookup tables are used to simulate the electrical, magnetic and mechanical characteristics of the motor. Simulation results are shown to closely agree with practical current and rotor position responses under locked and free rotor conditions. It is also explained how the validated simulation models will be used in the future design of a power converter and nonlinear torque (current) and speed control loops.
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Design and performance evaluation of a medium power PM-assisted reluctance synchronous traction machine using bonded PM-sheets by SE Sibande, MJ Kamper and R Wangc
Abstract: This paper describes the optimum design of a permanent-magnet-assisted reluctance rotor of a 110 kW reluctance synchronous traction machine. Previous studies show that the performance of the pure reluctance synchronous machine drive deteriorates fast in the flux-weakening speed region. To address this problem, thin bonded permanent-magnet sheet material is used inside the flux barriers of the reluctance rotor to improve the performance of the drive, especially in the flux-weakening speed region. A design optimization algorithm is
implemented to minimize the volume and hence the cost of the permanent-magnet material, subject to voltage and torque constraints. The calculated and measured results show clearly that the performance of the reluctance synchronous traction machine with a minimum amount of permanent-magnet material in the rotor compares favorably with the performance of the conventional induction machine drive at both rated and maximum speeds.
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Stability analysis of multicell converters using Gerschgorin circles by RH Wilkinson, TA Meynard and H du T Mouton
Abstract: The multicell converter topology possesses a natural voltage balancing property. This paper uses models obtained previously to perform a stability analysis of multicell converters. It uses a generic model in steady-state- and time constant analyses to determine the stability of multicell converters. In this paper stability refers to balanced cell capacitor voltages, whereas instability refers to unbalanced cell capacitor voltages. The stability analysis is performed for both sinusoidal modulation as well as fixed duty-cycle modulation. The model used in the stability analyses in this paper is valid for any modulation method. The conditions that lead to stability of the cell capacitor voltages as well as that leading to instability are presented. Theoretical results are included to verify the presented stability analyses and properties.
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The impact of closed-loop power flow control strategies on power system stability characteristics in a single-generator system by A Ally and B S Rigby
Abstract: This paper presents a theoretical study into the influence of closed-loop control of ac power flow on the small-signal and transient stability characteristics of a single-generator study system. Both the constant power and constant angle modes of power flow control are examined for a range of controller response times. The results indicate that the effect of a power flow controller on system stability is dependent on both the mode of the controller and its response time.
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Torque performance of optimally designed three- and five-phase reluctance synchronous machines with two rotor structures by ET Rakgati and MJ Kamper
Abstract: In this paper the torque performance of optimally designed three- and five-phase reluctance synchronous machines with different normal laminated rotor structures are studied. Both the round rotor with internal flux barriers and salient-pole rotor with no internal flux barriers but only cut-outs are investigated. The effect on the torque performance by adding third harmonic current component to the phase currents in a five-phase reluctance synchronous machine is also studied. The magnetostatic finite-element field solution with skew taken into account is used directly by an optimisation algorithm to optimise in multi-dimensions the design of the machines under same copper losses and volume. It is found that the torque increase due to third harmonic current injection is only 4% in the case of the five-phase machine with salient-pole rotor; the three-phase machine with round, internal-flux-barrier rotor is shown to outperform this machine in terms of torque by 28%. The measured torque results of the three-phase machine with round, internal-flux-barrier rotor are presented and compared with calculated results.
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Linear model of a closed three-shaft Brayton cycle by CR van Niekerk, JF Pritchard, G van Schoor and MA van Wyk
Abstract: A linear model of a closed three-shaft Brayton cycle at normal power operation is developed. The model is intended to give an understanding of the dominant dynamic behaviour in a nuclear power system that utilizes a closed three-shaft Brayton cycle. The insights gained from the model can be used for system design as well as for the design of control algorithms. A conceptual linear model is constructed and it is explained how this model can be developed starting with the models of the turbines and compressors followed by models of the shafts and the volumes inside the circuit to finally obtain the linear state space equations. A time domain comparison is made between the linear system model and the responses predicted by the thermal-fluid analysis software program, Flownet.
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REGULAR PAPERS

Model for the apportionment of the total voltage drop in combined medium and low voltage distribution feeders by CG Carter-Brown and CT Gaunt
Abstract: The maximum allowable voltage variation is a major and often primary constraint in the planning and design of distribution networks. In South Africa the apportionment of the allowable voltage drop between the MV and LV networks is not standardised and often not managed. The best voltage apportionment depends on network and load characteristics, including distribution transformer tap settings. A model is developed to calculate MV and LV voltage and voltage drop limits based on selected network and load characteristics. Comparing the results of the model with load flow simulations show the model to be suitably accurate for application to feeder calculations.
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Voltage management by the apportionment of total voltage drop in the planning and operation of combined medium and low voltage distribution systems by CG Carter-Brown and CT Gaunt
Abstract: The maximum allowable voltage variation on the MV and LV networks is a major and often primary constraint in the planning and design of the distribution feeders, which cannot be considered in isolation from each other. Assumed or assigned volt drop limits affect the capital and lifetime costs of the networks. This paper presents a new approach, based on network classes and transformer tap zones, to:
• identify suitable maximum voltage drop limits for planning, designing and operating MV and LV networks, taking into account the characteristics of urban and rural networks,
• improve voltage regulation on existing networks, and
• reduce the costs of extending and strengthening MV and LV networks.
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Rainfall rate modeling for LOS radio systems in South Africa by MO Fashuyi, PA Owolawi and TJ Afullo
Abstract: The cumulative distributions of rain intensity for 12 locations in South Africa are presented in this paper based on 5-year rainfall data. The rainfall rate having an integration time of 60 minutes is converted to the ITU-R recommended integration time of 1 minute for Durban in South Africa. Consequently, values of coefficients a and b so determined are used to convert 60-minute into 1- minute rain rates for other regions of South Africa. The resulting cumulative rain intensities are compared with the relevant ITU-R Recommendation P837. Based on this, an additional three rain zones are determined alongside the five ITU-R rain zones for South Africa.
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A public key management scheme and threshold-multisignature scheme for mobile ad hoc networks by J van der Merwe, DS Dawoud and S McDonald
Abstract: Mobile ad hoc networks offer communication over a shared wireless channel without any pre-existing infrastructure. Threshold digital signatures are an important cryptographic tool used in most existing key management schemes for mobile ad hoc networks. This paper proposes a thresholdmultisignature scheme designed specifically for mobile ad hoc networks. The signature scheme allows a subset of shareholders with threshold t, to sign an arbitrary message on behalf of the group. The group signature is publicly verifiable and allows any outsider to establish the identity of the individual signers. The paper proposes a self-certified public key issuing protocol that allows negotiation between a single entity and a distributed certificate authority for an implicit self-certified public key. The main objective of the paper is to find a key management scheme that is suitable for mobile ad hoc networks. As a solution, the paper proposes a public key management scheme, called Ad Hoc Public Key Management (AdHocPKM). AdHocPKM integrates the advantages of distributed key generation, threshold-multisignatures, self-certified public keying and self-certificates to yield a secure, trustworthy key management service with a high availability feature. The proposed scheme is operated solely by the end-users and does not require any offline trusted third party or a priori sharing of keying material. This addresses the major problem in providing key management services for mobile ad hoc networks, without compromising security.
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Design and implementation of a fractional-N frequency synthesizer for cellular systems by P J Venter and S Sinha
Abstract: Frequency synthesis presents one of the most challenging subsystems in a monolithic transceiver implementation. This paper presents a PLL (phase-locked loop) frequency synthesizer based on fractional-N frequency synthesis techniques, employing direct charge injection for fractional spurious tone reduction and an integrated VCO (voltage-controlled oscillator) with an active inductor resonant tank. A 0.35 µm CMOS process is used for implementation.
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