Thesis (MScEng)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: This thesis discusses the analysis and design of a voltage regulator based on an AC-to-AC converter. A background study was performed on the best topology for the purpose. The chosen topology was analysed and the converter was designed in detail. A voltage sign-detector and an over-current detector were designed and built. They were used for control and protection. Three methods of control were investigated. The first was a slow but reliable method of computing the RMS value of the input voltage and then using that value and the RMS value of the desired output voltage to compute the duty ratio of the converter. The second method was fast and is an open-loop control method, where the measured input voltage and a reference value of the desired output voltage are used to compute the duty ratio. The third method is a closed-loop control method in which the input voltage, output voltage and the same reference values used in the second method are used to compute the duty ratio. All of these methods were implemented and tested. / AFRIKAANSE OPSOMMING: Hierdie tesis bespreek die analise en die ontwerp van 'n spannings reguleerder wat gebaseer is op 'n WS-na-WS omsetter. 'n Ondersoek was ingestelom die beste topologie te vind vir die doel. Die topologie wat gekies is, is toe geanaliseer en die omsetter is in detailontwerp. 'n Spannings-tekendetektor baan as ook 'n oorstroombeskermings baan was ontwerp en is gebou. Hierdie bane word gebruik vir die beheer en die beveiliging van die stelsel. Daar is drie metodes van beheer wat ondersoek is. Die eerste metode is stadig, maar betroubaar. Die metode bereken die WGK waarde van die intree spanning en gebruik dan die waarde en die WGK van die gewenste uittree spanning om die diens siklus van die omsetter uit te werk. Die tweede metode van beheer is vinnig en is 'n oop-lus metode van beheer. Hierdie metode maak gebruik van die gemete intreespanning en 'n verwysing van die gewensde uittree spanning om die dienssiklus uit te werk. Die derde metode is 'n geslote-lus beheer wat van die gamete intreespanning, die gemete uittreespanning en die verwysing soos die in die tweede beheermetode gebruik maak on die diens siklus uit te werk. AI die metodes was geimplementeer en getoets.

Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: As control problems become more and more complex, techniques are required that surpass the capabilities of simple controllers that are linearized about certain parametric set points. Controllers that can operate over a large range of model parameter variations and even controllers that are largely model-independent are becoming more valuable and necessary. In this control application, voltage regulation is done on a direct AC-AC medium voltage regulator, making use of a type of regulated autotransformer configuration. The fifth order system is shown to be prone to oscillations on the input bus. This, together with the control requirement of robustness to load variations, provides a challenging control problem that is rarely addressed in literature. This thesis solves the control problem by means of applying sliding mode control on voltage regulator module level, and supervisory control on system level. / AFRIKAANSE OPSOMMING: Soos die soeke na oplossings vir hedendaagse beheer probleme al hoe meer uitdagend raak, word die behoefte vir model onafhanklike en robuuste beheerders dienooreenkomstig groter. Eenvoudige beheerders wat gelineariseer is om ’n parametriese werkpunt raak ondoeltreffend vir vandag se vereistes vir doeltreffende beheer ongeag van parametriese veranderinge. In hierdie tesis word spanning regulasie toegepas deur ’n direkte WS-na-WS medium spanning reguleerder in te span. Hierdie toestel maak gebruik van ’n tipe van outotransformator opstelling waar die sekondêre wikkelings gereguleer word deur die skakelaksie van die drywingselektroniese regulasie modules. Die vyfde-orde stelsel se intree bus is geneigd om onstabiel te raak, en moet dus aktief gedemp word terwyl die uitreespanning reguleer word. Die vereiste dat die beheer boonop robuus ten opsigte van las veranderings moet wees maak hierdie probleem ’n monster van ’n uitdaging wat skaars in die literatuur aangeraak is. Hierdie tesis los die probleem van robuuste beheer op deur glymodus beheer toe te pas op reguleerder module vlak, en ook deur toesighoudende beheer op stelsel vlak toe te pas.

Thesis (DTech (Electrical Engineering))--Cape Peninsula University of Technology, 2016. / Electric power systems are being operated closer to their designed stability limits due to the constraints caused by the continuous increase in system loading, and the lack of new power stations and transmission network infrastructure to support this increase in system loading. This coupled with the practice of long distance bulk power transmission and cascading contingencies, makes system instability and consequently blackouts inevitable. In such scenarios, system instabilities like voltage instability becomes a serious threat to the secure operation of the power system, and voltage collapse (system-wide blackouts) are prone to occur. This is often compounded by the unavailability of real-time system measurements for situational awareness from the existing Supervisory Control and Data Acquisition (SCADA)/Energy Management System (EMS) platforms which are usually based on unsynchronized SCADA measurements with a slow reporting rate of 1 measurement every 2-10 seconds. This Doctoral thesis proposes non-iterative algorithms and methods of solution based on the IEEE C37.118 synchrophasor measurements from Phasor Measurement Units (PMUs) with a high reporting rate of up to 200 measurements every second (200 fps) for voltage stability assessment and automated wide area Centralised Protection/Control (CPC) against catastrophic voltage instabilities/blackouts in power systems. Extended formulations are proposed for the Optimal Placement of PMUs (OPP) in power systems with respect to voltage stability assessment. The impact of zero injection buses, critical buses, and PMU redundancy is considered in the formulation of the OPP problem solution. The extended formulations made use of Binary Integer programming (BIP) and Modal Participation Factors (MPFs) derived from the eigenvalues of the power flow Jacobian.

M.Ing. (Electrical and Electronic Engineering) / Long transmission lines in power system require high line loading in order to lower voltage limits due to line losses. For relatively long lines, line charging is high and thus higher voltage limits reached at low loading. It follows then that it is a challenge to maintaining the voltages between the acceptable limits for relatively long lines. This dissertation highlights the problems experienced when load varying from very low to very high is supplied by very long parallel lines of different impedance characteristic. When the load is extremely high, there are low voltages experienced which are solved by use of shunt capacitors and/or adding more lines. When the load is extremely low, there are high voltages experienced which are solved by use of shunt reactors and/or switching some of the lines off. The type of solutions to this two loading extremes as indicated above, can be problematic, in that; new lines requires servitudes which can take too long, shunt capacitors and reactors in this type of the network is not desirable since the introduction of too many of these devices have maintenance implications and they would require continuous switching to maintain acceptable voltages, resulting in complicated operation of the network. This research proposes the use of a phase shifting transformer located on one of two parallel corridors supplying power to a load located remotely from the rest of the system. The transformer is able to rearrange the active power flows to vary loadings of the corridors and the improvements in voltage regulation can be realised during both low and high load conditions.

Indiana University-Purdue University Indianapolis (IUPUI) / The inner structural characteristics of non-minimum phase DC-DC converters pose a severe limitation in direct regulation of voltage when addressed from a control perspective. This constraint is reflected by the presence of right half plane zeros or the unstable zero dynamics of the output voltage of these converters. The existing controllers make use of one-to-one correspondence between the voltage and current equilibriums of the non-minimum phase converters and exploit the property that when the average output of these converters is the inductor current, the system dynamics are stable and hence they indirectly regulate the voltage. As a result, the system performance is susceptible to circuit parameter and load variation and require additional controllers, which in turn increase the system complexity. In this thesis, a novel approach to this problem is proposed for second order non-minimum phase converters such as Boost and Buck-Boost Converter. Different solutions have been suggested to the problem based on whether the converter is modeled as a linear system or as a nonlinear system. For the converter modeled as a linear system, the non-minimum phase part of the system is decoupled and its transfer function is converted to minimum phase using a parallel compensator. Then the control action is achieved by using a simple proportional gain controller. This method accelerates the transient response of the converter, reduces the initial undershoot in the response, and considerably reduces the oscillations in the transient response. Simulation results demonstrate the effectiveness of the proposed approach. When the converter is modeled as a bilinear system, it preserves the stabilizing nonlinearities of the system. Hence, a more effective control approach is adopted by using Passivity properties. In this approach, the non-minimum phase converter system is viewed from an energy-based perspective and the property of passivity is used to achieve stable zero dynamics of the output voltage. A system is passive if its rate of energy storage is less than the supply rate i.e. the system dissipates more energy than stores. As a result, the energy storage function of the system is less than the supply rate function. Non-minimum phase systems are not passive, and passivation of non-minimum phase power converters is an attractive solution to the posed problem. Stability of non-minimum phase systems can also be investigated by defining the passivity indices. This research approaches the problem by characterizing the degree of passivity i.e. the amount of damping in the system, from passivity indices. Thus, the problem is viewed from a system level rather than from a circuit level description. This method uses feed-forward passivation to compensate for the shortage of passivity in the non-minimum phase converter and makes use of a parallel interconnection to the open-loop system to attain exponentially stable zero dynamics of the output voltage. Detailed analytical analysis regarding the control structure and passivation process is performed on a buck-boost converter. Simulation and experimental results carried out on the test bed validate the effectiveness of the proposed method.

This thesis investigates the use of real-time phasor measurements for voltage and transient stability monitoring and control. Taking advantage of the ability of a Phasor Measurement Unit (PMU) placed at a bus to sample at a fast rate the voltage and current phasors of that bus) various schemes for placing PMU's are considered and evaluated. These schemes include coherency based methods and pilot point placement techniques for system controllability. A novel scheme is proposed which places a minimal set of PMU's so as to make the system measurement model observable, and thereby linear. This placement scheme is derived from the topological observability theory. It concerns the building of a spanning measurement sub-graph across the system with actual or pseudo-measurement assigned to each of its branches. The minimal PMU set is found through a dual search algorithm which uses both a modified bisecting search and a simulated annealing-based method. The former fixes the number of PMU's while the latter look for a placement set that leads to an observable network. In order to accelerate the procedure, an initial PMU placement is provided by a graph-theoretic procedure which builds a spanning measurement sub-graph according to a depth-first search. From computer simulation results performed on various test systems, it appears that only one fourth of the system buses need to be provided with PMU's in order to make the system observable. In an effort to reduce the computing time of transient stability assessment, a dynamic equivalent is presented, which results from the elimination of the load buses provided with voltage--dependent loads. The elimination is performed through a new version of the Ward equivalencing method. In this approach, the equivalent current injections are expressed in terms of the retained bus angles and a sensitivity matrix. The non-linearity of the load flow model is accounted for through piecewise linear approximations by updating the sensitivity matrix whenever the operating point moves beyond the validity of the linearization. The expressions of the incremental changes in the generator electric power is derived. The study also investigates the possibility of using the telemetered data provided by the PMU's during pre-fault and post-fault operating conditions in conjunction with a. new version of the Extended Equal Area Criterion (EEAC) method. The latter is able to handle complex loads through the dynamic Ward equivalencing method. The performance of the approach is illustrated on three test systems which have been reduced to the internal generator nodes. / Ph. D.

The work presented in this text concentrates on three aspects of voltage stability studies: analysis and determination of suitable proximity indicators, design of an effective real-time monitoring system, and determination of appropriate emergency control techniques. A simulation model of voltage collapse was built as analytical tool on 39-bus, 10-generator power system model. Voltage collapse was modeled as a saddle-node bifurcation of the system dynamic model reached by increasing the system loading. Suitable indicators for real-time monitoring were found to be the minimum singular value of power flow Jacobian matrix and generated reactive powers. A study of possibilities for reducing the number of measurements of voltage phasors needed for voltage stability monitoring was also made. The idea of load bus coherency with respect to voltage dynamics was introduced. An algorithm was presented which determines the coherent clusters of load buses in a power system based on an arbitrary criterion function, and the analysis completed with two proposed coherency criteria. Very good agreement was obtained by simulation between the results based on accurate and approximate measurements of the state vector. An algorithm was presented for identification of critical sets of loads in a voltage unstable power system, defined as a subset of loads whose changes have the most pronounced effect on the changes of minimum singular value of load flow Jacobian or generated reactive powers. Effects of load shedding of critical loads were investigated by simulation and favorable results obtained. An investigation was also done by sensitivity analysis of proximity indicators of the effects that locations and amounts of static var compensation have on the stability margin of the system. Static compensation was found to be of limited help when voltage instabilities due to heavy system loading occur in power systems. The feasibility of implementation of the analyses and algorithms presented in this text relies on development of a feasible integrated monitoring and control hardware. The phasor measurement system which was designed at Virginia Polytechnic institute and State University represents an excellent candidate for implementation of real-time monitoring and control procedures. / Ph. D.

Advances in transistor-integration technology and multi-core technology of the latest microprocessors have driven transient requirements to become more and more stringent. Rather than relying on the bulky output capacitors as energy-storage devices, increasing the control bandwidth (BW) of the multiphase voltage regulator (VR) is a more cost-effective and space-saving approach. However, it is found that the stability margin of current-mode control in high-BW design is very sensitive to operating conditions and component tolerance, depending on the performance of the current-sensing techniques, modulation schemes, and interleaving approaches. The primary objective of this dissertation is to investigate an advanced multiphase current-mode control, which provides accurate current sensing, enhances the stability margin in high-BW design, and adaptively compensates the parameter variations. Firstly, an equivalent circuit model for generic current-mode controls using DCR current sensing is developed to analyze the impact of component tolerance in high-BW design. Then, the existing state-of-the-art auto-tuning method used to improve current-sensing accuracy is reviewed, and the deficiency of using this method in a multiphase VR is identified. After that, enlightened by the proposed model, a novel auto-tuning method is proposed. This novel method features better tuning performance, noise-insensitivity, and simpler implementation than the state-of-the-art method. Secondly, the current state-of-the-art adaptive current-mode control based on constant-frequency PWM is reviewed, and its inability to maintain adequate stability margin in high-BW design is recognized. Therefore, a new external ramp compensation technique is proposed to keep the stability margin insensitive to the operating conditions and component tolerance, so the proposed high-BW constant-frequency control can meet the transient requirement without the presence of bulky output capacitors. The control scheme is generic and can be used in various kinds of constant-frequency controls, such as peak-current-mode, valley-current-mode, and average-current-mode configurations. Thirdly, an interleaving technique incorporating an adaptive PLL loop is presented, which enables the variable-frequency control to push the BW higher than proposed constant-frequency control, and avoids the beat-frequency input ripple. A generic small-signal model of the PLL loop is derived to investigate the stability issue caused by the parameter variations. Then, based on the proposed model, a simple adaptive control is developed to allow the BW of the PLL loop to be anchored at the highest phase margin. The adaptive PLL structure is applicable to different types of variable-frequency control, including constant on-time control and ramp pulse modulation. Fourthly, a hybrid interleaving structure is explored to simplify the implementation of the adaptive PLL structure in an application with more phases. It combines the adaptive PLL loop with a pulse-distribution technique to take the advantage of the high-BW design and fast transient response without adding a burden to the controller implementation. As a conclusion, based on the proposed analytical models, effective control concepts, systematic optimization strategies, viable implementations are fully investigated for high-BW current-mode control using different modulation techniques. Moreover, all the modeling results and the system performance are verified through simulation with a practical output filter model and an advanced mixed-signal experimental platform based on the latest MHz VR design on the laptop motherboard. In consequence, the multiphase VRs in future computation systems can be scalable easier with proposed multiphase configurations, increase the system reliability with proposed adaptive loop compensation, and minimize the total system footprint of the VR with the superior transient performance. / Ph. D.

MScEng / Thesis (MScEng (Electrical and Electronic Engineering))--Univerity of Stellenbosch, 2009. / ENGLISH ABSTRACT: As the number of distributed generation units connected to a distribution network increase, the possibility of island formation increases. An island is formed when distributed generation units continue to energize local loads within a section of the grid, which has been disconnected from the main distribution network. These islands pose significant danger to maintenance personnel as well as to members of the public. In this study, an investigation is done into various anti-islanding methods. The modes of operation of these methods are discussed, as well as their strengths and weaknesses. The slip-mode frequency shift method and the Sandia voltage shift method, in combination with over/under voltage and frequency protection, are simulated and tested to confirm their functionality. The results obtained show that it is possible to prevent distributed generation units from energizing local loads when the grid is disconnected. / AFRIKAANSE OPSOMMING: Die moontlike toekomstige toename in die aantal verspreide generasie eenhede gekoppel aan die verspreidings netwerk, verhoog die moontlikheid van eiland vorming. ‘n Eiland word gevorm wanneer verspreide generasie eenhede energie aan lokale laste voorsien nadat die netwerk ontkoppel is. Dit hou groot gevaar in vir onderhouds personeel asook vir die publiek. In die tesis word ‘n studie gedoen oor die verskillende metodes om die vorming van ongewensde eilande te voorkom. Die glipmode-frekwensieskuif metode en die Sandia spanningskuif metode word gekombineer met die oor/onder spanning en frekwensie beskerming metodes. Die kombinasie van metodes word dan gesimuleer en eksperimenteel getoets. Die verkrygde resultate toon dat dit moontlik is om die vorming van ongewensde eilande effektief te voorkom.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The use of power electronic converters provides a more efficient, accurate and dynamic solution to reactive compensation. In this thesis the application of power electronic converters to shunt reactive compensation will be discussed. In particular voltage dips and voltage unbalance are considered as both can be mitigated by means of shunt reactive compensation. A pre-existing uninterruptible power supply is adapted to operate as a shunt reactive compensator. The uninterruptible power supply consists of a 250 kVA three phase voltage source inverter. The modifications are limited to software and control algorithms that do not alter the normal operation of the uninterruptible power supply. Control algorithms are designed and discussed in detail. A typical double loop control strategy is implemented on the power electronic converter. The inner loop consists of a dead-beat current controller. The outer loop consists of three proportional and integral controllers controlling the DC-bus voltage, AC voltage and voltage unbalance respectively. Voltage dips and unbalance are compensated for using only reactive power. Focus is placed on producing a result can be used easily in practice. / AFRIKAANSE OPSOMMING: Drywings elektroniese omsetters wat gebruik word vir newe reaktiewe kompensasie lewer meer effektiewe, akkurate en dinamiese resultate. In hierdie tesis word die toepassing van drywings elektroniese omsetters vir newe reaktiewe kompensasie bespreek. Daar word meer spesifiek na spannings duike en spannings wanbalans gekyk aangesien albei met newe reaktiewe kompensasie verminder kan word. ’n Bestaande nood kragbron is aangepas om as n newe reaktiewe kompenseerder te funksioneer. Die nood kragbron bestaan hoofsaaklik uit ’n 250 kVA drie fase omsetter spanningsbron. Die aanpassings is beperk tot sagteware en beheer algoritmes wat nie die oorspronklike funksionaliteit van die nood krag bron beinvloed nie. Beheer algoritmes word ontwerp en deeglik bespreek. ’n Tipiese dubbel lus beheer strategie word op die drywings elektroniese omsetter toegepas. Die binnelus bestaan uit ’n voorspellende stroom beheerder. Die buite-lus bestaan uit drie proportioneel en integraal beheerders wat onderskeidelik die GS-bus spanning, WS spanning en spanning wanbalans reguleer. Spannings duike en wanbalans is verminder deur slegs reaktiewe drywing te gebruik. Die doel was ook om ’n prakties bruikbare resultaat te lewer.