In industry, adjustable speed drives (ASDs) are widely employed in driving AC motors for variable speed applications due to the high performance and high energy efficiency obtained in such systems. However, ASDs have an impact on the power quality and utilisation of AC power feeds by injecting current harmonics and causing resonances, additional losses, and voltage distortion at the point of common coupling. Due to these problems, electric power utilities have established stringent rules and regulations to limit the effects of this distortion. As a result, efficient, reliable, and economical harmonic mitigation techniques must now be implemented in practical systems to achieve compliance at reasonable cost. A variety of techniques exist to control the harmonic current injected by ASDs, and allow three-phase AC-line-connected medium-power systems to meet stringent power quality standards. Of these, the broadband harmonic passive filter deserves special attention because of its good harmonic mitigation and reactive power compensation abilities, and low cost. It is also relatively free from harmonic resonance problems, has relatively simple structural complexity and involves considerably less engineering effort when compared to systems of single tuned shunt passive filters or active filters and active rectifier solutions. In this thesis, passive broadband harmonic filters are investigated. In particular, the improved broadband filter (IBF) which has superior overall performance and examples of its application are increasing rapidly. During this research project, the IBF operating principle is reviewed and its design principles are established. As the main disadvantage of most passive harmonic filters is the large-sized components, the first proposed design attempts to optimize the size of the filter components (L and C) utilized in the existing IBF topology. The second proposed design attempts to optimize the number and then the size of filter components resulting in an Advanced Broadband passive Filter (ABF) novel structure. The proposed design methods are based on frequency domain modelling of the system and then using a genetic algorithm optimization technique to search for optimal filter component values. The results obtained are compared with the results of a linear searching approach. The measured performance of the optimal filter designs (IBF and ABF) is evaluated under different loading conditions with typical levels of background voltage distortion. This involves assessing input current total harmonic distortion, input power factor, rectifier voltage regulation, efficiency, size and cost. The potential resonance problem is addressed and the influence of voltage imbalance on performance is investigated. The assessment is based on analysis, computer simulations and experimental results. The measured performance is compared to various typical passive harmonic filters for three-phase diode rectifier front-end type adjustable speed drives. Finally, the broadband filter design’s effectiveness and performance are evaluated by involving them in a standard IEEE distribution network operating under different penetration levels of connected nonlinear total loads (ASD system). The study is conducted via detailed modelling of the distribution network and the linked nonlinear loads using computer simulations.

In recent years, the wireless industry has witnessed tremendous development for the defense and commercial segments. The explosive growth in the modern radio frequency and microwave systems leads to an increased interest in the research of miniaturized microwave circuits with superior performance. Broadband components, in particular band pass filters (BPFs) and couplers are some of the widely used components in the modern communication systems, software defined radios, cognitive radios, imaging systems and positioning radars. In order to meet these requirements, the use of innovative geometries, a thorough understanding of their behavior by appropriate analytical techniques and the use of appropriate fabrication approaches are essential. This thesis is an effort in this direction. In this thesis work, an aperture in the ground plane is used to achieve the tight coupling in the edge and broadside-coupled coupled lines which may be otherwise difficult due to the fabrication limitations. Since microstrip lines with an aperture in the ground plane are found to be very useful in various MIC and MMIC components, closed form analytical expressions developed here will be useful for their initial synthesis. The performances of components using these are enhanced using open/short circuited resonators, spurlines and stubs. A quasi-static approach has been investigated to obtain simple closed form expressions for a microstrip line with a rectangular aperture in the ground plane. The effect of a rectangular aperture in the ground plane has been incorporated in the commonly used expressions of a regular microstrip line by introducing the concept of an equivalent effective height. The expressions for the effective height microstrip geometries with defected ground are obtained and this has been further extended to various possibilities of two-conductor and three-conductor coupled lines. Analysis of the filters and the couplers are mainly based on the even and odd mode propagation characteristics of coupled lines. This approach is extended in this thesis for three conductor coupled lines. Novel broadband BPFs and couplers and dual band pass filters employing various coupled line configurations and defected ground have been developed in this research work. Most of these components have been implemented on a regular microwave laminate or LTCC medium (planar or multi-layer) and tested for the required RF performances. The experimental results were compared against the analytically computed results based on the circuit models and the full wave simulations using electromagnetic (EM) simulations for the validation. The results are in good agreement. With practical requirements of the organization in mind, additional design elements such as open circuit stubs have been incorporated in some of these designs to achieve the desired performance. It is expected that the wideband filter (3.0GHz to 3.8GHz) and the broadband coupler (4GHz to 6GHz) developed in this thesis work would be deployed in systems developed at the Central Research Laboratory, Bharat Electronics Limited, Bangalore, India. In summary, the present doctoral work strives to (i) establish a simplified analysis method for the microstrip lines and coupled microstrip lines with a rectangular aperture in the ground plane, (ii) extend the even and odd mode analysis of the coupled lines for several new coupled line configurations, (iii) design novel broadband microwave filters, dual band filter and couplers using both these, (iv) fabricate these devices using the planar technologies including LTCC, and (v) validate the analysis and design with important practical applications.

<p>In many practical multi-channel active sound control (ASC) applications, such as active noise control (ANC), various constraints need to be satisfied, such as the robust stability constraint, noise amplification constraint, controller output power constraints, etc. One way to enforce these constraints is to add a regularization term to the Wiener filter formulation, which, by tuning only a single parameter, can over-satisfy many constraints and degrade the ANC performance. Another approach for non-adaptive ANC filter design that can produce better ANC performance is to directly solve the constrained optimization problem formulated based on the <em>H</em>₂/<em>H</em>_inf control framework. However, such a formulation does not result in a convex optimization problem and its practicality can be limited by the significant computation time required in the solving process. In this dissertation, the traditional <em>H</em>₂/<em>H</em>_inf formulation is convexified and a global minimum is guaranteed. It is then further reformulated into a cone programming formulation and simplified by exploiting the problem structure in its dual form to obtain a more numerically efficient and stable formulation. A warmstarting strategy is also proposed to further reduce the required iterations. Results show that, compared with the traditional methods, the proposed method is more reliable and the computation time can be reduced from the order of days to seconds. When the acoustic feedback path is not strong enough to cause instability, then only constraints that prevent noise amplification outside the desired noise control band are needed. A singular vector filtering method is proposed to maintain satisfactory noise control performance in the desired noise reduction bands while mitigating noise amplification.</p> <p><br></p> <p>The proposed convex conic formulation can be used for a wide range of ASC applications. For example, the improvement in numerical efficiency and stability makes it possible to apply the proposed method to adaptive ANC filter design. Results also show that compared with the conventional constrained adaptive ANC method (leaky FxLMS), the proposed method can achieve a faster convergence rate and better steady-state noise control performance. The proposed conic method can also be used to design the room equalization filter for sound field reproduction and the hear-through filter design for earphones.</p> <p><br></p> <p>Besides efficient filter design methods, efficient filter implementation methods are also developed to reduce real-time computations in implementing designed control filters. A polyphase-structure-based filter design and implementation method is developed for ANC systems that can reduce the computation load for high sampling rate real-time filter implementation but does not introduce an additional time delay. Results show that, compared with various traditional low sampling rate implementations, the proposed method can significantly improve the noise control performance. Compared with the non-polyphase high-sampling rate method, the real-time computations that increase with the sampling rate are improved from quadratically to linearly. Another efficient filter implementation method is to use the infinite impulse response (IIR) filter structure instead of the finite impulse response (FIR) filter structure. A stable IIR filter design approach that does not need the computation and relocation of poles is improved to be applicable in the ANC applications. The result demonstrated that the proposed method can achieve better fitting accuracy and noise control performance in high-order applications.</p>